84R-

13

PLANNI
NGANDACCOUNTI
NGFOR
ADVERSEWEATHER
 AACE® International Recommended Practice No. 84R-13

PLANNING AND ACCOUNTING FOR ADVERSE WEATHER

TCM Framework: 7.2 – Schedule Planning and Development
8.1 – Project Control Plan Implementation

Rev. September 15, 2015

Note: As AACE International Recommended Practices evolve over time, please refer to web.aacei.org for the latest
revisions.

Any terms found in AACE Recommended Practice 10S-90, Cost Engineering Terminology, supersede terms defined in
other AACE work products, including but not limited to, other recommended practices, the Total Cost Management
Framework, and Skills & Knowledge of Cost Engineering.

Contributors:
Disclaimer: The content provided by the contributors to this recommended practice is their own and does not necessarily
reflect that of their employers, unless otherwise stated.

Delbert E. Bearden, PSP (Primary Contributor) Hilal A. Itani, PSP

Christopher W. Carson, CEP DRMP PSP Patrick M. Kelly, PE PSP
(Primary Contributor) Vera Lovejoy, CCP PSP
Edward E. Douglas, III CCP PSP Donald F. McDonald, Jr. PE CCP PSP
Patrick B. Egger Jose Noe
Fouad H. Elfaour, PSP Zartab Z. Quraishi, PE CCP PSP
Lisa A. Enloe Michael E. Stone
Marc S. Glasser, PSP Dr. Stephen P. Warhoe, PE CCP CFCC
Gokulan Govender, PSP Daniel A. Weis
Dennis Read Hanks, PE CCP DRMP Ronald M. Winter, PSP
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AACE® International Recommended Practice No. 84R-13
PLANNING AND ACCOUNTING FOR ADVERSE
WEATHER
TCM Framework: 7.2 – Schedule Planning and Development
8.1 – Project Control Plan Implementation

September 15, 2015

TABLE OF CONTENTS

Table of Contents ..........................................................................................................................................................1

Introduction ...................................................................................................................................................................1
Overview ........................................................................................................................................................................2
Background ....................................................................................................................................................................3
Identification of Sources for Historical Data ..............................................................................................................3
Interpretation of Historical Data ...............................................................................................................................6
Non-Work Averages Method .....................................................................................................................................6
Statistical Analysis Method ........................................................................................................................................7
Blended Approach – Statistical Analysis Provided by USACE ....................................................................................8
Methodology for Planning for Predicted Adverse Weather ......................................................................................8
Description of Industry Practices ...............................................................................................................................8
Weather Impact Days: Before and After a Weather Event .....................................................................................14
Recommended Practice ...............................................................................................................................................14
Identification of Sources for Historical Weather Data .............................................................................................14
Interpretation of Historical Weather Data ..............................................................................................................15
Methodology for Planning for Predicted Adverse Weather ....................................................................................15
References ...................................................................................................................................................................15
Contributors.................................................................................................................................................................15

INTRODUCTION

Planning for adverse weather and incorporating that into the project schedule is an essential practice for
successful projects. This recommended practice (RP) for adverse weather planning is intended to provide a
guideline, not to establish a standard. It provides planning guidelines developed primarily for engineering and
capital construction projects but can be adapted to any type of project or program where weather can influence its
outcome.

This RP is intended to:

• Identify methodologies for adverse weather planning.
• Provide sources for historical data and provide recommendations for their use.
• Explain and critique weather planning methods.
• Recommend approaches for managing actual weather documentation compared to what was planned.

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Positive and negative aspects for each weather planning method should be considered in the context of specific
projects elements, including contractual requirements. This RP does not address legal ramifications or forensic
schedule delay analysis considerations that may need to be addressed when adverse weather delay is in
contention.

This RP serves to provide guidance in the absence of contractual requirements as well as potential guidance in
writing specifications for adverse weather planning. If adverse weather planning is addressed in the contract,
planning and modeling should be in compliance.

OVERVIEW

Weather is defined as the “state of the atmosphere with respect to heat or cold, wetness or dryness, calm or
storm, clearness or cloudiness”[4]. Weather conditions fluctuate constantly and may not always be considered
severe.

The following weather-related terms and definitions are used throughout this RP:

NORMAL WEATHER —Weather that is expected for a period of time based upon the historical weather conditions
of a particular locale. Normal weather includes weather that is and is not adverse as would be expected for a
particular location.

ADVERSE WEATHER — Normal weather events that negatively affect the productivity of workers and/or which may
affect a project’s critical path or consume float. What deems weather as being adverse is defined differently for
each project depending on several factors, including location of a project, the project scope, and even the terms of
union construction worker labor agreements. The definition of adverse weather cannot be standardized for all
projects or locations, but represents conditions that should be expected during project execution that may impact
work progress.

SEVERE WEATHER — A weather event, which is in itself severe and can be of violent nature. If the average weather
over time is significantly different from the normal then it is said to be other than normal. In either case, if such
weather affects the job and causes a delay, it may be excusable and form the basis for a contract adjustment for
time and possibly money once all relevant contract clauses are considered.

WEATHER EVENT — A storm or significant weather condition that stops or appreciably hinders work until it has
passed or the effects of the weather condition have dissipated. This may include rain, rising water, snow, ice,
extreme cold, high winds, extreme heat and/or high humidity, or other weather related occurrence. The weather
event may not be localized at the site as in the case of flood water from an upstream rain event or preparations for
a hurricane that does not actually pass through the site.

WEATHER DAY — A work day that was stopped and/or had appreciably hindered work progress due to a weather
event.

PLANNED ADVERSE WEATHER DAY — Expressed as the number of days within a period of time (typically specified
month) that a project can be expected to be affected by adverse weather. The number of planned adverse
weather days is calculated by a review of historical weather data obtained from a reliable weather source
supplemented or validated by actual experience at or near the work site.

UNUSUALLY ADVERSE WEATHER DAY — Expressed as the number of days of actual adverse weather that stopped
or impacted a project above what was originally planned prior to the start of the project. From a contractual delay
analysis standpoint, those days are compared to the number of days originally planned for in the schedule to

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identify excusable time extensions; i.e.: weather that negatively affects the project production in excess of the
expected normal or adverse weather.

WEATHER PREPARATION DAY — A day where planned work progress ceases going forward or is significantly
impeded while the project makes preparation for an upcoming weather event, e.g. boarding up windows before a
hurricane, making the site safe by securing loose materials and unfinished structures and dismantling or securing
cranes from expected high winds to mitigate any potential damage to the work.

WEATHER RECOVERY DAY — A day where a project is unable to resume work due to the after-effects of a weather
event such as excessive flooding and mud after a rain storm. This would also include the time necessary to
duplicate the status just prior to the weather event.

Planning for adverse weather consists of:

• Identifying sources for historical weather data applicable to the project site.
• Interpretation of that historical data and rendering it useful for planning and schedule modeling.
• Examining various planning approaches for adverse weather conditions to determine the most
appropriate method that models the expected periods and durations of weather events in order to
produce reasonable and appropriate dates in the schedule.
• Accounting for actual adverse weather to provide an accurate as-built project schedule as well as the
basis for consideration of possible time extensions due to unusually adverse weather.

The purpose of weather planning is to establish a consistent approach to ensure that the as-planned schedule
appropriately addresses adverse weather days during project execution. The planned adverse weather days may
be based on historical norms, relevant location experience or as may be dictated by a contract condition.

The planner or scheduler provides assistance to the project management team in choosing the appropriate
methodology for implementing adverse weather planning. He or she provides input so that the as-planned
schedule includes appropriate time allowances for adverse weather to ensure reasonableness, and provides an
approach to account for actual adverse weather, allowing for analysis of unusually adverse weather and any
entitlement to time extensions due to those unusually adverse weather conditions.

BACKGROUND

This recommended practice is intended to provide guidance for any project that has scope of work that is subject
to delay or disruption due to adverse weather conditions that is not already provided for in the project’s contract
documents.

Identification of Sources for Historical Data

Historical data includes sources of objective, factual information recording actual weather conditions, generally at
least daily, in a widespread range of locations, from professional weather bureaus.

Historical data sources are used to plan for future weather in a schedule under the assumption that the average
weather in the past will represent a reasonable model of similar future weather conditions. It is important that
legitimate sources of actual weather conditions are used for the basis of planning so the plan will be as realistic as
possible.

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Further, it is important to document in the schedule basis documentation the data used, the process used to
gather the data, any analysis performed, weather calendars used and assumptions made about the inclusion or
exclusion of weather days in the schedule.

From a dispute resolution standpoint, the choice of data sources is important as it may be questioned in the event
of a dispute. Use of a less accurate source or questionable data acquisition process could undermine the legitimacy
of a time extension request. For that reason, it is important to document the basis and assumptions made during
weather planning in the schedule basis documentation[3].

Historical adverse weather data sources would be used when a project has activities that will be performed that
can be influenced by weather conditions. This includes all forms of precipitation including rain, snow, hail, sleet, as
well as any other weather influences such as high or low humidity, high winds, high or low temperatures,
shortened daylight times, and even airborne particle conditions such as dust and pollution.

Secondary effects from the primary adverse weather conditions could include a variety of issues such as rising and
flooding waters, limited visibility, the effects of changing tides, mud conditions limiting accessibility or halting
earthwork operations, mold or mildew conditions requiring abatement, snow and ice removal needs, masonry,
concrete, or roofing work stoppages due to low temperatures, and reduced crane operating hours due to high
winds. These secondary effects can create conditions that cause the same lost time as primary conditions and, as
such, should be included as part of the adverse weather planning.

Adverse weather is variable based on locations, time of year, and other local conditions (natural and man-made).
Since weather is locality driven, the adverse weather planning data should be captured from a source that
experienced weather conditions similar to that expected at the project’s location.

Within the project’s critical path method (CPM) model, those activities that are weather dependent or related
should be identified such that they can be filtered out separately from the non-weather related activities. This will
allow weather planning to be applied only to the activities that are likely to suffer from adverse weather. Extensive
weather planning is not normally used for projects that are unlikely to be affected by adverse weather, such as
indoor renovation projects, or projects in stable climate regions. Care should be taken in these cases to ensure that
no secondary weather conditions might affect the work, such as humidity that could affect indoor finish work. It is
important to note in the schedule basis document any assumptions or exclusions concerning planning for adverse
weather.

While the weather data sources noted within this RP are taken from examples within the United States, it is
important to note that there are many sources around the world from both government and private organizations
that provide historical weather data.

The most common and readily available source for historical weather conditions in the United States is the
National Oceanic and Atmospheric Administration (NOAA), an agency within the US Government’s Department of
Commerce. NOAA operates the National Weather Service (NWS) that produces forecasts and warnings, as well as
radar tracking of storms for regions across the United States. NWS produces contemporaneous precipitation
analysis, including archives of records, based on readings taken at the NWS offices across the country. The NWS
also publishes the National Weather Service climate pages that provides local weather information.

NOAA also operates the National Climatological Data Center, which is the world’s largest archive of climate data
and provides both free and subscription-based services.

The internet is a rich source of weather data. Countries, universities and private organizations are constantly
collecting, analyzing and publishing global weather data. Be sure to validate any source used. Representative sites
include:

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• www.worldclimate.com
• www.weather.org
• www.bom.gov.au
• www.climate-charts.com
• www.wunderground.com

There are private organizations that record and provide weather data such as Weatherbase
(www.weatherbase.com) with its database of information for over 15,000 cities, the Washington Post
(www.washingtonpost.com/weather) providing information for 2,000 cities, and World Climate
(www.worldclimate.com) offering worldwide climate data in a database of over 80,000 records.

The US Army Corps of Engineers (USACE) provides a summary of non-work days per month due to predicted
adverse weather for various regions around the country based on USACE project experience combined with their
analysis of historical weather data. The USACE uses and analyzes NOAA data, and summarizes it into a reasonable
expected loss of time due to adverse weather.

Other sources of local historical weather data include the United States State Departments of Transportation
(DOT), providing adverse weather days by month; local Associated General Contractors (AGC) offices, providing a
count of rain days; airport records providing wind and rain data; and state meteorologists who may be affiliated
with universities that collect and analyze data on an ongoing research basis.

In the United States, only NOAA can provide records certified by the US Government such as might be required as
supporting documentation when providing analysis related to delays or disruption due to adverse weather. (In
Canada, the Environment Canada climate service provides similar records.) Any well-established weather station
with an uninterrupted appropriate history can be used to establish a history in a particular location in order to
calculate normal weather averages and establishing a planned adverse weather calendar that will assist in project
planning. There may be instances in which non-NOAA weather stations are closest in proximity to the project
location.

The data compiled from NOAA can be voluminous and hard to read and interpret. The data should be evaluated
and understood in advance in order to effectively use the information for planning. For any weather data source, a
decision will need to be made as to what constitutes a historical non-work weather day due to adverse weather.

Further, if the data is to be used in a claim or litigation, it may be necessary that a project-level statistical analysis
be supplemented by a professional statistical analysis that can survive legal scrutiny.

Weather records supply the amount of precipitation but they do not indicate if it was possible to work during that
time. For example, precipitation of more than 1/10 of an inch may be considered significant enough to halt
activities that are under the influence of that weather. When precipitation accumulates in the range of one inch, it
may create wet conditions such that work on the day(s) following the weather event will be impacted. This is
sometimes called a weather recovery day (or a mud day), which indicates the precipitation has halted but the site
is still affected by the wet conditions.

Another consideration is a weather preparation day, where progress is stopped or significantly hindered in order
to prepare for a forecasted weather event such as a hurricane. These conditions and limitations should be taken
into account when evaluating the historical data for use in planning for adverse weather.

All weather events are dependent upon location; 1/10 of an inch of rain in Los Angeles will affect work differently
than the same amount of rain in Portland, Oregon. Similarly, extreme cold and wind speeds will be treated
differently in Chicago than in Atlanta. The definition of a weather event needs to be decided early and should
consider only regional weather and how it affects local conditions and projects.

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A decision should be made to determine the period of time that is considered statistically significant when
selecting historical weather data. Generally, an average of five to ten years of data will provide a sound basis for
prediction of future adverse weather. Other credible and verifiable procedures may be used to select high
probability future non-work days due to adverse weather.

Interpretation of Historical Data

Interpretation of historical data involves taking the available statistical data and rendering it useful and relevant in
planning for adverse weather in the as-planned schedule.

The purpose of interpreting historical weather records is to provide legitimate and relevant anticipated adverse
weather data for the planned project duration. This data may be used to make appropriate allowances for the
work periods during which the project should expect to be affected by adverse weather. Such analysis translates
historical records into reasonable, usable, and predictive data to be integrated into the as-planned schedule.

Interpretation of historical weather records is only used if the contract does not define either an average expected
non-work time for adverse weather on a periodic basis to be used in the as-planned schedule or some other
weather planning basis.

There are two basic ways to interpret the historical weather records; by use of simple non-work averages and by
use of statistical analysis. Both approaches require use of historical weather records for the same location as the
project being planned.

Non-Work Averages Method

Historical weather data may supply an average number of work days lost to adverse weather in the period being
analyzed including the average daily precipitation, the average wind speeds at different times of day, humidity,
temperature, visibility, and other adverse conditions. The planner/analyst should provide parameters for each of
these conditions that may result in adverse weather in the region where work is to occur; e.g. weather that affects
production of activities that are either critical or near-critical. Examples of parameter values that may qualify as
adverse weather conditions are:
• Precipitation
• More than ½” accumulation of water equivalent precipitation that affects exterior work/weather
sensitive activities.
• Mud conditions on an exposed site can cause increased non-work time that extends
beyond the precipitation exposure. Judgment should be used in determining what is
reasonable to expect; for example, on a stripped earthwork site with poor drainage, it is
likely that one day of more than 1” accumulation of precipitation will create muddy
conditions for several days and may require significant rework to put the project back to
where it was before the weather event. The greater the amount of precipitation,
combined with the conditions after the precipitation, the more mud/recovery days that
will follow. Working in mud conditions may also damage existing site locations or
installations.
• Wind speeds
• Winds over 40 to 45 mph are generally a cause that requires tower cranes to be shut down.
• Lower speeds may still affect tower crane use or other hoisting devices (e.g., the lifting
of lightweight materials or flat materials that may act as a “sail” in winds may be
affected or even halted until the wind subsides).

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• Work in desert or beach conditions may be affected by sand and dust storms.
• Wind speeds above 20 mph may cause disruption in the installation of roofing material, glazing,
siding, wall forms, framed walls and sheer walls.
• Humidity
• Humidity in the range of 90% or higher may make work conditions seem much hotter than the
ambient temperature. This condition will likely impact worker productivity, and can cause
moisture-sensitive materials to swell and impede efficient installations.
• Low humidity in the range of 25% or less may make it very difficult to install concrete pavements
due to high dust, and may cause moisture-sensitive materials to shrink such that severe swelling
should be anticipated once the humidity level returns to normal.
• Temperature
• Low temperatures below 40 degrees F may prevent installation of materials that include
adhesives, concrete, or asphalt without the use of mitigating work methods. Extreme low
temperatures may cause problems with productivity and workers health, safety, and morale.
• High temperatures ranging above 95 degrees F may cause problems with installation of concrete
and coatings. Extreme high temperatures may cause problems with productivity and workers
health, safety, and morale.
• Other conditions should be reviewed for their potential impact to the project.

Safety is always a primary consideration, and all of the above identified weather concerns should take into account
health and safety considerations that may stop work and/or significantly hinder progress.

Statistical Analysis Method

A statistical approach to weather analysis requires review and analysis of historical weather data to provide
meaningful predictions used for project planning purposes. This approach requires the use of NOAA or similar data
in detail, converted to a spreadsheet or database in order to perform data filtering and calculations. Once the data
is extracted into the database or spreadsheet, the historically anticipated “normal” conditions can be summarized
to a value by day by year, for instance, precipitation in inches by day by year.

Once the anticipated weather conditions are statistically organized, there is still a subjective identification of what
is significant in adverse weather for the type of condition of the current project. This allows for identification of
specific days and the probability of the condition occurring in planning periods. Then a threshold must be chosen
to limit the days based on the probability of the condition occurring, and that provides a chart of days of adverse
weather per month, identifying the probable days with the adverse weather condition such as precipitation.

The project team needs to define what constitutes a “weather event” for the current project. This could include
information as shown in Table 1. It is relative to the project’s location and what is considered normal adverse
weather.

Number of Days with Rain Above 0.10” (2.54 mm)

Number of Days with Wind Speeds Above 10 mph (16kph)
Number of Days with Cold Temperatures Below 0˚ Fahrenheit (-18 C)
Number of Days with Hot Temperatures Above 110˚ Fahrenheit (43 C)
Table 1 – Sample Weather Event Thresholds

With the adverse weather days defined, the next step is to compare the average available work days with the
historic average adverse weather days. Once these two data points are correlated, a non-work calendar of
predicted adverse weather can be compiled.

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Blended Approach – Statistical Analysis Provided by USACE

The third approach is a blend of the first two performed by the USACE in their publication of monthly non-work
days due to adverse weather in various office locations. If this data is available, use of the results is economical,
reasonable, and likely to stand up in a technical forensic analysis. This specific data is only available for USACE
Districts and may not be relevant outside of the United States. The USACE offers a publication in the Unified
Facilities Criteria (UFC) system called “Design Engineering Weather Data[1],” which describes historical data records
and the process used for planning construction and building operation.

The time of day for adverse weather should be reviewed because it could have a significant reduction or increase
of impact on work activities. For example, if an area typically experiences most of the adverse weather near or
after the end of the work day, this phenomenon might reduce the effect of that adverse weather on project
performance, or could simply move the effect to the next day.

The determination of the parameters to use in planning is clearly subjective, so the goal is to be as reasonable as
possible and provide rationale for the determination, especially if it may appear to be biased towards more or less
adverse weather planning. All this analysis and these determinations and assumptions must be fully documented
in the schedule basis.

Methodology for Planning for Predicted Adverse Weather

Methodology for planning for adverse weather is the strategy and implementation used to develop a schedule that
results in reasonable and historically consistent dates that take into account the adverse weather conditions that
would be expected for a project in a specific location during a specific time frame.

Planning for adverse weather helps establish reasonable schedule dates so that it is resilient enough to be useful in
a specific location and time frame. Adverse weather may easily reduce the productivity of work on a project to a
greater extent than planned, including complete shutdown of the project’s weather-sensitive work. There is a high
likelihood of slippage of planned dates, with the resulting reduction in credibility of the schedule, if adverse
weather is not adequately planned. Planning involves choosing a methodology that is credible, reasonable, and
replicable, while requiring minimal schedule revisions.

Planning for adverse weather using a transparent and reasonable methodology provides a clear baseline for the
project expectations for adverse weather losses. Unusually adverse weather, defined as weather that is worse than
the historical records would suggest, can be analyzed for extensions of time requests compared to this benchmark
of planned adverse weather. Generally, unusually adverse weather would entitle the contractor to an excusable
time extension.

These recommendations are made in order to appropriately plan for weather delay, and therefore avoid situations
where a delay to the end date is caused by weather that should have been anticipated through historical data
analysis. This avoids situations where legitimate claims for time and potentially money are rejected because of a
general lack of consideration of weather as an expected factor in contract completion.

Description of Industry Practices

Owners can request that contractors mitigate for adverse weather during their project planning; and sometimes
owners are willing to fund any additional costs due to that mitigation. This should be carefully analyzed before

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including contractual language related to mitigation requirements as it increases the risks on the contractor and
could greatly impact the contract price.

Practices that are often used in the construction industry when planning for adverse weather include:
• Increasing activity durations.
• Using weekends for “make-up” days.
• Using a weather allowance activity just prior to substantial completion to accommodate adverse weather
time for the entire project.
• The use of weather calendars.

Some projects may use a combination of these approaches. The choice may be influenced by contract
requirements and risk shifting provisions related to how the parties deal with weather risk.

For critical path delay issues related to adverse weather, the most common impact is from work activities that fall
within good weather periods in the baseline schedule, but with updates and changed conditions are shifted into
periods of higher risk for adverse weather. With good weather planning incorporated into the schedule, the
schedule should show a delay immediately upon updating or incorporation of an unanticipated condition in the
schedule. This identifies the impact of work that is shifted into future periods that may have a higher risk for
adverse weather. Analysis of the update may result in the need to prepare a request for an adverse weather time
extension. The methodology and considerations in developing a proper delay and time impact analysis is beyond
the scope of this recommended practice.

Use of Increased Activity Durations

The use of increased activity durations for activities that are exposed to adverse weather in the as-planned
schedule is a potential methodology for planning. It is simple, accommodates bar chart planning, and appears to
be a legitimate approach. However, the use of increased activity durations in a CPM model does not provide
verifiable and repeatable weather planning. This approach makes it difficult to accurately model the dynamic
nature of actual work progression and effects of weather on individual weather sensitive activities. This approach
is not recommended since it does not accommodate the dynamic nature of the critical path.

Other problems with this approach include:

• Weather planning is placed in the as-planned schedule during the appropriate season or expected
weather conditions, but as soon as the schedule is updated and compressed or prolonged, the activities
that had durations increased as a result of planned adverse weather may no longer be planned to execute
during that planned weather condition.
• Activities that did not have durations extended for adverse weather that subsequently advance or slip due
to progress will no longer contain adequate durations for the adverse weather.
• Evaluation of the duration is difficult for those activities that have been artificially inflated.
• Use of artificially increased activity durations affects the accuracy of progress monitoring, reporting and
controlling, and therefore the accuracy of earned value reporting and analysis.

Use of Extended Shifts or Planned Non-Work Days as Make-up Days

The use of planned non-work days (often weekends and holidays) to make up for lost weather time on critical or
near-critical path work is a popular methodology for contractors because it is simple and requires minimal work to
implement, but does not reflect effective weather planning. The approach can be appropriate when the climate
does not demonstrate large variations in adverse weather. In this discussion, a five day workweek is assumed with
weekends providing planned non-work days.

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Problems with this approach include:

• The weekend may not provide adequate replacement for situations where required planned non-work
days exceed the number of weekend days, or where adverse weather occurs for more than two
consecutive days.
• As the project nears completion weekend weather mitigation work is less likely to suffice for recovery
from the time lost due to weather.
• An owner may take the position that the contractor has planned for two days of adverse weather in each
five-day workweek.
• Due to schedule constraints, weekend days may not be available to allow for make-up time.
• Overtime premiums for craft and supervision may be required when weekend work is necessary.
• Owner furnished supervision or oversight should typically be available if weekend work is required.
• Inspections by owners or local jurisdiction may not be available to inspect on weekends.
• Weekends may not account for seasonal variations.
• Reduces the contractor’s opportunity to use weekends to mitigate low productivity periods caused by
non-weather related issues.
• Poor morale and fatigue issues leading to worker safety and productivity problems if weekend work
occurs over multiple weekends in a row.

Use of an Allowance Activity to Store Predicted Adverse Weather Time

The methodology of using a dedicated activity (commonly referred to as a "weather allowance activity") just prior
to the substantial completion milestone or the dry-in milestone to store adverse weather time is common. This
methodology includes summing all the expected adverse weather non-work days and using that total as the
duration for a weather activity that is inserted into the project immediately prior to the substantial completion or
dry-in activity.

This duration is typically monitored on a monthly basis and reduced as necessary to reflect the amount of actual
adverse weather experienced during that month.

Actual adverse weather is generally monitored by the project team, with the owner/CM and the contractor
determining each week the total number of days that were lost due to adverse weather and summing those to
account for the total in the reconciliation each month. This includes determining if critical activities were unable to
progress as planned due to adverse weather conditions including collateral results (e.g. muddy site). A report is
usually issued that identifies the actual adverse weather and authorizes the appropriate reduction of the weather
allowance activity duration.

Often, the project team analyzes the remaining time in the weather planning activity duration to determine if it is
adequate for the completion of the project.

Problems with this approach include:

• There may be a failure to provide adverse weather planning for site development work that is outside the
path of the dry-in activity.
• The inclusion of the weather-planning activity at the end of those paths containing non-weather-related
activities will artificially reduce float values along those paths, making those activities more likely to
appear on the critical path.
• The critical path is less reliable since the network does not calculate properly for those activities that are
falsely affected by the weather bank activity.
• An owner may reject the schedule because it artificially sequesters float on the non-weather related
activities.

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• The weather allowance activity requires additional monitoring and adjustments in monthly updates.
• Except for the weather allowance activity, the balance of the baseline schedule activities are built as
though there will be no weather interruptions to the project so that all early dates may be overly
optimistic. The only activity in the entire schedule that has dates adjusted by weather planning is the
milestone that is a successor to the weather allowance activity. The baseline schedule activities
themselves do not reflect weather planning.
• This approach does not allow the schedule to automatically and immediately predict delay when activities
are shifted into heavier weather periods.
• A claim for an adverse weather delay may only be made once the weather allowance has been consumed.
• Guidelines may not have been established for the use and removal of weather days from the allowance
activity.
• Some projects may have an allowance for a large number of weather days and parties are tempted to use
those days to reduce legitimate delays not related to weather.

Recommended Planning Technique for Adverse Weather

In order to reduce the problems created by using a weather allowance activity, the following chart is
recommended as it represents a distribution of the weather day allowance over each month of the year. The ‘x’
under each month in the table below represents the forecasted number of adverse weather days for the month
based on historical weather records for the area where the project will occur. The consumed actual weather days
from the allowance activity should be monitored to ensure that the remaining as-planned schedule still contains
sufficient reasonable adverse weather planning.

J F M A M J J A S O N D

x x x x x x x x x x x x

• The weather allowance activity should be linked to all weather affected activities and the completion
milestone associated with the project schedule.
• Non-weather sensitive activities should not be linked to the weather allowance, but instead to the
completion milestone.
• Careful consideration should be made when creating the schedule that the path of activities with the
potential for weather delay be linked in a manner that will end with the weather allowance before
completion, whether this is the longest path or not. There should be no “dropped” logic with weather
sensitive activities that are not tied into the path to the weather allowance activity. (An example would be
grading, foundations, structure, roof & exterior, site work, paving, planting, etc.)
• Non-weather activities that cannot start until dry-in need to be linked to the individual dry-in activities for
the area (For example: Installation of drywall may need to be linked to roofing, and exterior plaster).

Maintenance of Actual Adverse Weather (Removal of “weather days” from the Weather Allowance)

When a defined weather event interrupts a critical activity (meaning the weather sensitive activity must be on the
critical path in order for the allowance to be used) the following maintenance process should be implemented,
ensuring that it complies with any contract language:
• Documentation in the schedule basis to note the weather day removal from allowance

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• Weather days are recorded by adding an activity on the day of the event, log notes, or other notations
should be made in the schedule update narrative
• Unused “weather days” for each month are removed from the weather allowance activity after the month
has passed without weather interruption. The use of weather allowance days should not be used directly
by any party to mitigate a delay not caused by weather. Instead, unused weather days should be returned
to the schedule as float to the project where they would become available for use as needed. This is done
by an appropriate reduction in the weather allowance planned for the time period under analysis.
• For example: There are 10 weather days allowed for January, but there were only weather
events on 3 days. The remaining 7 weather days should be removed from the weather allowance
activity duration in order to release that float back to the project.
• When a project has been delayed, and a time extension granted, the weather allowance activity could
have additional weather days added in order to provide any additional adverse weather planning included
in the time extension.
• For example: The original project time was scheduled to complete by December 31, and an
additional 21 calendar days been added by change order. The new completion date is January 21.
Three weeks have been added to the end of the project to accomplish the work scope added.
Within the change order the contractor might consider requesting additional days be added to
the weather allowance activity to allow for weather events during the extended time of the
project.

Use of Weather Calendars

The use of weather calendars to model adverse weather is a popular method of weather planning for the as-
planned schedule. Calendars should show non-work days on a monthly basis, with the non-work days selected at
random across the weeks of the monthly calendar, using the average number of days as determined from review
of historical adverse weather data. While truly random dates are difficult to create, it is possible to use
spreadsheet formulas if required. The assignment of the non-work days should be over a seven-day week since
weather records are compiled on seven-day weeks, which will cause some of the non-work days due to weather
occurring on weekends.

Use of random allocation of expected adverse weather days in the CPM schedule calendars allows the CPM
network to automatically calculate and restricts the adverse weather planning to the appropriate season, forcing
weather-related activities, as they shift due to changing conditions, to take on the appropriate non-work time of
the season. This approach accommodates delay analysis and provides accurate predictive results as a result of
adverse weather in any conditions of delay and disruption.

In planning for an average number of weather days to include in an as-planned schedule using historical weather
databases, for example, the following chart represents adverse weather predictions for Tidewater, Virginia,
collected and analyzed from the National Weather Service’s 10-year average for adverse weather days in this
region. These numbers are specifically pulled from the US Army Corps of Engineers (USACE) UFGS for the Norfolk
District, but the numbers are very similar to those used by other federal and state contracting agencies for this
location. The USACE captures historical data on the quantity of what it considers adverse weather days. Using
USACE data for a particular location maybe useful because extracting adverse weather days directly from the NWS
requires more judgment of how much precipitation is required to define a non-work day at this location. Based on
their own experienced-based judgement, the USACE has converted inches of precipitation per day into non-work
days for a particular location. Be mindful that USACE’s typical project for which it defines anticipated weather days
may not be sufficiently similar to the project to be planned.

Considering a construction project in Norfolk, Virginia area, using National Weather Service data for estimating
purposes, the contractor could plan for the following number of adverse weather days:.

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Month J F M A M J J A S O N D
Adverse Weather Days 9 9 9 6 7 6 7 7 5 6 7 9

It could be argued that adverse weather days in excess of these numbers are deemed unusually adverse weather
days and as such might be subject to a legitimate time extension. In order to track these planned normal adverse
weather days, and plan for the activities that they affect, the following process would be appropriate within the
terms of the contract documents:
• Develop the baseline schedule using a workweek duration that does not consider weather.
• Identify all activities that are subject to weather and code them for easy filter selection.
• Develop a separate project calendar (the “weather calendar”) within the scheduling software, showing
the estimated number of adverse weather days per month. Ensure that this calendar aligns with the main
project calendar.
• Using the predicted days of adverse weather per month, apply the count of days randomly across either
the month or, assign them in the weekly proportion across each week. Spread the days out so they are
not contiguous because that will tend to show gaps in the work and confuse users of the schedule.
• Include the weekends in the full week of assigning non-work days, since weather databases track
calendar-week weather, not work-week weather.
• Apply this calendar to the activities affected by weather activities, identified in step (2).
• Calculate the new finish date and compare to the benchmark. If the project shows a prolonged
completion, check to see if the extended date is due to the added activities to model adverse weather
conditions.
• Review planned non-work days that may appear in a contiguous fragnet, such as between completion of
formwork and the concrete pour. If the schedule user looks at this carefully, it could appear confusing.

In this way, should an unusual series of adverse weather days occur, the project manager now has the means to
request a project time extension since he has reasonably and responsibly planned for weather in the submitted
and approved project schedule. It is important to note that in order to request a time extension in the case of
“unusually adverse weather,” the weather must not only have had occurred (and be documented), it must also
have had affected the completion of a critical path activity (i.e. an activity with no float and/or on the longest path)
such that the project completion was unavoidably prolonged.

It is also necessary to define what a lost weather day really means; the job records must show that work ceased, or
manpower was effectively reduced, or the work was shut down for the day or a large part of the work day, and
that the work cessation was not at the end of the workday. If the construction project is being performed by union
contractors, the labor agreement typically will define what comprises a lost weather day. Lost weather time can
also include conditions such that work could not occur, such as mud days when the site is too muddy to put
equipment to work. Review of the historical weather conditions can help in planning for the effects of secondary
conditions on the project.

Not properly defining or accounting for lost weather days, recognized as authoritative by both federal and state
contracting agencies, will have two potential impacts. First, the schedule could be considered to be flawed and not
realistically represent when the work will be performed, and will potentially mislead the contractor and/or the
owner. Secondly, the impact of any delay will be masked because of the inaccurate calendar, and in absence of any
reasonable plan, a claim will likely be rejected.

The problems with weather calendar approach include:

• When using weather calendars, there is an effect on the float path from non-work time programmed in
weather calendars as activities move from a period of full production into a period when the calendar has
more non-work days to model the planned weather, during updates of the schedule.

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• If a schedule is organized by total float, there will likely be a jump in the total float value when the
activities on the same path are driven by weather calendars which increase the number of days of non-
work to model the adverse weather.
• If the weather calendars are not actualized, then actual durations for those activities that are affected by
weather will not be accurate.
• There may be a lack of credibility if a standard number such as “three days per month, every month, all
year” is used, as it indicates that no analysis techniques were employed to arrive at the conclusion. Some
projects may be appropriate for statistically significant approaches.

Weather Impact Days: Before and After a Weather Event

As noted previously, a significant weather event may result in lost time or productivity impacts before or after the
event itself. The most commonly used example is when a cyclone or hurricane event draws near a project site.
Days may be spent evacuating the site and preparing it to be as safe as possible in the eventuality of the storms
passing through the area is a weather preparation day. If the storm does pass through the area, there will be “mud
days” or “recovery days” spent getting the site back to where it was before the weather event became an issue.
For this recommended practice a “recovery day” is synonymous with a “mud day.”

It is presumed that the weather event from a contractual perspective is not treated as a force majeure event in
part or in whole. In case of force majeure, the contract will govern how the parties have agreed to deal with the
issue of excusability and compensability for the event. Such contractual considerations are outside the scope of
this recommended practice, but are a consideration the project team must understand when planning for weather.

As it relates to weather preparation and recovery days, the following considerations should be taken into account:
• There are two trains of thoughts on how best to address, or not, the issue of a recovery day. If modeling
with use of non-work days in a CPM calendar or using weather allowance activities in front of milestones,
some include preparation and recovery days while others do not as a general principal unless it is clear
that historically the activities involved will require such non-work days.
• Preparation and recovery days should be used only as they relate to historically significant weather. If the
default is to include preparation and recovery days with each weather event, then the project will have
assumed too much risk in the plan and it will improperly elongate the work (activity durations and/or
weather allowance activities).
• Recovery days represent the time necessary to return the project to its point of progress just prior to the
weather event. This would also include reestablishing pre-weather event production and productivity
levels.

RECOMMENDED PRACTICE

Identification of Sources for Historical Weather Data

The most widely accepted and validated source of historical adverse weather data for the United States is NOAA
through the National Climatological Data Center. As NOAA does collect data for a variety of international locations;
when lacking better data, the NOAA data may be appropriate to use internationally.

Since the USACE handles the statistical weather analyses of the NOAA climate data and makes recommendations
on what constitutes an adverse weather day, using the data supplied by USACE is the quickest and easiest way to
provide average historical adverse weather data in the US or other areas where available.

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Interpretation of Historical Weather Data

If the USACE-provided data in their published specifications are available, the recommended practice is to use
those non-work days per month. If USACE records are not available for the location of the project or the USACE
recommendations do not fit the needs of the project, then using simple non-work averages is recommended due
to the simplicity of interpretation and ease of understanding by the project participants and other audience.

Methodology for Planning for Predicted Adverse Weather

Of the several methods to plan for weather, the use of weather calendars has the most advantages and least
disadvantages. The next best approach is the use of a weather allowance activity to house planned non-work time
within the CPM schedule model.

In either case, proper implementation of the practice is essential to provide appropriate, meaningful, and
reasonable planning for adverse weather.

REFERENCES

1. USACE Publication, Design Engineering Weather Data, 28 February 2003, UFC 3-400-02
2. Dictionary, M.-W. (n.d.). Retrieved April 7, 2012, from http://www.merriam-webster.com/dictionary/weather
3. AACE International, Recommended Practice 38R-06, Documenting the Schedule Basis, AACE International,
Morgantown, WV, (latest revision)
4. Merriam-Webster Dictionary, 2015

CONTRIBUTORS

Disclaimer: The content provided by the contributors to this recommended practice is their own and does not
necessarily reflect that of their employers, unless otherwise stated.

Delbert E. Bearden, PSP (Primary Contributor)

Christopher W. Carson, CEP DRMP PSP (Primary Contributor)
Edward E. Douglas, III CCP PSP
Patrick B. Egger
Fouad H. Elfaour, PSP
Lisa A. Enloe
Marc S. Glasser, PSP
Gokulan Govender, PSP
Dennis Read Hanks, PE CCP DRMP
Hilal A. Itani, PSP
Patrick M. Kelly, PE PSP
Vera Lovejoy, CCP PSP
Donald F. McDonald, Jr. PE CCP PSP
Jose Noe
Zartab Z. Quraishi, PE CCP PSP
Michael E. Stone
Dr. Stephen P. Warhoe, PE CCP CFCC
Daniel A. Weis
Ronald M. Winter, PSP

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