Drought-Buster Storms This Week

 The latest U.S. Drought Monitor graphic suggests that the only area with drought over the Northwest is on the western side of the Washington Cascades (see below).

Whether the term drought is appropriate can be debated.   For example, reservoir levels for Seattle, located in the moderate drought area, are near normal (see below)

And most westside rivers are near normal (green colors below).

But whether you think we are in a drought right now, we won't be in a week, as a series of storms and a potent atmospheric river will hit our region.

Consider the forecast accumulated precipitation through next Tuesday at 5 AM (below). 

Clearly, divine intervention.

Huge amounts in the mountains, with the greatest precipitation where the Drought Monitor suggested that drought is occurring.  We are talking up to TEN INCHES of liquid water at the highest elevations (see below). 

Much of the precipitation will occur over the weekend as a potent atmospheric river moves into our region from the southwest.  To illustrate, the graphic below shows you the amount moisture transport at 5 PM Sunday, with the red and white colors indicating a massive horizontal movement of moisture into our region.

No wonder there will be a deluge in the mountains as massive amounts of warm air are forced to rise on the terrain.

The strong southwesterly flow associated with the atmospheric river will move warm,  moist subtropical air into our region, resulting in temperatures at SEATAC surging into the 60s from Monday through Wednesday (see below).   

It will feel like spring.  And yes, spring began early this morning.  The atmosphere must be watching the calendar.

But rain will return later next week.   

Rivers will surge higher.  For example, the Snoqualmie River will go from below-normal levels right now to well above normal flows, even breaking some daily records on March 28.

As I have stated before, heavy rain or snow in late March and early April is golden....far more valuable than earlier in the season.  The water is all retained (no flooding worries), the ground is moistened before the dry summer, and the snowpack is topped off.

Super Convergence Zone

 If you ask any Western Washington meteorologist about the most important local weather phenomenon, the answer is immediate:  the Puget Sound Convergence Zone.

Today, we had a spectacular example....let me show you.

For those of you who are unfamiliar with this feature, such convergence zones occur when the low-level winds on the coast are westerly or northwesterly (from the west to northwest). 

 The air is deflected around the Olympics and then convergences somewhere over Puget Sound (see figure below).    Converging air at low levels forces upward motion, resulting in clouds and prediction.  

A convergence zone is born!

We typically get 10-20 convergence zones per year, with the greatest frequency from March through June.

A strong convergence zone occurred this afternoon, with associated heavy rain quite evident on the weather radar around 3 PM (see below).    You see the east-west precipitation band (yellow and green colors)/   That is the convergence zone, which extends into the western foothills of the Cascades.

 

The satellite image at the same times shows the cloud band and a clear zone to the south....we often see this.   It can be pouring over the city, but clear and sunny to the south.

If the air is relatively unstable (prone to cumulus and cumulonimbus clouds caused by temperatures declining rapidly with height), convergence zones can produce lightning and thunder. Many convergence zones often produce a single lightning flash and today's event was no different.

The location of the flash is shown below (again, about 3 PM).  We call this a "one-flash wonder"

Rain was heavy in the convergence zone and nearly absent to the north and south.  To prove this, below is the 6-h precipitation total ending at 5 PM.  Around a half-inch in the convergence zone but nearly nothing to the north and south.

When I was a graduate student many years ago, the origin of the convergence zone was not known.  Weather Service forecasters thought there was a strong band of rain offshore that sometimes moved in off the ocean and then took up housekeeping over the central Sound.

Today we understand the critical role of terrain and particularly the Olympics.

But there is something else that changed:  the ability of high-resolution models to predict convergence zone precipitation well in advance.  For example, below is the predicted  3-h precipitation ending at 5 PM on Monday.   

When it is sufficiently cold the convergence zone can produce a very well-defined band of snow across Puget Sound, something that happened in 1990.   Even yesterday, there was convergence zone snow above approximately 750 ft ASL.   Here is a picture sent to me by Dr. Peter Benda at around 1200 ft in the hills above Bellevue (3 inches of snow!).

Under very rare conditions, the snow band can get very, very narrow 😈