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 😈

Can We Deal with the Snoqualmie Pass Snow Closures Better?

Late Thursday afternoon, moderate snow led to multiple crashes and trucks/cars sliding off I-90.   As a result, the pass was closed for several hours, allowing the removal of vehicles and letting the WSDOT plows improve the drivability of the roadway.

Now, here is my frustration.    

The snow was foreseeable.  Weather information is so good now that we can warn folks in real time of the approaching snow band so that they can slow down!

The key modeling tool is the NOAA/NWS HRRR model, which runs at high resolution EVERY HOUR.  

The 11-h snow forecast for 3-4 PM snow below.  Snow around the pass!

The 12-h snow forecast for 4-5 PM is here.  Slippery conditions!

You get the idea....snow was predicted on the western side of the Cascades and the Cascade passes well ahead of time.

In real time, the weather radar showed the snow falling at 4:30 PM.

A few years ago, I offered to build a system for WSDOT that would provide state-of-the-art prediction of snow in the pass with real-time updates provided by weather radar, satellite imagery, and other weather assets.  

This real-time snow guidance could be used to update reader boards on the side of the highway.

The goal was to stop the snow closures and keep folks safe.  Unfortunately, they were not interested.

Another example of valuable information available from meteorologists not being used effectively by society.