Are Tornadoes and Severe Thunderstorms Coming to the Northwest on Wednesday?

I have gotten nearly two dozen emails about it.  The media are calling.  The amateur websites are going crazy. 

All have heard rumors of the potential for severe thunderstorms tomorrow in the Northwest.   How much is truth and how much is hype?    This blog will examine the situation

The NOAA Storm Prediction Center in Norman, OK is responsible for putting out official severe convection/thunderstorm storm warnings for the US.  Their severe storm warmings for tomorrow (Wednesday) predict the main action over the Northwest, which is unusual (but not unprecedented).

They have a 15% chance of hail in the Willamette Valley and SW Washington.

And a 2% chance of tornadoes.   Needless to say we are not twister country.  But there are typically a few weak tornadoes each year over the region.  One even damaged Bill Gates old home in Seattle.

One of key reasons folks are getting so excited is because of the great instability predicted tomorrow, with large instability promoting thunderstorms (also called convection)

The most familiar measure of the potential for vertical instability is called CAPE, Convective Available Potential Energy.    Drop such terms at a party and folks will be impressed.😀

Around here, meteorologists get excited when CAPE gets into the low hundreds.  Tomorrow, it will exceed 1000 in some locations.   For some perspective, in tornado-alley areas in the midwest, values can rise to 3000-4500.

Here is the NOAA HRRR model forecast for CAPE for tomorrow at 3 PM.  Huge values (yellow and red colors), mainly near the western slopes of the Cascades. Wow.

To release this CAPE, you need some uplift get the convection going.  In this case, it will be supplied by a weak trough of low pressure moving up the coast (the situation at 500-hPa, 18,000 ft)  around 8 PM is shown below.

My group at the UW runs the highest-resolution forecast model in the region and strong thunderstorms are exactly where it shines.  Let me show you a sequence of simulated satellite images based on the latest forecasts.  You will see the development of some potent thunderstorms!

By 5 PM, thunderstorms are breaking out (the white blobs are the upper cloud shields of the storms)

At 8 PM, thunder and rain will spread into NW Washington and thunderstorms will be revving up east of the Cascades.

And by 11 PM, boomers will over widespread over eastern Washington

Expect localized heavy rain and gusty winds. And there will be lightning.  

This event will not be unprecedented so I hope the hypesters will keep things realistic.

Super Rain Shadow

There is a lot of attention given to the heavy precipitation that falls on the windward side of Northwest mountains.  But perhaps the most remarkable aspects of Northwest meteorology is not where it is raining but where it is not.

Our region is also the land of rain shadows....some of the most profound in the world.

Most are aware of the classic climatological rain shadows of our region:  northeast of the Olympics and east of the Cascades (see below).  This is why folks retire to Sequim and Port Townsend--Los Angeles rainfall in the Northwest!

But during the past few days, one of the most dramatic rain shadows in memory has occurred in a different location than normal:  over central Puget Sound.

This blog will describe and explain why.

Consider the accumulated rainfall over the past 48 hours (ending 6 AM this morning) shown below.

Over 5 inches over the windward (western) side of the Olympics, but only 0.01 inches over Puget Sound.

You read that right.  Over FIVE HUNDRED TIMES more precipitation on one side versus the other.  Mama Mia....that a world-class rain shadow.   

The Cascades were almost as impressive.   Roughly 5 inches on the western side and 0.01 inch on the other.

The weather radar images have been stunning in showing the profound rain shadow effect (see below for a few samples).

You could even see a rain shadow "cloud hole" in the visible satellite image from yesterday afternoon....some luck folks around the Hood Canal were able to see the sun, while rain was falling around them.

Why did the rain shadow move over central Puget Sound?  And why was it so strong?

Rain shadow location and strength are dependent on the wind speed and direction of the air approaching  the associated mountain barrier.

As air approaches a  barrier it rises, resulting in enhanced precipitation.   As it sinks on the downstream (leeward) side it dries out, producing a rain shadow.

For most of the winter, moist air approachies the Olympics is from the southwest, producing a rain shadow to the northeast of the barrier.  Thus, the most significant drying is around Sequim and Port Angeles.

But yesterday, the winds approaching the barrier were not only strong, but coming almost directly out of the west, thus producing an intense rain shadow nearly due east of the Olympics. 

It illustrate, here are the winds around 5000 ft during mid-day Sunday (the barbs show wind direction and speed...almost 50 kt at the level).   Strong westerly winds hitting the Olympic crests and then descending over Puget Sound.

Enjoy the weather contrasts....it is why meteorologists love this region.