Severe Thunderstorm Potential for Washington State

Portions of Washington State, particularly from the Cascade crest eastward, may see some very strong thunderstorms starting tomorrow morning and extending into the evening.    Storms that may bring heavy rain, hail, lots of lightning, very strong winds and even the chance of a tornado-packing supercell thunderstorm.

And western Washington may get some heavy rain and perhaps some thunder. 

The NOAA/NWS Storm Prediction Center in Norman, Oklahoma has an unusual severe storm outlook for Saturday, with the greatest threat in central Washington and Oregon. You don’t see that very often!

Key ingredients will be in place on Saturday in our region: very unstable air and an approaching upper level disturbance that will provide upward motion that will initiate the convection (thunderstorms). 

To put it another way: we will have both the fuel (unstable air) and the match (the approaching disturbance).   And we will also have strong wind shear (wind change with height), which is important element for long-lived severe thunderstorms.

Tomorrow morning, the existence of a trough offshore and higher pressure inland, will produce a southerly (from the south) flow of warm,  moist air over the region (an upper level map for around 18,000 ft is shown below).  This will foster the development of instability AND strong wind shear–both important ingredients.

And to the south an intense upper low is lurking over California.  That feature…the match— will move northward during the day and will initiate strong thunderstorms.    But there is more, this upper low will have a very strong low pressure center associated with it, and by 8 PM Saturday, the surface low will be positioned over eastern Washington (see below). Wow…there is a HUGE pressure gradient over central WA at this time….that will result in strong winds.

This evolution is going to produce extraordinarily unusual levels of instability in the atmosphere….the potential for the atmosphere to convect.  A measure of the potential for instability is know as CAPE (Convective Available Potential Energy).   Values tomorrow will be very high over eastern Washington, with some values getting about 2500 (that is very, very high for our region)….see forecast for tomorrow at 4 PM.
The UW high-resolution forecast model is predicting substantial precipitation over the region through 5 AM Sunday (see below), with much of western WA being drenched by .5-1.2 inches of rain.

And the UW model is predicting strong thunderstorms, something evidence in some of the cloud simulations shown below for 6 AM and 6 PM Saturday.  The white ovals are strong thunderstorms.  Huge.

The NOAA/NWS HRRR model forecast for 7 PM Saturday predicts some supercell thunderstorms with threatening hooked echos the Columbia Basin (simulated radar composite radar reflectivity is shown).

And I haven’t even discussed the simulated winds, which gust to 60 mph near some of the forecast thunderstorms!  There will be blowing dust and potential fallen trees.

It should be an interesting day. (Cliff Mass Weather Blog)

2020-05-29 19:41:38

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