September 18, 2021

Big Winds, Heavy Rain, and Now Thunderstorms

 Nearly 100,000 Washington State power customers lost power last night as strong winds first hit Northwest Washington and then spread across the remainder of the western interior as a powerful front crossed the region during the late evening.  Trees are very vulnerable to the first windstorm of the season, particularly if they are fully leafed out.

Winds gusted to around 60 mph over Northwest Washington, 40 mph over the lowlands of the South Sound, and reached 103 mph at Camp Muir on Mount Rainier.

The power outage map from Puget Sound Energy last might shows the damage, centered around NW Washington and the south Sound towards the Cascades.

Here are the top wind speeds on Friday (click to expand).  A gust to 63 mph a Whidbey Island Naval Air station, nearly the same at Port Townsend, and nearly 50 mph on the San Juans.   These winds occurred during the afternoon before the front made landfall.   

Strong winds over NW Washington are very typical for winter weather systems around here,  with the powerful winds responding to the strong pressure difference (gradient) produced downstream of the Olympics.  This is illustrated by the forecast pressure (brown lines are isobars, lines of constant pressure) and winds (color shading) at 5 PM yesterday (below).   Look carefully and you will see a large change of pressure over northern Whidbey Island....this accelerated winds from the southeast.

Over the south Sound, the winds gusted to 40-50 mph last night over the lower elevations, 70-80 mph on lower peaks, and over 100 mph on Rainier.

These winds occurred during the later evening as the strong front went through....the feature I warned about in my last blog.  The radar image at 11:11 PM last night showed a line of strong radar return with the front (the arrow shows you the feature).

The frontal winds came up very quickly and dropped very quickly, as illustrated by the winds at the USDA RAWS site in Enumclaw, southwest of Seattle.  A gust to 36 mph with the front.

And this was a particularly strong front with a VERY large temperature change associated with it.  To prove this, here are the temperatures at Enumclaw.  Temperatures rose before the front to 72F and then crashed to the lower 50s within a few hours.  Don't see that kind of large temperature changes with fronts very often around here!

Precipitation?   This system brought plenty, ranging from 4-6 inches in the mountains to around a half-inch in the rain shadowed areas.

Rivers are way up right now, with many Washington rivers above normal levels.  But no flooding because they started at below-normal to normal levels.   

Part II. Thunderstorms and showers

The excitement is not over yet!  With an upper-level trough/low approaching and cooler air moving in aloft, the atmosphere will be primed for convection and thunderstorms.  So get your lightning rods ready.

The latest visible satellite image shows the front moving through right now (the solid band of clouds) but also displays lots of convection (instability showers of cumulonimbus) offshore (these are the popcorn-appearing features).  Those showers are strong and have our names on them.  But most of you will have a break this morning and early afternoon--so have fun outside this morning if you can.

A particularly potent band will come through this afternoon and evening (see the simulated radar image at midnight tonight below).   Many of you will hear the rumblings of thunder.  And more shower action on Sunday.  

It is good to have active weather back...and I note that it is quite typical for this time of the year.

Forecast radar reflectivity at midnight tonight.

September 17, 2021

A Potential Intense Feature Moving Through Tonight. My New Podcast. And Why are Northwest Summers so Dry.

A potentially VERY intense frontal feature will move through western Washington tonight, one that could produce intense rainfall and localized urban flooding.

A very strong front will move through this evening, crossing Puget Sound sometime between 8 PM and 11 PM.  This front will bring INTENSE, heavy rain for a short period (less than an hour) that will cause localized flooding and strong winds.  There could be a thunderstorm with it.   

To illustrate this threat, here is a simulated radar image for 10 PM tonight.  Wow. The orange/red colors indicate very heavy rainfall.  The NOAA HRRR model is doing the same thing.  

This feature will pass through the coast a few hours earlier. 

I would not be driving during this event and if you live in a basement apartment with flooding issues, I would be watchful.  The City of Seattle used to have the RainWatch system that provided warning capabilities for such events, but since they dropped it a few years ago,  I am providing the heads-up manually. 

The total precipitation for the storm follows the previous guidance (the total through 5 AM Sunday is shown below).  Even eastern WA gets good rain, with over 5 inches in portions of the Olympics and Cas Cascades.

In western Washington, the rain will be very heavy tonight, with showers into Sunday.  Good chance of thunderstorms on Saturday and Sunday.

I talk about the forecast in-depth in my podcast.   

In the second section of the podcast, I explain why the Northwest has some of the driest summers in the nation.  In fact, the entire West Coast is very dry (the plot below shows you the average July precipitation).  By the end of the podcast, you will know the reason.

You can listen to the podcast below or through your favorite podcast server.

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Big Winds, Heavy Rain, and Now Thunderstorms

  Nearly 100,000  Washington State  power  customers lost power last night as strong winds first hit Northwest Washington and then spread ac...