Falling Trees, Thunder, Strong Winds and a Problem Forecast

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This morning I didn't plan on biking home in strong winds, rumbling thunder, light rain and a blocked bike trail....but it happened.  During the second half of the afternoon thunderstorms developed over and to the west of the Cascade crest and then moved over the Puget Sound lowlands.  Strong outflow from the thunderstorms produced gusts of 30-40 mph, causing branches from the well-leafed trees to break off and descend upon surprised local residents. (the leaves enable the winds to do more damage to the trees than would have been the case a few months ago).

This is what was waiting for me on the Burke Gilman trail around 7 PM...and much of the trail was covered by small branches and torn off leaves.

The winds came up suddenly as the thunderstorm cells approached...here is the wind observations on the Evergreen Point floating bridge:

 It should also be considered that there is considerable danger when there are strong wind and you are near big trees.  The strong winds were over when I entered the wooded portion of the trail...and I was still a bit nervous.  People have been killed and injured by falling trees--even in cars--so you got to be watchful.

Thunderstorms often have strong winds associated with downdrafts produced by rain falling out of the storms.  Not only does the rain drag the air downward, but evaporation cools the air, making it more dense and thus heavy.....allowing the air to accelerate towards the surface where it is forced to spread out as a fast current.  The transition to these strong outflow winds is called the gust front.   Not only can it knock down branches or trees, but there can be danger on the water if you are in a sailboat.   I was involved in a legal case once where some fell off a sailboat when a gust front hit--there didn't make it.

The cam on top of the atmospheric sciences roof showed the building cumulus over the Cascades at 3:15 PM--that was a sign of what was to come.

The visible satellite imagery illustrates the development from space.
First, at 1 PM, cumulus started developing over the mountain, but the lowlands were clear and sunny.

By 4 PM, the convection had intensified and started to shift over the eastern side of the lowlands.  North Bend was getting a thunderstorm at this time.

The Camano Island radar at 3 PM and 5 PM picks up the cells moving west.  For most locations the showers were light and and some the rain evaporated before reaching the surface.

The national lightning network picked up quite a number of discharges...here is a sample over a thirty-minute period ending 5 PM.

The precipitation reaching the ground was pretty minimal, although a few locations in the foothills got a tenth or so (precip for the six hours ending 9 PM Friday)

I found a wonderful video from Snohomish showing precipitation falling out of the clouds...called virga.  Find it here.

The UW WRF model clearly underplayed the precipitation (see below)...it appears it underestimated the amount of convection---something that we need to diagnose and perhaps improve.   Convection is hard ... particularly weakly forced convection like this.


The NWS NAM model and the European Center model had a bit more, but failed to move it over the lowlands as well.  Because the models missed the intensity of the convection and its movement over the lowlands, the official forecast this AM gave little clue to the storms that reached the metro area.  We've  got more work to do!

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