Super Convergence Zone Dumps Heavy Rain

One of the most striking peculiarities of western Washington weather is the Puget Sound convergence zone  (PSCZ), which can produce a heavy band of precipitation across northern Puget Sound and into the Cascades.

The PSCZ  results when low-level winds on the coast are westerly (form the west), move around the Olympics, and then converge together over Puget Sound, producing upward motion, clouds and precipitation.

This is exactly what occurred Saturday evening, as illustrated by the plot of surface observations at 7 PM (a large scale and close up view is shown below)

The air was relatively unstable Saturday evening and an upper level trough was producing additional uplift....so the convergence zone really revved up.    

Now let me impress you...here is a radar image around 7 PM Saturday.  Red indicates very heavy rain, and yellow is moderate to heavy. Intense convergence zone band.  Note the sharp southern edge of the rainfall, which is very typical of strong convergence zones.

There was substantial convection and thunderstorms in that heavy precipitation band, and lots of lightning was noted by the regional lightning sensors (see below-24h lightning ending 1 AM Sunday)

Seattle RainWatch combines radar and observations to provide a good estimate for rainfall.   Here is the RainWatch 24-h total ending 5 AM Sunday.  A substantial area got 1-2 inches, which even more in limited regions.  Pouring over Snohomish County but downtown Seattle was dry.   You got to love living around here.

Local rain gauges had 24-h rainfall totals that were consistent with RainWatch, with over 2 inches around Monroe.

I know someone that had a wedding celebration near Monroe...thankfully they had tents, but it was a soggy affair.

Now the really exciting part for me is how well this event was forecast....our high-resolution models was very skillful, days ahead of time.  To illustrate, here is the forecast from the UW WRF high-resolution (4/3 km grid spacing) model for the 24h precipitation ending 5 AM Sunday.  

Nice convergence zone.  It underplayed the total amounts a bit...but it was clear that Snohomish County was going to be drenched, which downtown Seattle would be dry.

from Cliff Mass Weather and Climate Blog https://ift.tt/2xC8jtn