Cool, Cloudy, and Wet Period Ahead

Cooler, wet weather is ahead, something that the models picked up on over a week ago.  And this intrusion of clouds and rain is highly welcome in many ways--for agriculture, home gardens, and to reduce fire danger, particularly in British Columbia and northeast Washington.

The spigot will turn on slowly starting late Wednesday, with Friday being the wettest and coolest day. 

Let me begin by showing you the forecast accumulated precipitation through 5 PM Saturday from the UW WRF model (below).  Precipitation will occurr over the entire region, with the exception of the Yakima area in the rainshadow of the Cascades.  As much as 1-2 inches in the Washington Cascades, but what is really impressive are the totals north of the border.  The values in Alberta are very large (2-5 inches), which should knock down the fires in Alberta.   Rain in British Columbia is also important, since last year the BC fires were a major source of high-level smoke in the Northwest.

 A closer-in view of the accumulated precipitation through 5 PM Saturday using the European Center model (graphics by WeatherBell Inc.) predicts 1-1.5 inches in the WA Cascades and appreciable amounts over most of western WA, which would lessen the dry condition a notch, particularly over SW Washington.

Why this wet bounty from the sky?   Its origin is the approach of a relatively deep trough for this time of the year over the region.

Let me illustrate this by presenting a series of upper level (500 hPa, about 18,000 ft above sea level) charts.  These charts show the height of that pressure surface above sea level, but you can think of them as pressure at roughly 18,000 ft.  Winds are parallel to the lines and are stronger when the lines are closer together.

At 10 AM today, there will be  strong southwesterly flow over our region and a trough of low pressure/heights (the "L") offshore.  An approaching trough will bring upward motion, clouds, and precipitation.

By 5 PM Wednesday, the trough is closer, which will bring increasing chances of clouds and showers.

By 5 PM Thursday the trough has deepened over the West Coast and extended down to California.  Yes, those poor, sodden California could get more rain.   And western Oregon will definitely get some.

And by late Friday, the trough has pushed right over us.

So enjoy today-- there will be plenty of sun after the low clouds burn off.   Tomorrow, clouds will start streaming in aloft, and the chances of shows will increase.  And don't be shocked by the occurrence of a thunderstorm on Friday--the models are suggesting the possibility of that.  Warmer and drier conditions should be expected by Sunday-- so plan to use that day for outdoor activities.

Finally, lets look back on the spring so far here in Seattle (last 12 weeks).  Temperatures (red lines below) were normal, or cooler than normal, in April, but generally warmer than normal in May.  My tomatoes are doing very well.  The huge temperature spike in early May really sticks out.

Precipitation?   We had a very wet period in early April, but minimal rain in May, leaving us about 2.5 inches below normal for the period.    We should catch up a bit during the next few days.