Northwest Rain During the Driest Time of the Year

 We are now in the driest period of the year in the Pacific Northwest.   Below is the daily climatological probability of a trace or more of precipitation in Seattle.

Just amazing: we bottom out from roughly July 5 through August 15th, with the lost few days of July (NOW) the most arid.

Crossing the Cascades to Sunnyside, we find a similar dry period...although the general level is much lower in the arid Columbia Basin.

The reason for this climatological Northwest dry period is clear:  the jet stream and storm track are not only weakened in mid-summer, but are also positioned to our north during this period

Furthermore, with cool waters offshore, we don't get many thunderstorms, unlike much of the U.S. 

Interestingly, the models are now suggesting that many will experience precipitation during the normally dry period.....let me show you.

 Starting with the latest UW WRF model forecast for accumulated precipitation over the next five days, one notes substantial precipitation from northern California into eastern Oregon, with a few showers making it into the southern WA Cascades.  

Most of this precipitation is from thunderstorms, as shown by the simulated precipitation on Thursday around 6 PM.

This precipitation pattern has been persistent over the past few weeks, with portions of northern CA and eastern Oregon wetter than normal (see below, green and blue colors).

A number of model runs are showing a significant change in the pattern of the past few weeks with a series an energetic troughs of low pressure pushing south into British Columbia, shifting the rain shield south.

The latest UW extended precipitation total through Monday afternoon (August 4th) shows the moisture.  Impressive for this time of the year, with welcome precipitation over the wildfire region east of the Cascade crest.  

Puget Sound will be rainshadowed by the Olympics.

The ensemble systems of many forecasts concur with this moist forecast, and the NOAA Climate Prediction Center 6-10 day precipitation forecast is for above-normal precipitation over our region.

This moist period will be associated with cooler-than-normal temperatures and will help restrain the wildfire threat over the region.