Saturday, May 25, 2019

Memorial Day Weekend Forecast: Moist to Dry

This weekend is going to be a mixed bag over the Pacific Northwest, with Saturday being the coolest and wettest, improvement on Sunday in the west, and a glorious day on Memorial Day.

Let's start with today.  An upper level trough/low is moving through now--and headed for California where it promises declining weather during the next few days.  The forecast map for 5 PM today shows the low center sufficiently offshore that the most substantial precipitation will be along the coast.  Puget Sound will see only a few sprinkles today.  And the central and north Cascades will enjoy rain showers (which is good for keeping the wildfire threat down).


The  9 AM radar shows showers over NW WA and the northeast Cascades


Looking at the latest NOAA/NWS HRRR model, the accumulated precipitation through 11 AM is limited to those coastal/NE WA locations.

And through 5 PM, there is a substantial moistening of the Olympics, SW Washington and the northeast slopes of the Cascades.   Exactly where moisture is needed. Relatively dry over Seattle the the western slopes of the Cascades.  But there will be generally cloudy and breezy conditions.


The 24h precipitation ending 5 AM Sunday, reflects the pattern, with the dry conditions along the western slopes of the Cascades due to downslope (easterly) flow associated with the coastal trough.


Sunday will be an improving day in the west, but not in the eastern part of the state.  Moisture will wrap around from the southeast into eastern WA, bringing showers and thundershowers.  The forecast  three-hour precipitation ending 2 PM Sunday illustrates this ((below).   The precipitation extend over the Cascades before dying over the western slopes of the mountains.


Why is everything reversed, with eastern WA and eastern Cascade slopes so wet?  Because there is persistent easterly (from the east) flow on Saturday and Sunday.

The 24-h precipitation ending 5 AM Monday shows impressive amounts over eastern WA and the SE Cascades.   Extremely beneficial precipitation.


But then there is Monday, when major improvement will occur.  High pressure will build into the eastern Pacific,  dry conditions should prevail, the sun should be out, with temperatures zooming into the 70s in western WA and a perfect day for memorial day activities should be enjoyed by all.