The Subtleties of Warmth

For many in western Washington and Oregon, today was the warmest day of the past seven months... this was true at Sea-Tac Airport, where temperatures rose to 77F this afternoon (see plot of temperatures since 30 September...today was the warmest). 

The plot of high temperatures today (Tuesday) shows some interesting contrasts:  mid-50s near the water.  70s over land around Puget Sound, and 80s near the coast and in the warm Willamette Valley. 

Why so warm along the coast?  Because there was strong easterly flow aloft.  This is shown by a plot of winds over time above Sea-Tac Airport (red are temperatures, blue are wind barbs, time is in UTC/GMT with 24/18 indicating 11 AM this morning, the y-axis is height in pressure with 850 being about 5000 ft)  Strong easterly flow developed in the lower atmosphere last night and this morning.

Easterly flow pushes the cool marine influence offshore and produces downslope warming on the west side of terrain (like the Cascades and coastal mountains).  The coast enjoys the downslope warming benefits of two mountain barriers and thus gets very warm.

Another interesting oddity is the minimum temperatures this morning.  72F near Black Diamond in the eastern foothills of the Cascades and 60s nearby.  Why so warm?  Because of downslope warming off of nearby terrain!

 Tomorrow will be warm, but not as warm as today.  Why?   Because the easterly flow has weakened and remain weaker on Wednesday.  The sea level pressure forecast for 2 PM tomorrow shows a thermal trough of low pressure extending into southwest Washington and less east-west pressure difference than today.  Such a pattern tends to give us more northerly than easterly flow over the Sound...thus, a cooler pattern.

Thursday at the same time, the trough has pushed northward to Bellingham and the flow is more easterly over the Cascades--thus I expect warmer conditions (upper 70s that afternoon around Seattle).  But the end of warmth will be upon us soon.   With the approach of a low offshore, a ridge of high pressure builds along the coast and a strong coastal pressure gradient is building.

The result will be an onshore push of marine air late Thursday/early Friday, with considerable cooling on Friday. 

So enjoy the next two days....precipitation and cooler temperatures are in store for the weekend.



from Cliff Mass Weather and Climate Blog http://cliffmass.blogspot.com/2018/04/the-subtleties-of-warmth.html