1. The West Coast Thermal Trough

2. The Most Important Summer Mesoscale Feature of the West Coast of North America
An inverted pressure trough based out of the Central Valley of CA that can extend into the Pacific Northwest
925 hPa heights and temperature

4. JJA
-36h
925mb
925mb
500mb
500mb

5. JJA
-12h
925mb
925mb
500mb
500mb

6. JJA
Start
925mb
925mb
500mb
500mb

7. JJA
End
925mb
925mb
500mb
500mb

8. JJA
+12h
925mb
925mb
500mb
500mb

9. A Case Study
The WRF model was initialized at 0000 UTC 13 May 2007 using NCEP’s GFS model and the model ran for 72 hours.
Nudging toward the GFS was turned off so this run (Control) may be compared to runs with terrain modifications and without surface heat and moisture fluxes.

13. WRF km terrain

14. WRF 36-km heights and wind at 500 hPa

18. Some interesting details
Warmest weather is the last warm day, when trough is overhead and about to move east of the Cascades
When trough is to the south, northerly flow pulls cool air in form the north.
Strong easterly flow can cool Seattle because of troughing on western slopes of Cascades.