Importantly, the new circulation will bring substantial precipitation to eastern Washington and the eastern Cascade slopes, regions that desperately need the precipitation.
The UW WRF model precipitation for the next 72 hour shows some substantial totals for May, ranging up to 1-3 inches in the Cascades, particularly over the north Oregon terrain. Eastern Washington is quite wet, as is the Willamette Valley.
Precipitation continues for the next 72 hr, again with LOTS of rain over and east of the Cascades.
So why the big shift? Take a look at the upper level (500 hPa) forecast map at 5 PM today (Monday). An upper trough over Oregon!
4 PM Thursday, the trough deepens and extends southward over California. Folks, this is NOT a dry pattern for the West Coast...and not a warm one either.
Interestingly, the atmosphere seems to be locked in such a configuration for a while. Not the kind of weather tomato plants like.
Want to really be impressed?--- here is the total precipitation over the western U.S. for the next 16 days (from a relatively low resolution model, the GFS. Four to 10 inches in some locations. This has to make Northwest farmers and water managers smile.
Tuesday AM Update: The radar at 5 AM shows bands of moderate precipitation circling around the trough, with particularly heavy rain (yellow colors) over SW Washington and NE Oregon. This precipitation should move northward today, with moderate precipitation along the eastern slopes of the south-central Cascades.