April 28, 2021 – Dry and much warmer weather through the rest of the week.
Discussion
A +2 to 3 5H height anomaly (strong ridge) along the W coast of NOAM shifts inland the next couple of days in the wake of the Desert SW upper low. This will lead to well above normal max temps through the rest of the week, generally peaking on Thu in Norcal.
Max temps forecast to warm some 6 to 12 degrees today with additional warming up to 8 degrees on Thu. 850 mbs temps warm into the mid teens deg C on Thu with max temps nearing 90 in the Valley, some 13-15 degrees above normal in the Valley, and 15 to 20 degrees above normal in the mountains. (NBM Viewer shows a 46% chance of at least 90 at DTS, 39% for KRDD, 20% for KSCK, on Thu.)
An increase in cloud cover on Fri associated with an approaching short wave trof moving into the Pac NW may limit max temps a couple of degrees on Fri. This will be followed by onshore flow and minor synoptic cooling on Sat as the trof weakens the ridge as it moves through the western states.
In the near term, warming aloft will lead to the formation of thermal belts in the foothills/mtns, seemingly favoring the BCCONSRAW/ALL min temp guidance. Forecast looks dry for now as precip is forecast to stay north of CA, save for the ECMWF which brings some light precip south of the CA/OR border on Sat. Localy breezy winds over the Sierra this morning should decrease by midday as gradient weakens with the departing upper low.
Extended discussion (Sunday through Wednesday)
Short wave trough with limited available moisture pushes through interior NorCal early Sunday. Most deterministic and ensemble solutions favor a dry forecast with this feature. Consensus in EPAC upper ridge building back into the area early next week.
This will result in warming temperatures with some gusty north to east wind. High temperatures expected around 10 to 15 degrees above normal through the extended forecast period.