April 2, 2021 – Dry weather with above normal temperatures trending cooler into next week. Unsettled weather with light precipitation may return early next week.
Discussion
Upper ridge axis forecast to build inland over the Desert SW area today and into the weekend allowing onshore flow to increase resulting in a slight cooling trend over NorCal today and continuing through the weekend. However, it will still be a warm day with max temps some 8 to 16 degrees above normal today.
Nighttime fog product shows stratus returning to the coast as the synoptic flow turns onshore. NAM Bufkit forecast soundings for KOAK shows a deepening of the marine layer over the weekend so the cooling trend is expected to continue through the weekend. Forecast max temps on Saturday will range from 6 to 13 degrees above normal, then 3 to 12 degrees above normal on Sun, and 0 to 8 degrees below normal on Monday.
Over the weekend, the marine layer is likely to deepen enough for some low clouds to intrude into the southern Sacramento Valley during the morning hours. The HREF low cloud ceiling probability prog suggests a little less than 50/50 Sat morning, and over 70% in the Carquinez Strait. These probabilities should increase for Sun morning. Our SKY grids hold off on the low clouds for Sat morning for now, and will take another look at it in later forecasts as they become available.
Deterministic models trending similar for Mon with the trof off the B.C. coast dropping into the Pac NW, but differing on the details and chance of precip for a portion of our CWA, and the amount of cooling for the CWA. Ensemble 5H cluster guidance heavily weighted to the GFS show a deeper/wetter-looking trof/closed low than the ECMWF which is drier and milder. The 5H cluster mean which is still biased to the ECMWF at least shows a weaker trof. For the time being we have “blended” the forecast to show some low PoPs over the Nrn mtns and Sierra while trending the temps cooler as well.
Extended discussion (Tuesday through Friday)
Models including ensembles continue to struggle with the extended period and look quite a bit different from yesterday on the night shift. We see the trough split over the west on Monday with the one piece of energy digging over the west and the other piece of energy closing off over the Pacific. The energy over the Pacific will become cut off for a bit and as it does it will bring short wave ridging to NorCal with dry weather for Tuesday and Wednesday.
Troughing will then dig out of the Gulf of Alaska late week and will encompass the energy from the closed off/cut off low over the Pacific. There is a ton of variability in timing and placement of this trough but some light shower chances look possible towards the end of the next week. With less troughing now, we should see highs a bit warmer then before with them slightly above average.