April 29, 2021 – Dry and above normal weather are forecast to prevail through the rest of the week. Gusty offshore winds possible Sunday and Monday.
Discussion
Upper level ridging remains over California today bringing warmer temperatures to the forecast area. Current satellite imagery shows light upper level clouds streaming over the ridge and into northern California. These clouds along with the upper level ridging will likely keep minimum temperatures fairly mild- generally in the 50s across the Valley, 40s to 50s in the mountains, and 50s to 60s in the foothills due to the thermal belt. Daytime temperatures today will soar to around 15 to 20 degrees above normal. Valley temperatures are expected to peak in the upper 80s to low 90s with 60s to 80s in the mountains and foothills.
Ensembles in good agreement of a weather system approaching the West Coast Friday and moving onshore through the weekend. This will flatten the upper level ridge ahead of the weather system on Friday, allowing some cloud cover over the area through the weekend. Temperatures still expected to remain well above normal Friday with Valley temperatures in the upper 80s. Temperatures then forecast to cool a few degrees each day through the weekend as the weak weather system moves through the forecast area, though still will remain around 5 to 15 degrees above normal. No precipitation is expected with this weather system as there will not be much moisture associated with it. The main impact will be locally increased onshore flow on Saturday.
As the weather system moves to the east on Sunday, ensemble guidance continues to suggest the low will quickly move southeast into the Desert Southwest with upper level ridging rebuilding over the Eastern Pacific. This will cause surface pressure gradients to tighten, resulting in gusty offshore winds Sunday. Daytime relative humidity values are forecast to remain extremely low through the forecast period, even with the weak trough passage. The gusty offshore (north to east) winds along with continued dry conditions could bring increased fire weather concerns Sunday into early next week.
Extended discussion (Monday through Thursday)
Ensembles continue to favor strong ridging over NorCal early next week resulting in a continuation of the unseasonably warm and dry pattern with breezy north to northeast winds at times. Clusters indicate only about a 50 percent chance that the ridge will begin to weaken and shift east after mid-week, so we’ll lean toward NBM with only slight cooling forecast on Thursday (but low confidence of this materializing).