April 30, 2021 – Dry and above normal weather are forecast to prevail through the rest of the week. Gusty offshore winds possible Sunday and Monday could bring elevated fire weather concerns.
Discussion
An trough approaching the West Coast is beginning the flatten the upper level ridge overnight. Satellite imagery is showing upper level clouds streaming over Northern California along the flow. Cloud cover is keeping temperatures very mild overnight with most Valley locations remaining in the low to mid 60s as of 1 am.
Morning lows are expected in the low to mid 50s across the Valley, 50s and 60s in the foothills, and 30s to 50s for the mountains. The upper ridge will continue to flatten over California today with the axis shifting to the east.
Overall, high temperatures will be similar to yesterday in the Valley with some mountain and foothill locations a few degrees lower than yesterday due to continued cloud cover and the shifting ridge. However, temperatures across the forecast area will still be 10 to 15 degrees above the daily normal with Valley temperatures near 90 degrees again.
Good agreement remains between ensembles and previous forecast runs in the upper trough moving through Northern California Saturday. The forcing and moisture are fairly weak with this trough over the CWA, so precipitation chances look to remain just to the north. However, recent model runs indicate a very slight chance of isolated showers in Shasta county and Lassen county mountains.
Temperatures are expected to lower another few degrees again for the weekend, though they will still remain well above normal. Valley temperatures are forecast in the mid to upper 80s with 50s to 70s in the mountains and foothills. Onshore flow will continue today into Saturday, peaking Saturday as the trough moves through the area. Locally gusty winds forecast, mainly in the Delta influenced areas.
Ensembles then continue to show the trough digging southeast into Nevada and then the Desert Southwest Sunday into Monday with upper level ridging building back over the Eastern Pacific. Winds will quickly switch from onshore to offshore. Surface pressure gradients expected to tighten producing breezy north to east winds Sunday and Monday.
Gusts up to 20 to 35 mph are forecast in the western Sacramento Valley, Northern Sierra, and the foothills. Daytime relative humidities will also be extremely low, especially for early May, with widespread values in the single digits to the teens across the Valley.
These dry conditions and breezy winds could create elevated fire weather concerns Sunday and Monday. Burning is not recommended, and outdoor activities that could produce sparks over dry grass should be avoided.
Extended discussion (Tuesday through Friday)
Another warm-up expected to continue into the middle of next week as strong ridging builds over NorCal. Highs may climb into the lower to mid 90s across the Central Valley on Wednesday. Still quite a bit of uncertainty on the evolution of the ridge and possible trough late next week.
A majority of ensemble clusters favor maintaining the ridge, although weaker than mid-week, allowing for minor cooling (but still around 10-15 degrees above average). However, a few are pointing toward a stronger trough brushing through next weekend which could bring another brief cool down followed by breezy north winds in its wake.