Critical fire weather conditions through this evening due to gusty winds and low humidity across portions of the Sacramento Valley. Areas of Moderate HeatRisk today, then cooling trend begins Friday with below normal temperatures through early next week. Increased chances for showers and isolated thunderstorms Friday into the weekend.
Trough that moved through NorCal Tuesday has moved into the northern Rockies early this morning, with gusty northerly winds in its wake. Peak wind gusts so far have ranged from 15-30 mph along the west side of the Sacramento Valley, as the northerly surface pressure gradient has tightened (MFR-SAC is up to around ~ 11 mbs, and the RDD-SAC is 2.6 mbs). Latest HRRR smoke continues to show areas of haze and wildfiresmoke pushing down the Valley this morning, with current visibilities down to 4 to 6 miles across portions of the Sacramento Valley. Areas of haze and smoke may linger across the northern San Joaquin Valley through early this afternoon. Lingering north winds may result in additional smoke intrusion across portions of the northern Sacramento Valley tonight.
Expect Valley temperatures in the upper 90s this afternoon, with upper 80s to 90s in the foothills, and 70s to low 90s in the mountains. Areas of Moderate HeatRisk are expected across the Delta and portions of the Sacramento Valley. Sensitive groups should practice heat safety.
The Extreme Forecast Index (EFI) continues to highlight the elevated offshore winds today. Peak wind gusts of 25 to 35 mph can be expected across the northern, central, and west side of the Sacramento Valley; strongest winds this morning through early this afternoon. Cannot rule out the potential for some localized wind gusts near 40 mph this morning.
The combination of gusty northerly winds and low humidity will result in critical fire weather conditions across portions of the northern, central, and western Sacramento Valley through this evening.
Breezy winds are forecast to decrease Wednesday evening with an increase in onshore flow anticipated Thursday afternoon.
Ensembles and clusters advertise an unseasonably strong upper low dropping south into NorCal late this week. This will lead to a significant cooling trend, with Valley highs cooling into the 70s to low 80s Friday and Saturday (10-25 degrees below seasonal normals).
There is also the potential for showers and isolated thunderstorms for the Coastal Range, southern cascades, northern Sierra, and the northern Sacramento Valley Friday and Saturday. There is a lesser chance for thunderstorm development as cloud cover may inhibit convection (15-25% probability).
Latest NBM shows precipitation moving into portions of Shasta County as early as late Thursday night. Latest WPC guidance indicates generally 0.25-0.75 inches of rain over the mountains (locally higher amounts possible in Shasta County), and 0.10-0.40 inches across the northern Sacramento Valley. The EFI also depicts elevated onshore winds Thursday afternoon into Friday (strongest winds Friday).
Extended Discussion (Sunday through Wednesday)
Upper low forecast to push through interior NorCal Sunday as it weakens. Feature will keep a threat of showers/thunderstorms over higher elevations of the Sierra Nevada. High temperatures Sunday look to be around 5 to 10 degrees below normal. AMS stabilizes Sunday night as short wave ridging moves through.