September 17, 2020 – Localized fire weather concerns thru Friday as a weather system passes through the area. Main concern will be from winds across the northern Sacramento Valley this afternoon, then over the northern Sierra higher terrain late tonight into Friday. A few showers will be possible Friday over the northern Sierra and across the Coast Range and northern mountains.
Discussion
Skies are clear across the region early this morning other than areas of smoke and haze continuing from the ongoing wildfires. IR difference imagery shows extensive stratus along the coast extending inland through the Bay Area. Similar to 24 hours ago, a narrow band of stratus is nosing into the Central Valley through the Carquinez Strait, and Travis AFB is now carrying a low ceiling. We may see some low clouds around sunrise across the Sacramento area and portions of the southern Sacramento Valley.
Satellite imagery shows the closed low off the coast of the PacNW with a good fetch of south/southwesterly flow out ahead of it across NorCal. This flow is drawing high clouds northward from the remnants of Karina which will move across central California later today.
The low off the PacNW coast is forecast to pivot toward NorCal today. This will push a mid/upper level front into NW California this afternoon which will then move southeast overnight tonight into Friday. Strong wind fields at 850/700 mbs forecast to accompany the front as it moves through, and surface pressure falls ahead of the front are expected to induce a pretty strong southerly gradient across the Sacramento Valley this afternoon. Gusts in the 20s mph will be possible across the northern half of the Sacramento Valley by mid-afternoon continuing into the evening.
RH’s expected to rise somewhat across the valley and foothills today, but will remain in the teens across the higher elevations of the southern Cascade Range and northern Sierra Nevada, and combined with afternoon breezes, will result in elevated fire weather conditions.
Trough moves across the area on Friday with a chance of showers across the Coast Range and northern mountains. Some of the moisture from Karina may pop a few showers across the northern Sierra. Temperatures are expected to be around 10 degrees cooler compared to today, and RH’s are forecast to be considerably higher across the region.
Ridging returns over the weekend bringing a return of warmer and drier conditions with generally light to moderate north to northeast winds.
Extended Discussion (Monday through Thursday)
Warm and dry pattern early next week under zonal flow. Ridge then amplifies into Wednesday, resulting in a slight warming trend. Near normal temperatures are expected, with Valley highs generally in the 80s and low 90s.
Ensemble guidance is hinting at another trough approaching the region late next week, with possible cooler and breezier conditions. Confidence is low in exact timing/impacts of trough in the extended.