NWS SF Saturday: Very cold air will spill into the Great Basin setting up a strong surface pressure gradient across the Sierra. Pressure gradients rapidly increase to about 18-19 mb over the next 24 hours. Initial shot of north winds will work down the Sac Valley today and should reach the Napa hills by early afternoon. Strong wind gusts in excess of 50 mph will hold off until 4-7 pm.
We dont often get to see true synoptic frontal boundaries that are so clearly defined. The airmass change will be abrupt as very dry air filters over the region and it wont discriminate between the hills and valleys.
The northeast winds will howl across much of the Bay Area. Models show strongest wind potential will be with the initial burst of winds. Model cross sections show very strong winds aloft with downward sinking air.
There wont be any inversion of note to keep the strong winds from mixing down. In terms of magnitude if memory recalls the models were showing perhaps stronger wind strength during the 2017 wine country and 2019 Kincade fires but this event looks to be more widespread. Another noted difference is the strong east winds over the Sierra region west of Tahoe. During these setups those winds take a straight beeline towards Mt Diablo and then under the right conditions will drop down into the East Bay hills region above the 880 corridor. Wind damage still appears likely overnight as the high momentum wind gusts will take out drought and fire weakened trees/limbs/powerpoles across the region.
Unlike normal offshore wind events we dont expect the strongest winds to be confined to the hills as the atmospheric profile will be conducive to mixing those winds down.
Wind gusts 40-50 mph at lower elevations, downwind of the hills (East Bay hills, coastal Marin, Pacifica/Half Moon Bay) with expectation for gusts 60-70 mph for the higher elevations of of the North and East bay hills, roughly above 2500 feet. Heavy fuels remain at or near record dry levels.
Long range spotting, record dry fuels, wind gusts 50-70 mph and humidity in the teens would all be valid model inputs along with a highly turbulent/unstable boundary layer. Expect egress to be blocked and normal comms to potentially be disrupted.
