September 21, 2018 – Continued warm with improving humidity levels with return of onshore flow today. Cooling trend continues through the weekend with locally breezy winds over the Sierra. Warming above normal next week with breezy north winds and drying on Monday. No precipitation expected over interior NorCal through the extended period.
Winds a little lighter over the northern Sacramento valley and over the coast range now that northerly surface gradient has relaxed. Surface winds should shift to mainly onshore today as the upper ridge over the west coast slips inland this afternoon. By this evening the upper ridge axis should be over the Great Basin but daytime highs today should still come in about the same as yesterday. RH values should creep up a bit today thanks to the more onshore flow.
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A low pressure system moving out the the Gulf of Alaska is predicted to move into the Pacific Northwest on Saturday. Main impact of this system will be a drop in daytime temperatures over the weekend and breezy winds over the higher Sierra elevations. Any precipitation should remain well north of the forecast area with just a few clouds expected over the northern most zones. Just a bit more cooling is expected on Sunday as the upper trough axis shifts into the northern Great Basin. Breezy southwest winds will continue over the Sierra.
Breezy north winds will return in the wake of trough on Monday bringing drying again especially over the northern zones and down the coast range. These breezy north winds and a several degree warm up will likely increase fire danger. Highs on Monday will likely climb to a few to several degrees above normal as high pressure aloft begins building in over the west coast.
Extended Discussion (Tuesday through Friday)
The highly amplified ridge over the eastern Pacific will likely build into California on Tuesday, with strong subsidence remaining over Northern CA through midweek. NAEFS ensemble analysis suggests this system will bring warmer than normal temperatures, but isn`t particularly anomalous for this time of year. Current blended guidance suggests temperatures will be in the 90s across the Valley Tuesday through Thursday, with 70s to 80s over the mountains. The closed upper low associated with this Rex Block will very slowly approach the West Coast. Current guidance suggests that this low will move inland late next week, but model performance of these closed lows is notoriously poor in the medium-range.