September 29, 2016 – Near normal temperatures today then a cooling trend through the weekend with daytime highs 15 to 25 degrees below normal by Sunday. Chance of showers and isolated thunderstorms Sunday into Tuesday. Drier by the middle of next week. A warming trend next week.
Discussion
The cooling trend continues today as a low pressure system in the Gulf of Alaska drops southeast towards the coast. Highs today should come in right around normal for this time of year. A moderate delta breeze has brought coastal stratus into the delta this morning. As the upper trough drops into the eastern Pacific still more cooling is expected Friday. A continued delta breeze on Friday will bring the most cooling in the delta influenced areas. Still more cooling is expected on Saturday with daytime highs dropping to around 10 degrees below normal.
The upper low centers around the northwest corner of the state by Sunday afternoon. Enough moisture and instability associated with this low for a threat of precipitation over nearly the entire forecast area.
GFS UVV and most unstable cape show the best chance of any thunderstorm activity will be Sunday afternoon and evening over the Sierra cascade crest, northern mountains and northern Sacramento valley and coast range.
Still more cooling will make Sunday the coolest day of the forecast period with daytime highs forecast to drop to between 15 and 20 degrees below normal.
Extended discussion (Monday through Thursday)
The upper level low quickly moves through the region, shifting east into the Great Basin by Monday. Cool northerly flow aloft takes hold across the region on Monday, and there may be enough residual moisture for a few showers as well. Overnight temperatures will likely be on the cool side for a few nights, with 40s across the Central Valley and 20s possible over higher terrain.
Drier weather is expected by the middle of next week as the eastern Pacific ridge rebuilds into California. High temperatures are expected to warm back to near normal levels, though overnight lows may remain on the cooler side.