Heat flares back up, then weekend will be on the cool side

September 26, 2018 – Hot and dry weather continues for the next couple of days. Cooler with a slight chance of showers by the weekend.

Discussion

Clear skies across interior NorCal as subsidence dominates downstream from the Rex Block over the eastern Pacific. While satellite imagery indicates stratus along the entire north coast, the marine layer remains suppressed at around 1000 ft or below. It remains very dry across the region away from the limited Delta Breeze influence with many areas reporting dewpoints in the upper teens and 20s. Current temperatures range from the 30s in the mountain valleys to around 70 across the mildest portions of the Central Valley. Little change in the hot and dry pattern expected today and Thursday, though the north wind pattern will continue to subside. Warmest portions of the forecast area will once again see highs around the century mark.

By Friday, the closed low that makes up the southern half of the Rex Block is forecast to edge closer to the coast as stronger energy upstream drops southward into the Pacific from the western Aleutians nudging the pattern along. This will initiate a strong cooling trend across the region heading into the weekend. A few showers may even be possible on Saturday as the low lifts out across far northwestern California, though uncertainty exists on how far south the shower potential will extend.

Extended Discussion (Sunday through Wednesday)

Upper low continues to track into the Pacific Northwest early Sunday but latest model runs have it fairly far north so impact on CWA weather will likely be minimal. At this time, forecast has just slight chance pops over the northern mountains Sunday morning. Daytime high temperatures Sunday afternoon should come in similar to Saturday highs under similar airmass and variable cloud cover.

Next Pacific low on track to approach the north coast on Monday bringing a chance of rainfall to the coast range early in the day. Models diverge pretty significantly at this point. GFS remains more progressive of the extended models pushing this next system through NorCal during the day Monday bringing a chance of precipitation to most of the CWA north of about Sacramento.

GFS then progresses this system east of the state and into the Great Basin on Tuesday. ECMWF scenario drops the low southward off the coast on Monday and Tuesday allowing it to tap into moisture from tropical storm Rosa. This moisture tap brings rain chances mainly to the southern half of the state but possibly into our forecast area as well. GFS solution looks a bit more realistic and have weighed with it for the extended forecast.

Either way, a significant change in the weather pattern is indicated with daytime temperatures remaining below normal.

Wednesday looks mainly dry with temperatures warming back up to near normal under building upper ridge over the eastern Pacific. Model differences continue to be an issue, however, so confidence level remains low in the extended period.