Mostly clear skies prevail across interior Northern California early this morning as the Rex Block remains nearly stationary over the eastern Pacific and Gulf of Alaska. Current temperatures are a little milder across most of the region compared to 24 hours ago and range from the 30s in the mountain valleys to the upper 50s to around 70 across the Central Valley and surrounding thermal belts. Little change is expected today as the region remains under the influence of high pressure. The area of strongest subsidence has lifted to the north and occasional high clouds are expected which may edge temperatures down just a bit.
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 over the weekend as the low lifts out across far northwestern California, though uncertainty exists on how far south the shower potential will extend.
Extended Discussion (Monday through Thursday)
Upper-level energy off the West Coast will approach interior NorCal on Monday. Main impacts expected to be a chance of precipitation across the forecast area, and below normal temperatures. Model guidance continues to struggle in terms of the overall progression of the system, as well as precipitation timing and amounts.
Regardless, best chances of precipitation appear to be Monday afternoon into Tuesday, with lingering activity over the Sierra into Wednesday afternoon. In terms of daytime highs, the coolest temperatures over the next week are expected to occur on Tuesday as the upper system moves overhead.
Look for Valley highs in the 70s, and 50s to 60s over the mountains on Tuesday. Temperatures will be warming back up to near normal by the end of the forecast period as upper ridging rebuilds across the area.