October 19, 2018 – Warm and dry conditions through the weekend with daytime temperatures a few to several degrees above normal. A cooling trend starting Monday with a chance of precipitation northern forecast area Tuesday through Thursday. High pressure rebuilding with warming and drying at the end of next week.
Discussion
A Rex Block type pattern is forecast to develop today with high pressure over the Pacific Northwest and a weak low centered off the Socal coast. Overall airmass warms a few to several degrees today as the upper ridge portion of the block centers along the coast. Surface high pressure digging into the great basin will bring about a 4mb easterly surface gradient across the Sierra tonight. This combined with upper east flow will bring breezy east winds over the Sierra tonight through Saturday morning. West facing wind prone canyons could see gusts up to about 25 mph.
As the upper ridge shifts inland on Saturday winds will decrease with a slight cooling likely as the upper ridge axis shifts east of the region. Even with the cooling, daytime highs Saturday will still come in several degrees above normal.
By Sunday, the SoCal upper low opens and moves inland over the state. The effects of this will be minor but most areas will see at least a little cooling. A bit more cooling occurs on Monday as the upper low remains over the southwest U.S. but high temperatures still remain slightly above normal.
Extended Discussion (Tuesday through Friday)
Pac frontal system approaches the West Coast Tuesday, shifting inland Tue afternoon into night. Baroclinic zone weakens as it moves inland under diffluent upper level flow. EC showing stronger system and more widespread precip over CWA, while GFS indicating little if any precip. EC has also trended drier from previous run. Given synoptic pattern, forecast leans more towards drier solution with low POPs mainly over the Coastal and Shasta mountains Tue into Wed.
Another disturbance moves into northern portions of the forecast area Thursday. Lingering showers possible over mainly the Shasta mountains Friday otherwise upper ridging strengthens over Interior NorCal. Below normal high temps forecast Tuesday through Thursday, warming to slightly above normal Friday.