October 2, 2018 – Areas of light to locally moderate precipitation possible through Thursday. Drier Friday into next weekend with breezy north wind. Below normal temperatures expected through the forecast period.
The upper level low is now about 350 miles off the coast of CA with the associated cold front 50-100 miles off the coast. Showers currently moving through the wester part of the CWA and Coastal Range are out ahead of this cold front. We will continue to see widespread showers mainly over the Coast Range and west of I-5 during the morning. Shower activity will start to become less widespread during the afternoon and evening as the upper level low starts to track south and the cold front weakens. Rain totals through this evening will remain light with the heaviest totals over the Coastal Range. As we head into the overnight the upper level low will continue to push south and east and we will see some moisture advection ahead of the weakening cold front move north. This will bring a slight chance for scattered showers overnight with the best chances in the San Joaquin Valley, totals will remain light. We will also start to see some instability build into the region overnight with the increased moisture. As we head into Wednesday afternoon we will see CAPE values reach 200-500 j/kg across the entire CWA and this will bring the chance for t-storms. This activity is not expected to be widespread as the better forcing will remain to our south but will be a bit more widespread in the higher elevations. The upper level low will become an open wave early Thursday and track east as a trough digs south and tracks over NorCal. This will keep shower activity in the forecast but should be limited to the higher elevations. Instability looks to be limited on Thursday but still can`t rule out an isolated t-storms during the afternoon. Temperatures will continue to run below average with the coolest day expected on Thursday.
Extended Discussion (Saturday through Tuesday)
Pattern amplifies later this week into early next week with strong ridging over the eastern Pacific and a deepening trough over the western US. Embedded short-waves in the northerly flow over NorCal will lead to occasional shower chances over the mountains, otherwise the remainder of the forecast area is expected to remain dry. Breezy north winds are expected over the weekend as N-S pressure gradients tighten which could potentially result in increased fire weather concerns. North winds slacken Monday.