Unsettled weather this week with chance of rain and cooler than normal temperatures

October 1, 2018 – Areas of light to locally moderate precipitation possible Monday into Thursday. Drier Friday into next weekend with breezy northerly wind. Below normal temperatures expected through the forecast period.

Discussion

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Water vapor satellite this morning is showing an upper level low about 650 miles off the coast of CA. This upper level low will track south and east today developing an area of warm air advection over the northern part of the area. This will bring the chance for showers starting in the coastal range mid morning and spreading across the rest of the northern part of the CWA during the afternoon. Rain accumulation through the afternoon is expected to remain light.

The upper level low will continue to track south down the coast and push a cold front into the region later Monday into Tuesday. This will bring more widespread shower activity mainly to the coastal range and far western part of the CWA with scattered showers over the remainder of the area.

Models are indicating some instability building into the far western part of the CWA and isolated thunderstorms will be possible late morning Tuesday into the afternoon. The upper level low will track over SoCal on Wednesday and become an open wave later in the day. This will keep shower activity in the forecast Wednesday but the greatest chances will be in the Sierra.

We also see some instability build into the higher elevation during the afternoon Wednesday and this will bring the chance for a few t-storms. The upper level low tracks east on Thursday but we will see a short wave trough dig south into far northern CA and the PacNW. This will keep shower chances in the higher elevations on Thursday.

Rain totals today through Thursday are looking to remain under 0.25″ in the valley and generally between 0.25-0.75″ in the higher elevations. A short wave ridge builds in on Friday and this will bring drying conditions. Temperatures will be cool this week generally running 5-15 below average for this time of year. A warming trend will begin on Friday.

Extended Discussion (Friday through Monday)

Upper ridging builds inland late this week as short-wave energy digs to the east developing a trough over the Rockies. Surface high pressure also builds from the eastern Pacific into the PacNW resulting in and enhanced N-S pressure gradient and breezy north to east wind over portions of Interior NorCal that continues into early next week. This could result in increased fire weather concern depending on amount of precipitation received earlier in the week. Temperatures will be close to average.