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May 11, 2020 – Precipitation chances and cooler temperatures are expected this week. High mountain snow is possible with light accumulations at pass levels bringing some slippery road conditions. Thunderstorms are possible Tuesday afternoon and evening, mainly for Shasta County. Breezy southwest winds continue through Tuesday.

Discussion

Infrared satellite shows a wrapped upper low centered off the northwestern California Coast. High clouds are spreading inland ahead of this system. The upper low will shift move to the northeast off the coast of Oregon and Washington by tonight. A cold front will move inland over NorCal today, with increasing mid level clouds across the area this afternoon. Rain showers will gradually spread into the Coastal Range this morning along and ahead of the front, reaching areas north of I-80 by this afternoon. Heaviest showers are expected over northern Shasta/eastern Butte County and northern Shasta County. Models show showers reaching the Sacramento area, northern San Joaquin Valley and the northern Sierra by this evening. The potential for showers across the area continues overnight.

An additional shortwave rotating around the low will bring additional showers on Tuesday. Afternoon and evening thunderstorms are possible over Shasta and Tehama counties. Instability does not look high, but afternoon heating could be enough if there are breaks in the clouds post frontal environment. NBM projects an 18% potential for thunderstorms for Redding. CAMs models show heaviest rainfall is expected from over northern Shasta County. Scattered showers are expected elsewhere.

Rain totals through Tuesday are looking to be up to 0.60″ in the far northern Sac Valley to minimal accumulation in the northern San Joaquin Valley and 0.50″ to 1.00″ in the mountains north of Highway 50 with locally higher amounts to 1.50″ over the Shasta County mountains.

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A few inches of accumulating snow are possible up at Sierra pass levels through Tuesday night, locally higher over peaks. Warm road temperatures should limit accumulations on highways, but periods of slippery conditions are possible in heavier showers. Lassen Peak could see up to a foot of snow above 10000 feet. Several more inches of snow could accumulate there as another shortwave continues showers on Wednesday and Thursday. The precipitation will mainly be over the mountains north of I-80 and the northern Sacramento Valley. Northern Shasta County could see up to an inch of rain, with as much as a half inch over the northern Sacramento Valley.

Temperatures trend much cooler this week, with highs in the Sacramento Valley and Delta in the 70s, around 80 over the northern San Joaquin Valley. By Tuesday and Wednesday, highs in the 60s’s to lower 70s are expected.

Gusty south to southwest winds are expected to continue through Tuesday, with the strongest winds today along with the main frontal passage. Most areas can expect gusts around 15 to 25 mph in the afternoon and evenings, with gusts up to 30 mph in the Delta. Today expect gusts 25 to 35 mph and up to 40 mph over higher terrain.

Extended Discussion (Friday through Monday)

Dry Friday with near to slightly above normal highs as upper ridging moves through. Pacific frontal system approaches early Saturday with precipitation spreading into northwestern portions of the CWA by the afternoon, and across the remainder of interior NorCal Saturday night into Sunday morning. Snow levels look to initially be above pass levels Saturday, lowering behind the front to around 5500 to 6500 Sunday morning. Unsettled weather continues through Monday as associated upper low remains off the West Coast, sending embedded vorticity maxima through the forecast area.