April 17, 2020 – Unsettled conditions with near normal temperatures into early next week. The best shower and thunderstorm chances will be over the mountains, but a few could move into the Valley this afternoon.
A closed low off the coast will move inland into SoCal today. This will bring increased clouds and cooler temperatures into the weekend. Showers and isolated thunderstorms will be possible, mainly over the mountains and foothills.
Showers this morning are possible on the northern fringe of the low over the deformation axis, south of I-80. CAMS models show showers and isolated thunderstorms spreading northward up the Sierra and into the northern Coastal Range. CAPE levels look higher than yesterday, and expect more thunderstorms than we saw over the Sierra on Thursday. Some convective activity could spread into the northern San Joaquin Valley as well, with some showers possible into eastern portions of the southern and central Sacramento Valley.
On Saturday mountain showers continue, with a slight chance spreading northward into Shasta County. Moisture looks limited and snow levels are at and above pass levels, so snow should have little impact. Another low drops down from Canada on Sunday, continuing showers over the northern mountains. The overland trajectory will bring limited moisture. By Monday, this system tracks into the Great Basin with drier weather briefly returning.
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A shortwave from the eastern Pacific tracks inland on Monday, bringing the potential for widespread precipitation. Ensembles continue to favor minor rainfall and snowfall amounts and have trended even lower, though, so impacts are expected to be limited. Temperatures Monday afternoon are forecast to be below normal, remaining in the upper 60s for the Valley. Snowfall amounts near the Sierra Crest are currently projected to be just an inch or two at this point.
Extended Discussion (Tuesday through Friday)
Ridging builds in behind the early week system on Tuesday, resulting in dry and warm conditions. A weak system may brush across the far north by midweek, and bring some light showers to the northern mountains. QPF amounts should remain on the light side. Depending on how this system tracks above the mentioned ridge, some increase in northerly winds could occur in its wake. Ensembles suggest wind gusts in the 20-30 mph range. Ridging will result in stable conditions and above normal temperatures late in the period, with Valley highs in the upper 70s to low 80s. These forecast highs will be 5 to 10 degrees above seasonal normals.