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April 25, 2018 – Mainly dry and warm this week except for a chance of a few afternoon showers or thunderstorms near the Sierra Crest. Cooler weather Friday into the weekend with increasing chances for mountain showers and thunderstorms.

Discussion

Skies are clearing out as the initial band of high clouds shifts east. Current temperatures range from the mid 30s to lower 40s in the mountain valleys to the 50s to lower 60s across the Central Valley.

Ridge will remain over the area over the next 24 hours as the offshore low remains quasi-stationary off the coast. Temperatures will remain warm inland today, but some cooling will begin near the Delta as onshore flow increases. Cap may weaken enough by late this afternoon to allow for isolated thunderstorm development along the Sierra crest.

Low is forecast to shift closer to the coast Thursday and Friday before moving inland Saturday. Temperatures will cool to average or below by Friday and Saturday and chances for mainly mountain showers and thunderstorms will increase.

Extended discussion (Sunday through Wednesday)

Extended models keep upper low over the Pacific Northwest and NorCal as the prominent weather feature on Sunday. Still enough instability indicated for isolated thunderstorms in the afternoon. Upper trough axis should have shifted east of the state by Sunday so shower and thunderstorm threat should remain confined to the northeastern half of the forecast area. Cooler air ushered into the region will keep daytime highs just a bit below normal. Models differ going into early next week but a slight shower threat continues at this time over the Sierra Cascade range in the western most portion of the cyclonic flow of the low. Most areas should see northerly flow on Monday as upper ridging begins to build over the eastern Pacific. This should bring up daytime highs Monday to a few degrees above normal. As usual with upper lows models struggle with progression of this one. Have left a slight chance of showers over the northern Sierra on Tuesday but the rest of the forecast area remains dry. Airmass continues to warm with highs by Tuesday pushing to more than 10 degrees above normal. Upper ridging will bring more warming on Wednesday with northerly flow and subsidence bringing and end to any precipitation threat over the Sierra.