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April 26, 2018 – Warm and dry today except a slight threat of isolated mountain showers or thunderstorms. Cooler Friday through the weekend with increasing chances for mainly mountain showers and thunderstorms.

Discussion

One last day of above normal temperatures expected today as the west coast remains under upper level high pressure ridging. Main ridge axis has shifted east of the state however ahead of a Pacific low pressure system edging towards the coast. Models are indicating a slight threat of convective showers over the Sierra and Coast range this afternoon as the north state edges under cyclonic flow.

A fairly sturdy delta breeze has developed and is expected to continue through the day. Therefore temperatures will cool today in the delta and near delta areas more significantly than elsewhere while still remaining a little above normal. Main upper low is forecast to push into the Pacific Northwest and NorCal on Friday bringing cooling to all areas.

Daytime highs will drop to a little below normal thanks to cooler airmass and cloud cover especially over the northern most portion of the state. Areas closest to the low center over the northwest portion of the state will see another threat of afternoon showers. This threat will shift eastward Friday night as the upper low continues to shift inland. Stability progs indicate enough instability for a chance of thunderstorms as well.

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The upper low continues to shift inland on Saturday bringing still more cooling and spreading the shower threat over most of the CWA. Stability progs indicating most unstable areas Saturday afternoon will be far north state and over the Sierra Cascade crest so have limited thunderstorm threat to these areas. Only minor changes are expected on Sunday as the upper low continues to slowly lift northeastward. Showers or thunderstorms will remain a possibility over the northeast half of the forecast area Sunday but areas southwest of about Marysville should remain dry.

Extended discussion (Monday through Thursday)

Northern California will likely remain under cyclonic flow on Monday and potentially into Tuesday, with a longwave trough centered over the Pacific Northwest. As such, the chance for a few high Sierra showers and thunderstorms will persist.

This upper troughing will shift eastward toward the middle of next week as the eastern Pacific ridge builds into California. This will result in warmer and drier weather across the region. Longer range models hint at a broad area of low pressure returning to the West Coast late next week, bringing a modest cooling trend and potentially a few mountain showers.