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May 28, 2020 – High pressure will continue hot weather today with moderate to high heat risk. A pattern change begins Friday which will lead to cooler temperatures and a chance of showers and thunderstorms over the weekend.

Discussion

Clear skies cover interior NorCal early this morning. IR difference imagery shows shallow stratus has returned to the coast and the Fort Ord profiler indicates a current depth of around 1k ft. Surface gradients are beginning to trend onshore and Travis AFB is currently seeing southwesterly gusts approach 30 mph, so the first signs of the end of the heat episode are showing up.

Hot weather will continue today as the strong ridge remains overhead, though the upper ridge axis will begin to shift east by this afternoon. While temperatures will remain hot today, most areas will see a few degrees of cooling as gradients continue to trend toward onshore. The exception will be the north end of the Sacramento Valley and over the mountains where readings will be similar to Wednesday, or a few degrees warmer.

The Excessive Heat Warning continues into this evening. If you have outdoor plans, practice heat safety and stay hydrated.

Friday we will begin to see our weather pattern change as a closed off upper level low begins to approach NorCal. This will bring noticeably cooler temperatures to the southern part of the forecast area (8-13 degrees cooler compared to today) as onshore flow invades the Central Valley. The northern areas and mountains will remain hot with highs similar, or slightly lower, to today’s.

We will also see instability build in over the higher elevations. Forcing will be weak over the area on Friday but we could see some terrain driven thunderstorms in the afternoon but they are expected to be isolated. The soundings have an inverted V look and are quite dry under 500 mb. Any thunderstorms that do develop will produce little rain and will potentially bring gusty winds.

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The closed off upper level low will push over NorCal Saturday morning into the early afternoon. This will bring widespread scattered showers to the area. Instability will be in place mainly from Sacramento north and a few thunderstorms are likely. Vertical moisture will increase Friday night and the low precip thunderstorm threat will turn into a brief heavy rain threat by Saturday. The NBM indicating potential for some 1/2 to 1 inch QPF across the northern half of the forecast area Friday night into Saturday. The shower and thunderstorm threat will diminish by late Saturday afternoon and evening. Highs will be much cooler on Saturday and will be running 5-15 degrees below average.

Temperatures will recover somewhat on Sunday as the low exits to the north and clouds and showers diminish.

Extended Discussion (Monday through Thursday)

A highly amplified pattern is in place to start the work week as an elongated longwave trough sits off the California coast while a strong ridge occupies the middle of the country. Embedded shortwaves advancing into northern California will ignite scattered showers across the foothills and mountains, especially on Monday and Tuesday. Ensembles gradually cut off this trough in the vicinity of southern California although its exact position is debatable given the typical forecast spread at these ranges. Some showers may linger over the higher terrain, mainly south of Highway 50, during the middle of next week. Upper ridging curling up and around the low center should prompt a notable warm up after starting a few degrees below climatology. High temperatures are likely to return to the upper 80s/low 90s by mid-week.