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May 23, 2020 – An extended period of excessive heat is expected next week, with moderate to very high heat risk. Dry weather is expected through next week, except for a few showers along the Sierra crest south of HWY 4 by mid-week. More widespread mountain showers and possible thunderstorms late next week.

Discussion

Less northerly wind and warmer temperatures expected today as the Pacific ridge builds into the area. Today`s highs will be in the mid to upper 80s in the Valley, and will likely climb into the 90s by Sunday. These forecast highs will be 5 to 10 degrees above climatology. The heat risk will run in the moderate category for portions of the Valley and the Delta on Sunday. Sensitive groups should prepare for hot conditions and stay hydrated.

An amplified ridge of high pressure will dominate the pattern next week. This anomalous pattern should result in a prolonged period of excessive heat across portions of interior NorCal. Memorial Day is shaping up to be a hot one, with Valley highs approaching the century mark. Heights are forecast to continue building next week, with further warming on Tuesday.

Forecast 850mb temperatures in the 22-25C range should support the first widespread triple digits of the season, with Valley highs generally topping around 103 degrees on Tuesday. Overnight relief might be limited during this prolonged heat event, with morning lows in the mid 60s to mid 70s. Some locations may tie or exceed daily high temperature records on Tuesday. NBM shows a 66% chance of tying and/or breaking record maximum temperatures on Tuesday at Sacramento Executive AP, 77% at Stockton and 61% at Modesto.

Thus, an Excessive Heat Watch remains in effect for the Valley, the Delta and foothills from Monday through Thursday. The general population should take precautions to prevent heat related illnesses given the potential for moderate to very high heat risk. Area waterways continue to run very cold and fast, and precautions should be taken if heading to the water.

Extended Discussion (Wednesday through Saturday)

Wednesday and Thursday the West remains in an Omega Block pattern, with a pumped up ridge between low pressure over the eastern Pacific and the southern Plains. Very hot weather is expected Wednesday, with widespread triple digits and daily record high temperatures likely to be set. Little relief is expected overnight, with thermal belt areas forecast minimums remaining in the low to mid 70s, and Valley locations in the upper 60s to lower 70s. These factors and the multiple days of heat combine to have heat risk levels projected to be High Risk to the entire population for much of the Valley and Delta, with Very High Risk for much of the west side of the Central Valley. This will bring dangerous levels of heat, especially to vulnerable populations.

Little change is expected on Thursday, with some locations potentially even a little hotter. The Heat Risk continues to be High for much of the Valley, with an expanded area pf Very High for the central and western Valley. Ensemble NAEFS return intervals how 850 mb temperatures in some locations are outside the range of climatology, which highlights the unusual nature of the heat.

Conditions will be dry except for the Sierra Crest south of Highway 4, where are a few showers and possibly thunderstorms are possible in afternoon heating. This is due to the potential for strong southerly flow to bring in some subtropical moisture.

The Pacific low moves inland later Friday and Saturday, putting an end to the heat and bringing potential precipitation. Latest NBM and ensembles show a trend towards wetter weather, so have bumped up chances of mainly mountain precipitation. Instability levels are looking sufficient for some afternoon and evening thunderstorms. Temperatures Friday will cool with onshore flow and increasing clouds, with highs 10 to 15 degrees less than Thursday. By Saturday, highs should be down to normal levels.