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June 25, 2020 – The prolonged period of hot weather continues through Saturday. Moderate to very high heat risk is expected today and Friday in the Valley and foothills, with little overnight relief. Slight chance of afternoon and evening thunderstorms mainly over the mountains today. Significant cooling trend Sunday and into early next week, with a slight chance of some mountain showers.

Discussion

Positively tilted upper level trough shifts a little southeast today, with eastern Pacific ridging building into NorCal. High temperatures this afternoon will be very hot, generally 4 to 8 degrees higher than yesterday. The hottest area is expected to be the northern Sacramento Valley, with Redding at 110, while the Valley to the south will be in the 100-107 range. Except for the marine/Delta influenced areas, overnite lows will continue to be quite warm. Foothill elevations within the temperature inversion and in strong thermal belt areas will experience some very warm overnights, with lows in the upper 70s to near 80. Friday highs look similar to Thursday, maybe slightly less hot in Delta Breeze areas, but a little hotter elsewhere. Redding is the forecast hot spot at 112, just shy of the daily record of 113.

Thunderstorms are possible over northern Sierra this afternoon and early evening, and also over Lassen Peak. The best potential is south of I-80. NAM soundings show high CAPE levels 3000-4000+ j/kg, favorable for hail growth. SARS Sounding Analogs even give some potential for significant hail along the Crest south of Highway 50. Mountain convection continues into the early evening, with dry weather returning by midnight across the area.

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Saturday will see the beginning of the end of the heat wave as a developing trough over British Columbia moves into the Pacific Northwest. The cooling Delta Breeze should return keeping temperatures in the upper 80s to mid90s in the marine/Delta influenced areas. Although still a little toasty up north, max temperatures on Saturday should range from about 10 degrees cooler up north to little change south of Modesto. The Heat Advisory remains in effect through 8 pm Saturday evening.

Sunday looks significantly cooler, with Valley and Delta highs only in the 80s, finally reaching below normal levels. Some showers could spread into the mountains Sunday evening.

Extended Discussion (Monday through Thursday)

A broad trough will slowly be moving eastwards and into the intermountain west this period. This will usher in a slightly cooler airmass to the region. High temperatures are forecast to be near normal for this time of year; however, an upward trend is forecast for the middle of the week which will place Valley highs in the 90s. Breezy winds are forecast with this pattern change as well, with north winds expected on Monday. Finally, the chance for some mountain showers remains in the forecast for the early portion of the week.