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July 25, 2018 –  Upper ridge of high pressure expected to retrograde slightly and bring increasingly warm temperatures through Friday. Latest consensus model blends have increasing afternoon high temperatures today through Friday. Updated forecast to extend heat advisory through Friday, added portions of the lower Sacramento Valley to the heat advisory, and upgraded to an excessive heat warning for the northern and central Sacramento valley through Friday.

Discussion (Today through Friday)

Upper ridge of high pressure over the southwest U.S. continues to edge westward. A shortwave disturbance rotating over this ridge brought evening thunderstorm activity to the north and east of the CWA but this activity has ended and skies are clear this morning. Overall airmass will warm slightly today so should see an edge up in temperatures so heat advisory will remain in place.

Most locations this morning seeing temperatures a degree or two above 24 hours ago. Moderate delta breeze continues which will allow overnight lows to drop into the 60s in delta influenced areas as well as moderate daytime temperatures so these delta influence areas will stay out of the heat advisory. Most of the forecast area under stable southwest flow but could see a little monsoon moisture work its way up the Sierra so a slight threat of afternoon thunderstorms continues south of about highway 50.

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Upper ridge edges back westward even farther on Thursday bringing still more subsidence and warming over NorCal and therefore expect yet another edge up in temperatures in non delta influenced areas. This will likely make Thursday the warmest day of the heat spell. Isolated afternoon thunderstorms will still be possible over the northern Sierra.

Upper ridge still forecast to start shifting back eastward on Friday and this should bring the start of a very slow cooling trend. Even with cooler temperatures, daytime highs will remain well above normal. A more established southwest flow aloft should keep any afternoon thunderstorm threat east of the region. Only minor changes are expected in the forecast for Saturday with upper ridge forecasting very little movement.

Extended discussion (Sunday through Wednesday)

A ridge of high pressure will remain a fixture in the forecast through the period with dry conditions and above normal temperatures. Little change is expected in daytime temperatures, with highs in the 90s to lower 100s across the Valley, and upper 70s to 80s to 90s over higher terrain. Onshore flow will keep overnight temperatures in the low to mid 60s for most of the valley, up to the low 70s in the northern Sacramento valley.