July 24, 2020 – Slightly below average temperatures continue, trending a little above average over the weekend. A few late day thunderstorms are possible over the Sierra Crest south of I-80, becoming a bit more widespread across the Sierra by Sunday. Breezy southwesterly winds to continue near the Delta while spreading into the Valley.
Discussion
Early morning GOES-17 satellite imagery and surface observations show the marine layer has stayed confined to the Bay Area with a depth of around 3,000 feet per the Fort Ord profiler. Current temperatures are running close to where they have been with upper 50s around the Delta and nearby influenced locations. The warmer spots remain in the northern Sacramento Valley into the thermal belts with temperatures in the low to mid 70s.
Across the more synoptic scales, a longwave trough remains a fixture across western Canada extending into the Pacific Northwest. Farther south, a weak, elongated trough continues its presence over California extending into the eastern Pacific. While one embedded impulse advances into Nevada, additional waves have tended to evolve maintaining high Sierra convection chances.
Based on the latest output from the 06Z NAM and 08Z HRRR, scattered showers and thunderstorms are likely to develop by 21-22Z (2-3 PM) today, generally south of I-80. These should gradually weaken by the early evening given the loss of diurnal heating. High temperatures will run about 3 to 5 degrees below average in the Delta influenced locations, generally in the upper 80s, while a bit cooler around the Delta.
Mid-level heights and 1000-500 mb thicknesses come up over the weekend which will bolster temperatures, particularly across the northern Sacramento Valley with highs between 102 and 105 degrees. On Sunday, the broad mainstay trough over the region will spin up another wave which encourages another round of mountain showers and thunderstorms on Sunday, possibly continuing into Monday. An increasing Delta breeze should lead to an uptick in southwest to southerly winds across the Sacramento Valley. This may increase cloud cover over the region late Sunday into Monday.
Extended Discussion (Tuesday through Friday)
Long wave troughing begins to develop over NorCal Tuesday, continuing through most of the week. This will bring drier weather to the area with temperatures ranging from near to above normal through the work week.
Slightly above average highs Tuesday cool to normal values Wednesday afternoon as a weak shortwave rounds the base of the main trough. The northern Sac Valley may finally reach temperatures below the 100 degree mark mid week, with this trend continuing into Thursday. Lows each night look to also be near average through Friday, with the warmest low temperatures in the typical areas of the north Sac Valley and the thermal belts of the foothills. Looking a bit further into the extended forecast, models suggest dry weather continuing through next weekend.