Hot and dry weather this week with moderate heat risk during the middle of the week. Increasing chances of late day Sierra thunderstorms late in the week.
Satellite imagery shows some high clouds beginning to spread across the region from the south as high level moisture moves northward between the cutoff low offshore and the strong high centered over Nevada. Profiler data show the marine layer remains very shallow (less than 1k ft deep), and only at weak Delta Breeze is present at Travis AFB. Current temperatures are milder compared to 24 hours ago and generally range from the mid 60s to mid 70s across most of the Central Valley. Milder spots along the edge of the valley are still still seeing temps around 80.
GEFS and EC ensemble are in good agreement keeping strong ridging over the area through mid-week before only gradually lowering heights aloft beginning Thursday. This will keep the marine layer suppressed the next several days. Highs will inch up today, then warm to around 10-15 degrees above average on Tuesday and Wednesday with the hottest portions of the Central Valley expected to be around 105-108.
Overnight lows will also be very mild, in the mid 60s to mid 70s across most lower elevation locales. The result will be moderate to high heat risk across much of the area.
Thursday will likely be quite hot too, but ensemble spreads widen a bit lending a little more uncertainty (perhaps an increase in mid and high-level moisture from the south, or an increase in wildfire smoke).
Late day thunderstorm chances may return to the Sierra south of Tahoe beginning Thursday as some higher level moisture spreads north and the mid-level cap weakens just a bit.
Extended Discussion (Friday through Monday)
Ensemble guidance continues to struggle with the overall weather pattern for the extended period. An upper level ridge axis is still forecast over the Intermountain West with a shortwave trough approaching the coast on Friday. Uncertainty remains with the fate of the shortwave. The latest GEFS has the wave essentially dissipating as it reaches the PacNW coast. The Euro ENS wants to move the shortwave through Northern California. This could affect whether the daytime temperatures remain warmer on Friday or if they begin to cool.
For now have gone with deterministic NBM which indicates temperatures cooling into the mid to upper 90s across the Valley and 70s to upper 80s in the mountains and foothills. Monsoon moisture is expected to continue moving along the western edge of high pressure into the forecast area on Friday.
Another round of afternoon and evening thunderstorms is possible along the Sierra crest. Moisture may begin waning Friday compared to earlier in the week, so thunderstorm chances could be limited to near the crest with significant rainfall not expected.
Upper level flow is expected to become more zonal over the area on Saturday with ensemble guidance suggesting the potential for another weak shortwave to graze northern California Sunday into Monday. This would allow temperatures to cool to near normal by Sunday.