August 20, 2020 – Hot and dry weather with areas of smoke persist for at least the remainder of the week. Thunderstorm chances return to the Sierra Nevada early next week.
Discussion
Numerous large wildfires depicted on IR satellite imagery across NorCal. The strongest heat signatures currently for fires in the CWA are occurring over western Stanislaus and western Glenn Counties. Weather conditions remain warm and dry, though not as extreme as early Wednesday. RH’s along the western edge of the Central Valley are in the teens and 20s (up around 10 percent compared to 24 hours ago) and westerly wind gusts are considerably lighter over most of that area. Coastal profiler data indicate the marine layer remains shallow (under 1000 ft in depth), but that’s an improvement compared to yesterday and IR difference imagery shows some areas of stratus along the coast.
High pressure retreats a bit today and Friday as a series of short-waves pivot through the PacNW. This will bring minor synoptic cooling to the region and maintain some onshore flow with locally breezy conditions at times, especially in the afternoons and evenings. RH’s will remain low, but trend up slightly for most areas.
Smoke from the numerous wildfires will continue to blanket the region, so air quality will be a real problem through at least the end of the week. The smoke will also continue to have significant impact on temperatures, both highs and lows.
Widespread triple digit heat forecast to return to the Central Valley over the weekend as the Southwest ridge rebuilds over NorCal. We’ll also be keeping an eye to the south as moisture from Genevieve works its way around the periphery of the Southwest high potentially bringing another round of elevated convection as early as Sunday.
Extended Discussion (Monday through Thursday)
The upper level ridge will be pushing to the east for the start of the extended period. This will begin to push some moisture from now Hurricane Genevieve into NorCal and will bring an increase in instability mainly over the higher elevations. Afternoon mountain thunderstorms will be possible over the Sierra on Monday. Drier air will start to push in on Tuesday as a trough tracks east into the PacNW. This will keep the forecast dry for the rest of the extended period. Temperatures will be cooler but they will still be a bit above average for this time of year.