August 21, 2020 – Hot and dry weather with areas of smoke likely persist into next week. Thunderstorm chances potentially return to NorCal Sunday into Tuesday.
Numerous large wildfires continue to be indicated on IR satellite imagery across NorCal. The strongest heat signatures currently for fires in the CWA are occurring over southwestern Stanislaus, and over western Glenn and Tehama Counties where drier conditions persist. Current temperatures are generally lower by around 5 degrees across most of the Central Valley and foothills compared to 24 hours ago, while the mountains are running about the same as early Thursday. Significantly higher humidity has spread into the southern half of the foothills of the Sacramento Valley where most areas are currently seeing readings of 40-60 percent.
IR difference imagery shows stratus has become well established along the coast, extending locally inland (a few patches indicated around Napa). However, coastal profilers show the marine layer remains quite shallow (less than 1000 ft deep).
Little change in the overall pattern expected today as high pressure remains centered over northern Arizona, and short-waves continue to pass to our north through the PacNW. This will maintain relatively weak onshore flow into the Central Valley with locally breezy conditions at times, especially this afternoon and evening. RH’s are expected to continue to trend up a bit today for most areas.
Smoke from the numerous wildfires will continue to blanket the region, so air quality will be a significant problem today, and likely through the weekend. The smoke will also continue to have significant impact on temperatures, both highs and lows.
Slight warming possible over the weekend into early next week as the Southwest ridge extends westward, and we possibly see some thinning of overall smoke conditions.
Moisture from the remnants of Genevieve is working its way northward along and west of the Baja peninsula. We’ll likely begin to see some high clouds, above the smoke, on Saturday. Appears there is increasing potential for another elevated convective event early next week. At this point, it’s looking like Sunday night into Monday morning will be the best chance for dry thunderstorms, though poorly resolved remnant disturbances originating from large thunderstorm complexes over Arizona and northern Mexico may bring more isolated lightning potential as early as Sunday. We’ll continue to monitor.
Extended Discussion (Tuesday through Friday)
For the start of the extended forecast mid-level moisture will be in place over NorCal from the current tropical system Genevieve. The upper level pattern has us in between the four corners high and troughing over the PacNW. Ensembles agree on a short wave trough tracking through the region late Monday night into Tuesday afternoon. Soundings look a bit drier Tuesday compared to Monday even at the mid-levels but there still looks to be a chance for a few isolated to scattered thunderstorms for the northern half of the CWA. Moisture exits the area Tuesday evening as the troughing over the PacNW starts to slowly push east. This will bring dry conditions for the rest of the forecast. Despite seeing some troughing over the area temperatures look to remain above average for this time of year.