Red Flag Warning, Wind Advisory and Excessive Heat Warning

September 7, 2020 – Dangerous, record-breaking heatwave will impact the region through Labor Day. Critical fire weather concerns will return tonight into early Wednesday with gusty north to east wind and low humidity. Smoke and haze from wildfires will continue to impact air quality and temperatures. Excessive Heat Warning until 10 PM PDT this evening. Red Flag Warning from 10 PM this evening to 8 AM PDT Wednesday. Wind Advisory from midnight tonight to 6 PM PDT Tuesday

Discussion

Skies remain mostly clear across the region early this morning. Strong inversion remains in place with warm current temperatures in the upper 60s and 70s across the Central Valley, with 80s to lower 90s over the surrounding foothills. Winds are generally light with surface gradients trending toward offshore.

Strong high pressure weakens slightly today, but continuing subsidence will further warm the lower level airmass across NorCal today. 1000-500 mb thickness increases about another 30 m today despite approximately 50 m height falls at 500 mb. Highs today expected to be similar to Sunday’s, or slightly warmer, but wildfire smoke will likely have some impact. Daily record highs, along with September records, will be jeopardy once again today.

Changes develop later tonight and Tuesday as the strong short- wave, presently over southeastern BC, slides southward through the eastern Great Basin. Both the EC ensemble and GEFS continue to trend a little further to the west with this feature as it pushes a dry cold front south through the Great Basin.

NorCal will be on the fringe of cooling behind the front on Tuesday, but we’ll see a period of strong north to east winds later tonight and Tuesday, especially along the western edge of the Sacramento Valley and across the foothills and west slopes of the northern Sierra. Local gusts of 40-50 mph will be possible as both the MFR-SAC and RNO-SAC gradient is forecast to get to 12-14 mbs Tuesday morning.

Surface pressure gradient forecast to relax by later Tuesday, but enough easterly gradient remains Tuesday night for locally gusty east winds to persist in the foothills and northern Sierra. Wind advisories have been posted.
The gusty winds along with a very dry airmass (widespread RH in the single digits and teens) will result in a prolonged period of critical fire weather conditions lasting into Wednesday morning.

Less breezy/windy conditions are expected Wednesday and Thursday as upper ridge and offshore flow gradually weaken. Temperatures won’t be as hot with highs cooling to around only 5 degrees above average. Overnight lows will be cooler as well, but still above average.

Extended Discussion (Friday through Monday)

Upper ridge over the area Friday slowly shifts into the Great Basin through the weekend. Short wave trough approaches the West Coast on Monday. Dry weather expected with high temperatures remaining about 5-10 degrees above normal through the extended forecast period.