September 4, 2020 – Dangerous heat wave with widespread record temperatures during Labor Day weekend into early next week. Possible north Valley winds and downslope east winds over the foothills will increase fire weather concerns Tuesday and Wednesday. Smoke and haze from wildfires will continue to impact air quality.
Discussion
Warm temperatures continue today with readings about 5 degrees above normal, generally from 95 to 100 over the Valley. Much hotter conditions Saturday and Sunday with upper level high strengthening near the Great Basin Region and moving over NorCal Sunday. Winds will remain light at most locations except near the Delta, where local gusts to 30 mph will be possible.
Main weather concern in the near term will be record breaking heat wave Saturday through Tuesday. Temperatures are expected to be from from 15 to 20 degrees above normal during this timeframe. High temperatures are forecasted to approach 110 degrees at Sacramento, Stockton and Modesto.
These readings would break all- time September high temperatures records for these locations. Also low temperatures are expected to be very warm only cooling into the 75 to 80 degree range.
Hot conditions continue Monday with Valley high temps from 105 to 110 degrees. These extreme temperatures will cause dangerous heat impacts and an excessive heat warning will continue Saturday through Tuesday.
There is the potential for very high heat risk during this time, which could impact the general population, especially those spending an extended time outdoors.
Extended Discussion (Tuesday through Friday)
Euro and GFS continue to come into better agreement with track of trough dropping south through the eastern Great Basin and Rockies Tuesday and Wednesday.
A track of this type is often too far east to deliver significant winds to NorCal. However, this situation is a bit different, as strong surface pressure gradient will be the driver of enhanced winds in the absence of upper level wind support.
Strong surface high pressure dropping south from Montana into the central Rockies will create the strong gradient with surface low pressure off the the northern California coast Tuesday through Wednesday.
Currently expecting 15 to 20 mph north valley winds and east downslope Sierra and foothills winds with gusts up to 30 mph.
The lack of much upper level wind support above 850 mb will likely limit gust potential. These winds combined with very dry conditions will likely bring critical fire weather conditions Tuesday through Wednesday.
High temperatures in the upper 90’s to low 100’s Tuesday and Wednesday will cool slightly back to low and upper 90’s Thursday and Friday. Dry weather will prevail through the extended.