August 17, 2020 – Prolonged heat wave will impact the region through the week. Daytime temperatures will be 10 to 20 degrees above normal with limited overnight relief. Isolated dry thunderstorms are possible today.


A very active weather pattern for August over NorCal with impacts from a historic heat wave and thunderstorms many of which may be dry. Unseasonably strong 600 dm 500 mb upper high pressure system remains centered near the Utah/Nevada boarder with heights in the 594-597 dm range extending over Norcal. This in combination with very warm 850 mb temps in the 25-29 C/deg range has lead to a record breaking heat wave over much of Northern California. Several long standing records have already been broken including the hottest August day on record which was broken at Sacramento 112, Stockton 113 and Modesto 111 on Sunday. The current stretch of 2 consecutive days above 110 ties the all-time record at Downtown Sacramento with the last similar stretch of extreme heat in 1996.

Current forecast suggest possibly two more days of extreme heat with temperatures potentially reaching or exceeding 110 degrees, which would turn this heat wave into possibly the longest stretch of extreme heat for many locations. Most Valley locations are also only seeing low temps cool into the 75-80 degree range at night, which is magnifying the extreme nature of this heat wave. It is possible that today could be a few degrees cooler then the past two days, but this is highly dependent on increased cloud cover and any extended period of extensive afternoon sunshine will likely push temps to near 110 degrees at most Valley locations. Tuesday looks to be the hottest day of the heat wave, as there should be very little cloud cover and 850 temps peak to near 29 C.

Nocturnal thunderstorms developed just after midnight over the Sacramento metro area with many cloud to ground lightning strikes, very little rain fell with all gauges under 0.10 inches. Very high levels of moisture and modest instability continue to rotate into northern California around the western edge of upper high. Also small embedded vort disturbances are acting to initiate isolated thunderstorm development. Next plume of moisture and instability is poised to move into the area this morning with preciptable water amounts near 1.50 inches. These levels are on the extreme side during August over NorCal. Despite these high PWAT values the lower part of the atmospheric column remains very dry, so there is significant evaporation of the precip before reaching the ground, which means most of the storms are on the dry side.

Mesoscale models suggest the best thunderstorm coverage with advancing moisture plume will be over the Northern/Central Sacramento Valley, especially Shasta county and the northern Coastal range.

If significant lightning activity does develop in these areas numerous fire starts will be likely and the current red flag warning has been extended to 9 pm this evening. Flow should become more westerly Tuesday and Wednesday, pushing the thunderstorms activity into Nevada, although areas south of I80 near the crest could still see a isolated storm.

Extended Discussion (Friday through Monday)

Upper high from the Desert SW continues to influence interior NorCal during the extended forecast period. Triple digit heat continues in the Central Valley with highs generally around 100 to 105 degrees with mainly 80s and 90s for the mountains and foothills. At this time dry weather is forecast. However, progs indicate another tropical system off the Baja coast with associated moisture that could be entrained northward. Therefore, this will need to be watched with future model runs.