August 18, 2020 – Prolonged heat wave will impact the region through the week with a brief break in the extreme heat Thursday and Friday. Daytime temperatures will be 10 to 20 degrees above normal today then 5 to 10 degrees above the rest of the week.
Extreme heat will be the main story today, as thunderstorm activity diminishes with coverage restricted to high mountain areas near the crest south of highway 50. Strong monsoon season upper high pressure system remains centered near the Utah/Nevada boarder with unseasonably high heights extending over Norcal. This in combination with very warm 850 mb temps in the 27-29 C/deg range will lead to one more record breaking day in the extreme heat wave that has sweltered much of Northern California for the past 6 days. Many Valley locations will make a run at 110 degrees today, as the cloud cover that held temps a few degrees lower Monday is replaced by more sunshine.
Upper high will weaken slightly and slip south over northern Arizona Wednesday. This will allow upper heights to lower a bit over NorCal with high temps a few degrees cooler, but still about 10 degrees above with readings near or slightly above 100 degrees. Delta breeze of some magnitude is likely late Wednesday afternoon and evening with southwest winds 25-30 mph progged through the Delta. This will finally give some brief relief from the current heat wave, especially overnight lows which will fall back to 60 to 65 for many locations Wednesday night and Thursday night. High temps Thursday and Friday will be 95 to 100 about 5 to 10 degrees above normal. This combined with the overnight cooling will bring heat risk back to low/moderate levels. It is also possible Delta breeze is a bit stronger leading to more cooling, but this will be dependent on marine layer depth, which we should have a better handle on over the next few model runs. Drier westerly flow above upper high dropping into Arizona will keep thunderstorm activity east of the area through Friday.
Extended Discussion (Saturday through Tuesday)
Upper ridging from the Desert SW high extends over interior NorCal this weekend. Heights then lower slightly early next week as short wave troughing enters the PacNW. Above normal temperatures continue expected through the extended forecast period. Triple digit heat is forecast in the Central Valley and portions of the Delta. Guidance suggests highs will be around 100 to 105 with mainly 80s and 90s for the mountains and foothills. Dry weather expected for now. Progs however indicate another tropical system off the Baja coast over the weekend, lifting NW early next week. Associated moisture could be entrained in the upper level flow and drawn into NorCal leading to increased potential for convective storms. This could also impact temperature forecast early next week. Will need to continue to monitor this possibility with future model runs.