An extended period of dangerous heat is expected for the Labor Day holiday weekend into next week. Above normal temperatures are expected to continue each day through the middle of next week, with the most significant heat forecast for Sunday through Wednesday. Excessive Heat Warning from 11 AM Sunday to 8 PM PDT Wednesday.
Discussion
The upper level shortwave trough is moving onshore over Northern California early this morning. Ensemble guidance has quickened the pace of the trough and indicates it will move through Northern California quickly, exiting the state by midday as it moves into the Northern Rockies. The trough has pushed the main ridge axis east over the Great Basin and will continue to lower heights over Northern California. This will dampen temperatures over the area, especially for the northern Sacramento Valley and higher elevations.
Daytime temperatures today are forecast to peak around 98 to 105 degrees in the Valley. Widespread moderate heat is forecast, with some areas of high to very high heat risk.
Also associated with the trough, locally breezy winds are being observed across the northern Sacramento Valley and mountains early this morning and are expected to continue through this evening.
Local gusts up to 15-25 mph are possible which combined with very low humidities will cause elevated fire weather concerns. In addition, the extended period of very hot and dry conditions will bring elevated fire weather concerns through the middle of next week. Know your wildfire risk and have an emergency plan in advance.
High pressure will build back in quickly over Northern California by Sunday, with temperatures warming significantly Sunday into early next week. Both the ECMWF EFI and NAEFS Return Interval suggest anomalous heat during this time frame, especially on Monday and Tuesday which are expected to be the hottest days of this event.
Valley and foothill temperatures are forecast to reach 100 to 109 degrees Sunday, warming even more for Monday and Tuesday. Confidence is increasing in Valley temperatures reaching 110 to 113 degrees both Monday and Tuesday with 850 mb temperatures peaking around 30-32 degrees Celsius.
Not only are daily records in jeopardy, but so are all-time September records. Some locations may even approach the overall all-time temperature records.
Ensemble guidance shows another upper level trough approaching the West Coast Wednesday. Although there remains uncertainty in cluster analysis on exact timing of this trough, ensembles indicate heights will begin to lower over Northern California.
Temperatures are forecast to lower from Tuesday, though they will still remain hot with a similar forecast to Sunday.
In addition to the hot daytime temperatures, overnight temperatures will remain warm with limited relief through Wednesday morning. Expect morning lows in the Valley and foothills to only fall to around 68 to 85 degrees, warmest in the foothills. The combination of very hot daytime temperatures and warm overnight temperatures will cause dangerous heat with widespread high to very high heat risk.
An Excessive Heat Warning remains in effect Sunday through Wednesday. These temperatures increase the chance of heat- related illness for the general population, especially those who are sensitive to the heat, have outdoor plans, or do not have air conditioned homes.
Drink plenty of water, seek air conditioning, and learn the symptoms of excessive heat exposure.
Extended Discussion (Thursday through Saturday)
By Thursday, the significant heat is expected to break across interior Northern California. Ensemble guidance indicates the upper level trough will make its way onshore at some point late next week, though cluster analysis indicates uncertainty in exact timing. Nevertheless, temperatures will likely cool each day Thursday into the weekend as heights lower over Northern California.