Warming trend through the Labor Day Holiday weekend.
Tail end of a positively-tilted trof over Norcal will be exiting the area early this morning allowing high pressure over much of the Srn portion of NOAM to expand Wwd into CA today, then NWwd over our CWA this weekend. This will weaken onshore flow and large scale subsidence will “squash” the marine layer yielding a warmer temps throughout the CWA. Ft Ord profiler shows the beginning of a less deep marine layer tonight and we expect the marine layer to become much shallower over the weekend.
As high pressure begins to build Nwd into our CWA the wild fire smoke may begin to spread NNWwd tonight and early Sat before turning more to the ENE Sat afternoon. We expanded the area of haze/smoke into the Sierra foothills during that time frame to account for the subtle shift of the prevailing winds. With increasing subsidence and stronger thermal belts developing over the weekend, areas of haze/smoke may return to areas that had a pleasant break over the past couple of days from the onshore/SW flow.
Forecast highs today will trend near normal and about 4 to 10 degrees warmer than yesterday in the Valley and up to 5 to 7 degrees over the higher terrain. Sat’s highs will be up to 8 degrees above normal with a return of warmer/hotter temps Sun/Mon. By then forecast highs will warm 5 to 12 degrees above normal over much of the forecast area with warmest/hottest day on Mon. Although no record max temps are forecast per the NBM Record Display Table, triple digits are forecast to return Sun and Mon over much of the Valley with an increase in the areal coverage of moderate heat risk on Mon. With the 5H Cluster Analysis continuing to show strong ridging over the region, albeit the monopole near the location of the ridge would indicate some uncertainty in its amplitude, there is little, if any, EOF variance in the ridge over our CWA and leads an individual to lean to the anomalous temp fields Sun/Mon, and higher end of the range of the max temp forecasts. This is more pronounced or evident on Mon. Seemingly only the impact of the wildfire smoke would adversely effect the max temp forecasts.
As high pressure over the Srn Plains States expands WNWwd Sun nite through Mon morning we could see vestiges of subtropical/monsoon moisture spreading Nwd into our CWA as S/SEly flow develops around the high pressure cell reforming over NV. By afternoon the moisture is forecast to lift Nwd, mostly exiting our CWA. Instability is forecast to remain shunted well to the south of our CWA, over SOCAL. Onshore flow should return Tue with an approaching trof mentioned in the EFP portion of the AFD.
Extended Discussion (Tuesday through Friday)
Upper high centered over the Great Basin as short wave trough lifts through northern third of CA Tuesday afternoon into night. Increasing onshore flow and some synoptic cooling associated with this wave. Ensembles support deepening of long wave trough along the West Coast Thursday into Friday will suggest continued cooling trend through end of the week.