Significant warming up until Sunday, another cool down could bring showers and thunderstorms starting Monday

September 12, 2019 – Increasing temperatures and dry weather expected through the remainder of the week. Low pressure will bring cooler weather and the possibility of showers and thunderstorms early next week.

Discussion

Clear skies cover the region early this morning. Current temperatures are running a little milder compared to 24 hours ago across the mountains, with little change to slightly cooler observed across the valley. Readings range from the mid to upper 30s across the mountain valleys to the upper 50s to mid 60s through the Central Valley.

Hot and dry weather is expected the next several days as ridging strengthens across the region and short-waves pass to the north. Friday and Saturday look to be the peak of the heat with warmest valley locales expected to be near the century mark.

There’s still reasonably good agreement between the ensembles and latest operational model runs depicting a significant pattern change beginning early next week. Cooler weather begins Sunday with highs forecast to be around 5-10 degrees below Saturday’s highs. Precipitation chances expected to hold off until Sunday night across the far northern portion of the forecast area.

Extended Discussion (Monday through Thursday)

Ensemble data indicates a deep trof with a minus 2 to 2.4 height anomaly and a source region from the Bering Sea will bring cooler and showery wx to Norcal early in the week. The GEFS mean continues to generally trend a little wetter at BLU, but the QPF hit a resistance (ceiling) at RDD in this evening’s run, although the max member continued the wetter trend. Generally we are looking at mean QPFs at RDD and BLU around a third of an inch, a tenth to a quarter inch around SAC, and around a tenth at MOD, although we have forecast precip that far south in our CWA at this time.

The source region and negative height anomaly will bring some unseasonably cold temps to our CWA. Monday’s highs look to be some 10 to 20 degrees below normal, moderating to 8 to 15 degrees below normal on Tue, then returning to normal/slightly above normal by the middle of the week. The cold temps could lead to an inch or two of snow over the higher peaks of the Sierra. The synoptic pattern and the forecast CAA will lead to steepening lapse rates with the potential for thunderstorms over the Sierra mainly Mon/Tue afternoons and evenings. In addition, Onshore/SW flow will continue breezy/windy conditions into Mon (which began on Sun), followed by decreasing winds on Tue.

Deterministic runs vary as to the duration of the precip in Norcal with the ECM ending the precip by 18z Tue. The 5H anomalies suggest the trof axis will still linger over the Sierra on Tue and we have continued some PoPs through Tue.