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November 10, 2020 – Dry weather and below average temperatures continue through mid- week. Freezing Valley temperatures are expected this morning and Wednesday morning. The chance for rain and snow returns late in the week and into the weekend.

Discussion

NW flow will prevail over NorCal through the mid-week period in the wake of the Upper level low moving into/across the Plains States. Mid to high cloud cover embedded in the NW flow will periodically move overhead the next couple of days with the timing of the clouds possibly having an effect on temps, i.e. more clouds during the day limiting maxes and less clouds at night allowing for more radiational cooling. Weakening fronts today and Wed will approach our forecast area, but precip is expected to remain to the north.

Although NW flow aloft will moderate, below normal temps will continue into Fri with a chance of precip late Thu into Fri. Model differences result in lower than normal confidence, but the GFS/ECMWF ensemble Prob of IVT plumes illustrate relatively “weak” TPW plumes in the mid latitudes nearing the W coast beginning day 4 (Fri the 13th) and into the weekend which will impact NorCal. The deterministic runs have slowed the onset of precip for NorCal with the first wave to the 00z-06z Fri time frame. The ECMWF is now slower than the GFS, reversing the timing forecasts from yesterday. (Thus the lower than normal confidence.) The second wave Fri nite/Sat looks to be a wetter than the first with greater impacts for NorCal.

Generally, the GFS has now followed the ECMWF with much milder temps for the weekend. Model precip timing differences continue.

Extended discussion (Saturday through Tuesday)

Short wave trough moves through Saturday with some showers possible, best chances over the eastern foothills and mountains. Some overrunning precip advertised Sunday over northern portions of the CWA. Drier weather expected on Monday per deterministic models although NBM continues to show POPs. Forecast uncertainty increases Tuesday as models differ significantly. GFS brings widespread precipitation into NorCal while EC maintains upper ridging over the area.