Red Flag Warning in effect today, strongest winds will be over the mountains and foothills tonight

September 24, 2019 – Hot and dry weather with breezy northerly winds will bring critical fire weather conditions thru mid-week. Significant cool down with a chance of showers expected for the weekend.

Discussion

Clear skies cover the region as the eastern Pacific ridge extends into NorCal. North and east surface pressure gradients have increased to around 10 mbs MFR-SAC and to about 7.5 mbs RNO-SAC.

Local gusts of 30-40 mph are occurring in the foothills and northern Sierra Nevada, while gusts along the western edge of the Sacramento Valley have mostly been in the 15-25 mph range.

Current temperatures are mostly milder compared to 24 hours ago and range from the upper 30s in the colder mountain valleys to around 80 in the warmest breezy portions of the Central Valley.

Critical fire weather concerns will continue into Wednesday. The strongest northerly winds for the Sacramento Valley will occur this afternoon and early evening, and again on Wednesday, where gusts of 20-30 mph will be possible. The strongest winds will be over the mountains and foothills tonight, especially across the exposed ridges and northeast oriented canyons of the Sierra Nevada and foothills.

Hot weather is expected today and Wednesday when highs warm to 10-15 degrees above average, with warmest Valley readings around 100. Some cooling will be possible by Thursday as short-waves moving through the Pacific Northwest flatten the ridge and induce a return of onshore flow. Stronger cooling will begins Friday with the approach of the deep trough along the West Coast.

Extended Discussion (Saturday through Tuesday)

Large 543 DM upper low centered over the PacNW with troughing extending into NorCal will be main synoptic feature through bulk of extended forecast period. Unsettled weather is expected with unseasonably cool temperatures, upwards of 15 to 25 degrees below normal. As individual vort maxes rotate around main low, periods of showers can be expected Saturday into Monday with highest POPs forecast over the mountains. As colder air is ushered into the CWA over the weekend snow levels will lower to 5500 to 6500 feet with some accumulations possible over higher terrain. By Tuesday, it appears low will transition into the Intermountain West as upstream EPAC ridging spreads increasing subsidence over the CWA.