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September 23, 2019 – Hot and dry weather with breezy northerly winds will lead to elevated fire weather concerns early this week. Significant cool down with a chance of showers late in the week.
Satellite imagery indicates the upper trough is over western Nevada early this morning and is in the process of closing off as a strong vort drops southeastward near Lake Tahoe. Skies have cleared behind the vort, other than some lingering low clouds over eastern Shasta County and northern Plumas County, but remain partly cloudy over the southern part of the forecast area.
Current temperatures are milder compared to 24 hours ago and generally range from the upper 30s and lower 40s in the mountain valleys, to the upper 60s across milder portions of the Central Valley. Northerly surface gradients have already begun to tighten (KMFR-KSAC over 8 mbs at 09Z) resulting in local wind gusts in the teens to lower 20 mph range over the northern Sacramento Valley and surrounding foothills.
The low will move south to near Yuma early Tuesday, and into northern Baja by early Wednesday, allowing ridging from the eastern Pacific to build into NorCal in its wake. Along with dry conditions and hotter temperatures, breezy to locally windy northerly flow will develop elevating fire weather concerns into mid-week.
The strongest northerly winds for the Sacramento Valley will occur during each afternoon and evening thru Wednesday where gusts of 25-35 mph will be possible, but the strongest winds will retreat to the western edge of the valley each night. Locally gusty east winds will also be likely each night across the Sierra Nevada and foothills.
Temperatures will only warm a few degrees today compared to Sunday, but hotter weather is expected Tuesday and Wednesday when highs warm to 5-10 degrees above average with warmest valley readings around 100. Some cooling will be possible by Thursday as short-waves moving through the PacNW flatten the ridge and induce a return of onshore flow.
Extended Discussion (Friday through Monday)
Anomalously deep troughing will settle over the western U.S. late this week into early next week, bringing significantly cooler temperatures and precipitation chances across interior NorCal. The highly amplified setup over North America has been rather well advertised as a powerful ridge across the Gulf of Alaska forces a deep trough to position itself over the West. During the Fri-Sun timeframe, forecast highs could drop 15 to 25 degrees below seasonal normals. Valley locations should see temperatures in the upper 60s to low 70s, while 30s and 40s would be more commonplace across the mountains. At this point, precipitation coverage and amounts remain uncertain, but it appears best chances will be over higher terrain. Snow levels could lower to around 5,500 to 6,500 foot over the weekend, so cannot rule out the potential for some snow over the higher elevations.