NEVADA CITY, Calif, September 13, 2019 – Above normal temperatures continue today & Saturday, with temperatures 5 to 15 degrees above normal. Elevated fire weather concerns on Sunday due to breezy winds and low humidity. A weather system will bring wet and unseasonably cool temperatures to NorCal early next week. Precipitation will spread across NorCal north to south throughout Monday. Light snowfall is possible over higher Sierra passes. Thunderstorms are possible in the mountains & northern Valley, mainly north of I-80. Light showers could linger over the Sierra Tuesday.
Discussion
Upper level ridging remains over NorCal today through early this weekend. Hot and dry weather continues today and Saturday with high temperatures expected to be around 5 to 15 degrees above normal. Early afternoon temperatures are in the low to mid 90s across the Valley and should peak near the century mark today and tomorrow.
Ensemble models continue to depict a deep trough moving through the West Coast Sunday through early next week. Winds will increase Sunday as the trough begins to move onshore, creating enhanced fire weather concerns. The trough will also bring a cooling trend with daytime temperatures Sunday around 5 to 10 degrees cooler than Saturday.
The trough will move onshore Monday, bringing wet weather and continued cooling. Ensemble models are in pretty good agreement on widespread rain chances Monday. At this time, rain showers are expected to move north to south throughout the day as the trough moves through the area. The NAM and GFS are also suggesting thunderstorms are possible across the mountains and Valley along and ahead of the surface front. Right now thunderstorms are expected to remain mainly north of I-80 in the afternoon, then spreading along the mountains in the evening. However, model consistency between runs is low for the location of storms at the moment. Forecast rainfall totals are around 0.1 to 0.5 inches in the Valley and 0.5 to 1 inches in the foothills and mountains. Light snowfall is also possible Monday evening over the higher mountain elevations, though accumulations would be less than an inch.
Extended Discussion (Tuesday through Friday)
Unseasonably cool weather is expected next week as a series of cool troughs move through. Ensembles and latest model runs continue trend of moving the first system out on Tuesday, with just some lingering rain showers over the Sierra and southern Cascades during the day. A weak shortwave then brushes by the far north late Tuesday, bringing a slight chance of light precipitation to the Shasta County to Lassen Park area Tuesday night into Wednesday. Snow levels will be high enough that this be in the form of rain except maybe for Lassen Peak. Tuesday high temperatures will be 6 to 15 degrees below normal, in the upper 70s to around 80.
Models diverge Wednesday through Friday, with deterministic runs out of phase and some uncertainty in temperatures. NBM and ensembles suggest a slight warming and drying trend, so will go with that for the current forecast. Will watch this time period closely to see if confidence in specific details increases. Overall, it looks like a fairly typical fall, progressive pattern is expected late next week with periods of unsettled, relatively cool weather.