559 FXUS66 KSTO 192142 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 242 PM PDT Tue Oct 19 2021 .SYNOPSIS... Rain begins tonight with periods of precipitation continuing through early next week. Moderate to locally heavy rainfall may be seen at times. Significant snow possible early next week. && .DISCUSSION... Mid level clouds have begun streaming across interior Northern California early this afternoon, ahead of a trough located just off the coast. Rain is expected to move inland this afternoon, reaching the western portion of the CWA this evening. Rain will overspread the area during the overnight hours, with the heaviest rainfall expected over the Coast Range, Southern Cascades, and northern Sacramento Valley. HREF values indicate only a 30 percent chance or less of 1 hour rainfall greater than 0.5", and burn scar issues are not anticipated. South of Highway 50, only a few hundredths to a few tenths may be seen with this initial system. With this push, snow levels actually rise through the night which will limit any winter impacts overnight. That said, an inch or so of snow will still be possible with the onset of precipitation in locations above 6.0-6.5 kft. By mid-morning, snow levels look to be above pass levels. Breezy southerly winds are also expected tomorrow morning with some gusts from 20 to 30 mph, locally higher over the mountains. Precipitation lightens up a bit Wednesday afternoon into Thursday and may become more showery, but chances do continue for much of the CWA during this time. A wetter system will push into the area Thursday into Friday, with the heaviest precipitation expected in generally the same areas as tonight's system. North of I-80 in the Sierra Nevada, Southern Cascades, and northern mountains, anywhere from 2 to 5 inches will be possible during this time, locally higher. Portions of the northern and central Sacramento Valley can see from 0.5 up to 2 inches. Snow levels continue to be above 9 kft and no significant snow impacts are expected. The wave that brings this Thursday into Friday precipitation quickly moves out of the area, briefly clearing precipitation in the Valley in the evening. More rain on the way for the weekend into early next week. && .EXTENDED DISCUSSION (Saturday THROUGH Tuesday)... After a brief lull early Saturday, consistent ensemble agreement continues to bring high confidence in a very wet pattern setting up for the weekend into Monday. This is due to a strong (170+ kt) zonal polar jet extends across the Pacific bringing copious subtropical moisture into Northern California. This Atmospheric River (AR) will be supported with strong upward motion from upper level diffluent flow. This will lead to significant amounts of precipitation across the region, especially Sunday into early Monday, especially during the Sun/Mon time frame when IVT exceeds 500kg/(ms). Liquid precipitation estimates at this time for the period 12z Sat through 00z Tue equate to about 1 to 4 inches in the Central Valley with 3 to 7 inches in the foothills and mountains. Snow levels remain above pass levels over the weekend, lowering to around 5000 to 6000 feet Mon/Tue. There is the potential for several feet of snow over higher mountain terrain by early next week. This has the potential for significant travel problems, especially at Sierra pass levels. Periods of moderate to heavy rain could bring potential impacts to recently burned areas, such as ash flow. This depends on rain intensity levels which are difficult to predict this far out. Currently convective activity is not forecast for the area with warm advection being the main driver of the precipitation. EK && .AVIATION... VFR conditions over interior Northern California will gradually give way to local MVFR/IFR conditions tonight as a cold front spreads precipitation from NW to SE. SE surface winds could gust to 20 to 25 kt in the Central Valley after 06z tonight over the northern Sacramento Valley. Gusts to 30 to 40 kts are possible over higher terrain. Snow levels are at or above 6000 feet AMSL. && .STO WATCHES/WARNINGS/ADVISORIES... None. && $$