FXUS66 KSTO 302008 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 108 PM PDT Wed Oct 30 2024 .SYNOPSIS... First notable precipitation event of the Water Year arrives this evening, bringing widespread precipitation, mountain snow, breezy winds, and thunderstorm chances through Saturday. Breezy northerly winds develop on Sunday and into Monday. .DISCUSSION... Key Points: -First notable precipitation event of the Water Year will bring TWO WAVES of widespread measurable rainfall and mountain snow accumulations this evening thru Saturday. -First Wave (this evening-Thursday), Second Wave (Friday PM- Saturday) -Following this system, north winds increase through the Valley on Sunday & Monday -A Winter Weather Advisory has been issued for elevations greater than 6000 feet in the northern Sierra from 11 PM this evening through 5 PM Thursday. Snow levels will start around 5000 feet Thursday AM before rising to around 6000 feet throughout the day No real updates to the forecast needed today as things still on track for 1st widespread rain and high level snow event over the next 3 days. First system still looking to be the weaker of the two systems. First wave of molisture is from a large upper level trough which has most of it's energy well to the north in the Pacific Northwest. This system is now bringing rain to the coastal areas of northern California at this time. The precipitation will spread south overnight. Main band of preciptation should be in norther forecast area by late afternoon/early evening then work south to the Sacramento area around and a bit after midnight. Snow with the system looks to be falling mainly in the advisory criteria in the 2" to 6" range above 6000 feet. NBM showing some modetate (50 to 70%) chance of a foot of snow at highest peaks. Also coming across during the night on a weekday which should help over impacts. Thursday...looks like we will be in between the 2 systems with most of the rain out of the valley in the morning but some upslope showers in the foothills and mountains. Friday through Saturday...another wave will rotate around the pre- mentioned Gulf of Alaska upper low and bring a better round of moisture to the area. Once again from north to south and best energy from it Friday into early Saturday morning. With upper low location should be able to get fair enough forcing to out due westerly downsloping flow off of the coastal range and would think most of the valley will be moderate measurable widespread rain. NBM showing high probabilities (over 80%) of a half inch and respectible values (30-40%) chance of an inch...so overall thinking half inch to inch across valley with higher in the north valley. We did condider issuing another WWA product for the snow that will also be with this system. With probabilities of a foot of snow a big larger near the I80 corridor but still a fairly small area of that risk. Do think there will be more snow with second system but at this time overall impacts and some confidence that system may take a drier easterly route will hold off as it is looking like another Advisory vs a warning criteria system. && .EXTENDED DISCUSSION (Sunday THROUGH Wednesday)... Confidence increasing with NBM wind gust probs and overall placement of upper low as it moves east will set up a risk for northerly winds Sunday and into Monday. Overall fire risk at this time seem lowered with expected rain/snowfall but we will have to take a closer look as it gets closer for possible fire weather impacts from the wind. Temperatures will likely be a the cooler side of normal through Wednesday. && .AVIATION... General VFR conditions are expected to continue over the next 24 hours, except for local MVFR/IFR after 22Z this evening from a weather system moving across Northern interior California from Northwest to Southeast. This system will give the potential of rain showers overnight into Thursday. Southerly gusts up 25-30 knots possible for the Northern Sacramento Valley. Southwesterly winds over the Sierra crest up to 30 knots with gusts up to around 45 knots. && .STO WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 11 PM this evening to 5 PM PDT Thursday for Burney Basin / Eastern Shasta County-West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park. && $$