FXUS66 KSTO 032130 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 130 PM PST Mon Feb 3 2025 .SYNOPSIS... Active weather continues through much of the week ahead as the last two systems in this series impact the region. Impacts from moderate to heavy rain and mountain snow and gusty winds are expected through midweek. Additional impacts from moderate rainfall, heavy mountain (and potentially upper foothills) snowfall, and breezy winds are then anticipated across the end of the week. Major mountain travel impacts are expected as a result. && .DISCUSSION... Precipitation is gradually spreading across interior NorCal from north to south today as the next shortwave trough is approaching the region. Radar returns are primarily confined to areas north of Interstate 80 at this time, with some portions of Shasta County already seeing 0.5 to 1.5 inches of rainfall since around 6 am PST. Gusty winds are expected to follow a similar southward progressing pattern, with southerly gusts to 40 mph being realized across portions of the northern and central Sacramento Valley at this time as well. As a result, a Wind Advisory remains in place through 4 pm Tuesday for portions of the northern and central Sacramento Valley, and goes into effect from 10 am to 4 pm Tuesday for the remainder of the Sacramento Valley and portions of the Delta and northern San Joaquin Valley. The initial push of precipitation will through this afternoon and early evening is expected to remain primarily moderate as the shortwave is still approaching the region. As the shortwave moves inland late tonight through Tuesday, attendant precipitation rates are expected to increase, with gusty southerly winds becoming more widespread as well. Additionally, while a highly conditional threat, there are some 10 to 20 percent probabilities for thunderstorm development, primarily along and south of Interstate 80 Tuesday afternoon and evening. All these factors considered, periods of moderate to heavy rainfall are expected over the next 36 hours with this system and as such, a Flood Watch remains in effect through 4 am Wednesday for most Valley, Delta, and foothills locations. Current forecast rainfall totals of 2 to 4 inches in the Valley, 3 to 8 inches in the foothills, and 5 to 10 inches in the mountains below 5500 feet are anticipated. Snow levels are expected to remain around 6000 feet until the shortwave becomes more centered over interior NorCal as it makes an inland push. While there is some uncertainty, in exact timing, the more pronounced drop in snow levels to as low as 2000 feet for the Coast Range/Shasta County mountains and to 3000 to 4000 feet along the Sierra/southern Cascades is expected late in the day Tuesday into Wednesday morning. Similarly to the rainfall, heaviest snowfall is expected to occur tonight through Tuesday, where snowfall rates of 1 to 2 inches per hour will be possible at times. Resultant forecast snowfall totals of 1 to 3 feet are anticipated along the Sierra/southern Cascades above 5500 feet, with up to 5 feet possible over peaks. Additionally, snowfall totals of 4 to 8 inches and up to 1 foot over peaks will be possible across the Coast Range/Shasta County mountains. As a result, a Winter Storm Warning remains in effect above 5500 feet along the Sierra/southern Cascades through 10 am Wednesday, with a Winter Weather Advisory in place from 10 pm tonight through 10 pm Tuesday for the Coast Range/Shasta County Mountains above 3500 feet. Be prepared for Major mountain travel impacts from this system through Tuesday evening. As the shortwave ejects eastward, precipitation is expected to become less widespread and generally light for much of Wednesday. Gusty winds also look to become primarily breezy through the middle of the week as the system departs as well. Some portions of the southern Sacramento and northern San Joaquin Valley may even see a few rays of sunshine on Wednesday as well! Depending on the timing of this potential clearing, some chilly morning temperatures will be possible throughout the Valley on Wednesday morning, with generally 30 to 50 percent probabilities of low temperatures less than 35F. Despite the relative lull for much of Wednesday, precipitation chances are once again expected to increase late Wednesday night as the last wave of precipitation begins to move toward interior NorCal. && .EXTENDED DISCUSSION (Friday THROUGH Monday)... Ensemble guidance is coming into a little better agreement on the progression and phasing of the expected shortwave/longwave trough interactions that are expected to eventually bring an end to this wetter weather pattern by the weekend. Before drier weather settles in though, the more vertically stacked and deeper nature of this trough progression will introduce another round of moderate to heavy precipitation and colder temperatures Thursday into Friday. While overall available moisture is expected to be lesser compared to the early/midweek system, an attendant weak atmospheric river plume will still accompany the strong forcing associated with this system. As a result, probabilities of rainfall exceeding 1 inch only sit around 20 to 40 percent across the Delta, Valley, and lower foothills. Still, 2 to 4 inches of liquid precipitation are anticipated across the Sierra/southern Cascades and into the upper foothills however. With minimal recovery in snow levels expected during the brief reprieve on Wednesday and a deeper system bringing an additional push of colder air through the end of the week, forecast snow levels around 3000 to 4000 feet on Thursday are expected to fall even further to 2000 to 3000 feet by Friday. While snowfall accumulation into the upper foothills (potentially as low as 3000 feet) will be possible, exact snowfall totals in these areas remains uncertain, particularly attributed to uncertainties with regards to the magnitude of the expected cold air push. As a result, probabilities of snowfall exceeding 1 inch down to 3000 feet sit around 30 to 50 percent at this time. Moving toward higher elevations, this next round of heavy snowfall looks to bring an additional 1 to 3 feet of snowfall above 5000 feet, with some 6 to 12 inches of snowfall possible between 4000 and 5000 feet through Friday evening. This means that additional mountain travel impacts are expected through the end of the week and possibly lingering into the early weekend, so be sure to take time now to prepare if you have mountain travel plans ahead! In addition to precipitation, periods of gusty southerly winds are also expected to continue. For the weekend, ensembles continue to show a trend toward a drier pattern. && .AVIATION... Widespread MVFR conditions with periods of IFR/LIFR conditions are expected to drop southward from the Northern Sacramento Valley to Southern Sacramento Valley this afternoon/evening and forecast to persist into the overnight hours. An atmospheric river is forecast to persist with moderate to heavy precipitation in the area for the next 24 hours. Southerly surface winds across the Valley floor are expected around 15 to 20 knots with gusts 25 to 40 knots at times. Snow levels are currently around 6000' and expected to drop to around 5000' by tomorrow evening. Southerly wind gusts in the Sierra are expected to be up to 55 knots for the next 24 hours. && .STO WATCHES/WARNINGS/ADVISORIES... Flood Watch through late Tuesday night for Carquinez Strait and Delta-Central Sacramento Valley-Motherlode-Mountains Southwestern Shasta County to Western Colusa County-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-Northern San Joaquin Valley-Shasta Lake Area / Northern Shasta County- Southern Sacramento Valley-West Slope Northern Sierra Nevada- Western Plumas County/Lassen Park. Winter Weather Advisory from 10 PM this evening to 10 PM PST Tuesday for Mountains Southwestern Shasta County to Western Colusa County-Shasta Lake Area / Northern Shasta County. Wind Advisory until 4 PM PST Tuesday for Carquinez Strait and Delta-Central Sacramento Valley-Motherlode-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-Northern San Joaquin Valley-Southern Sacramento Valley. Winter Storm Warning until 10 AM PST Wednesday for West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park. && $$