Area Forecast Discussion
Issued by NWS Sacramento, CA
Issued by NWS Sacramento, CA
823 FXUS66 KSTO 182140 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 240 PM PDT Wed Sep 18 2024 .Synopsis... Cooler, unsettled weather through this evening with scattered showers and thunderstorms, mainly over the mountains and foothills. Lingering showers over the Sierra Nevada Thursday, otherwise drier and warmer conditions into early next week. && .Discussion... A cool upper level low from the Gulf of Alaska is moving along the California coast. Southerly flow form this low is bringing moisture and unstable conditions. Latest radar shows showers and thunderstorms developing over much of the Sierra crest and extending into eastern Shasta County. There is 30-40% chance of scattered showers and thunderstorms for eastern portions of the Park Fire burn scar the rest of this afternoon and evening. The main concern is for these storms moving from east to west into that area, bringing the potential for heavy rainfall, which could cause debris flows and flooding impacts there. A Flash Flood Watch remains in effect for the Park Fire burn scar through 10 pm PDT today. Convective Allowing Models (CAMs) currently show the some of the strongest cells developing late this afternoon into the evening. Easterly flow around the northern portion of the low would track these storms into eastern portions of Tehama County and northern Butte County. Forecast rainfall amounts are around 0.10-0.50" over the Sierra, southern Cascades and foothills. Highest rainfall is expected over the Sierra crest, where up to an inch is possible. In addition to rain, small hail is possible. Caltrans webcams are currently showing some small hail on the ground on Highway 50 at Echo Summit. The Valley has 10-20% chance of light rain showers this afternoon and evening, with a 25% chance for the Delta. Rainfall amounts in the Valley should be limited to just a trace to a few hundredths of an inch. As this system moves further into Southern California Thursday, There will be another round of afternoon and evening thunderstorm chances, but these should more limited to the Sierra crest, mainly from I-80 southward. NBM probabilities have 15-30% along the Sierra crest, with the highest chances south of US 50. Given the center of the low bring further south, this will result in more southeasterly steering flow and more downslope storm motion. High pressure begins to build in Friday as eastern Pacific ridging builds eastward. Temperatures begin to warm to near normal by Saturday, with highs above normal on Sunday. Friday will see afternoon highs warm to the mid 80s to low 90s in the Valley with upper 60s to mid 80s across the mountains to foothills areas. There will be some warming temperatures and some occasionally breezy north winds in the Valley through the weekend, daytime humidities lower back to upper teens to 20% over the Northern Sacramento Valley Sunday into Monday. && .EXTENDED DISCUSSION (Sunday THROUGH Wednesday)... Dry weather is expected through the extended forecast period as cluster analysis and ensemble guidance indicates high pressure extending inland over Northern California. Temperatures will warm up through the weekend and into early next week with highs in the mid to upper 90s across the Valley and Delta. Areas of Moderate HeatRisk return to the Delta, Valley, and foothills of the Sierra Monday through Tuesday of next week. Light to breezy northerly flow across the more wind-prone areas of the Sacramento Valley (i.e. the northern Sac Valley and along the I-5 corridor in the Sacramento Valley) Sunday through Tuesday, with gusts to 15 to 22 mph. Daytime humidities will decrease gradually through this period into the upper teens to upper 20s, which may lead to localized fire weather concerns in the northern Sacramento Valley and adjacent foothills. && .AVIATION... Generally VFR conditions expected across interior NorCal next 24 hours. However, MVFR/IFR possible in any stronger showers/thunderstorms through this evening, especially along the Sierra and into adjacent foothills locations. Gusty and erratic winds will be possible in the vicinity of thunderstorms as well. Otherwise some gusts 20-30 kts along the Sierra after 06z, with winds generally less than 12 kts elsewhere. && .STO WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch until 10 PM PDT this evening for Central Sacramento Valley-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-Western Plumas County/Lassen Park. && $$