Area Forecast Discussion
Issued by NWS Sacramento, CA
Issued by NWS Sacramento, CA
111 FXUS66 KSTO 200933 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 233 AM PDT Sat Jul 20 2024 .SYNOPSIS... Hot weather persists into next week. Thunderstorm chances along and east of the Sierra/Cascade crest into next week as well. && .Discussion... Clear skies are being observed early this morning across interior NorCal. Temperatures in the Valley as of 1:30 AM are hovering in the mid 70s to mid 80s, with temperatures in the upper 60s to low 70s closer to the Delta. Daytimes highs this weekend are forecast to be a few degrees cooler compared to yesterday. This minor, short-lived cooling trend is a byproduct of a shortwave trough that will clip the northwestern corner of Northern California this weekend, boosting the Delta breeze and bringing slightly cooler daytime highs. The trough will also tap into a swath of monsoonal moisture and instability present over the Great Basin ridge. As a result. there is a 15-30% chance for isolated afternoon/evening showers/thunderstorms along the Sierra Crest and a a 10-15% chance over the Coastal Range in western Shasta/Tehama counties. Hotter weather returns early next week as an upper-level high builds westward on Monday, as depicted in cluster analysis and ensemble guidance. This high will remain in place over the Great Basin and strengthen through mid-week. As a result, areas of triple digit high temperatures will become more prevalent throughout the Valley. HeatRisk is expected to increase to widespread moderate and areas of major in the Northern Sacramento Valley/foothills by Monday. By Tuesday, widespread major HeatRisk is expected throughout the Valley & foothills with isolated pockets of extreme in the foothills. Alongside these warmer temperatures, thunderstorm chances along the Sierra crest will remain present as monsoonal moisture continues to circulate around the high. Isolated afternoon/evening thunderstorm chances of 15-30% are expected, with the highest chances located south of US 50. && .EXTENDED DISCUSSION (Wednesday THROUGH Saturday)... Ensembles and clusters continue to display high confidence that this upper-level high will remain in place over Northern California through Thursday. NBM probabilities suggest the hottest day of the week will be Tuesday, with a 75-100% chance of highs greater than 105 through the northern Sacramento Valley and 20-60% chance in the southern Sacramento and northern San Joaquin valleys. A combination of high temperatures and elevated overnight recoveries will result in widespread moderate to major HeatRisk, with isolated areas of Extreme HeatRisk (as a result of thermal belts in the foothills) on Wednesday. In addition to the warmer weather, maximum relative humidity values will be in the 40s-60s (modest overnight recovery) through Wednesday before troughing influence works to improves overnight recovery (max RHs in the 50s-70s). Overnight recovery will still be poor in the thermal belt regions of the foothills with max RH values in upper 20s to low 40s. The high pressure will weaken and slowly be replaced by upper level troughing, which will start to cool things down slightly for the region on Friday. Monsoonal moisture advecting through the high pressure, coupled with the Sierra as the forcing mechanism, could result in isolated thunderstorm chances along the Sierra Crest through Friday, before the threat subsides as a result of troughing and dry air moving in from the west. && .AVIATION... VFR conditions next 24 hrs. Isolated thunderstorms along and east of the Sierra/Cascade crest 21Z thru about 02Z Sunday. Surface wind gusts generally below 12 knots except local southwest 15-30 kts vicinity west Delta and areas of southerly gusts 15-20 kts after 20Z Sacramento Valley and surrounding terrain. && .STO WATCHES/WARNINGS/ADVISORIES... None. && $$