Area Forecast Discussion
Issued by NWS Sacramento, CA
Issued by NWS Sacramento, CA
509 FXUS66 KSTO 191011 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 311 AM PDT Thu Sep 19 2024 .Synopsis... Warmer temperatures today compared to yesterday, with chances for scattered showers and thunderstorms along the Sierra mountains south of I-80, with a few lingering cells in the higher elevation foothills. Warm and dry weather returns Friday into next week, with areas of moderate HeatRisk and elevated fire weather conditions Monday & Tuesday across the Delta, Valley, and foothills. && .Discussion... Today`s thunderstorm threat will remain confined to the Sierra mountains and higher elevation foothills south of I-80, where NBM probabilities show a 30-40% chance of development. The center of the low pressure system will gradually move across Southern California later today, drawing in moisture across the Sierra in an east to west orientation. A few scattered showers developing during the late morning hours will precede afternoon thunderstorm initiation along the foothills. East-southeasterly steering flow aloft will allow storms to propagate downslope of the Sierra shortly after their initiation. Storms will generally weaken as they move downslope, but more robust cells will hold together longer bringing potential thunderstorm impacts further into the Sierra foothills where only a 20-25% chance of thunderstorms exists. Remnants of these storms have the potential to bring light showers/virga to eastern portions of the Valley, particularly the eastern portions of the northern San Joaquin Valley. Dry conditions are expected everywhere else. A few light showers may reach the Park Fire burn scar, but no debris flow impacts are expected today. 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. The NBM shows a 60 to 90% chance of high temperatures greater than 90 deg F on Friday over the central and northern Sacramento Valley. Probabilities extend into the Delta, southern Sacramento Valley, and northern San Joaquin by Saturday and Sunday. This warming trend continues into early next week, accompanied by a drying trend. As the previous system propagates eastward, a steepening pressure gradient develops as ridging begins to move over Northern California. As a result, periodically breezy north winds will permeate throughout the Valley and easterly/downslope winds over the Sierra over the weekend and into early next week. Gusts will generally be 15-20 mph with daytime relative humidities will be in the mid teens to mid 20s, initially over the northern Sacramento Valley and adjacent foothills Sat-Sun resulting in localized elevated fire weather conditions. && .EXTENDED DISCUSSION (Monday THROUGH Thursday)... Warmer and dryer weather is expected Monday and Tuesday, as we see the return of Moderate HeatRisk throughout the Valley and foothills. The NBM is also showing a 20-50% of high temperatures greater than 100 degrees around the Greater Sacramento Metro Area. Alongside these warmer temperatures, daytime relative humidities in the mid teens to low 20s will be present throughout the Valley, Delta, foothills, and mountains. This is also when were seeing indications of breezier 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) on Monday with gusts 15 to 25 mph. On Tuesday, increased flow will be located along the Sierra foothills and mountains where breezy downslope/easterly winds and gap winds gusting 20 to 30 mph will be present. These conditions look to bring more widespread elevated fire weather conditions to the region, especially in the Valley and foothills where RH values will be lowest. By Wednesday and into Thursday, troughing begins to develop offshore resulting in increased onshore flow and higher humidities through the Delta and Sacramento area. However temperatures will still be 5 to 10 degrees above normal for this time of year. && .AVIATION... Exiting weather system will keep MVFR/IFR conditions along the foothills and Sierra/Nevada Mountains near showers and thunderstorms for the next 24 hours. Gusty and erratic winds near storms should be expected. Elsewhere VFR conditions with variable winds 12kts or less is expected. && .STO WATCHES/WARNINGS/ADVISORIES... None. && $$