Area Forecast Discussion
Issued by NWS Sacramento, CA
Issued by NWS Sacramento, CA
876 FXUS66 KSTO 192222 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 322 PM PDT Thu Sep 19 2024 .Synopsis... A cool pattern is over interior NorCal today, brining gusty winds to the Delta, along with rain showers and a chance for isolated thunderstorms over the Sierra (primarily south of I-80). Drier weather starts tomorrow with an overall warming trend beginning. Increased high temperatures with Moderate HeatRisk and elevated fire weather concerns return to the Valley and foothills early next week. && .Discussion... An upper lverl low off the coast is bringing in moisture from east to west over the Sierra and foothills, bringing one last day of showers and thunderstorms to that area this afternoon and evening. While a few showers and thunderstorms have developed over western Plumas County this afternoon, most thunderstorm development has been south of I-80. A few light showers may reach the Park Fire burn scar, but no debris flow impacts are expected today. Remnants of Sierra storms have the potential to bring light showers/virga to the far eastern portions of the Valley, particularly the eastern portions of the northern San Joaquin Valley. Dry conditions are expected elsewhere. The center of the low pressure system will gradually move in Southern California Friday, with dry weather returning to interior Northern California. 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 build 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 Saturday-Sunday resulting in localized elevated fire weather concerns. && .EXTENDED DISCUSSION (Monday THROUGH Thursday)... Early next week, we`re expecting drier and warmer conditions as ridging builds in over the west coast; we`ll also see a return of Moderate HeatRisk to much of the Valley and foothills. NBM is currently showing a 60 to 85% probability of temperatures over 95 in the Valley on Monday and Tuesday, and 20 to 40% of temperatures above 100 (though the risk of this is limited to the northern San Joaquin Valley and Sacramento Metropolitan area). In addition to the warm temperatures on Monday, with the positioning of the ridge just over an upper low to the south, the resulting pressure gradient will bring breezy north winds to parts of the Sacramento Valley (primarily along the I-5 corridor and in the northern Sacramento Valley). Gusts are currently expected to fall around 15 to 25 mph. Tuesday, as the upper level pattern continues to progress, breezy winds will shift to the Sierra and foothills, with easterly downslope winds gusting around 20 to 30 mph. These winds combined with lower relative humidities in the teens will bring elevated fire weather conditions early in the week. Midweek is expected to cool a couple degrees as weak troughing begins to develop offshore, though remaining above normal for the time of year. NBM currently has a decent amount of spread in possible temperatures on Wednesday and Thursday, due in large part to uncertainty within ensembles and clusters about the timing of the trough moving into the area. That said, the NBM is projecting a 25-50% probability of temperatures below 90 degrees on Wednesday and Thursday in the Valley, which would put Valley locations back near climatological normal. && .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. && $$