Area Forecast Discussion
Issued by NWS Sacramento, CA

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000 FXUS66 KSTO 202112 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 212 PM PDT Tue Apr 20 2021 .SYNOPSIS... Cooler today with the chance of mainly mountain showers and thunderstorms. A few showers possible Wednesday in the Sierra south of Tahoe. Otherwise dry and warmer weather returns through the end of the week. Chance of precipitation returns this weekend. && .DISCUSSION... A robust short wave and vort max is begining to push into the NW part of the state this afternoon. This vort max has brought showers to far northern CA this morning and we are starting to see these showers spread south. Mesoscale Analysis has generally 100-250 j/kg of uncapped CAPE from about Chico north and models build in as much as 500+ j/kg of CAPE by the evening mainly from Sacramento north. Showers and a few thunderstorms will be possible even in the Valley given the CAPE as the vort max pushes over the region but the best chances will remain over the higher elevations. Current PWATS are between 0.20 and 0.60" and we will see moisture continue to build into the region this evening with PWATS reaching 0.75" in some areas. This will bring a heavy rain threat with any convection but soundings due have an inverted V look to them in the Valley and that should limit the heavy rain potential for lower elevations and bring more of a gusty wind threat. With more moist low levels over the higher elevations that is where the heavy rain threat will be greatest. Rain rates are expected to remain below flash flood guidance over the burn scars but up to 0.60" of rain in an hour will be possible over portions of the North Complex, Camp Fire Burn Scar and August Complex. With limited low level shear storms will likely pulse up and quickly pulse down. Shower chances will continue to push south into the evening but dry low levels and limited CAPE will really limit activity as it reaches the Sacramento area. Mainly dry conditions can be expected overnight. The upper level trough will push into NV during the morning tomorrow and we will see some CAPE build in over the Sierra from I-80 south. This will bring a threat of a few showers and thunderstorms once again for tomorrow but coverage will be very limited to I-80 south over the Sierra. Wind will be the other impact with this system and we are seeing some breezy northerly winds over the northern Sacramento Valley this afternoon. These winds will calm some overnight before picking back up tomorrow mid to late morning into the afternoon. Most of the region is seeing onshore flow today and that is keeping humidity levels up but that will not be the case tomorrow and some single digit humidity reading look possible over the northern Sacramento Valley during the afternoon. This will bring some increased fire weather concerns given the increased wind with it. Upper level ridging builds in on Wednesday and that will bring a period of quiet weather into Friday with less wind. A short wave trough will approach the northern part of the state Friday night and we could see a few light showers develop over the Shasta County Mountains overnight. Highs will continue to run 6-12 degrees above average this week with today being the coolest day in the short term. -CJM && .EXTENDED DISCUSSION (Saturday THROUGH Tuesday)... Cluster analysis has come into better agreement about an unsettled weather pattern impacting Northern California this Sunday into next week. We will likely see cooler temperatures as a trough makes it way into the region. We are also forecasting widespread precip chances across Northern California Sunday into early next week. Best chances will be for higher elevation locations, however Valley rain will be possible. Snow showers will likely bring travel impacts to Sierra Passes particularly on Sunday. Some uncertainty still exists with timing and amounts as depicted by cluster analysis so check back for updates. && .AVIATION... Widespread VFR with MVFR possible due to passing shower activity. Isolated thunderstorms could impact Sacramento Valley and Sacramento Metro area TAF sites. IFR possible under any isolated heavy showers that may develop. Locally gusty northerly winds 20 to 30 kts over Northern Sacramento Valley and Southwesterly gusts up to 25 kts through Delta region decreasing after around 4 UTC. && .STO WATCHES/WARNINGS/ADVISORIES... None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.