Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Nov 21 2024 06:01:35 AWUS01 KWNH 210601 FFGMPD CAZ000-211800- Mesoscale Precipitation Discussion 1162 NWS Weather Prediction Center College Park MD 100 AM EST Thu Nov 21 2024 Areas affected...Northern California Concerning...Heavy rainfall...Flash flooding possible Valid 210600Z - 211800Z SUMMARY...Nearly stationary atmospheric river will maintain heavy rainfall across northern CA through Thursday morning with increasing concerns for widespread areal flooding and possibly some burn scar flash flooding. DISCUSSION...The latest GOES-W Airmass RGB satellite imagery in conjunction with CIRA-ALPW data shows a well-defined atmospheric river and an associated front currently stalled over northern CA and the adjacent offshore waters extending southwestward off the West Coast. Strong warm air advection and moisture transport focused in close proximity to this front continues to overrun northern CA with rather widespread moderate to heavy rain impacting the region. Strong low to mid-level southwest flow around the south side of a powerful deep layer cyclone west of Vancouver Island will be locked in place overnight and through Thursday morning which will maintain enhanced atmospheric river activity over northern CA. Multiple weak waves of low pressure are expected to ride northeastward along the front over the next 6 to 12 hours which will take aim on the northern CA coastal ranges, and this will be facilitated by some backing of the mid to upper-level flow as additional shortwave/jet energy digs through the base of the larger scale trough offshore of the West Coast. Enhanced 850/700 mb moisture flux coupled with the deeper layer warm air advection pattern and orographic ascent/upslope flow over the higher terrain of northern CA will focus a persistence of moderate to heavy rain, with IVT magnitudes north of the Bay Area reaching 500 to 750+ kg/m/s. This will support elevated rainfall rates which based off the 00Z HREF guidance should occasionally reach well into the 0.50" to 0.75"/hour range. Some isolated rates potentially exceeding 0.75"/hour will be possible and especially over southern parts of Humboldt County where the southwest-facing slopes will see stronger orographic ascent coupled with very close proximity of the aforementioned front. The persistence of these elevated rainfall rates over the next 6 to 12 hours will likely result in additional rainfall amounts by 18Z (10AM PST) Thursday morning of 3 to 5 inches over the coastal ranges and the foothills of the northern Sierra Nevada, with some isolated 6+ inch amounts possible. These rains will be in addition to the already several inches of rain that have fallen for the event, and thus isolated some storm total amounts by late Thursday morning may well be 10+ inches. Lesser amounts of as much as 1 to 3 inches will generally be expected for the interior valleys including the northern Sacramento Valley. Expect increasing concerns for widespread areal flooding impacts from the additional rainfall going through Thursday morning, and this may include some concerns for burn scar flash flooding at least locally. Debris flow and landslide activity will be a notable concern as well from these heavy rains. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!9BjNe-FW4HxpKd13oWUWc_JNgylrROGd6XTEv4pWKbm3bvcktYTNiMqJ079UYAML0jex= TIjq5wZE6gguTaoK9-EWBdE$ for graphic product... ATTN...WFO...EKA...MFR...MTR...STO... ATTN...RFC...CNRFC...NWRFC...NWC... LAT...LON 41972347 41602315 41142289 41142210 40672151=20 40082122 39472046 39052031 38842061 38542159=20 38122193 37882259 38322329 39262410 40662456=20 41842417=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .