Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 06 2025 00:22:02 FOUS30 KWBC 060020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 720 PM EST Wed Mar 5 2025 Day 1 Valid 01Z Thu Mar 06 2025 - 12Z Thu Mar 06 2025 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF SOUTHERN CALIFORNIA AND NORTHEAST... ....0100 UTC Update... ....Northern Mid-Atlantic to New England... Latest GOES-16 water vapor and longwave IR Loops show the back edge of the Warm Conveyor Belt (WCB), or 'limiting streamline', nudging east toward the DELMARVA and east of central PA-NY. Largely due to the (a) progressive nature of any pre-frontal squall lines and (b) lack of deep layer instability (elevated CAPEs under 250 J/Kg), peak hourly rainfall rates are struggling to exceed 1" in most areas. For the rest of the evening into the overnight, strong deep-layer moisture flux ahead of the increasingly negatively- tilted mid-upper level trough will support solid overrunning=20 stratiform rainfall across frozen/snow covered ground, allowing=20 for increased run-off potential. While not truly exceeding FFG=20 values, the risk of flooding from the combination of rain on snow=20 is covered by the Marginal Risk. Hurley/Gallina ....1600 UTC update... ....Southern California... No significant changes were made out in California with the Marginal Risk; lighter than average onshore/upslope flow is counter-balanced with slightly higher than average 1.25"+ total PWat values will support 300 kg/m/s IVT flux should mitigate intense rain-rates but slow eastward progressions may allow for solid, hopefully more beneficial rainfall totals. A Marginal Risk still seems appropriate. Gallina Day 2 Valid 12Z Thu Mar 06 2025 - 12Z Fri Mar 07 2025 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF SOUTHERN CALIFORNIA... ....21Z Outlook Update... The previous forecast is on track, with lingering heavy rain potential continuing from D1/Wed especially in terrain-favored areas of the Transverse and Peninsular Ranges of southern California. Latest guidance has trended wetter in the latter half of the forecast period (00Z-12Z Fri) especially across San Diego County. Isolated instances of flash flooding remain possible as areas of 0.75-1.5 inch rain amounts fall over/near burn scars within the Marginal Risk area. See the previous discussion for more information. Cook ....Previous Discussion... Persistent in-flow trailing the mean trough propagating into the Western CONUS will allow for continued moderate precip in-of the Southern California terrain, mainly the Transverse and Peninsular ranges within the Los Angeles and San Diego WFO domains. Areal moisture flux will be trending down in terms of anomalies with the highest PWATs centered further inland. Despite the evolution in question, this is mainly a continuation from the period prior leading to ongoing flash flood potential within the terrain where several burn scars are present. Fortunately, the setup degrades further by the second half of the period, leading to mainly scattered shower instances as we move into the overnight period Thursday through early Friday morning. Majority of guidance has precip between 0.5-1" within the mountains outside LA and San Diego, enough to continue any flood prospects that occurs on D1. The previous MRGL was relatively unchanged as guidance maintains run-to-run continuity. Kleebauer Day 3 Valid 12Z Fri Mar 07 2025 - 12Z Sat Mar 08 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Cook/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9T634LbzFpAp3crNevZw5e_fgT8myAsMd0UcHFMuoWQL= IYnoW5-KfrmrMfeLH3otWxKkb3Klfkto1xNHLTYpl7C-rkw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9T634LbzFpAp3crNevZw5e_fgT8myAsMd0UcHFMuoWQL= IYnoW5-KfrmrMfeLH3otWxKkb3Klfkto1xNHLTYpc5LgU5k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9T634LbzFpAp3crNevZw5e_fgT8myAsMd0UcHFMuoWQL= IYnoW5-KfrmrMfeLH3otWxKkb3Klfkto1xNHLTYpX3PRnJM$=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) .