Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 05 2025 15:41:15 FOUS30 KWBC 051541 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1041 AM EST Wed Mar 5 2025 Day 1 Valid 12Z Wed Mar 05 2025 - 12Z Thu Mar 06 2025 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF SOUTHERN CALIFORNIA, NORTHEAST, AND MID-ATLANTIC... ....16z update... ....Southern California... No significant changes were made out in California with the=20 Marginal Risk; lighter than average onshore/upslope flow is=20 counter-balanced with slightly higher than average 1.25"+ total=20 PWat values will support 300 kg/m/s IVT flux should mitigate=20 intense rain-rates but slow eastward progressions may allow for=20 solid, hopefully more beneficial rainfall totals. A Marginal Risk=20 still seems appropriate. ....Mid-Atlantic to New England...=20 South of the Mason-Dixon line, the risk for excessive rainfall will be associated with slowly strengthening squall/convective line that is starting to cross into the Piedmont. Low level flow will=20 continue to strengthen with 65-70kt southerly 850mb flow, rapidly=20 increasing deeper layer moisture along/ahead of the cold front,=20 this will bring total PWats over 1.5" nearing the NC/VA boarder=20 over the next few hours (this combination results in 1200-1400=20 kg/m/s IVT stream). The higher moisture/WAA along with steepening=20 lapse rates aloft may allow for some weak instability 250 J/kg may=20 reach 500 J/kg MUCAPE by 18z into NC/S VA to help fuel greater=20 vertical depth and expansion of the line northward with time. Rates should steadily come up to 1"/hr but given flux and convergence=20 along the line, may allow for scattered cores within the line to=20 produce higher rates and streaks of 1-2" and may reach as far north as DC/S Central MD. These rates/totals are at the limit of FFG=20 value mainly north of the VA/NC line with greatest potential for=20 flooding in urban locations, but given progressive nature of the=20 line a Marginal Risk seems appropriate for coverage. North of the Mason Dixon line, the moisture flux will support solid over-running stratiform rainfall across frozen/snow covered ground allowing for increased run-off potential. While not truly=20 exceeding FFG values, the risk of flooding from the combination of rain on snow is covered by the Marginal Risk. A few convective cells within the cold-air aloft across the Upper Ohio River Valley may induce highly localized showers/hail and pose some similar risk, so have adjusted the the Marginal Risk a bit further west and north to account for this potential.=20 Gallina ---Prior Discussions--- ....Mid-Atlantic States through New England... Potent mid-latitude cyclone over the Midwest will progress further into the Great Lakes during the period with a strong southerly moisture advection regime located over the Eastern CONUS. Fairly textbook meridional push of warm, unstable air will press northward amidst a potent 850-700mb layer of winds between 60-80kts with PWAT anomalies pushing the 95-99th percentiles extending from the Carolinas all the way into Southern New England. Sharp cold front pivoting underneath the primary cyclone will press eastward through the Southeastern U.S with the fronts northern latitudinal placement shifting through the Mid-Atlantic and Northeast. The combination of deep layer moisture, modest theta_E advection, and strong surface/low-level convergence will create a relatively progressive line of convection later this morning over Appalachia, moving eastward through the Piedmont creating a period of heavy rain potential within the urban corridors extending from Philadelphia to the Triad of North Carolina. Area FFGs are most prone to flood prospects within the DC/Balt, Richmond, and Philadelphia metros with the Raleigh-Durham corridor signaling a much higher threshold which should limit the potential despite the convective signature present. 00z HREF neighborhood probs for >1" are very high (80%) over much of the Mid-Atlantic and Northeast, but probs for >2" are much more muted outside some spots within the Blue Ridge of VA up through PA. The urbanization factors will create the greatest prospects for localized flash flood concerns, especially given the 1-2"/hr rates anticipated as the primary axis of convection blows through during the afternoon. Southeastern VA across the Hampton Roads area will be another area of concern with much of the latest CAMs output fairly aggressive for the local 1-2" totals over a very short span of time. Considering the heightened risk for this area due to the urbanization factors, the previous MRGL risk was extended south and southeast to include those places like Norfolk/VA Beach within the Tidewater, as well as the Richmond metro where hi-res depictions for heavy rainfall within a corridor of greater instability is increasingly likely. Further north into New England, heavy rainfall coupled with snow melt will allow for greater flood concerns, along with ice jam concerns coupled with the rain over portions of Downeast ME. The previous MRGL issuance was relatively unchanged in those areas with the biggest addition focused over VA and Southern MD. ....Southern California... Troughing over the Eastern PAC will make headway into the West coast with sights on CA as we move through period. A formidable moisture advection pattern is forecast across Central and Southern CA with the strongest anomalies (95-99th percentiles) focused over the Southern CA terrain thanks to prevailing southwesterly flow providing a but of an orthogonal component to the wind field. PWATs will be generally between 0.75-1" with locally higher within the confines of the coast. This is generally an output that puts the coastal terrain within the LA and San Diego domains at risk for localized flash flood concerns normally due to the complexity of the terrain. We add in the positioning of several major burn scars lingering over the Transverse and Peninsular Ranges and you create a better opportunity for those flood concerns, especially considering the threshold is hovering ~0.5"/hr at best. Thankfully the rates will be mainly relegated to the 0.5-1"/hr at peak intensity during the height of impacts this evening. This was enough to warrant a continuation of the previous MRGL in place with an extension to the Southern Sierra Foothills where heavy precip totals between 2-3" are forecast within complex terrain just east of the Valley. Kleebauer 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... 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. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9z1aqcj9hQn6q7t67KsjRexBABLa7AiwxbZsLh0bxJE7= 5Pm00nwPX2Pe4nB193jB_hlk7G292lcu1Tg8TlWhqfeY--E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9z1aqcj9hQn6q7t67KsjRexBABLa7AiwxbZsLh0bxJE7= 5Pm00nwPX2Pe4nB193jB_hlk7G292lcu1Tg8TlWhifgtXgk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9z1aqcj9hQn6q7t67KsjRexBABLa7AiwxbZsLh0bxJE7= 5Pm00nwPX2Pe4nB193jB_hlk7G292lcu1Tg8TlWh576uCks$=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) .