Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 29 2025 19:40:59 FOUS30 KWBC 291939 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 339 PM EDT Sat Mar 29 2025 Day 1 Valid 16Z Sat Mar 29 2025 - 12Z Sun Mar 30 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR SOUTHEAST LOUISIANA... A convergence zone/segment of the polar front across the northern Gulf and southeast TX is expected to lift northeast and north into Louisiana with time and act as a focus for convection today into tonight. Aloft, a mid- level shortwave is moving across southeast AR and acting as a mechanism for divergence aloft near the Gulf Coast. Precipitable water values of 1.75"+ are expected within an environment of 5640-5670 meter 1000-500 hPa thickness, implying complete saturation. ML CAPE is expected to rise to 2000 J/kg or so during the day today. A north-northeast/south- southwest oriented=20 convergence zone at 850 hPa moves from west to east across=20 Louisiana and Mississippi today and tonight, which should provide=20 an uptick to low-level winds and effective bulk shear, values of=20 which are already sufficient for organized convection in and near=20 southern LA & southern MS. Hourly rain totals to 2.5" should be easy to achieve in this environment -- should storms backbuild/train for a prolonged period, even higher hourly totals would be possible. The 12z HREF showed a 40%+ chance of 8"+ totals across southeast LA -- there is the potential for local amounts in the 10" range. Offshore=20 convection continues to fade offshore the upper TX Coast while the axis of 850 hPa convergence moving across LA and its former MCV=20 spawn new convection inland, which was hinted at in the 12z HREF=20 probability fields of 0.5"+ in an hour which seem to be handling=20 the situation fairly well, thus far. When cell training or backbuilding establishes itself across=20 southern LA, after roughly four hours it could bore a hole in the=20 instability field. Once this happens, backbuilding would become=20 more likely and the 1000-500 hPa thickness field suggests a=20 southeast propagation to some portion of the mass of convection=20 would be expected -- this should cause a northward bound on the=20 location of highest rainfall potential. A significant enough threat exists for high totals this afternoon through tonight that an=20 upgrade to a Moderate Risk appeared prudent based on the above,=20 which was coordinated with LIX/the Slidell LA forecast office and=20 also supports their ongoing flood watch. The potential for=20 significant impact/inundation from flash flooding over urban areas=20 can't be ruled out. Roth Day 2 Valid 12Z Sun Mar 30 2025 - 12Z Mon Mar 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE=20 ARKLAMISS... Central Gulf Coast through the Ohio Valley into the Great Lakes... A pair of shortwave troughs emerge from the Intermountain West=20 into the Central U.S. on Sunday, eventually partially phasing with=20 the polar jet (and an associated longwave trough) by Sunday night.=20 This complex interaction is expected to result in a broad area of=20 convection across much of the eastern half of the CONUS, including=20 portions of the MS, TN, and OH Valleys.=20 Despite the complexity of the synoptic and mesoscale details,=20 models remain in fairly good agreement in indicating convection=20 breaking out across a broad warm sector. Broad inflow from the Gulf with precipitable water values of 1.25-1.75" and ML CAPE=20 potentially as high as 4000 J/kg is supportive of discrete and=20 multi-cell clusters initially, and likely transitioning to more a=20 more linear QLCS orientation along the front into the evening.=20 Hourly rain totals to 2.5" are supported by the ingredients above,=20 with the 12z Canadian Regional indicating the potential for 6",=20 which would be possible where storms train or two or more=20 mesocyclones align. The model guidance is coming up on amounts as=20 anticipated/we get more into the mesoscale model window, with=20 global guidance in the 2-3" range and mesoscale guidance higher.=20 With probabilities of 3"+ maximized in the vicinity of the Mid-=20 South, went ahead with an upgrade to a Slight Risk from the AR/LA=20 border into central MS. Florida Peninsula/Southeasternmost Georgia...=20 A frontal boundary at 850 hPa slowly lifts through the state,=20 bringing precipitable water values to 1.5-1.75". MU CAPE maximizes above 2000+ J/kg in South Florida. There appears to be enough=20 effective bulk shear for thunderstorm organization as well. The model guidance --- mostly mesoscale -- indicates the potential for local amounts in the 5" range. This led to the introduction of a new Marginal Risk area. Roth/Churchill Day 3 Valid 12Z Mon Mar 31 2025 - 12Z Tue Apr 01 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST & NORTHEAST... Southeast... The aforementioned pair of shortwave troughs are progged to fully separate by Day 3, as the northern stream shortwave continues to phase with the parent longwave trough into the Mid-=20 Atlantic/Northeast, while the southern shortwave likely hangs back=20 into the Mid-South and Lower MS Valley. The best moisture and=20 instability lies over portions of the Southeast (where forcing=20 from the secondary shortwave should still be sufficient for=20 organized convective activity). The inherited Marginal risk was=20 generally maintained for this portion of the Southeast (including=20 much of AL/GA with surrounding portions of TN/NC/SC and MS), where=20 GEFS/ECENS exceedance probabilities suggest the best potential for=20 2-4" localized totals.=20 Northeast... Thunderstorms with heavy rainfall are expected during the afternoon and evening hours ahead of a northern stream shortwave as MU CAPE=20 peaks around 1000 J/kg and precipitable water values peak in the=20 1.25-1.5" range. Sufficient inflow and effective bulk shear is=20 available for organized convection for much of the period, with=20 activity expected to be in decline past Tuesday 06z. Hourly rain=20 totals up to 2" are possible where convection trains, which would=20 challenge the flash flood guidance across urban portions of the=20 Mid-Atlantic States. This led to the introduction of a new Marginal Risk area. Near the western CA/OR border...=20 A cyclone bring low-level inflow up to 40 kts and some limited=20 instability, up to 250 J/kg MU CAPE, into the region. The guidance=20 generally agrees on ~3" of QPF this period. However, precipitable=20 water values should stay below 0.75" and freezing levels are low,=20 generally 2000-4000 feet above ground level in westernmost OR and=20 northwesternmost CA. Mostly as a nod to continuity, a Marginal Risk area was not re-introduced here, but there is a non-zero chance of 0.5"+ rain totals, which would be problematic in and near burn=20 scar locations. Roth/Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5d8ivkAwFdsqxZUh3vjzq4puHf-pu-DFEBIWH-BSya14= 7CKx6mdlNThd7L8Z7s0ZotvSLwlGZGBWHEvzYG0J_YMcc5o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5d8ivkAwFdsqxZUh3vjzq4puHf-pu-DFEBIWH-BSya14= 7CKx6mdlNThd7L8Z7s0ZotvSLwlGZGBWHEvzYG0JU2g32mo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5d8ivkAwFdsqxZUh3vjzq4puHf-pu-DFEBIWH-BSya14= 7CKx6mdlNThd7L8Z7s0ZotvSLwlGZGBWHEvzYG0JY2z_klQ$=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) .