Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 29 2025 15:52:56 FOUS30 KWBC 291552 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1152 AM 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=20 tonight. Aloft, a mid- level shortwave is moving across southeast=20 AR and acting as a mechanism for divergence aloft near the Gulf=20 Coast. Precipitable water values of 1.75"+ are expected within an=20 environment of 5640-5670 meter 1000-500 hPa thickness, implying=20 complete saturation. ML CAPE is expected to rise to 2000 J/kg or so during the day today...recent GOES-East imagery shows pockets of clearing over Louisiana and southwest Mississippi at the present=20 time which should allow for an additional 1000 J/kg or so of ML=20 CAPE to form today (sitting presently at 500-1000 J/kg across=20 southern LA and the southwest half of MS). A north-northeast/south- southwest oriented convergence zone at 850 hPa moves from west to=20 east across Louisiana and Mississippi today and tonight, which=20 should provide an uptick to low-level winds and effective bulk=20 shear, values of which are already sufficient for organized=20 convection in and near southern LA & southern MS. Hourly rain totals to 2.5" should be easy to achieve in this=20 environment -- should storms backbuild/train for a prolonged=20 period, even higher hourly totals would be possible. The 12z HREF=20 showed a 40%+ chance of 8"+ totals across southeast LA -- there is the potential for local amounts in the 10" range. Convection south of High Island TX and west of Galveston TX has been difficult to=20 dislodge, with back building convection evident. The usual=20 expectation would be for the offshore convection to fade in the=20 next couple hours and send outflow boundaries ashore or that the axis of 850 hPa convergence moving across LA would force new=20 convection inland, which is hinted at in the 12z HREF probability=20 fields of 0.5"+ in an hour and this expectation may be starting to materialize in recent radar imagery.=20 There is concern about the exact location and track of any=20 organized convective rainfall areas. When cell training/=20 backbuilding establishes itself across southern LA, after roughly=20 four hours it could bore a hole in the instability field. Once this happens, backbuilding would become more likely and the 1000-500=20 hPa thickness field suggests a southeast propagation to some=20 portion of the mass of convection would be expected -- this should cause a northward bound on the location of highest rainfall=20 potential. A significant enough threat exists for high totals this=20 afternoon through tonight that an upgrade to a Moderate Risk=20 appeared prudent based on the above, which was coordinated with=20 LIX/the Slidell LA forecast office and also supports their ongoing=20 flood watch. The potential for significant impact/inundation from=20 flash flooding over urban areas can't be ruled out. Roth Day 2 Valid 12Z Sun Mar 30 2025 - 12Z Mon Mar 31 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS... A much broader inherited Marginal risk area (relative to Day 1) has been largely maintained, as a pair of shortwave troughs emerge from the Intermountain West into the Central U.S. on Sunday, eventually partially phasing with the polar jet (and an associated longwave trough) by Sunday night. This complex interaction is expected to result in a broad area of convection across much of the eastern half of the CONUS, including portions of the MS, TN, and OH Valleys. Despite the complexity of the synoptic and mesoscale details, models remain in fairly good agreement in indicating convection breaking out across a broad warm sector (characterized by PWATs of 1.5-2.0", SB CAPE of 1000-2000 J/kg, and effective bulk shear of 40-60 kts) supportive of discrete and multi-cell clusters initially, and likely transitioning to more a more linear QLCS orientation along the front into the evening (as best depicted by the 00z FV3 and NAM-nest, which are the only CAMs which extend through the full period at 00z). While WPC areal average QPF calls generally for only 1-2" totals, most deterministic solutions from the global models are likely well underdone on localized totals, as CAMs will be needed to better model the discrete convective initiation that is anticipated prior to the main QLCS passage (with the FV3, NAM- nest, and CMC-reg all indicating the potential for 3-5" totals, with probabilities likely maximized in the vicinity of the Mid-South and MS Delta regions where instability and moisture are more ideal... and this is where a targeted Slight Risk is most likely to be introduced in future updates, possibly as early as later today). 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... The aforementioned pair of shortwave troughs are progged to fully separate by Day 3, as the northern stream shortwave continues to phase with the polar jet (and associated parent longwave trough) into the Mid-Atlantic/Northeast, while the southern shortwave likely hangs back into the Mid-South and Lower MS Valley. This is anticipated to result in a weaker setup overall for excessive rainfall on Monday into Monday night, as the best dynamics and upper-level forcing over the Northeast and Mid-Atlantic is displaced from the best moisture and instability over portions of the Southeast (where forcing from the secondary shortwave should still be sufficient for organized convective activity). The inherited Marginal risk was maintained for this portion of the Southeast (including much of AL/GA with surrounding portions of TN/NC/SC and MS), where GEFS/ECENS exceedance probabilities suggest the best potential for 2-4" localized totals. Farther northeast into the Mid-Atlantic and southern portions of New England, while total tropospheric moisture will certainly be lower (~1.25" versus 1.5"+), these values are either equally or more anomalous than in the Southeast (90th percentile or higher). Despite this, have decided not to introduce a Marginal risk at this time, due to the combination of marginal instability and progressive storm motions. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6mHmWvuZZ0xXBIOq9AK3p-S1jIiLihAsaYu0Cv4Iuzvl= _E-iaIL2ulRwGL7A-CBdd84tUVcmgHJxetgx4lBsQTZqx_w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6mHmWvuZZ0xXBIOq9AK3p-S1jIiLihAsaYu0Cv4Iuzvl= _E-iaIL2ulRwGL7A-CBdd84tUVcmgHJxetgx4lBsw_GDuzg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6mHmWvuZZ0xXBIOq9AK3p-S1jIiLihAsaYu0Cv4Iuzvl= _E-iaIL2ulRwGL7A-CBdd84tUVcmgHJxetgx4lBsPyAgQ7Q$=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) .