Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 30 2025 00:51:41 FOUS30 KWBC 300050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 850 PM EDT Sat Mar 29 2025 Day 1 Valid 01Z Sun Mar 30 2025 - 12Z Sun Mar 30 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR SOUTHEAST LOUISIANA... ....01Z Excessive Rainfall Discussion... Maintained the Moderate Risk area over southeast Louisiana in deference to the 18Z HREF probabilities showing additional convection across southeast Louisiana overnight...despite some=20 very short- term decreases in rainfall rates that correlated with=20 decreases in instability in the Hi-Res CAMS. Slow moving thunderstorms along and near I-10 earlier this afternoon had a history of 2.5 to 3 inch per hour rates. However...radar was beginning to show rainfall rates diminishing as the SPC Mesoanalysis page was beginning to show weakening instability in that part of the state. If convection behaves as the HREF shows...storms will be moving back into a region characterized by a nearly saturated environment with a tall...skinny instability profile. The 00Z sounding from Slidell...to the east of the=20=20=20 afternoon convection...was nearly saturated through a deep layer=20 with Precipitable Water value of 1.8 inches. With radar and=20 satellite imagery showing at least two convectively induced=20 vorticity centers and the support of an upper level jet to=20 potentially support additional convection that overlaps with=20 convection earlier in the day...maintained the Moderate risk area.=20 There were a few modifications to the western side of the Marginal=20 and Slight risk areas based on early evening satellite and radar=20 imagery...with few changes made elsewhere. Bann ....16Z Excessive Rainfall Discussion... 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 convergence zone at 850 hPa moves from west to east across Louisiana and Mississippi today and tonight, which should provide an uptick to low-level winds and effective bulk shear, values of which are already sufficient for organized convection in and near 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 convection continues to fade offshore the upper TX Coast while the axis of 850 hPa convergence moving across LA and its former MCV spawn new convection inland, which was hinted at in the 12z HREF probability fields of 0.5"+ in an hour which seem to be handling the situation fairly well, thus far. When cell training or backbuilding establishes itself across southern LA, after roughly four hours it could bore a hole in the instability field. Once this happens, backbuilding would become more likely and the 1000-500 hPa thickness field suggests a southeast propagation to some portion of the mass of convection would be expected -- this should cause a northward bound on the location of highest rainfall potential. A significant enough threat exists for high totals this afternoon through tonight that an upgrade to a Moderate Risk appeared prudent based on the above, which was coordinated with LIX/the Slidell LA forecast office and also supports their ongoing flood watch. The potential for significant impact/inundation from 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 SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE ARKLAMISS... Central Gulf Coast through the Ohio Valley into the Great Lakes... 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. Broad inflow from the Gulf with precipitable water values of 1.25-1.75" and ML CAPE potentially as high as 4000 J/kg is supportive of discrete and multi-cell clusters initially, and likely transitioning to more a more linear QLCS orientation along the front into the evening. Hourly rain totals to 2.5" are supported by the ingredients above, with the 12z Canadian Regional indicating the potential for 6", which would be possible where storms train or two or more mesocyclones align. The model guidance is coming up on amounts as anticipated/we get more into the mesoscale model window, with global guidance in the 2-3" range and mesoscale guidance higher. With probabilities of 3"+ maximized in the vicinity of the Mid- South, went ahead with an upgrade to a Slight Risk from the AR/LA border into central MS. Florida Peninsula/Southeasternmost Georgia... A frontal boundary at 850 hPa slowly lifts through the state, bringing precipitable water values to 1.5-1.75". MU CAPE maximizes above 2000+ J/kg in South Florida. There appears to be enough 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- Atlantic/Northeast, while the southern shortwave likely hangs back into the Mid-South and Lower MS Valley. The best moisture and instability lies over portions of the Southeast (where forcing from the secondary shortwave should still be sufficient for organized convective activity). The inherited Marginal risk was generally 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. Northeast... Thunderstorms with heavy rainfall are expected during the afternoon and evening hours ahead of a northern stream shortwave as MU CAPE peaks around 1000 J/kg and precipitable water values peak in the 1.25-1.5" range. Sufficient inflow and effective bulk shear is available for organized convection for much of the period, with activity expected to be in decline past Tuesday 06z. Hourly rain totals up to 2" are possible where convection trains, which would challenge the flash flood guidance across urban portions of the Mid-Atlantic States. This led to the introduction of a new Marginal Risk area. Near the western CA/OR border... A cyclone bring low-level inflow up to 40 kts and some limited instability, up to 250 J/kg MU CAPE, into the region. The guidance generally agrees on ~3" of QPF this period. However, precipitable water values should stay below 0.75" and freezing levels are low, generally 2000-4000 feet above ground level in westernmost OR and 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 scar locations. Roth/Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_z6sqvtoLPB6353I0tZjPoS88_UQ-Zgb3DUKdS8627rz= wzZA_faYlFgIS2N8YRrWTUwWOW0mWLszjJxXoPyWrM39yEg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_z6sqvtoLPB6353I0tZjPoS88_UQ-Zgb3DUKdS8627rz= wzZA_faYlFgIS2N8YRrWTUwWOW0mWLszjJxXoPyWIcV8f34$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_z6sqvtoLPB6353I0tZjPoS88_UQ-Zgb3DUKdS8627rz= wzZA_faYlFgIS2N8YRrWTUwWOW0mWLszjJxXoPyWVyBxrpQ$=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. 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