Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 06 2025 00:30:44 FOUS30 KWBC 060030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 830 PM EDT Mon May 5 2025 Day 1 Valid 01Z Tue May 06 2025 - 12Z Tue May 06 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN HIGH PLAINS... ....Southern High Plains... Showers and thunderstorms capable of producing downpours and a risk of excessive rainfall will become more numerous across parts of=20 eastern New Mexico and spread south and east into parts of western Texas as moisture deepens within an upslope flow regime and=20 an upper low and surrounding shortwaves approach the region. With precipitable water values ranging from 0.75" in the Southern=20 High Plains to 1.5" at lower elevations to the east, a broad area=20 of 500-3000 J/kg of MU CAPE to the south, and effective bulk shear=20 of 40-70 kts, supercell development is likely to continue. An MCS=20 likely to form tonight as storms merge into a broader mass across=20 western Texas/eastern New Mexico. Hourly rain totals to 2.5" would be possible where storms backbuild, merge, or try to train, with=20 local totals to 5" possible given the non-zero probabilities of=20 such within the 18z HREF guidance. The signal in the 18z HREF=20 probabilities of the various precipitation thresholds through 12z=20 has shifted southward, which led to a southward shift in the Slight Risk area. Radar reflectivity and MU CAPE trends in southeast TX were also taken into account with the adjustments made to the Marginal Risk area. ....Northeast... There is a continuing risk of excessive rainfall as a closed low meanders over parts of the Ohio Valley and Great Lakes region...leaving the Northeast under diffluent flow at upper=20 levels while moisture continues to be transported into the region=20 at low levels. Adjustments were made to account for radar reflectivity trends and 18z HREF probabilities of various thresholds. ....Southwest... There is a Marginal risk of excessive rainfall over portions of the Southwest US as a deep layered upper low moves very slowly eastward. Between precipitable water values of 0.5 to 0.75 inches and cold temperatures aloft helping to create steep low/mid level lapse rates and saturated profiles...showers and thunderstorms=20 should continue into tonight with isolated downpours and locally=20 heavy rainfall amounts today. Adjustments were made to account for radar reflectivity trends and 18z HREF probabilities of various=20 thresholds. Roth/Bann Day 2 Valid 12Z Tue May 06 2025 - 12Z Wed May 07 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF EASTERN TEXAS INTO THE LOWER MISSISSIPPI VALLEY... 20z Update: Main adjustment was a southward shift of the MDT risk over eastern TX into the lower MS Valley based on trends seen in the 12z HREF and REFS. Highest 3" EAS probabilities (a smoothed point probability) stretch from east TX into much of LA, with 5" neighborhood probabilities getting into the 30-70% range. Thus seems likely that a swath of 3-5" of rainfall will occur, with localized totals over 6". 8" neighborhood probabilities drop to 20% or less, with the forward progression potentially limiting the exceedance of 8" and keeping this from being a High risk type of event. Nonetheless, even with a progressive system convection will be organized and widespread enough to result in significant rainfall totals, especially given what should be a very efficient environment for high rates. Numerous instances of flash flooding are expected, some of which could be locally significant in nature, especially if these higher totals end up over a more sensitive urban area. Chenard ....Previous Discussion... ....Lower Mississippi Valley to the Colorado Front Range... The inherited Moderate Risk area was greatly expanded with this update as global models get a better handle on instability and part of the event begins to be covered by high resolution CAMs. Heaviest rainfall and associated risk of flash flooding should be tied to deeper convection that forms and moves from parts of eastern Texas across central and northern Louisiana/far southern Arkansas into parts of Mississippi. 05/00Z guidance continued to depict amounts of 2 to 4 inches in the area with embedded amounts greater than 6 inches. Convection should start out at roughly a southwest to northeast orientation but become realigned more west to east Tuesday night. This increases the potential for repeating storms and training of cells. On the northern side of the complex of storms will allow for continued lighter rains in northern areas that were hard hit earlier in the evening, which will continue or worsen ongoing flash flooding as a result of the earlier storms. Should there be significant increases with future runs, that are expected to be more concentrated, then it's not completely out of the question that a targeted High Risk may be needed with future updates. Even though the heaviest rains will be associated with the moderate Risk area in portions of the lower Mississippi Valley, a good amount of that moisture and instability will be drawn northwestward ahead of a strong shortwave trough associated with the slow-moving upper level low over the 4 Corners region with some of the heavier rainfall amounts moving over areas hard hit with prior flooding rains, particularly over Oklahoma and the Panhandles. ....Northeast... A slow moving upper level low over the Midwest will continue to pump Atlantic moisture into New England on Tuesday...a continuation of periods of rain from today. Instability is far from impressive, but it will still be enough to allow for convective showers and maybe some embedded thunderstorms to move north up the coast and into New England but model QPF remained in check given the progressive nature of the forcing. Bann Day 3 Valid 12Z Wed May 07 2025 - 12Z Thu May 08 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE OVER PORTIONS OF THE GULF COAST... 20z Update: Only minimal changes to the inherited Slight risk over southeast TX into LA. The environment remains conducive for potential training convection early Wednesday, but some question whether this activity ends up just offshore or stays onshore. Our latest WPC deterministic QPF would support a MDT risk over LA...however several pieces of guidance are trending towards a more offshore solution by this time...including the AIFS and the 12z RRFS. Given this uncertainty we will leave the risk at a Slight level for now. Either way these areas of coastal TX into LA have a flash flood risk...just a question as to whether the main risk is late on day 2 or on day 3. Future shifts will be able to continue to fine tune these timing details. Chenard ....Previous Discussion... Locally heavy rainfall looks to be lingering along portions of the Gulf coast and Upper Texas coast. Most of the rainfall should be along and south of a cold front left behind by the system that brought heavy to excessive rainfall to the area on Tuesday. Even after the cold passes south over the waters of the Gulf...broad isentropic upglide from broad southwesterly flow should keep at least as risk of rain. However...the 00Z suite of guidance suggested that the convection may be more widely scattered and more offshore than shown by the 01Z and 05Z NBM and may end. Later outlooks will keep an eye on whether this trend continues. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5xU1V2FDVGNwyqLiXaGlYjGNXT8W033uo4Hlg14IaqZq= Nv4zVFlqudizb2Wizd9WD3D9hKr0pR3s2Wdg8uRNJhFcPIo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5xU1V2FDVGNwyqLiXaGlYjGNXT8W033uo4Hlg14IaqZq= Nv4zVFlqudizb2Wizd9WD3D9hKr0pR3s2Wdg8uRNk41atAs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5xU1V2FDVGNwyqLiXaGlYjGNXT8W033uo4Hlg14IaqZq= Nv4zVFlqudizb2Wizd9WD3D9hKr0pR3s2Wdg8uRNmw-oLoM$=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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