Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Oct 31 2024 20:21:16 FOUS30 KWBC 312020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Thu Oct 31 2024 Day 1 Valid 16Z Thu Oct 31 2024 - 12Z Fri Nov 01 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST... 16Z Excessive Rainfall Discussion Update... Only changes was a slight westward expansion of the Marginal risk area near the Gulf of Mexico in far southeast Texas in response to radar trends late this morning. There has been an area of enhanced moisture flux convergence from far eastern Texas into northwestern Louisiana during the morning which has led to some locally enhanced rainfall rates with MRMS showing 3-hour totals at or above 2 inches in eastern Texas...with 3+ inches depicted in Fort Bend and Brazoria counties. Showers and thunderstorms were already anticipated in the Lower Mississippi Valley as mid- and upper-level height falls and an associated surface cold front moves across the area...with the potential for localized heavier rainfall rates already anticipated from slow moving super cells...so the expansion of the Marginal Risk area does not represent a significant shift in forecast reasoning but merely accounting for short term conditions. Bann 0830Z Excessive Rainfall Discussion... As an upper-level trough advances through the Upper Midwest an attendant cold front will makes its way from the Southern Plains into the Lower and Middle Mississippi Valley. The front will slow and/or stall most significantly into the Lower MS Valley, where instability is expected to be maximized (while the flow and associated forcing weakens farther north). Convection is expected to be ongoing along and ahead of the cold front through morning, but should increase in coverage and intensity during the afternoon hours (while locally slowing the most from the Mid-South into portions of the Deep South and Central Gulf Coast.With PWs nearing 2" (well above the 90th percentile, per SPC sounding climatology) with storm motions locally as slow as 5-10 kts, the 00z HREF 40-km neighborhood probabilities for 3"/3-hr reach as high as 20-40%. While localized totals of only 1-2" are generally expected, the HREF PMM QPF indicates the potential for isolated 3-4" totals (in as little as 2-3 hours, nearing associated FFG thresholds). Churchill/Campbell Day 2 Valid 12Z Fri Nov 01 2024 - 12Z Sat Nov 02 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND ADJACENT SOUTHERN ROCKIES... 2030Z Update... Introduced a targeted Slight Risk area in southeast New Mexico where an uptick in model QPF and model-derived ingredients overlap with an area that was soaked less than 2 weeks ago and remains hydrologically sensitive...with the expectation that the risk is best late in the period and continuing into the Day 3 period. The maximum Areal average rainfall amount looks to be around an inch...which normally wouldn't warrant a Slight (especially with the given the high flash flood guidance being so high). But with the trends of increasing precipitable water, thetae and model QPF=20 over over and near the site of the flash flood...felt a Slight was=20 appropriate here while a Marginal remained more appropriate=20 elsewhere from the plains of southeast New Mexico to western=20 Oklahoma. Bann 0830Z Excessive Rainfall Discussion... Another shortwave trough will move inland across the West on Friday as ridging stays in place over much of the Southeastern US. Warm Gulf moisture will be advecting northward into the southern High Plains ahead of the trough. Meanwhile, the strongest instability (1000+ J/kg of SB CAPE) will likely be in place over West Texas and eastern New Mexico. Areal coverage of thunderstorms is forecast to increase during the overnight hours (after 00z) into portions of southeastern NM and the TX Panhandle, propagating eastward into portions of western OK Oklahoma and far southern Kansas. Maintained the Marginal Risk from prior cycles, given PWs increasing to 1.0"+ (above the 90th percentile, per SPC sounding climatology). While most CAMs still do not reach out to the timeframe of interest, the ones that do (the FV3 and CMCreg) suggest the potential for localized 3"+ totals, as well as support from the downscaled ECMWF. Decided to maintain the Marginal for this cycle, given considerable uncertainty and dry antecedent soil conditions. Churchill/Campbell Day 3 Valid 12Z Sat Nov 02 2024 - 12Z Sun Nov 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... 2030Z Update... Few changes were needed given fairly decent run to run agreement with the scenario of a much more potent longwave trough digging into the Southwest US that results in more organized convection capable of producing heavy rainfall. Opted to not to jump all the way to the full extent suggested by the ML ERO guidance...but felt an expanded areal coverage was warranted based on the placement suggested by spaghetti plots of QPF from the ensembles as well as the global models combined with the idea that the rainfall on Day 3 will function to prime the soils for more runoff/flooding potential on Day 4 and beyond. Will continue to watch how much overlap there is and upgrade if necessary. Until then...thinking is that the risk from Saturday into early Sunday is a high-end Slight Risk from parts of Oklahoma into far north Texas. Bann 0830Z Excessive Rainfall Discussion... A much more potent longwave trough is progged to dig into the Southwest US on Day 2, as leading shortwave impulses are expected to result in much more organized convection over much of the Southern Plains. Continued southerly low-level flow ahead of the impulses should result in PWs pushing 2" (near record levels, per SPC sounding climatology) with ample forcing (left exit region of 100+ kt subtropical jet streak) for organizing convection. The WPC QPF calls for areal average totals of 2-3", which is still well below the 90th percentile of the PQPF (~4-5"). Given strong ensemble support, the inherited Slight Risk was expanded a bit (with the expectation of verification on the higher-end of the Slight Risk probability spectrum from Northwest TX/Big Country into south-central OK). Given very dry antecedent conditions, a Slight Risk was deemed sufficient (but will need to be reevaluated for upgrade potential with future updates). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9tc5csKwVqasJcVi2mOW2ZLzgu62FCEPDpEOkA90rPPK= eZBYI7RDqJvHfg7vecSU3wSKf17JFGEbjfbf2BJNAgwevZE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9tc5csKwVqasJcVi2mOW2ZLzgu62FCEPDpEOkA90rPPK= eZBYI7RDqJvHfg7vecSU3wSKf17JFGEbjfbf2BJN4yCbD3M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9tc5csKwVqasJcVi2mOW2ZLzgu62FCEPDpEOkA90rPPK= eZBYI7RDqJvHfg7vecSU3wSKf17JFGEbjfbf2BJN1HlI_hA$=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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