Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 31 2024 08:24:06 FOUS30 KWBC 310823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Sat Aug 31 2024 Day 1 Valid 12Z Sat Aug 31 2024 - 12Z Sun Sep 01 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE GULF COAST FROM GALVESTON TEXAS THROUGH THE MISSISSIPPI COAST AS WELL AS FOR=20 THE CENTRAL APPALACHIANS... ....Texas to the Mid-Atlantic and New England... A slow-moving cold front over the Midwest will move into the=20 central Appalachians this afternoon. Surface dew points in the 60s=20 and rising precipitable water values to around 1.75 inches (+2=20 sigma or 98th percentile) will support both a severe weather threat (see SPC outlook) and a heavy rain threat. Though convection=20 should be progressive with the front (or pre-frontal trough), FFG=20 values are around 1-2"/hr and 00Z HREF neighborhood probs show=20 values rising to 40-80% for >1"/hr and 10-40% for >2"/hr. Focus of=20 highest threat will lie from eastern WV northward through central=20 PA and into the Southern Tier of NYS where the complex terrain can=20 add to the flash flooding sensitivity coincident with slightly=20 higher than average soil moisture. Maintained the Slight Risk from=20 the previous forecast with little change in shape. Along the front=20 in general, a Marginal risk was maintained from New England back=20 through the mid-MS Valley/Mid-South into TX where convection will=20 be a bit less coherent and the threat for any flash flooding will=20 be lower, but non-zero.=20 ....Central Gulf Coast... Decaying mid-level vort max will drift eastward along the coast,=20 detached from any surface trough just offshore. However, in-situ=20 moisture remains highly anomalous -- precipitable water values=20 around 2.25 inches which is about +2 to +3 sigma. Most of the hi- res guidance keeps the heaviest rainfall today along and just off=20 the coast but hint at drawing some heavier cores inland a bit=20 toward the I-10 corridor in LA. Maintained the Slight Risk outline=20 for southern Louisiana given the continued threat of heavier rain=20 today.=20 ....Florida... Above normal moisture (PW > 2") will linger over southeast FL=20 where afternoon convection could support some local downpours=20 (2-3"/hr rates) that may induce some flooding over the urban areas=20 from ~PBI to MIA/HST and perhaps into the Keys.=20 ....Arizona... Added a Marginal Risk for the Mogollon Rim as a weak mid-level=20 vort center pushes through the region. Though precipitable water=20 values are around normal, sufficient instability this afternoon=20 could support some isolated heavier rain cores with 1"/hr rates. Fracasso Day 2 Valid 12Z Sun Sep 01 2024 - 12Z Mon Sep 02 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NEW MEXICO/TEXAS, THE NORTHERN GULF COAST, AND THE MID-ATLANTIC... ....Southeastern New Mexico and West Texas... 250mb upper trough moving out of northern Mexico and southern=20 AZ/NM and mid-level vort max (perhaps weakly closed low) will creep closer to far western Texas day 2. Sufficient moisture (though=20 near normal) will be in place over the region to sustain some=20 isolated excessive rainfall via afternoon convection.=20 ....Gulf Coast... Meandering weakness in the mid/lower-levels will again help drive=20 some heavier rainfall along the Upper TX Coast into southern=20 Louisiana. Maintained a Marginal Risk for now given the uncertainty in the day 1 period with respect to heavier rain placement (on or=20 offshore) and amounts. Again, the highest rainfall may linger just=20 off the coast but potential is certainly there to nudge inland. ....Eastern Tennessee Valley and Mid-Atlantic region... Cold front will continue to push through the area to the east and=20 south, squeezing the best forcing into southern Virginia and into=20 North Carolina where afternoon convection may yield 1-2" amounts=20 and a localized threat of flash flooding. There and to the west=20 over western VA into WV, FFG guidance is lower and some lingering=20 rain may eclipse those values as the front works through.=20 Fracasso Day 3 Valid 12Z Mon Sep 02 2024 - 12Z Tue Sep 03 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR TEXAS AND=20 THE CAROLINAS... ....Texas... Mid-level vort and upper trough will move into West Texas on=20 Monday atop increased moisture from the southeast. Guidance=20 continues to show modest QPF of 1-5" in various locations but the=20 spread is quite large -- nearly covering the whole state in their=20 QPF max region among the deterministic, ensemble, and AI guidance.=20 The CSU machine learning first guess fields suggest a broad Slight=20 Risk over the Hill Country which is, perhaps, a best consensus=20 placement, but prefer to hold off an upgrade to a Slight Risk for=20 now until at least the ensembles come into better agreement.=20 ....Carolinas... Cold front will sink into the Carolinas which will nudge the=20 excessive rainfall threat south from day 2. Best dynamics move=20 offshore with the trough axis but the Marginal Risk should cover=20 any isolated flooding threat due to afternoon convection.=20 Fracasso Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9eW3uSKoyAkjKFneZGiMtzWLreMFCBe8XUVrryK7aOy5= 7-72Cu8jQYB4e72d-i3lATpSw9BJJIK_KlKAYZd7mWBf-rw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9eW3uSKoyAkjKFneZGiMtzWLreMFCBe8XUVrryK7aOy5= 7-72Cu8jQYB4e72d-i3lATpSw9BJJIK_KlKAYZd7ZecnNPc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9eW3uSKoyAkjKFneZGiMtzWLreMFCBe8XUVrryK7aOy5= 7-72Cu8jQYB4e72d-i3lATpSw9BJJIK_KlKAYZd7Cz4sKkk$=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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