Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 04 2024 20:30:41 FOUS30 KWBC 042030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 PM EDT Wed Sep 4 2024 Day 1 Valid 16Z Wed Sep 04 2024 - 12Z Thu Sep 05 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MISSISSIPPI DELTA REGION AND ADJACENT GULF COAST... ....16Z Update... There was little change necessary given run-to-run consistency on the rainfall maxima forecast across the TX and Central Gulf coasts. Currently, surface low pressure is drifting eastward across Deep South TX with prevailing onshore flow in-of the Corpus Christi area up through the Upper TX coast. The setup will continue with onshore flow situated within the coastal plain with a secondary east- southeast steering component situated across Eastern LA and MS coasts. A tight instability gradient confined near the coast is within proxy of the quasi-stationary front bisecting much of the Gulf Coast leading to the delineation point of where the heaviest rain will occur. 12z sounding out of KLIX to KCRP were indicative of sufficient anomalous PWATs (2.3-2.6") and a deep warm cloud layer supportive of convective potential with efficient rates to drive flash flood concerns. A general 2-3"/hr will be possible within the strongest convective cores, especially this afternoon as further destabilization within the boundary layer will send SBCAPE indices between 2000-3000 J/kg during peak instability. This aligns well with a strong correlated signal within the latest HREF neighborhood probs for >3" in spots along the coast from KCRP over to KLIX. A general maintenance of the previous SLGT risk was adopted with only a minor extension further down the TX coast towards KCRP to align with recent radar/observational trends. Further east, a convergent pattern in-of of the Jacksonville metro has been well-documented over the past succession of runs with some flash flood warnings issued over the course of the morning between the I-10/95 junction. This signal has begun to wane as of the recent radar scans, but a few pockets of heavy rain are still possible through the remainder of the period thanks to a mesolow off the Northeast FL coast continues to meander providing that persistent focused convergent signature near the metro. The MRGL was maintained as a result. Kleebauer ...Previous Discussion.. ....Gulf Coast and Lower Mississippi Valley... The surface front will continue to be draped west to east across the South provide focus for convection to spread from south Texas to Florida Peninsula. The latest guidance has continued to favor a less progressive eastward advancement with the precipitation shield. A blend of CAMs/global guidance has shifted the focus for heavier rainfall to be along the Upper Texas coast where they will be located closest to an organizing and elongated area of low pressure in the western Gulf of Mexico. General consensus has 2 to 5 inches along the coast between Corpus Christi and Lake Charles. Southeast TX also still sports >80% soil moisture percentiles within the 0-40cm layer according to NASA SPoRT-LIS, suggesting soils there would be more sensitive to 2-3"/hr rainfall rates. Onshore flow across southeast Louisiana and southern Mississippi could yield 1 to 2 inches as well. Locally heavy rains are likely, especially from New Orleans east to Mobile. For the Florida Panhandle there will be an influx of anomalous PWs and modest instability along the stalled frontal boundary that may trigger additional thunderstorms capable of producing torrential downpours. Areas most at risk for possible flash flooding will be areas whose soils are more saturated following Tuesday's thunderstorm activity and as far east as the Jacksonville metro area. Campbell Day 2 Valid 12Z Thu Sep 05 2024 - 12Z Fri Sep 06 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ALONG THE CENTRAL GULF COAST... ....Gulf Coast and Lower Mississippi Valley... ....2000 UTC Update... Changes made to the Day 2 ERO were fairly minor, based on the latest (12Z) guidance/trends. Did expand the Marginal across the Southeast Coast, including GA and Central FL. Deep-layer instability may be lacking, however PWs of 2.2-2.4" will make for highly-efficient rainfall rates as the quasi-stationary front shifts a bit northward by Thu night as the upper shortwave trough dips across the western Gulf Coast.=20 ....Previous discussion... The latest guidance keeps a slower progression of the QPF=20 associated with an elongated area of low pressure in the northwest=20 Gulf of Mexico. As such, the Slight Risk area was expanded further=20 west along the Gulf Coast, between Houston and Corpus Christi, to=20 account for the westward trend. While dispersion is on the higher=20 side on Day 2, there is a better consensus for heavier rainfall=20 along the central Gulf Coast given the bulk of PWs >99th=20 climatological percentile residing along the Gulf Coast. The region is also closer to the surface-850mb southeasterly flow that will=20 sustain a steadfast regime of low level theta-e advection into the=20 nearby stationary frontal boundary. Additional forecast changes are likely given so much of this setup lies with the development of=20 low pressure along the Gulf Coast. ....New Mexico and Colorado... Convection is expected to fire up ahead of an approaching cold front thanks to PWs topping 0.75" and up to 500 J/kg of MUCAPE. Soils across this part of the region are sensitive given recent moisture and burn scars. The Marginal Risk area was maintained for the localized flash flood potential for the afternoon and evening. Hurley/Campbell Day 3 Valid 12Z Fri Sep 06 2024 - 12Z Sat Sep 07 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND EASTERN GULF COAST... ....Gulf states and Southeast... ....2000 UTC Update... Based on the latest guidance trends (including smart blends such as the NBM and WPC biased-corrected QPF), along with the most recent NBM exceedance probabilities and CSU UFVS-verified first-guess fields, have expanded the Slight Right across the Southeast given the digging trough across the western Gulf Coast region and increasing (and persistent) low-level frontogenesis along the=20 central-eastern Gulf Coast to the GA Coast. While narrowing with time on Day 3, the corridor of favorable thermodynamics (PWs of 2.2 to 2.5" with tall/skinny CAPE profile) and low-level FGEN along=20 the quasi-stationary surface boundary will favor the more intense=20 short term rainfall rates. This is essentially where the Slight Risk was drawn, which also correlates well with the highest 24hr QPF exceedance probabilities from the NBM.=20 Hurley ....Previous Discussion... The frontal boundary is expected to slowly progress eastward=20 during this period will draped along the Gulf Coast and the=20 Southeast. Deep moisture will continue to override the boundary=20 helping to concentrate the heaviest QPF along the central and=20 eastern Gulf coast. A Slight Risk remains in effect from southeast=20 Louisiana to the Florida Panhandle while a Marginal Risk area spans from south- central Louisiana to northeast Florida and northward=20 to South Carolina. Areal averages of 1 to 2 inches will be common=20 across the region with the possibility of very isolated maxes of 3=20 to 5 inches, especially along the coastline. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5JKPCros0GVdh8TaJ9Mk6FIXq9ATBOvHOZ98u4mPtBSB= L6ECWOaCgVOanskEPa41dYlRXWvze0B3_9wtIpyvo3d7JIE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5JKPCros0GVdh8TaJ9Mk6FIXq9ATBOvHOZ98u4mPtBSB= L6ECWOaCgVOanskEPa41dYlRXWvze0B3_9wtIpyvcP6lTxI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5JKPCros0GVdh8TaJ9Mk6FIXq9ATBOvHOZ98u4mPtBSB= L6ECWOaCgVOanskEPa41dYlRXWvze0B3_9wtIpyvVs-oyRo$=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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