Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 31 2024 20:07:31 FOUS30 KWBC 312007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 PM EDT Sat Aug 31 2024 Day 1 Valid 16Z 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 THE CENTRAL APPALACHIANS... ....16Z Update... ....Central Appalachians... No changes were needed with the Day 1 ERO risk areas. A slow moving cold front over the Midwest will move into the central Appalachians this afternoon. Both a severe and heavy rain threat are expected across this region. The highest threat will be in the Central Appalachians Slight Risk area where training storms interacting with the terrain as well as antecedent wet soil conditions in the WV Panhandle and central Pennsylvania will all support a flash flooding threat. ....Louisiana Gulf Coast... Also no changes were needed along the Louisiana Gulf coast as a stationary trough of low pressure continues to feed plentiful Gulf moisture into the region. Urban areas such as New Orleans are at higher threat for flash flooding. Saturated soil conditions are also present here, resulting in almost complete turnover of any heavy rainfall into runoff. ....Florida... Above normal moisture will linger over southeast FL where afternoon convection could support some local downpours with 2-3 inch per hour rates. That may induce some flooding over urban areas from West Palm Beach south through the Keys. ....Arizona... Signals remain elevated enough in the latest guidance to support the continued Marginal Risk along the Mogollon Rim. A weak vort center will push through the region, which combined with sufficient instability could support some isolated heavier rain cores with 1"/hour rates. Wegman/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... ....2030Z Update... No major changes were made to the risk areas on Sunday. For the mid-Atlantic, the signal into eastern West Virginia decreased in favor of an increased signal into the Delmarva. Thus, the portion of the Marginal in West Virginia was trimmed in favor of an expanded Marginal east of I-95. Along the Gulf Coast, guidance continues to keep heavy rain associated with the tropical low just off the Texas and Louisiana coasts. Given most of the heavy rain the past few days have only=20 resulted in flash flooding into urban areas such as New Orleans,=20 the consistent nature of the heaviest rain staying offshore has precluded another day with a Slight risk upgrade for now. That said, given that soils there are saturated, any northward shift in the guidance could mean a Slight may be needed. No major changes were needed in west Texas and New Mexico. Any rainfall that occurs Sunday will saturate soils as a precursor to Monday/Labor Day's rain. Wegman ....Previous Discussion... ....Southeastern New Mexico and West Texas... 250 mb upper trough moving out of northern Mexico and southern AZ/NM and mid-level vort max (perhaps weakly closed low) will creep close to far western Texas day 2. Sufficient moisture (through near normal) will be in place over the region to sustain some isolated excessive rainfall via afternoon convection. ....Gulf Coast... Meandering weakness in the mid-lower levels will again help drive some heavier rainfall along the Upper TX Coast into southern Louisiana. Maintained a Marginal Risk for now given the uncertainty in the day 1 period with respect to heavy rain placement (on or offshore) and amounts. Again, the highest rainfall may linger just 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 south, squeezing the best forcing into southern Virginia and into North Carolina where afternoon convection may yield 1-2 inch amounts and a localized threat of flash flooding. There and to the west over western VA into WV, FFG guidance is lower and some lingering rain may eclipse those values as the front works through. Fracasso Day 3 Valid 12Z Mon Sep 02 2024 - 12Z Tue Sep 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE HILL COUNTRY OF TEXAS... ....2030Z Update... ....Texas... Increasing rainfall forecasts and CSU first guess field input both have increased confidence in a Slight Risk upgrade across much of the Hill Country of Texas. Expected widespread rainfall totals Monday of 1-3 inches with locally higher amounts will likely result in widely scattered instances of flash flooding. Despite recent dry conditions, any rain from Sunday will prime some of the soils, but elsewhere, the expectation of heavy rain will likely cause localized flash flooding in the usually flashy areas. In coordination with SJT/San Angelo, TX; EWX/San Antonio, TX;=20 LUB/Lubbock, TX; and OUN/Norman, OK forecast offices, the Slight Risk upgrade was introduced with this update. ....Carolinas... Signals for heavy rain have been both shifting south and decreasing. The Marginal Risk has been shifted more towards the coast due to these changes in the forecast. Wegman ....Previous Discussion... ....Texas... Mid-level vort and upper trough will move into West Texas on Monday atop increased moisture from the southeast. Guidance continues to show modest QPF of 1-5 inches in various locations but the spread is quite large -- nearly covering the whole state in their QPF max region among the deterministic, ensemble, and AI guidance. The CSU machine learning first guess fields suggest a broad Slight Risk over the Hill Country, which is, perhaps, a best consensus placement, but prefer to hold off an upgrade to a Slight Risk for now until at least the ensembles come into better agreement. ....Carolinas... Cold front will sink into the Carolinas which will nudge the excessive rainfall threat south from day 2/Sunday. Best dynamics move offshore with the trough axis but the Marginal Risk should cover any isolated flooding threat due to the afternoon convection. Fracasso Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4fL58eXgZjIDghDgpZGFKn6VM7ejpdWjoVMZ1lrwjZ9F= rw-UtiJ-L2jHPmS33Us61cZGFhJ4LNXzrrvPykSjI1o1S9s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4fL58eXgZjIDghDgpZGFKn6VM7ejpdWjoVMZ1lrwjZ9F= rw-UtiJ-L2jHPmS33Us61cZGFhJ4LNXzrrvPykSjQHWBAvA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4fL58eXgZjIDghDgpZGFKn6VM7ejpdWjoVMZ1lrwjZ9F= rw-UtiJ-L2jHPmS33Us61cZGFhJ4LNXzrrvPykSjxvLymTs$=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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