Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 07 2024 07:33:33 FOUS30 KWBC 070733 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 333 AM EDT Sat Sep 7 2024 Day 1 Valid 12Z Sat Sep 07 2024 - 12Z Sun Sep 08 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... ....Gulf Coast and Southeast... Little has changed in the forecast from the previous update. A stalled out front and a similarly stalled out tropical low will both continue to haunt portions of the central Gulf Coast through the day today. Unlike previous days however, coverage of heavy rain will be quite a bit reduced. Since the front and the low have been stuck in the same place for the better part of a week now, instability has been rather meager due to extensive rainfall and cloud cover. The greatest instability will be over the warm waters of the Gulf. Once any showers and storms track ENE into the Gulf Coast, they will struggle to maintain their intensity going inland. Thus, the inherited Slight Risk from New Orleans east through the Emerald Coast of the Florida Panhandle remains largely unchanged. For Louisiana, a big contributing factor to the flash flooding potential is the multiple days of rainfall over much of the area, resulting in saturated soils. Meanwhile east into the Florida Panhandle, the area has also seen quite a bit of rain in recent days, but less so than in Louisiana. Thus, the area will be more dependent on heavy rainfall to result in flash flooding. That said, the predominant track for the heaviest rainfall is likely to set up into the Florida Panhandle, with somewhat less rain expected today around New Orleans. Thus both factors counter each other. For the rest of the Marginal Risk area across the Southeast and=20 the northern Florida Peninsula, somewhat less favorable antecedent conditions (i.e. drier soils, will preclude all but isolated flash flooding. Further, as mentioned above, rainfall rates should decrease markedly the further from the Gulf Coast any showers and storms track. The likelihood of less heavy rain will also diminish the flooding threat. ....Portions of the Southern Rockies... The Marginal risks in these areas remain largely the same, other than trimming out areas of southern CA and northwestern NM where less topographic influence should generally preclude any flash=20 flooding. The Marginals thus concentrate from the Peninsular Ranges of southern California over to the western Mogollon Rim of western Arizona, as well as for the Sangre de Cristo Mountains of southern Colorado and northern New Mexico. Any burn scars will locally=20 enhance the flash flooding potential due to a much greater=20 percentage of any rainfall converting to runoff. Wegman Day 2 Valid 12Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN ROCKIES, DEEP SOUTH TEXAS, AND THE SOUTHEAST... ....Southern Rockies... Southerly monsoonal flow out of the Gulf of California will push abnormally high moisture into portions of the Southwest on Sunday. By Sunday afternoon with diurnal heating, expect widespread shower and thunderstorm development across the Marginal Risk area, in particular on south facing slopes of the mountains. The fact the area has been baking in recent days should only increase the amount of instability that any storms in the area can feed on to increase their potential ability to produce heavy rains. Urban areas, arroyos, slot canyons, and other low-lying flood prone areas are where any isolated flash flooding will be most likely. ....Deep South Texas... An approaching tropical wave and potential surface low will move=20 across the western Gulf and impact deep south Texas on Sunday and especially Sunday night. The strong front draped along the coast will act as a focus for heavy rain. Coastal areas of Deep South Texas have seen rounds of heavy rain in recent days, resulting in some depression of FFGs in the area. A tropical wave and its associated heavy rainfall when combined with those antecedent soil conditions could result in isolated flash flooding. A major point of uncertainty in the forecast is how far west the slug of heavy rain will get to. There is significant disagreement in the guidance as to the answer to that question. A notable trend has been to shift the heaviest rain a bit to the east and more offshore. This in turn is both increasing uncertainty and lowering the flash flooding threat. For now, the Marginal risk area is maintained unchanged, but even a small additional eastward nudge would require a downgrade of the risk area. ....Southeast... The eastward extent of the same front as impacting Deep South Texas will remain across northern Florida (both Panhandle and Peninsula) as well as southern Georgia. The same pattern will remain on Sunday as on today, in that instability will be greatly lacking with any distance from the coast, so the greatest threat for flooding will be along the immediate Gulf Coast of Florida. Wegman Day 3 Valid 12Z Mon Sep 09 2024 - 12Z Tue Sep 10 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP SOUTH TEXAS... ....Coastal Deep South Texas... A tropical wave moving across the Gulf will continue to spread heavy rains along much of the Gulf Coast on Monday, continuing from Sunday. The bulk of the heaviest rain associated with the wave is likely to impact coastal Deep South Texas and adjacent coastal northeast Mexico. Thus, there is higher confidence that on Monday very heavy rainfall will occur along the coast of Deep South Texas as compared with Sunday. However, as mentioned in the Day 2 discussion, there is increasing uncertainty as to the track of the tropical wave, which greatly impacts the forecast and the potential for flash-flood-producing heavy rains. Guidance has been trending east/offshore with the heaviest rain, so as on Sunday, any further eastward shifts will require downgrades to the ERO risk areas.=20 For now, the Slight risk upgrade is contingent on at least 2 days (today and Sunday's) rains further lowering FFGs in advance of the main rainfall event in the area on Monday. If Monday is the third consecutive day with heavy rainfall, as it's highly likely to be the wettest of the 3 days, then it stands to reason compounding influences by increasingly saturated soils will quickly increase the potential for flash flooding.=20 ....The Rest of the Gulf Coast... No change to the pattern is forecast for the Gulf Coast from upper Texas east into Florida. As on previous days, most of the heaviest rain should remain offshore of these areas, with a predominant light ENE flow along the front allowing some of the heavier rains to move into FL and GA from the Gulf, though quickly weakening with increasing distance from the Gulf. Once again, this area will be swamped with multiple days of light to occasionally heavy rainfall. Thus, it's quite possible a Slight risk upgrade may be needed with future forecast in the areas hardest hit with heavy rains both today and on Sunday. The inherited Marginal was greatly expanded to the north with this update to cover for all intents and purposes the entirely of the US Gulf Coast. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!80AkbmucbybLESqkDE7RVebB2nGLIxxfTfhOVGKJzUf5= QGYAOs1InGej6SLIgO7Lc4lCsU_9ayxz3aCpbnwI-JkOWdc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!80AkbmucbybLESqkDE7RVebB2nGLIxxfTfhOVGKJzUf5= QGYAOs1InGej6SLIgO7Lc4lCsU_9ayxz3aCpbnwIWf4YVek$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!80AkbmucbybLESqkDE7RVebB2nGLIxxfTfhOVGKJzUf5= QGYAOs1InGej6SLIgO7Lc4lCsU_9ayxz3aCpbnwIj4cjr-U$=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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