Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 07 2024 15:50:57 FOUS30 KWBC 071550 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1150 AM EDT Sat Sep 7 2024 Day 1 Valid 16Z Sat Sep 07 2024 - 12Z Sun Sep 08 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE SOUTHEAST AND WESTERN U.S... ....16z Update Summary... The SLGT risk was removed from the previous update in coordination with the impacted offices. A MRGL risk remains for parts of the Southeastern U.S, with a focus along the coastal plain of Southeast LA and the Emerald coast of FL. MRGL risks remain steadfast across portions of the Western U.S. Kleebauer ....Gulf Coast and Southeast... 16Z Update: The pattern from this morning have devolved into something less robust with continued rainfall, but paltry rates to suffice much of a flash flood concern. Recent mesoanalysis has only furthered the argument for less of a risk of heavy rainfall as MUCAPE indices remain relatively benign with the coastal plain of Southeastern LA over into the FL Panhandle only subject to ~500 J/kg of MUCAPE, at best. Analysis of radar trends indicate a limit to the overall convective output this morning, correlating well with the <1" totals over the past 6 hrs for areas it has been raining through that short window. Some diurnal destabilization will be had for portions of FL and Southern GA, but the coastal=20 plain will be bordering on low-end probability for any instability=20 regime to really overcome the prevailing cloudiness and the convergence axis shifting off the coast as drier air advects in from the north. Despite continued rainfall this afternoon with scattered thunderstorm concerns for the immediate coast, the prospects for anything more than a rogue over-performer will limit the threat to a mere MRGL risk, thus necessitating the removal of the previous SLGT risk in place.=20 The northern extent of the MRGL was also trimmed away to tighten the gradient of the zone for best convective development. The setup may yield more flare further east as a better diurnal=20 destabilization regime could yield convective pattern with stronger cores capable of localized heavy rainfall >1"/hr as signaled=20 within the 12z HREF probs and associated CAMs. This setup is best aligned across the FL Big Bend into parts of Southern GA. This is still a lower end probability for flash flood concerns due to the higher FFG indices in place over the proposed area of interest. This was enough to maintain the MRGL risk across the northern half of FL.=20 Kleebauer ....Portions of the Southern Rockies... 16Z Update: Little to no change occurred from the previous MRGL risk forecast across portions of the Western U.S. Scattered convective signals in-of the Southern Rockies, Mogollon Rim of AZ, and the Peninsular Range of Southern CA will all benefit from a persistent mid-level ridge axis with a slightly higher than normal PWAT accompaniment as it migrates around the western and northern flank of the ridge. Modest instability coupled with the better moisture presence will provide just enough to create an isolated flash flood threat within the terrain, and over remnant burn scars located within both MRGL risk areas.=20 Kleebauer ...Previous Discussion.. 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 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 enhance the flash flooding potential due to a much greater 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 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. 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. ....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!6Y1OGziACftuMKRynH6yF7rBKxPbrtYkuyWdC1HDn_e2= 2Gw9ciHA-YEws8jqXs8fIFFKpo0508mic9TM-bz_-sVsVIs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6Y1OGziACftuMKRynH6yF7rBKxPbrtYkuyWdC1HDn_e2= 2Gw9ciHA-YEws8jqXs8fIFFKpo0508mic9TM-bz_x_7frP0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6Y1OGziACftuMKRynH6yF7rBKxPbrtYkuyWdC1HDn_e2= 2Gw9ciHA-YEws8jqXs8fIFFKpo0508mic9TM-bz_2vFAklw$=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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