Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 05 2024 15:53:38 FOUS30 KWBC 051553 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Sat Oct 5 2024 Day 1 Valid 16Z Sat Oct 05 2024 - 12Z Sun Oct 06 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP SOUTH TEXAS AND ACROSS THE SOUTHWEST AND SOUTHEAST FLORIDA PENINSULA... ....16Z Update Summary... The MRGL across South TX was maintained due to ongoing convection in a rich tropical environment, however the signal is expected to degrade by later this afternoon with a removal by next update. A MRGL risk was added across the Southwest and Southeastern FL coasts. Kleebauer ....Deep South Texas... 16Z Update: Numerous showers and storms continue to maneuver onshore of Deep South TX with a prevailing east-northeast flow advecting rich tropical moisture over the region. 12z KBRO sounding came in with a robust 2.66" PWAT output, a testament to a very favorable tropical regime with warm rain processes likely the driving factor for rainfall efficiency. Despite meager looking radar presentations, an axis of 2-4" with locally 5+" still managed to accumulate across the central and northern portions of Padre Island with some inland signatures of 2+" located further southwest into Harlingen, a more urbanized zone susceptible to flash flooding concerns. Obs are generally within reason when assessing the MRMS Multi-Sensor Pass, perhaps the MRMS running a bit hot, so the footprint of heavier precip likely has merit, despite a lackluster obs network north of the main population centers. With the rain ongoing, did not want to remove the MRGL risk for threat of any pulse convection potentially causing an isolated flash flood threat with the environment present. As a result, maintained general continuity, however there's a higher likelihood of the risk area being removed by the 01z update due to a degrading rainfall signature across all CAMs, shifting to a nil threat likely by sunset.=20 Kleebauer ....South Florida... 16Z Update: Prevailing easterlies across South FL have ushered in a significantly unstable, moist airmass with PWAT anomalies poking upwards of +2 to +2.5 standard deviations according to the latest ECENS forecast this past evening. 12z sounding out of KMFL indicated a PWAT output over 2.3", a satisfactory moisture output typically reserved for better flash flood prospects within the FL Peninsula. The signal for heavy rain chances has grown over the latest succession of CAMs output with a larger footprint and probabilistic depiction for the threat of locally higher than 5" of rainfall in portions of both the Southwest and Southeastern coasts of the state. Fort Lauderdale has already experienced the potential first hand with a cell dropping close to 5" of rain in 2-3 hours with intra-hour rates exceeding 3"/hr at times, averaging out over 2"/hr when factoring in the full hour. This is suggestive of an efficient warm rain process backed by a freezing level reading of=20 16.5K ft AGL, as well as a textbook tall, skinny CAPE signature=20=20 when assessing the 12z KMFL sounding. In any case, the threat for=20 locally >5" has grown considerably as suggested by the latest=20 neighborhood HREF prob (80+%) with a corridor of 35-45% for >8"=20 focused over the Southeast metro. The best risk will occur now=20 through the early evening prior to a potential break before ramping back up for heavy rain prospects as we move towards the very end=20 of the period. In coordination with the Miami WFO and with a strengthening signature for heavy rain over the urban metros on both coasts, a MRGL risk was introduced for the aforementioned area.=20 Kleebauer Day 2 Valid 12Z Sun Oct 06 2024 - 12Z Mon Oct 07 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE SOUTHERN FLORIDA PENINSULA... A trough developing ahead of a slowly developing tropical low over the Gulf will form over the southern Florida Peninsula on Sunday. The air mass the trough will be developing in will be incredibly moisture-rich...with PWATs anywhere from 2.2 to 2.6 inches. This will be close to the climatological maximum for this time of year...between the 95th and 99th percentile and more than 3 sigma above normal. The predominant mid-level wind flow will switch from northeasterly to southwesterly during the period, resulting in chaotic storm motions. The heaviest rainfall totals will be along both coasts, from Tampa south through Cape Coral on the Gulf Coast as well as from the Treasure Coast south through Miami on the Atlantic side. This bimodal distribution of QPF suggests organization will be somewhat lacking, depending more on mesoscale phenomena such as sea breezes or differential heating between the urban areas and their surroundings. This will likely be the first day of several where heavy rain will be common and widespread across the Florida Peninsula. In the increasingly likely event a tropical cyclone eventually develops in the Gulf and moves towards the Florida Peninsula mid to late week, this will be the first day of the predecessor rain event, or PRE. Given high FFGs areally and this being the first day of the PRE, a Marginal Risk was left intact for this area, but further increases in forecast rainfall may require a Slight Risk for some of the urban areas with future updates. Wegman Day 3 Valid 12Z Mon Oct 07 2024 - 12Z Tue Oct 08 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE FLORIDA PENINSULA... Conditions across the Florida Peninsula will continue to deteriorate on Monday. The PRE is expected to continue to intensify, making for a second full day of periodic heavy rainfall across the Peninsula. Both overall rainfall amounts and coverage north up the Peninsula will increase on Monday as compared with Sunday. All but the fastest guidance suggest that should a tropical cyclone form over the Gulf, it will remain well west of the Florida Peninsula through Monday night. This means the PRE will remain in full force across the Peninsula. Once again, there will be a bimodal distribution of the precipitation...but much more pronounced. The focus will remain along both coasts, albeit heavier and spread north. Given the expected heavy rainfall from Day 2/Sunday, by Monday most soils should be at or near saturation. This means most of the day's rains will convert to runoff in most areas. With heavier rainfall expected in the urban areas along both coasts, a Slight Risk upgrade was introduced with this update, in coordination with MFL/Miami, FL; TBW/Tampa, FL; and MLB/Melbourne, FL forecast offices. With better definition as to how and where any tropical cyclone may develop over the Gulf, this will improve the forecast for the PRE on Monday. This will apply both to where the heaviest rain may occur as well as how heavy the amounts will be. Further, with the hindsight of where Sunday's rainfall will occur, it's possible additional upgrades may be needed with future forecasts...though a preponderance of the guidance suggests any direct effects of a potential tropical cyclone will hold until after this period. Depending on how the heaviest rain areas align on Monday with Sunday, some areas may be approaching 2 day totals of 10 inches of rain by Tuesday morning. These amounts of rain in any urban areas could approach Moderate Risk level impacts. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!48DJE45MqMdSCoQkxAmKazy36eFNcVma4ds7bsuY9_MP= Frkwgvsee1ph87NyGzU7Vv8fwW0BB0L6DDobUwweWTDN0JQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!48DJE45MqMdSCoQkxAmKazy36eFNcVma4ds7bsuY9_MP= Frkwgvsee1ph87NyGzU7Vv8fwW0BB0L6DDobUwwerXsu0rA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!48DJE45MqMdSCoQkxAmKazy36eFNcVma4ds7bsuY9_MP= Frkwgvsee1ph87NyGzU7Vv8fwW0BB0L6DDobUwweklNH5bA$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .