Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Nov 07 2024 20:05:12 FOUS30 KWBC 072004 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 304 PM EST Thu Nov 7 2024 Day 1 Valid 16Z Thu Nov 07 2024 - 12Z Fri Nov 08 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AS WELL AS PORTIONS OF WEST TEXAS AND WESTERN OKLAHOMA... 16Z Excessive Rainfall Discussion Update... Maintained the Slight Risk areas with few changes across the Southeast and across portions of West Texas and western Oklahoma. The latest versions of the HRRR Conest and the HRRR both depict enough rainfall over the Southeast US that we maintained the Slight risk with the expectation that the flooding risk from intense rainfall is confined closer to the coastal portions of Georgia and South Carolina but any additional rainfall farther inland would exacerbate on-going flooding from areas soaked overnight. Farther west...the Slight risk area was expanded a bit northward and eastward based on the footprint of heaviest rainfall shown by the HRRR mainly later in the evening and during the overnight hours...NAM and NAM Conest. With an uptick in QPF...the area of north Texas into southwest Oklahoma is probably a higher-end Slight Risk. Bann 0830Z Excessive Rainfall Discussion... ....Southeast... Heavy rain continues across Georgia and South Carolina this morning. A nearly stationary front across the region is acting as a focus for that rainfall, which due to an influx of tropical moisture moving into the front from the Atlantic remains stuck. The moisture running into the front is allowing for convective development along the front in Florida and Georgia, which then advect northeastward into South Carolina. While convective coverage has been slowly diminishing, along with the storms with the heaviest rainfall, it's likely that the rain will continue into the day today. The inherited Slight and Marginal risks were largely left unchanged with this update, as the rainfall is generally behaving as expected. Since there will be a very slow drift of the heaviest rainfall towards the South Carolina coast, which due to the swampy terrain is generally much better equipped to handle heavy rainfall rates, a Slight Risk is sufficient to cover the flash flooding threat, rather than a continuation of the Moderate. ....West Texas and Western Oklahoma... A strong upper level low over the Southwest will interact with an LLJ of Gulf moisture and a developing surface low forming ahead of the upper level low. A large area of rain will develop from the Texas and Oklahoma Panhandles south and east over portions of the Cap Rock of Texas and much of Oklahoma west of Oklahoma City. Instability will be a significant limiting factor on short-term rainfall rates and the strength of any convection, but the fact that all of those synoptic systems just described will be very slow moving should increase the duration of steady rainfall. Since much of this region was hit hard with heavy rain a few days ago, FFGs remain low enough that they should be overcome in widely scattered instances in the Slight Risk area, and in isolated instances in the Marginal Risk region. Wegman Day 2 Valid 12Z Fri Nov 08 2024 - 12Z Sat Nov 09 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... 20z Update: Minimal changes to the inherited Slight risk area. Convection is likely to be ongoing at 12z Friday across north central TX into central OK, and rainfall today/tonight should result in more saturated conditions by 12z. Thus there is likely=20 to be some flash flood risk that continues into Friday...although by this time less instability and an increased easterly motion to=20 convection should limit the magnitude of the threat. Thus a Slight=20 risk should suffice. The convection over central TX will have more=20 instability to work with Friday afternoon and evening, but should=20 tend to be more progressive in nature. Although some cell mergers=20 could result in a Slight risk of flash flooding. Chenard ....Previous Discussion... The upper level low over the Southwest and its attendant surface low will both continue moving northeast across the Plains on Friday. With continued influx of Gulf moisture, the typical comma shaped precipitation footprint will develop from Texas to Oklahoma. Out west in Colorado and New Mexico the precipitation will be mostly in the form of snow. Where it remains all rain, the heaviest rain will be from north Texas across Oklahoma and into south central Kansas. As today, instability will remain the primary limiting factor for heavy rain in most areas, as MUCAPE values generally stay under 1,000 J/kg. For the Slight Risk area, there has been a general eastward/faster shift in the guidance, but that will at least be somewhat offset by the rainfall expected today over the Panhandles and western Oklahoma. The rationale for the Slight is two-fold. While somewhat lesser amounts of rain are expected overall for southern Oklahoma and Texas...greater instability there will favor at least some storms capable of heavy rainfall in that area. Meanwhile, north of Oklahoma City, expect a much longer duration rainfall as the surface low takes its time getting its act together. So while instantaneous rates will likely be lower than further south, it will be made up for by the longer duration of the rain. Wegman Day 3 Valid 12Z Sat Nov 09 2024 - 12Z Sun Nov 10 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE LOWER MISSISSIPPI VALLEY... 20z Update: Main change was a modest eastward shift to the Slight risk over MS, and a removal over portions of AR/MO following model trends. The highest threat of 3"+ rainfall totals still looks to=20 be across portions of LA and MS where moisture streaming north of=20 Rafael will interact with the front and upper jet. Still some=20 uncertainty on exactly how this interaction plays out, but there is a growing signal for the potential of 3-5" (locally higher) of=20 rain somewhere over the lower MS Valley. Chenard ....Previous Discussion... The upper level low that was driving the heavy rainfall the previous two days will move faster towards the northeast as it continues its return to the jet stream over the northern tier of states. The area will still have plenty of moisture from the Gulf to work with as Rafael (or what's left of it) remains over the central Gulf but the plume of moisture its embedded in continues to be drawn north ahead of the surface low's strong cold front. The front will likely be the dominant forcing for storms in this region. As on Friday, the southern areas of Louisiana and Mississippi will have the greater moisture and instability, while further north FFGs are much lower due to recent heavy rainfall. Thus, once again the Slight Risk is a two-fold combination of factors that individually increase the flooding risk. The front should still be slow enough moving that training storms tracking north along the front could still cause flooding problems, especially in Louisiana. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6nehDMRZ75cJy76cyyEZ8c3DMdEWd4ynXxYuv52Iy9HD= yWK4VDvocoYRof8t8n-SaopyEGWsRBeyFAtt0ZmghB_WJaQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6nehDMRZ75cJy76cyyEZ8c3DMdEWd4ynXxYuv52Iy9HD= yWK4VDvocoYRof8t8n-SaopyEGWsRBeyFAtt0ZmgTqqPe5Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6nehDMRZ75cJy76cyyEZ8c3DMdEWd4ynXxYuv52Iy9HD= yWK4VDvocoYRof8t8n-SaopyEGWsRBeyFAtt0ZmgB288_7g$=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) .