Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 19 2024 08:09:26 FOUS30 KWBC 190809 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 409 AM EDT Fri Apr 19 2024 Day 1 Valid 12Z Fri Apr 19 2024 - 12Z Sat Apr 20 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Texas into southern Oklahoma... The inherited Marginal Risk area from the Big Bend of Texas north and east to the Red River Valley has been downgraded with this update. Starting south and working north, model guidance is in very good agreement that any convection Friday afternoon and evening near the Rio Grande will largely stay on the Mexico side of the river. While it is probable that some remnant dying convection may make it over the Rio Grande to the Texas side from Eagle Pass north, it appears likely that much like today, the loss of terrain support should ensure any convection that tries to move off the mountains fizzles out quickly. However, these localized areas of convection can have a mind of their own, which is to say may develop embedded=20 mesolows, which can allow the convection to persist for far longer than expected. Since at the moment this appears to be a low possibility, the combination of high FFGs and highly localized convection should preclude flash flooding on the Texas side, so the Marginal was removed. Along the Red River, the area will be at the head of a rather modest LLJ that will advect some Gulf moisture northward. Much of the guidance shows an area of showers developing in response, but lack of instability largely holds the shower activity to that level, with relatively little embedded convection expected overnight Friday night. Any showers and the embedded thunderstorm or 2 should both move off to the east, and the limited instability will cap the strength of any storms. FFGs in this area are also quite high, so the flash flooding potential is low enough to not meet Marginal criteria. ....Portions of the Southern Appalachians... The remnants of the convection ongoing across portions of the Ohio Valley will make their way into the southern Appalachians Friday, particularly into the Virginias. Associated westerly flow will=20 upslope when the storms reach there, allowing them to maintain their strength up until the crest of the Appalachians. Despite the upslope, there will be little else supporting the showers...with limited atmospheric moisture and instability. The one factor causing just a hint of concern are the antecedent conditions, with low FFGs into the southern half of WV and southwest VA. Since the rain will really struggle to develop vertically into thunderstorms, much less be strong enough to support heavy rain, the threat here is also sub-Marginal. The area will continue to be monitored. Wegman Day 2 Valid 12Z Sat Apr 20 2024 - 12Z Sun Apr 21 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR CENTRAL AND EAST TEXAS THROUGH PORTIONS OF THE ARKLAMISS... A large but weakening shortwave trough will eject into the southern Plains from the Rockies on Saturday. Meanwhile, south to southeasterly flow from the Gulf will advect increasing amounts of Gulf moisture into south Texas ahead of the shortwave. At the surface, building High pressure will move into the central Plains, bringing a much cooler air mass into the Plains and Midwest. The contrast of these clashing air masses along with the upper level support from the shortwave will support shower and thunderstorm activity Saturday and Saturday night across much of eastern Texas, extending east into the Arklamiss. Some light rain on Friday may help saturate soils a bit across northern Texas, but for the most part as regards FFGs/antecedent conditions, the storms will be on their own. However, the building Gulf moisture will bring PWATs to 1.75 inches, which in some parts of Texas will be 3 sigma above normal for this time of year. That will be plenty of moisture to support strong, moisture-laden thunderstorms across the Slight Risk area. Instability will be a factor working against flash flooding for northern Texas...but as the storms push southward, instability will increase enough to support the storms. Portions of south Texas haven't had a soaking rain in a while, so FFGs are quite high across this area. Meanwhile the Slight Risk area was expanded east into the Arklamiss due to relatively more recent heavy rainfall. By Saturday evening, the upper level shortwave support will be somewhat diffuse, which will result in multiple rounds of showers and storms, further increasing the flash flooding threat due to increased probability of overlapping storm tracks. Portions of east-central Texas are in a high-end Slight category to account for the increased likelihood of multiple rounds of storms moving across this region. This area is little changed from previous forecasts, as well as the broader Marginal and Slight Risk areas. For now it appears the somewhat fast storm motions will preclude any areas picking up enough rainfall to result in a further upgrade to a Moderate, but the area will continue to be monitored. Wegman Day 3 Valid 12Z Sun Apr 21 2024 - 12Z Mon Apr 22 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... Convection ongoing Sunday morning across far east Texas and the Arklamiss will move eastward into the rest of Louisiana. While the storms will be weakening in the most climatologically-hostile time of day for the storms, expect the abundant moisture from the eastward shifting LLJ will still support storms capable of locally heavy rain. Recent rainfall events have made portions of eastern Louisiana more sensitive to flooding, along with urban concerns around New Orleans. The New Orleans metro is the highest concern within the broader Marginal. The LLJ will move eastward more quickly through Sunday afternoon, which with increasing westerly flow will allow the storms to also move more quickly eastward with time. Thus, the flash flooding threat Sunday night will be greatly diminished into Alabama, the Florida Panhandle and Georgia. Thus, the Marginal risk for southern Georgia and adjacent Florida and Alabama was removed with this update. Any heavy rainfall will likely be over or moving much too quickly across Georgia by Sunday night to pose a flooding threat, so the Marginal is largely for the daylight hours on Sunday. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5zhTZInDE303KtJXe-1V2Hw6Nokj-dnvt7H4QAwsUSWa= urGjq1GDfl3GtiXsvNb_ENtiMxc5CTzO8OUaVwgz5sWEIGo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5zhTZInDE303KtJXe-1V2Hw6Nokj-dnvt7H4QAwsUSWa= urGjq1GDfl3GtiXsvNb_ENtiMxc5CTzO8OUaVwgz6e4uf_8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5zhTZInDE303KtJXe-1V2Hw6Nokj-dnvt7H4QAwsUSWa= urGjq1GDfl3GtiXsvNb_ENtiMxc5CTzO8OUaVwgzciUSRt8$=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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