Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 23 2024 15:49:49 FOUS30 KWBC 231549 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1048 AM EST Tue Jan 23 2024 Day 1 Valid 16Z Tue Jan 23 2024 - 12Z Wed Jan 24 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN TEXAS INTO THE ARK-LA-TEX AND SURROUNDING PORTIONS OF NORTHWEST LOUISIANA AND SOUTHWESTERN ARKANSAS... ....16Z Update... Only minor deviations in the orientation of the risk areas compared to the previous forecast issuance. Stationary boundary bisecting portions of east TX into northern LA will be a focal point for low-level convergence ahead of amplifying longwave pattern upstream with the axis positioned over southeastern AZ. Heaviest rain will focus in-of of the Arklatex with the 12z HREF probability and blended mean fields correlating the best rainfall totals and rates within the I-20 corridor from Tyler to Shreveport, expanding 75 miles north/south of the aforementioned area. Rainfall rates will be generally 0.5-1" at their highest intervals, although some isolated rates greater than 1.25" are possible this evening as large scale ascent under a stronger mid-level perturbation moves overhead. Given the antecedent conditions from yesterday and this mornings rainfall, ground saturation across the Arklatex up through the area south of Little Rock has primed the area for enhanced runoff capabilities, and that's before even factoring in the hard grounds from the prolonged cold snap across the region north of I-20. General totals of 2-4" with up to 6" will be possible within the Moderate Risk area with some isolated higher amounts possible outside the MOD across the TX/LA Gulf coast thanks to better convective risks within the coastal plain as better theta-E's align closer to the Gulf waters. Overall, the HREF EAS probability of 3"/24-hrs is greatest within the corridor above with Shreveport the largest urban center positioned within the 35-50% maximum probability, so this will be a focal point through the course of the period. Over the southwest, broad upper trough continues to traverse slowly over AZ with scattered showers and a few embedded heavier cells focused over the terrain of southeast AZ. The heaviest precip will be located over the mountains with snow the primary ptype within elevations above 7000ft, but the adjacent hills and valleys will be the benefactor of up to 1" of precip today with the bulk occurring later this afternoon and evening as mid-level vorticity rotates overhead within peak diurnal instability. Considering the nature of the precip coverage and expected totals/rates, the MRGL over the area was sufficient from the previous forecast. Kleebauer ....Previous Discussion... ....Eastern Texas to the Mid-South... A brief window of relatively drier conditions is expected for much of the early morning as the shortwave impacting the region overnight lifts to the northeast through the mid Mississippi Valley. However, heavy precipitation is expected to quickly redevelop back to the west as upstream energy continues to amplify an upper trough moving across the Southwest. Amplifying southerly flow will support a resurgence of deeper moisture across eastern Texas and the lower Mississippi Valley, as a stronger shortwave trough ejects eastward into the Southern Plains. PWs over 1.5 inches (2.5 standard deviations above normal) will begin to extend north across eastern Texas and the lower Mississippi Valley late today into early Wednesday. This moisture (along with ample instability and large-scale ascent) will support the development of organized convection around midday across eastern TX. Rainfall rates of 1-2"/hr will be supported by the environment, as convection becomes linear and fairly progressive towards the east. Soils have become locally saturated over the past 24 hours with 2-4" of rainfall having already fallen (per MRMS estimates) resulting in FFGs as low as 1-2". As a result, numerous flash floods are likely from portions of eastern TX through the Ark-La-Tex. This area with the highest risk for flash flooding is generally encompassed by the Moderate Risk, and includes the region where 00z HREF exceedance probabilities are maximized (with 40-km neighborhood probabilities of up to 80% for 3" exceedance and between 20-40% for 5" exceedance). Given the potential for localized totals of 4-6" over already saturated areas, some locally significant flash flood events are possible (particularly over more sensitive low-lying areas and poor drainage urbanized terrain). ....Southeastern Arizona... An amplified upper trough along with anomalous moisture will continue to support precipitation across the region early in the period. While widespread heavy amounts are not expected, some additional locally heavy amounts (1"+) may produce some isolated runoff concerns early in the period. Churchill/Pereira Day 2 Valid 12Z Wed Jan 24 2024 - 12Z Thu Jan 25 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM FAR EAST TEXAS AND CENTRAL LOUISIANA THROUGH NORTH-CENTRAL MISSISSIPPI AND NORTHWESTERN ALABAMA... ....Central Gulf Coast through the Mid-south and Tennessee/Ohio Valleys... An upper trough over the Southwest will make meaningful progress eastward on Wednesday, as a 40-50 kt low level jet out of the Gulf will be racing northward up the Mississippi River Valley by Wednesday morning. A digging longwave trough will support the right entrance region of a 120 kt southerly jet centered over central TX and OK. The area east of it, where the western side of the Moderate Risk is located will be highlighted for the greatest atmospheric lift in this setup. Training thunderstorms stretching as far west as Houston will be ongoing over central and southwest LA northeast into western MS. These storms will be capable of 1-2"/hr rainfall rates, and localized 24-hr totals will likely reach 3-6" across the Moderate Risk area. When added to rainfall that occurred on prior days, the result will likely be numerous urban, small stream, creek, and small river flooding. Locally significant flash flooding will be possible, especially in those areas where the ongoing storms Wednesday morning are most persistent. This is mostly likely to be across central Louisiana, but could extend into surrounding portions of far eastern TX, northern LA, and central MS. On Wednesday afternoon through the overnight, the upper-level jet will continue strengthening to 130 kt over the mid-Mississippi River Valley, as the LLJ from the Gulf continues to stream air with PWATs as high as 1.75 inches into the Southeast. This will allow the storms that could be ongoing from the morning and afternoon hours to persist northeastward into northern Mississippi, Alabama, and potentially as far north as central Tennessee. A couple factors come into play in this part of the country...there's a bit more orographic assistance, especially into Tennessee, but this will be the first significant round of rain for this area, which is still warming up from the recent cold snap. The result is despite somewhat dry soils (below average for this time of year), thawing ground, high rainfall rates, and rainfall totals that could locally exceed 4 inches in 24 hours will likely be enough rainfall to overwhelm many small streams, creeks, and any urbanized areas to result in scattered to numerous instances of flash flooding. The thawing ground and vegetation still in "winter mode" should allow much of the rainfall to translate to runoff instead of being absorbed (as the soils act more like hydrophobic clay when frozen). Into the southern Appalachians, remaining snow cover from recent winter storms will contribute to additional runoff to the local river systems in the form of snowmelt. This factor has allowed for some expansion of the Slight and Marginal Risk areas northeastward, and additional expansions further east into the mountains and further northeast into the Ohio Valley are possible with future updates. Churchill/Wegman Day 3 Valid 12Z Thu Jan 25 2024 - 12Z Fri Jan 26 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL GULF COAST THROUGH THE SOUTHERN APPALACHIANS... Heavy rainfall may still be ongoing across portions the southeast into Thursday morning, though the intensity is likely to be waning as activity gradually begins to make some meaningful eastward progress. While the highest totals (locally 2-3") are likely to occur into the southern Appalachians (where a strong low-level jet will interact with terrain to enhance totals), additional totals of 1-2" farther southwest into LA/MS/AL could be more impactful given the potential 2-6" totals across much of those areas on Wednesday. For that reason, the inherited Slight Risk was expanded a bit westward, though the high-end Slight probabilities are currently confined to East TN, northern GA, and surrounding portions of northeast AL and western NC/SC. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5vOrV0QCf_7bjO8NnbqqnMVi8PE_lPaiEH3HhazKFoFS= 3yxZ1apaYb4otHlBfk0DZPHSeURKfqjzBl-a6z302tuBiAI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5vOrV0QCf_7bjO8NnbqqnMVi8PE_lPaiEH3HhazKFoFS= 3yxZ1apaYb4otHlBfk0DZPHSeURKfqjzBl-a6z30v8NQplE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5vOrV0QCf_7bjO8NnbqqnMVi8PE_lPaiEH3HhazKFoFS= 3yxZ1apaYb4otHlBfk0DZPHSeURKfqjzBl-a6z30n0swNpM$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .