Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 23 2025 19:21:34 FOUS30 KWBC 231921 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 321 PM EDT Wed Apr 23 2025 Day 1 Valid 16Z Wed Apr 23 2025 - 12Z Thu Apr 24 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PLAINS AND THE SOUTHEAST... ....16Z Update... Only minor changes were made to the large Marginal Risk area across the Plains and Southeast this morning. From a review of an ensemble of the CAMS to HREF probabilities, the Marginal Risk looks in really good shape. Ongoing convection across Texas, Kansas, and individual cells developing in the Southeast as well as expected dryline storms in the southern Plains and MCS development further north into NE/KS all remain in generally good agreement among the guidance. The smaller details of those features, however, are far from certain, lending to only "Marginal Risk" levels of confidence. Regardless, for the vast majority of the area, impacts should remain localized. Cell interactions and localized training may cause more widely scattered flash flooding in a few areas, but high uncertainty about where that may happen precludes any Slight Risk upgrades at this time. Wegman ....Previous Discussion... Persistent Gulf moisture advecting northward through the Gulf states over a stalled frontal boundary draped across the Southeast will continue to produce scattered to widespread convection capable of producing heavy rainfall. Rounds of showers and thunderstorms will fire up along the the boundary from Texas to the Carolinas with some training and backbuilding expected. Given there will be very little movement of the front the storms are likely to track over many of the same areas being impacted by ongoing showers and storms of late, which locally increases the flash flooding risk. A convective complex tracking east across the Hill Country during the very early hours of Wednesday is spreading 0.5 to 1 inch/hour rain rates along its path which may lead to isolated urban flash flooding. Refer to WPC mesoscale precipitation discussion #160 for more details. The latest guidance has better confidence that the wettest areas will likely focus over southern Nebraska and northern Kansas. This too will be the result of an MCS or clusters of thunderstorms developing and training multiple times over the same areas. The Marginal remains in place as the consensus of heaviest rains on the order of 1-2 inches (locally higher, of course) is in a portion of Kansas and Nebraska that has largely missed out on recent heavy rainfall, and the antecedent dry conditions should help to offset and delay the greatest impacts from any heavy rain. The area of the Marginal of least concern appears to be over Oklahoma, which while seeing some scattered shower and thunderstorm activity, should be between the MCSs/areas of more organized and persistent convection. However, due to heavy rainfall, lower FFGs over much of the state lower the threshold needed to achieve flash flooding, so the Marginal is largely unchanged there. Campbell/Wegman Day 2 Valid 12Z Thu Apr 24 2025 - 12Z Fri Apr 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... ....1930Z Update... The guidance surrounding the coverage and intensity of showers and thunderstorms across the country Thursday remains highly variable, which has resulted in some bigger changes to the Day 2 outlook. Generally, there have been increases in the forecast around Iowa, Oklahoma and North Texas, and the southern Appalachians and north Georgia. Meanwhile the main area of decreases was in Missouri and Arkansas. As a result of these changes, the Slight Risk area was trimmed on the eastern side largely out of Missouri and Arkansas, and expanded south to cover portions of north Texas, including the DFW Metroplex. For the Marginal, it too was expanded west to cover the rest of west Kansas and the eastern half of the Oklahoma Panhandle, the eastern Texas Panhandle (for initiation of dryline convection Thursday afternoon), and more significantly, was expanded north into eastern Kentucky from the southern Appalachians. All of these areas have had some heavy rain in recent days, resulting in depressed FFGs. For DFW, that heavy rain was as recently as this morning. Only an isolated flash flooding risk is expected in the Marginal Risk area...with the urban centers assuming the brunt of that risk. Wegman ....Previous Discussion... Multiple clusters of thunderstorms will likely be ongoing in different portions of the ERO risk areas at the start of the Day 3/Thursday time period. These MCS's will likely weaken and dissipate through the late morning, as MCS's typically do. However, with peak heating and the increased instability there will be renewed rounds of showers and thunderstorms. Since this is likely to occur over many of the same areas hit with recent heavy rain this will once again renew the potential for localized flash flooding over portions of the Southeast. Much of the guidance has areal averages of 1 to 2 inches with higher maximums possible falling over much of the same footprint in the day(s) prior. The potential for a new MCS to form Thursday night will keep the threat for flash flooding elevated. The inherited Slight Risk remained largely untouched as it continued to reflect the areas with the greater threat for flash flooding. In similar fashion as Day 1, the low level jet will keep a steady influx of Gulf moisture over the stalled boundary which will maintain support for heavier rainfall with the strongest storms, and greater coverage of showers and storms in general. A Marginal Risk covers the northern Gulf states and Southeast. Campbell Day 3 Valid 12Z Fri Apr 25 2025 - 12Z Sat Apr 26 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS, MID-MISSISSIPPI VALLEY, OHIO VALLEY, AND PORTIONS OF THE NORTHEAST... ....1930Z Update... A strengthening surface low developing ahead of a strengthening upper level shortwave trough will drive the storms from a forcing standpoint through the period. Starting in the Plains it will track northeast into the Great Lakes Friday night, with showers and storms forming along the cold front to the south and east of the low center. This will bring periodically heavy rainfall to portions of the Northeast, which has been in a drier period the past week. Gulf moisture continuing to advect in from the south will lead to shower and thunderstorm activity starting Friday afternoon across=20 the Tennessee and Ohio Valleys, and continuing into Friday night as the front lags behind (south and west) of the surface low. Meanwhile, the trailing (western) end of the cold front may be the trigger for additional thunderstorms into southern Oklahoma, north Texas, and the southern Texas Panhandle. For this reason (and previous days' rains), the Marginal was expanded to cover this area for an isolated flash flooding threat, though coverage of storms looks to be significantly decreased on Friday in this area as compared to previous days. Wegman ....Previous Discussion... Recent and ongoing rainfall over some portions of this broad region will make some areas susceptible to additional rains. During this period a surface low pressure system and its attendant front will likely track from the Mississippi Valley toward the Great Lakes region and interior Northeast. This system will usher in showers and thunderstorms to areas with increased soil saturation and lower FFGs. The broad Marginal Risk was maintained from the Tennessee Valley to the New Hampshire/Maine border given the increasing signal for heavier and more widespread QPF. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-VQpffFNBPwUCo9Fpzm93loIubdQpxH7wuCE2etXOL5R= PofQV7key4fDoAfTojhRn6XIdX2oEQheeR37vubq8jAGgiw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-VQpffFNBPwUCo9Fpzm93loIubdQpxH7wuCE2etXOL5R= PofQV7key4fDoAfTojhRn6XIdX2oEQheeR37vubqUgYRMio$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-VQpffFNBPwUCo9Fpzm93loIubdQpxH7wuCE2etXOL5R= PofQV7key4fDoAfTojhRn6XIdX2oEQheeR37vubqBjvew44$=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) .