Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 25 2025 15:57:20 FOUS30 KWBC 251556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Fri Apr 25 2025 Day 1 Valid 16Z Fri Apr 25 2025 - 12Z Sat Apr 26 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTH TEXAS AND THE TEXAS PANHANDLE INTO SOUTHWEST OKLAHOMA... ....16Z Update... No changes were made to the inherited ERO risk areas across the country. The latest 12Z CAMs guidance just in have not shifted significantly enough in any area to justify any changes, as an ensemble of the guidance remains remarkably consistent with previous runs, especially as compared with previous days. This consistency may be due to the forcing, weak as it is, finally shifting east into the Midwest and off the Plains, replacing the previous troughing with some weak ridging. The ridging stabilizing the upper levels through their warming is likely a contributing factor to the lessening coverage of storms over most of the Plains. There remains a notable convergence zone in the lower levels in the Slight Risk area, which is why additional convection will develop along the dry line this afternoon. Further east, the large Marginal Risk from the Mississippi River east is largely remnant convection with faster moving storms and generally weaker forcing, making any stronger signals that would favor a Slight largely nonexistent or too variable between=20 guidance to lock in on any one area.=20 Wegman ....Previous Discussion... ....Summary... Minor adjustments were made to the Slight Risk from yesterday's Day 2 ERO area across western portions of the TX/OK Red River Valley into the Cap Rock and Rolling Plains. Meanwhile, the Marginal Risk area farther east was extended to include more of the Lower MS Valley. ....North TX...Central OK...TX-OK Panhandles...Eastern NM Slight Risk area which was hoisted yesterday in the Day 2 ERO, in coordination with affected TX/OK offices, continues into today's Day 1 ERO with minor modifications. Upper level flow gradually becomes more diffluent today and tonight as the upper ridge axis pushes=20 east across the Southern Plains. Convection early this morning on=20 the leading edge of the outflow boundary south of the MCS will=20 continue to diminish south of the Red River, resulting in not much=20 rainfall from an areal-average standpoint. However, late this=20 afternoon and into the evening with the daytime heating and more=20 favorable forcing (increasing difluence aloft), additional storms=20 will develop along the dryline, likely growing upscale into an MCS=20 that will track slowly across the TX Panhandle and Red River Valley into the overnight hours. The activity will parallel a W-E=20 oriented surface frontal boundary that will become quasi-stationary overnight. The 00Z high-res guidance again indicated stronger=20 multicellular clusters and supercells, some of which become slow=20 moving per the simulated reflectivity guidance. Therefore there is=20 still concern that stronger storms/supercells within the MCS may=20 move more slowly or even stall, causing a more enhanced flash flood risk, while the MCS as a whole remains over an area hard hit with=20 heavy rain in recent days. Within the Slight Risk area, the latest=20 (00Z) HREF shows highest probabilities of >5" within 12hrs (40-50+=20 percent). Meanwhile, the Slight Risk area is also supported by the=20 latest UFVS-verified CSU ERO First Guess field. ....Lower-Mid MS Valley and TN Valley to OH Valley and Lower Great Lakes, Southern-Central Appalachians... The Marginal Risk inherited from yesterday's Day 2 ERO was expanded to include more of the Lower MS Valley and a little more of the Mid Atlantic Region. Over northern areas, deep-layer instability ahead of the cold front is largely elevated and rather meager (less than 1000 J/Kg). However given the higher relative soil moisture and lower FFGs over these areas, embedded convective elements may cause isolated instances of flash flooding. Farther south across the TN Valley and towards the Gulf Coast, instability will be more plentiful (Mixed-layer CAPEs of 1000-1500 J/Kg), while TPW values are a bit more favorable as well (between 1.5-1.7"). Despite the more favorable thermodynamic profiles, the more transient easterly progression of the shortwave energy will limit the duration of the heaviest rainfall. Still, more elevated 1-2"+/hr probabilities in these areas may lead to localized short- term runoff issues. Hurley Day 2 Valid 12Z Sat Apr 26 2025 - 12Z Sun Apr 27 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS INTO THE SOUTHERN OZARKS AND JUST WEST OF THE MISSISSIPPI DELTA... ....Southern Plains... The MCS that is expected to traverse the Texas Panhandle and along the Red River Valley Friday evening and overnight will persist into Saturday across the Marginal Risk area. Guidance continues to show more limited areal coverage of rain on Saturday as compared with Friday with any additional convection that develops along the dryline and the quasi-stationary west-to-east front. This stationary front will eventually lift northeast as a warm front Saturday night, taking the more organized/widespread convection with it. Uncertainty regarding MCS behavior on Saturday remains very high, and further updates and the move of this time frame into CAMs range may require a later upgrade to a Slight, particularly if heavy rain remains further west towards the Panhandle for longer. Regardless, additional convection is likely to form again Saturday afternoon along the dryline near the New Mexico border, which may further worsen flooding concerns in the areas where the storms develop. Hurley/Wegman Day 3 Valid 12Z Sun Apr 27 2025 - 12Z Mon Apr 28 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE GREAT BASIN AND FROM PORTIONS OF THE NORTHERN ROCKIES INTO THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY... Mid-upper trough across the southern Great Basin early Sunday will lift across the central Rockies by 12Z Monday. This in-turn will nudge the upper ridge axis east of the Northern Plains and Upper MS Valley late Sunday-Sunday night, with a quick onset of upper level divergence in its wake. Along with the more favorable dynamical forcing (upper divergence and deep-layer ascent), deepening southerly flow will tap the western Gulf and lead to an uptick in low-mid layer moisture transport, with TPW values climbing to 1.25+ inches. Deep-layer instability meanwhile will also be ramping up, to the tune of 1000-2000+ J/Kg MUCAPE per the 00Z GFS. Some of the guidance (particularly the ECMWF) shows an MCS developing along the western periphery of the upper ridge, then tracking east to eventually southeast along the 850-300 mb thickness. At this point (day 3 forecast) the forward-propagating Corfidi vectors would support a more progressive MCS, though much will depend on the strength of the LLJ. For now, have maintained a fairly broad Marginal Risk area given the potential of intense short-term (sub-3hr) rainfall rates. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!9ee4-E_sVrH0LwUk8cFzdzqc5L3z0p197W2wlnpMbhIvnOfXpG4RXhcnGnls2VZ= DNYV-OUYMnnNFAAgpBjJzUaLBTdo$ Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!9ee4-E_sVrH0LwUk8cFzdzqc5L3z0p197W2wlnpMbhIvnOfXpG4RXhcnGnls2VZ= DNYV-OUYMnnNFAAgpBjJzT0-49j0$ Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!9ee4-E_sVrH0LwUk8cFzdzqc5L3z0p197W2wlnpMbhIvnOfXpG4RXhcnGnls2VZ= DNYV-OUYMnnNFAAgpBjJzY2fQoYQ$=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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