FOUS30 KWBC 260051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Fri Apr 25 2025 Day 1 Valid 01Z Sat Apr 26 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... ...01Z Update... A majority of the changes made to the Day 1 Excessive Rainfall Outlook were based on short-term trends in satellite and radar imagery. Expanded the southern end of the Marginal risk area in Louisiana along a northwest-southeast convergence zone...with the idea that upscale growth will continue for a period in response to strengthening low level flow from the Gulf. Farther west...maintained the Marginal and Slight risk areas after expanding and realigning the western side of each outlook category to better match the satellite depiction. ...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 guidance to lock in on any one area. 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 east across the Southern Plains. Convection early this morning on the leading edge of the outflow boundary south of the MCS will continue to diminish south of the Red River, resulting in not much rainfall from an areal-average standpoint. However, late this afternoon and into the evening with the daytime heating and more favorable forcing (increasing difluence aloft), additional storms will develop along the dryline, likely growing upscale into an MCS that will track slowly across the TX Panhandle and Red River Valley into the overnight hours. The activity will parallel a W-E oriented surface frontal boundary that will become quasi-stationary overnight. The 00Z high-res guidance again indicated stronger multicellular clusters and supercells, some of which become slow moving per the simulated reflectivity guidance. Therefore there is still concern that stronger storms/supercells within the MCS may 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 heavy rain in recent days. Within the Slight Risk area, the latest (00Z) HREF shows highest probabilities of >5" within 12hrs (40-50+ percent). Meanwhile, the Slight Risk area is also supported by the 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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN OKLAHOMA AND FAR NORTH TEXAS... ...20Z Update... In coordination with OUN/Norman, OK forecast office, a Slight Risk upgrade was introduced with this update. The flooding rains in the Slight Risk area are expected to be from ongoing storms as part of an overnight MCS that will be impacting the area at the start of the period at 12Z/7am CDT. FFGs in the area have been lowered from recent heavy rains along the Red River, including for some areas as recently in the last few hours. Guidance remains in good agreement of the development of a new MCS from dry line convection this afternoon and evening, with the MCS persisting through the overnight and into the morning hours before dissipating. The good agreement with tonight's MCS stands in stark contrast to the poor agreement of the various MCSs over the past few days, so it lends to enough confidence between that and the lowered FFGs along the Red River to upgrade the area to a Slight. Elsewhere the surrounding Marginal remains largely unchanged. Wegman ...Previous Discussion... ...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... ...20Z Update... Very few changes were needed with the Marginal Risk areas on D3/Sunday. Both Marginals are low-confidence, with the area of Nevada struggling with both amounts and low snow levels, while most of the northern Plains is relatively difficult to flood in general, since the area is far enough from the Gulf to not have to contend with heavy rain that often. That said, the LLJ from the Gulf will extend that far north on Sunday, advecting in MUCAPE values between 1,000 and 2,000 J/kg, so the atmospheric ingredients remain present for storms to become strong enough to produce heavy rainfall. It remains to be seen whether that rainfall will be heavy enough to cause flooding. Given the relatively low FFGs, good instability, and a constant moisture stream from the LLJ, the Marginal in the area looks good. Wegman ...Previous Discussion... 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://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt