Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 03 2025 01:00:33 FOUS30 KWBC 030100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Fri May 2 2025 Day 1 Valid 01Z Sat May 03 2025 - 12Z Sat May 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS, LOWER MISSISSIPPI, TO THE SOUTHERN APPALACHIANS... 01Z Update... Biggest adjustment to the previous outlook was to trim away the northwestern extent of the previous outlooks across northern Texas=20 where the heavy rain and flooding threat has ended and where=20 additional development is not expected. A Slight Risk remains in place from Southwest through South- Central Texas into Southwest Louisiana where locally heavy showers=20 will continue through the evening hours -- producing additional=20 areas of flash flooding. Refer to WPC MPD #212 for additional=20 information regarding the near-term heavy rain and flooding threat=20 across this area. Further to the east, extended the previous Slight Risk area south into parts of southeastern Louisiana, southern Mississippi,=20 Alabama, and the far western Florida Panhandle. Recent runs of the=20 HRRR and some of the other CAM guidance show slow-moving convection developing later this evening along an axis of enhanced=20 convergence near the coast, with possible mergers with the upstream convection now moving across the lower Mississippi Valley. The 18Z HREF shows high neighborhood probabilities for evening/overnight=20 accumulations exceeding 3 inches centered near Mobile Bay. Similar to areas back to the west, the Slight Risk was trimmed out of the Tennessee Valley where the rainfall has ended. A Slight=20 Risk was maintained from central Alabama northeastward into the=20 southern Applachians where convection continues to develop and move ahead of an upstream front. =20 Pereira Day 2 Valid 12Z Sat May 03 2025 - 12Z Sun May 04 2025 .....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE GULF COAST REGION INTO PARTS OF THE NORTHEAST, EXTREME DEEP SOUTH TEXAS, AND THE SACRAMENTO MOUNTAINS OF NEW MEXICO... ....20Z Update... The Marginal Risk across portions of the East Coast was expanded east to include portions of the I-95 corridor through DC and Baltimore, the Piedmont of the Carolinas, and expanded west into east central Illinois. A new Marginal Risk was issued for the Sacramento Mountains of New Mexico. Due to uncertainties with potential training storms up the Blue Ridge of VA into PA, including impacts into the DC and Baltimore metros, a low-end Marginal expanded to include that portion of the I-95 corridor with this update. Most of the guidance suggests any heavier rains and training storms will be west of the metro, and the ongoing severe drought will mitigate most impacts, but an isolated flood can't be ruled out should training storms move over those cities. Given some instability expected into southern New England, the Marginal was also expanded into western MA and CT on the assumption that northeastward moving storms may persist longer into New England. Wraparound rain around the north side of a cutoff low over Illinois has resulted in a decently well agreed upon maximum of rainfall into east central Illinois, so the Marginal was expanded west for that as well. The Marginal in extreme deep south Texas was trimmed from the north to exclude areas with very high FFGs. It appears increasingly likely that the storms impacting that area, if they train, will be south of the border, but some potential remains into McAllen and Brownsville for training storms, so the Marginal was left in place for those urban centers. In coordination with ABQ/Albuquerque WFO, a Marginal was introduced for the Sacramento Mountains for the potential for nearly stationary storms tied to the terrain impacting old burn scar areas near Ruidoso, NM. Wegman ....Previous Discussion... The focus for excessive rainfall will shift eastward as a closed mid-level circulation center takes shape over the nation's mid- section. The mid- and upper-level flow backs with time from the Gulf coast northward across parts of the Tennessee and Ohio Valleys. Initially...this allows deeper moisture to stream northward. But it also results in the surface/low level front to move eastward more quickly later in the period...with generally a low-end risk of flooding from the Gulf coast northward. The 02/00Z suite of numerical guidance hint that there could be some 1 inch to perhaps 2 inch amounts where frontogenetic forcing aligns favorably in between the Gulf and the Ohio Valley...but the signal for excessive rainfall showed little run to run / model to model consistency. There is also a Marginal Risk of excessive rainfall over far southern portion of Texas...where the southern end of the cold has stalled its southward progression and return flow of Gulf moisture has begun. Several of the High Res ensemble highlight this area with QPF in the upper percentiles although the consensus is less than unanimous in amounts or placement. With soundings showing warm cloud processes through a deep layer and the CAPE profile is tall and skinny...introduced a Marginal risk here. Bann Day 3 Valid 12Z Sun May 04 2025 - 12Z Mon May 05 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE EASTERN UNITED STATES AND SOUTHERN ROCKIES ... ....20Z Update... The Marginal Risk across the East Coast was expanded greatly to include much of the Northeast, Mid-Atlantic and coastal Carolinas, with the other Marginal expanded well west to include portions of southwest Colorado, southern Utah, and eastern Nevada with this update. The Marginal across the east will be for continue training showers and storms into New England, with a maximum of rainfall likely near the Catskills and Poconos of NY and PA, respectively. Rates should remain low enough with some breaks in the rain that amounts remain within Marginal thresholds, but should the Day 2/Saturday rainfall overperform in this area or if the forecast rain comes up for Sunday, then a targeted Slight risk may be needed. Occasional storms with MUCAPE instability values between 500 and 1,000 J/kg are more likely to impact more of the I-95 corridor from Raleigh, NC through NYC. Drought conditions are very likely to temper all but the most persistent rains, but given the sensitivities in the urban corridor, the Marginal was expanded to account for potential training storms. The Marginal Risk out west was expanded with a deep but slow moving upper level low and trough moving across the 4-corners region, drawing some Gulf moisture well northwest and into the area, where terrain will cause localized upslope. Rocky terrain will also favor rapid development of flash flooding in the local areas where the rain is most persistent. The highest elevations will see mostly snow from this event, but most of the area should see rain, which is the rationale for the expansion of the Marginal. The San Juans will have some of the heaviest precipitation, which should lower snow levels locally enough to carve that area out of the Marginal with this update. Wegman ....Previous Discussion... The risk of excessive rainfall persists across parts of the Mid- Atlantic and Southeast US as a broad upper trough continues to make its way eastward on Day 3. Deepest moisture of 1.25 to 1.5 inches will be eventually be shunted off-shore late in the period. Until then...isolated downpours are possible with spaghetti plots of 2 and 3-inches of rain in 24 hours mainly being produced by models with the ARW core...but enough members from the NMM core and from the GEFS to warrant a Marginal. There is also a Marginal Risk area over parts of New Mexico again as moisture continue to spread northward and westward from the Gulf. After 00Z...the flow aloft between the westerly subtropical jet and a jet streak and low- and mid-levels become increasingly diffluent with height falls beginning ahead of an approaching closed low from the west. This...along with moisture and a low level boundary already in place...should support late day and evening storms that may produce locally heavy rainfall rates. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!81qhY1qFl3jBHGcMIsZVaTePDhnpfLJ1QJevNXchGRYF= cE9wf8Lz-4_tamSGOqTVUVK8YlWGJ0NqaqezhI_fv9xoAvQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!81qhY1qFl3jBHGcMIsZVaTePDhnpfLJ1QJevNXchGRYF= cE9wf8Lz-4_tamSGOqTVUVK8YlWGJ0NqaqezhI_fPaP7DdI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!81qhY1qFl3jBHGcMIsZVaTePDhnpfLJ1QJevNXchGRYF= cE9wf8Lz-4_tamSGOqTVUVK8YlWGJ0NqaqezhI_fhomXhaA$=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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