Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 08 2025 00:40:17 FOUS30 KWBC 080040 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 840 PM EDT Wed May 7 2025 Day 1 Valid 01Z Thu May 08 2025 - 12Z Thu May 08 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST FLORIDA COAST... Previous MRGL risk across the Southeast has decreased in size with a more focused area across Northeast FL where low-level convergence along a quasi-stationary front draped over the region has allowed for a repeating thunderstorm setup south of the Jacksonville metro. Recent mesoanalysis indicates a sharp theta_E gradient within the corridor mirroring the front with mean flow aligned parallel to the boundary. SBCAPE between 1000-1500 J/kg is located within the bounds of the area of interest with much of the convective redevelopment occurring on the western flank of a passing mid-level perturbation that is currently situated over the region. As the energy wanders east, we'll see a sharp cut off the convective threat with the remainder of the activity weakening due to lack of sufficient forcing aloft, as well as the loss of diurnal heat flux. This is a short term issue with the first 2-4 hrs. being the primary time frame of interest before conditions improve overnight. FFG's are thankfully very high within each 1/3/6 hr. intervals (4/5/6 inches) respectively for each exceedance marker, so not anticipating much outside a threat for isolated flash flooding, mainly within the confines of I-95 between Jacksonville to Saint Augustine.=20 Across the Mid-Mississippi Valley, a meander ULL will contribute to widespread showers and elevated convective concerns overnight culminating in a small axis of heavier precip across the east- central portion of MO where low to mid-level convergence and deeper moisture profiles will be present. Despite the better alignment for heavier precip, the maximum rate potential is capped due to the lack of an anomalous PWAT presence that typically is necessary this time of year for higher impacts. An isolated flash flood threat is still plausible across the area extending along and south of I-70 from Jefferson City across to St. Louis and north of the Ozarks of Southeast MO. The threat is non-zero, but under 5% for the necessary risk threshold, so decided to maintain a nil, but make mention of the very low-end potential where total rainfall will likely reach between 1-2", but rates will be lacking for sufficient flash flood concerns.=20 Kleebauer Day 2 Valid 12Z Thu May 08 2025 - 12Z Fri May 09 2025 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL GULF COAST & THE NORTHEAST... ....Central Gulf Coast & Southeast... 21Z update... The environment will be primed for scattered to widespread convection capable of producing rainfall rates of 0.50 inch/hr or higher. There is a decent amount of variance within the guidance on where the higher amounts are expected to occur however there was a notable decrease in amount for portions of the Lower Mississippi Valley while an increase across parts of the Tennessee Valley/Appalachians/Southeast. The Marginal Risk area was trimmed out of much of the Lower Mississippi Valley and increased to cover much of Kentucky and western portions of Virginia, North Carolina, South Carolina and Georgia. Campbell The stalled front draped north of the Gulf Coast will continue to act as a trigger for additional thunderstorms on Thursday. In addition to what is still a moist air-mass for early May, there will be the added assistance of an approaching positively-tilted 500mb trough to provide additional upper-level support. The central Gulf Coast remains most susceptible to flash flooding given their closer positioning to a >400 kg/m/s IVT and PWs above 1.75". These favorable atmospheric parameters are on top of what are now highly sensitive soils following multiple days worth of heavy rainfall. The Slight Risk remains in place over a small section of the Central Gulf Coast, while the expansive Marginal Risk that stretches from the Ozarks to as far east as the Southeast coast still looks to be in good shape given the dearth of moisture in place ahead of the approaching cold front and sufficient instability (MLCAPE between 500-1,500 J/kg). The FL Space Coast is also at risk for flash flooding given the similar setup to Wednesday and potential for more saturated soils following any thunderstorm activity on Wednesday. Localized flash flooding is possible, most notably in the region's more urbanized communities and where PWs are highest. ....Deep South Texas... 21Z update... Environment described below still expected so no major changes made for this issuance. Campbell A strong cold front pushing south out of central Texas will meet up with Gulf moisture, resulting in scattered thunderstorms across the area. These storms will have the benefit of a dual-jet streak setup aloft which when combined with the approach of a 500mb shortwave trough will help support thunderstorm clusters into Thursday night. Low-level easterly winds should lead to thunderstorm development over the Davis Mountains that could then propagate SE towards the Rio Grande Valley. With mean wind flow out of the west, any storms that may form over the Sierra Madre could also approach the Rio Grande River Thursday evening. As these initial round of storms generate cold pools, these new cold pools will invigorate additional clusters of storms Thursday night. Most areas will see at least some period of heavy rain, which will be capable of causing isolated instances of flash flooding. ....Northeast... 21Z update... The latest WPC QPF and model trends had a small uptick in QPF, particularly in the northeast Pennsylvania/northwest New Jersey/southeast New York corridor. With this increase the SLight Risk was adjusted further east across northwest New Jersey. The Marginal Risk saw a minor southward expansion across northern Virginia. Campbell A complex upper-level evolution will begin on Thursday over the Northeast that is trending towards a more wet and stormy setup across the Northeast. An anti-cyclonic wave break over southeast Canada is splitting a lobe of the positively-tilted upper trough over southern Ontario into a cut-off low by Thursday night. Guidance remains split on how far west the blossoming shield of precipitation will advance, but most guidance is coming into a consensus on a swath of the Northeast that stems from central PA and the southern tier of NY on east into areas just west of I-95. The area of greatest concern lies in northeast PA and into the Catskills where soils have grown exceptionally sensitive over the past several days thanks to multiple rounds of heavy rainfall. As the upper low deepens Thursday night, strong 200-500mb upper level divergence will work in tandem with strengthening 850-700mb FGEN and WAA to generate an axis of heavy rainfall. Soundings between 06-12Z Fri show highly saturated profiles and warm cloud layers as deep as 9,000ft deep closer to I-95. As of this discussion, 6-hr FFGs were as low as 1.00" along the northeast PA/NY state borders. While some recovery is expected, this highlights that it may not take much more than one inch of rainfall in 6 hours to cause flash flooding. For these reasons, a Slight Risk was introduced over parts of the Poconos and Catskills where there is a decent compromise for higher QPF and where soils are most sensitive. Mullinax Day 3 Valid 12Z Fri May 09 2025 - 12Z Sat May 10 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL GULF COAST... ....Gulf Coast & Southeast... 21Z update... The latest guidance and WPC QPF shifted the focus from far southeast Louisiana to the South Carolina/Georgia coast with the greatest amounts from the vicinity of the Mobile metro and across the Florida Panhandle. The Slight Risk area now spans from about Panama Beach to about Biloxi, Mississippi. The Marginal Risk covers southeast Louisiana to southern Carolina and south towards Melbourne, Florida. Campbell An amplifying and large upper low positioned over the Lower MS Valley will linger over the region through Friday night with heavy rainfall likely from the central Gulf Coast to the Southeast coast. Latest trends in guidance has been to sharpen up a 500mb low over the Lower MS Valley. The ECMWF SATs page shows an IVT >400 kg/m/s which is above the 90th climatological percentile. Soundings in the region also show highly saturated profiles, modest instability, and deep warm cloud layers. This combined with sufficient vertical wind shear to help sustain areas of organized thunderstorms is increasing the threat for Excessive Rainfall over the central Gulf Coast and FL Panhandle. This setup remains fluid given the recent trends in guidance to be wetter over the central Gulf Coast, with the EC- AIFS and GFS GraphCast showing similar trends. Given the growing soil sensitivities in the central Gulf Coast, the latest Day 3 ERO update now includes a Slight Risk. The more urbanized swath of the I-10 corridor is most at-risk for potential flash flooding on Friday. ....Northeast... 21Z update... No change to the Marginal Risk. Campbell The upper-low evolution stated in the Day 2 discussion holds true for Friday but unlike Day 2 when the heavy rain threat is confined to a 6 to at most 12 hour window, the Excessive Rainfall threat will impact not just the daytime hours on Friday but into Friday night. Out ahead of the upper low, guidance is showing a strengthening IVT surpassing 500 kg/m/s off the Northeast coast that will transport copious amounts of moisture into the Northeast. Moisture will wrap around the northern flank of the 500mb low into northern NY and central New England. Farther east, the warm sector will feature highly saturated and deep warm cloud layers that could contain some weak elevated instability. There remains some spread in where the axis of heaviest rainfall sets up, which is why a Slight Risk was not issued as of this forecast cycle. However, given trends in guidance are all pointing towards a cut-off low over the Northeast and soils throughout the region are highly sensitive, there may be the need for a Slight Risk upgrade in future forecast updates once confidence increases in where the heaviest rainfall takes shape. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_am5OHjYaZmuCH5B8StQ0M0XhPLNd5D_Ws20iUqH8k49= pf8Vsom56RFmmAxlq9m8LBMQV-5Qk9FrEMs2hFRyePqx1Ss$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_am5OHjYaZmuCH5B8StQ0M0XhPLNd5D_Ws20iUqH8k49= pf8Vsom56RFmmAxlq9m8LBMQV-5Qk9FrEMs2hFRyb4biIzs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_am5OHjYaZmuCH5B8StQ0M0XhPLNd5D_Ws20iUqH8k49= pf8Vsom56RFmmAxlq9m8LBMQV-5Qk9FrEMs2hFRyuzJQg6Q$=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) .