Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 07 2025 18:19:24 FOUS30 KWBC 071819 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 219 PM EDT Wed May 7 2025 Day 1 Valid 16Z Wed May 07 2025 - 12Z Thu May 08 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN LOUISIANA... ....Gulf Coast... 18Z Update... The heaviest rainfall has now pushed firmly off the Central Gulf coast with the theta_E alignment now running parallel, offshore of the coastal plain leading to remnant stratiform precipitation across Southeast LA. Some pockets of moderate to heavy rain are still plausible south of the I-10 corridor over LA leaving a low-end MRGL in place for the rest of the afternoon before the event fully subsides. Considering the above factors and limitations on the potential, the previous SLGT and MDT risks were removed in coordination with the local WFO with a small MRGL in place to cover for the remaining precip bands moving over the region.=20 Kleebauer 16Z update... The cluster of convection has mostly progressed out of Texas with intensity decreasing over western Louisiana noted as well. As such, the Marginal Risk area was reduced to the Galveston area and points east. The Slight Risk was trimmed out of southwest Louisiana. Currently there is a squall line over the central Gulf that will continue to impact south-central and southeast Louisiana over the next few hours. A Moderate Risk was already in place however was adjusted to include the city of New Orleans. The progressive nature of this convection may warrant a special update to address the reduction of threat for flash flooding as the storms dissipate and/or move out of the area. Campbell The warm front that lifted through the Upper Texas coast has stalled along the Gulf Coast and be the focus for widespread showers and thunderstorms through this morning. PWs remain above the 97.5 climatological percentiles from the Upper TX coast to southern LA. The flash flood threat remains greatest along the central Gulf Coast and Lower MS Valley regions where they are most commonly positioned at the nose of a >500 kg/m/s IVT and PWs that are approaching 2.0" along the LA coast. RAP soundings in southern LA Wednesday morning show low-mid level RH values >90% and warm cloud layers at least 11,000ft deep. These soundings also depict tall "skinny CAPE" profiles with MLCAPE >500 J/kg. Plus, hodographs still suggest the potential for organized convection given modest mid-level shear and low-level helicity (sfc-3km >100 m2/s2). The SWrly IVT will intersect the stalled frontal boundary in a way that supports training and back-building convection producing highly efficient rainfall rates >2"/hr over soils that are increasingly saturated. This supports the lingering presence of the Moderate Risk in southern LA through the first half of the day. Note that most model guidance has the bulk of the Day 1 Excessive Rainfall threat during the morning hours. There may still be some residual thunderstorm activity through the afternoon, but most guidance suggests the bulk of the flash flood threat should wind down some for the second half of the day. Further to the west, the Slight Risk remains in place for the Houston metro given the greater urbanization and susceptibility to flash flooding. There is the potential for another round of storms this afternoon that will track over a similar area that was hit by heavy rainfall yesterday, but any lingering flash flood threat should begin to wind down after sunset. ....Central Plains & Mid-South... The lingering mid-upper level low circulation of the Central Plains will funnel a narrow swath of 700-300mb moisture along the northern flank of the low, while weak 850mb FGEN and WAA over the Mid-South. Despite the lack of modest instability, RAP soundings show >90% saturated sfc-700mb profiles and warm cloud layers up to 9,000ft deep in some cases. With modest upper-level and mesoscale forcing present, as well as ample low-mid level moisture, efficient rainfall over saturated soils in parts of the region could result in additional areas of flash flooding. Aside from minor tweaks to the area given the latest QPF, the Marginal Risk remains on track. ....Georgia and northeast Florida... 16Z update... The latest CAMs have trended the convection a bit further north placing them from northeast Florida up the coast toward the South Carolina/Georgia line. Georgia was previously not included in the Marginal Risk area and has now been added. Portions of the central Florida coast was removed from the Marginal as well. Campbell A small Marginal Risk remains in place for urbanized portions of the I-95 corridor through north Florida along the Space Coast. A stalled out cold front to the north and typical sea breeze cycles will trigger rounds of thunderstorms that will have unusually high PWs available by early May climo in eastern FL. PWs are generally above 1.75" in northern FL, which is above the 97.5 climatological percentile according to ECMWF SATs. 00Z RAP soundings for Wednesday afternoon show highly saturated profiles (>90% within the sfc-500mb layer), skinny CAPE soundings with >1,000 J/kg of MLCAPE, and warm cloud layers of at least 12,000ft deep. Most rainfall will be a welcome sight for the region with severe to extreme drought having set in. However, the soils are so dry that rainfall rates >2"/hr would prove too much for the sandy soils of northern FL to handle, let alone the more urbanized corridor along I-95. Isolated instances of flash flooding are possible, with the more urbanized communities most at-risk. Mullinax 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... 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... 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... 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... 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... 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!-xxYvunSc3Nypsta0TXHvLCSiCxTYI1O_OyeXRoXL0dr= ui9U3MZ8KuYCfjIueHdXVuP3fMheFfFlMvYKQoIMnGoSM_Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-xxYvunSc3Nypsta0TXHvLCSiCxTYI1O_OyeXRoXL0dr= ui9U3MZ8KuYCfjIueHdXVuP3fMheFfFlMvYKQoIMK2bVtCQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-xxYvunSc3Nypsta0TXHvLCSiCxTYI1O_OyeXRoXL0dr= ui9U3MZ8KuYCfjIueHdXVuP3fMheFfFlMvYKQoIMpM95tVg$=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) .