Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 31 2025 00:54:16 FOUS30 KWBC 310054 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 854 PM EDT Fri May 30 2025 Day 1 Valid 01Z Sat May 31 2025 - 12Z Sat May 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY THROUGH THE MID-ATLANTIC STATES... ....01Z Update... The western most flank of the Marginal and Slight Risk areas were scaled back with heavy rainfall ending there. However, this storm is quite the dynamic system, highlighted by the 18Z and 21Z WPC=20 surface analyses showing the minimum low pressure areas dropping from 1003mb to 996mb. The storm will continue to deepen tonight as mentioned from day shift with the ECMWF SAT now showing MSLP=20 values over the Delaware Valley that are near the observed minimums for this time of year in the CFSR database. As the storm deepens,=20 intense 850-700mb vertical velocities along the developing occluded front and triple point will make for a narrow band of efficient=20 and heavy rainfall rates that could approach 1.5"/hr as it slowly=20 pivots over eastern PA tonight. RAP soundings north and west of the Philly metro area do show exceptionally saturated soundings at=20 low-mid levels that feature warm cloud layers nearly 10,000ft=20 deep. Instability will be lacking for the most part (<300 J/kg=20 MUCAPE), but given the robust synoptic-scale forcings at work and=20 ample moisture aloft, flash flooding very much remains a concern=20 from the northern Mid-Atlantic into the Northeast tonight, especially along complex terrain. Latest 18Z HREF guidance shows a swath of moderate-to-high chance=20 probabilities (40-70%) for 12-hr rainfall totals >3" from southeast PA on north through the Lehigh Valley, the Poconos, and into the=20 Lower Hudson Valley. FFGs are high enough to help limit the areal=20 extent of potential flash flooding, but recent instances of flash=20 flooding in northern WV, southwest PA, and near the DC/Baltimore=20 metro areas serve as a potential precursor to additional flash=20 flooding tonight. Complex terrain and nearby creeks and streams are most at-risk for flooding tonight, but urbanized flooding along and west of I-95 is possible as well. Note that WPC does have Mesoscale Precipitation (MPD 337) out for portions of the Mid-Atlantic through 0343Z this evening. Please read the MPD for additional flash flooding information. Mullinax --Previous Discussion-- The regional satellite imagery this morning shows a potent upper low moving across Ohio, and it is this feature which will help manifest the flash flood risk from the Ohio Valley into the Mid- Atlantic and Northeast today. As this low tracks northeast today, it will move along a baroclinic zone/surface front, helping to spawn a surface wave of low pressure. This low will track E/NE along the boundary, moving from eastern KY this morning to western New England by Saturday morning. As this low tracks northeast, it will encounter increasingly favorable thermodynamics, into which forcing will become more intense, leading to both an expansion and intensification of convection. The simulated reflectivity from the available CAMs are relatively similar with widespread showers and thunderstorms blossoming ahead of the low, and becoming most impressive from Maryland and points northeast into western New England. This is supported by more robust thermodynamics being drawn northward, characterized by a ribbon of elevated PWs of 1.25 to 1.5 inches, highest east of I-95, and MUCAPE above 1000 J/kg. This will support rainfall rates for which the HREF indicates have a high a moderate to high chance (40-70%) of exceeding 1"/hr, and 10-20% chance of exceeding 2"/hr, enhanced by increasingly impressive synoptic ascent aided by the LLJ and favorable LFQ of a strengthening jet streak. In general, convection which develops downstream of the low will be progressive on 0-6km mean winds of 20-30 kts. However, aligned Corfidi vectors suggest some short-term training is possible which could lengthen these rain rates, or result in repeating rounds in some areas. Additionally, as bulk shear surges to 40-50 kts, storms will likely organize into clusters, with an even better chance for some training along I-95 from Washington, D.C. into western New England. The simulated reflectivity suggests as the surface low wraps up a dry slot will pivot in behind the primary clusters, somewhat reducing the excessive rain risk, but this is lower confidence, and a stripe of heavy rain of 1-3" is expected, with locally more than 4" (or even 5") possible from Maryland through New York as reflected by both the NBMv5.0 and the HREF. The inherited SLGT risk was modified only slightly for recent guidance, and expanded a bit northeast into New England. Weiss Day 2 Valid 12Z Sat May 31 2025 - 12Z Sun Jun 01 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NEW ENGLAND... ....New England... Surface low pressure deepening in response to favorable ascent from the RRQ of an upper jet streak, height falls, and residual baroclinicity along a cold front. This surface low will intensify as it tracks northward through New England, while the low-level flow downstream advects greater moisture and instability northward. Despite the impressive system, thermodynamics are progged to remain modest as reflected by forecast PWs of 1-1.25 inches, around the 75th percentile according to the SPC sounding climatology, and MUCAPE of around 250 J/kg. This will support generally modest rainfall rates of 0.5"/hr, but locally higher is possible, especially within any organized convection on 30-35 kts of bulk shear. Storms will generally track northward at around 25 kts, but some short term training is possible which could additionally enhance the duration of any higher rainfall rates. This will produce swaths of 1-2" of rain as reflected by modest ensemble cluster probabilities, with a local maxima progged across VT where the deformation axis on the back side of this low will pivot, leading to longer duration of rainfall, despite more modest rainfall rates through Saturday evening, atop slightly wetter soils and sensitive terrain. After coordination with WFO BTV, opted to include a targeted slight risk where the greatest potential for persistent moderate to heavy rain overlaps the sensitive terrain, and where the highest HREF probabilities for more than 3" of rain exist. ....Southern Plains... Expansive trough centered over the Great Lakes will lead to lowering heights over the Plains, with an embedded but potent shortwave progged to track southward from Nebraska to Texas Saturday evening/night. This shortwave will impinge into a destabilizing environment reflected by a corridor of 1000 J/kg or more MUCAPE, collocated with PWs surging to around 1.25 inches as the SW LLJ increases to to 20-25 kts Saturday evening. Thunderstorms are expected to blossom along a decaying cold front being pushed south by this shortwave, with upscale growth and organization possible thanks to 40-50 kts of bulk shear. Rainfall rates of 1-2"/hr are progged within this maturing convection, leading to locally as much as 3" of rain (10-20% chance). This could produce isolated instances of flash flooding despite the general progressive nature of this convection. Weiss Day 3 Valid 12Z Sun Jun 01 2025 - 12Z Mon Jun 02 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST... The persistent closed upper level low off of the Baja Peninsula will begin to fill and slowly eject northeast on Sunday, reaching the low Deserts of CA and Arizona by the end of the forecast period. As this feature pivots northeast, it will produce subtle height falls, but more impressively draw tropical moisture northward as reflected by NAEFS PWs above 1.25 inches, or above the 99.5th percentile surging into AZ/CA/NV late D3. This will overlap at least modest instability, potentially as high as 250-500 J/kg, to support increasing convective coverage later D3. There remains some uncertainty into the timing of development of showers due to what could be enhanced cloud cover reflected by high 700-300mb RH. This may slow destabilization and reduce convective coverage, which explains why ECENS/GEFS probabilities for even 1" of rainfall are less than 5%. However, with the impressive PW anomalies in place, once showers to develop they could contain rain rates of above 0.5"/hr, which will support at least an isolated potential for flash flooding where storms move most slowly beneath the upper low. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5xrp87DpLj6LE-SyKe67ChGey3Kq-lZtLl9iNSg-QOPt= puNF7gtpOpwPQS1BqL8sQlR8lt8AAPf9aJNq7FaVqjK-14A$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5xrp87DpLj6LE-SyKe67ChGey3Kq-lZtLl9iNSg-QOPt= puNF7gtpOpwPQS1BqL8sQlR8lt8AAPf9aJNq7FaVVlGg0vY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5xrp87DpLj6LE-SyKe67ChGey3Kq-lZtLl9iNSg-QOPt= puNF7gtpOpwPQS1BqL8sQlR8lt8AAPf9aJNq7FaVRhRp_CU$=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) .