Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 21 2025 19:36:58 FOUS30 KWBC 211936 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 336 PM EDT Wed May 21 2025 Day 1 Valid 16Z Wed May 21 2025 - 12Z Thu May 22 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL APPALACHIANS AND EASTERN OHIO VALLEY... ....Ohio Valley through the Mid-Atlantic States... A series of shortwaves will interact and consolidate into a large upper low centered over the Great Lakes by the end of D1 /12Z Thursday/. During this evolution, impulses/shortwaves rotating around the amplifying low will spin northeast from the Ohio Valley into the Mid-Atlantic and Northeast, providing periods of enhanced ascent. This lift will encounter favorable thermodynamics characterized by PWs as high as 1.25 inches, overlapped by a plume of modest MUCAPE reaching above 500 J/kg, especially across western PA. Low-level flow at 850mb will vary in direction, but there is likely to be an area of enhanced convergence across the Central Appalachians as far north as northern PA where westerly flow south of the primary gyre meets locally backed SE flow from the Atlantic spreading inland from the NJ/DE coasts. This will result in enhanced convergence and moisture confluence, combining with at least subtle upslope flow to enhance lift across the region. Where temporally this combines with the greatest instability, which will likely peak within a narrow warm sector downstream of a surface low moving near Lake Erie, showers and thunderstorms with heavy rain are expected. Although activity is ongoing this morning, with clouds and some weak subsidence behind the primary surge of moisture will delay the convection this aftn at least a few hours, but the high res CAMs support a resurgence of convection with rain rates of 1-2"/hr at times this aftn. This activity is progged to congeal into a cluster or MCS moving across OH, PA, and WV this evening as convection becomes organized through 0-6km bulk shear of 30-40 kts, and lapse rates steepen beneath the developing upper low. Although this cluster, or clusters, will move rapidly on mean cloud-layer winds of around 25 kts, some short duration training is possible along the line to enhance the temporal length of these rain rates. Additionally, showers and thunderstorms blossoming behind the primary line and beneath the steepening lapse rates may produce repeated rounds of heavy rainfall across the same areas into tonight. FFG across this region is only 1-2"/3 hrs, which has a 10-20% chance of exceedance according to the REFS and HREF, supporting the inherited SLGT risk which was modified only cosmetically for recent guidance. Farther east towards the coasts of DE/NJ and the eastern shore of MD, the 24-hr ensemble probabilities, including rain occurring this morning, feature a low end risk (10%) for 3" of rainfall in a few rounds through tonight. However, instability is minimal which will generally limit the potential for any heavy rainfall rates. For this reason, the recently trimmed MRGL risk for these areas remains left out despite some areas likely picking up 2-3" of rainfall today. ....South Texas and Gulf Coast... The tail end of a cold front draped along the Gulf Coast will serve as the impetus for convection today, some of which has already developed across Louisiana. Plentiful ascent along this front through convergence combined with impressive PWs and high instability will support rainfall rates in excess of 2"/hr at times, which has already been occurring within an FFW over SW LA. Storm motions are progged to remain 10-15 kts along the front to the east, but will be aligned to the Corfidi vectors, and with upstream development likely, this will result in at least short- term training for parts of the region. Despite dry antecedent conditions and high FFG, training of these rates could support isolated flash flood instances today. After coordination with the metwatch forecasters and the Gulf Coast WFOs, a MRGL risk has been expanded from the Lower Texas Coast through eastern Louisiana. Farther SW, the setup is favorable for convection to blossom rapidly across the Sierra Madre of Coahuila, Mexico as a modest shortwave lifts eastward and interacts with extreme thermodynamics this afternoon and evening. PWs as high as 2+ inches and MUCAPE above 3000 J/kg will drift westward as modest 850mb inflow from the Gulf pushes onshore the lower Texas Coast and across the Rio Grande Valley. This will effectively upslope into the Sierra Madre, providing additional impetus for convective development in the terrain. With 0-6km bulk shear progged at 30-45 kts, this will quickly organize into clusters or an MCS which will then follow the moisture eastward into South Texas. Mean 0-6km winds of just 5-10 kts combined with aligned Corfidi vectors of 15-20 kts suggests some training is likely as this MCS pivots east, and with rain rates likely exceeding 2"/hr, this will result in areas of QPF of more than 3", with both the HREF and REFS probabilities indicating a low-end potential (10-15%) for 5+ inches along the Rio Grande Valley. This area has been dry recently (7-day rainfall less than 5% of normal according to AHPS) which will limit the flash flood risk, but these intense rain rates could still locally result in instances of flash flooding. Weiss Day 2 Valid 12Z Thu May 22 2025 - 12Z Fri May 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST, AS WELL AS SOUTHEAST FLORIDA... ....Northeast... Closed mid-level low centered over the Great Lakes will elongated and become aligned west to east in response to tugging via multiple spokes of energy /shortwaves/ rotating around the primary gyre. One of these shortwaves will intensify as it lifts from the Mid- Atlantic coast early Thursday, featuring a negative tilt as it moves near Cape Cod Thursday evening/night. In response to this evolution, a surface low will develop and deepen into a Nor'easter moving just east of the New England coast, with surface pressure drops aided by diffluence in the LFQ of an upper jet streak pivoting through the trough axis. The deepening low pressure will result in intensifying 850mb inflow onshore, reflected by NAEFS 850mb U-winds reaching below the minimum recorded in the CFSR database Thursday aftn. Although PWs will be modest, only exceeding 1" along the Cape and the Islands, this intense onshore flow will result in moisture flux that may exceed 3 standard deviations into Long Island and eastern New England.=20 This impressive moisture advection will efficiently converge into the region, offsetting what will be modest instability (MUCAPE 500 J/kg or less). This will still support rainfall rates that could reach 1"/hr (10-20% probabilities on the HREF/REFS). Additionally, Corfidi vectors are progged to become increasingly aligned against the mean 0-6km winds, suggesting training/backbuilding of even the more modest rainfall rates as cells continue to rotate onshore. This suggests that 3" of rain on D2 is likely (HREF and REFS probabilities > 70%), with locally as much as 5" possible (10% chance). This could result in isolated instances of flash flooding on Thursday, primarily across urban areas along I-95 between New London, CT and Portsmouth, NH. For this reason, a MRGL risk has been added and coordinated with the local WFOs. ....Southeast Florida... The tail of a cold front will sink into the southern Florida peninsula Thursday evening, driven southward by an expanding trough across the east, additionally pushing a jet streak southeast into the vicinity. The overlap of strong convergence along the front and RRQ diffluence in the pivoting jet streak will produce plentiful ascent into favorable thermodynamics resulting from PWs of more than 1.75 inches and MUCAPE above 2000 J/kg. The CAMs are in generally good agreement with thunderstorms blossoming during the aftn and then shifting slowly east across the Peninsula through the evening as 0-6km mean winds remain steady at around 10 kts to the east. Strong ascent into these thermodynamics will support rainfall rates in excess of 2"/hr (HREF probabilities above 40%), and some slowly of convection is likely where cells merge with the sea breeze, especially along the Gold Coast where rainfall probabilities are most significant. With slow moving storms moving atop the urban Gold Coast producing locally 3-5 inches of rain, a MRGL risk for excessive rainfall has been introduced. Weiss Day 3 Valid 12Z Fri May 23 2025 - 12Z Sat May 24 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM EASTERN KANSAS AND OKLAHOMA THROUGH THE OZARKS... Broad closed low over the Northeast and an expanding mid-level ridge blossoming across the Southern Plains will sandwich the Central Plains and Mid-Mississippi Valley between them. This will result in pronounced W/NW flow across the region on D3, within which embedded shortwaves will track quickly, leading to periods of enhanced ascent. At the same time, a jet streak digging around the trough to the northeast will at least peripherally place its RRQ across the region, especially late Friday/Friday night, leading to enhanced deep layer lift over the area. This deep layer ascent will impinge upon improving thermodynamics as return flow from the Gulf surges PWs to above 1.5" Friday night as the 850mb LLJ reaches as high as 40 kts. This will additionally transport elevated instability northward, and the overlap of this CAPE with the moisture and lift will lead to increasing thunderstorm coverage, especially Friday night. With Corfidi vectors aligned perpendicular to the mean 850mb flow, and 0-6km bulk shear forecast to exceed 45 kts, convection should rapidly grow upscale into an MCS with training of echoes from WNW to ESE through the night. This will likely result in axes of at least 2-3" of rain, with locally higher amounts possible.=20 At this time, there remains enough spread, both temporally and spatially, to preclude any upgrades to a SLGT risk. However, the MRGL risk was pulled a bit SW from inherited to best match the latest guidance, and to overlap regions of higher instability where convection may develop first the latter half of D3. It is possible a SLGT risk will be needed eventually, as this setup (continuing into D4) appears favorable for flash flooding. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-34w9KEsoKqWTZyntzUj4r0COi8UiUhIO9iDbsRTLxcq= srf1XFgSZOYaJVv6suMOUywazfZIiym57wLPdZAoTQSaqu8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-34w9KEsoKqWTZyntzUj4r0COi8UiUhIO9iDbsRTLxcq= srf1XFgSZOYaJVv6suMOUywazfZIiym57wLPdZAo4KWVBT4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-34w9KEsoKqWTZyntzUj4r0COi8UiUhIO9iDbsRTLxcq= srf1XFgSZOYaJVv6suMOUywazfZIiym57wLPdZAoK6J2jok$=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) .