Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 22 2025 15:46:35 FOUS30 KWBC 221546 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1146 AM EDT Thu May 22 2025 Day 1 Valid 12Z Thu May 22 2025 - 12Z Fri May 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NEW ENGLAND, THE SOUTHERN PLAINS, AND SOUTHEAST FLORIDA... ....Southeastern New England... Late season Nor'easter will strengthen and continue to move up the coast just east of Massachusetts today, spreading significant moisture onshore New England and eastern Long Island. Although=20 instability will be modest, generally less than 500 J/kg, and=20 highest right along the immediate coast, tremendous moisture flux=20 driven via 850mb U-wind below the 1st percentile (so easterly)=20 according to NAEFS will manifest as continuous moderate to heavy=20 rainfall today. Rain rates from the HREF are progged to generally=20 be around 0.5"/hr, although low probabilities (10-20%) for 1"/hr=20 exist across Southeast MA and the Cape/Islands. 0-6km mean winds of 10-20 kts will become increasingly anti-parallel to the Corfidi=20 vectors, and this suggests training of echoes will result in=20 repeating rounds of rainfall. Despite the overall modest rates,=20 this will cause as much as 3-5" of rain, especially across Rhode=20 Island, the Cape, and inside 495 around Boston. This could cause=20 localized flash flooding, especially in urban areas, and the MRGL=20 risk has been maintained with minimal changes. ....Southeast Florida... Thunderstorms will blossom along the tail of a cold front that will settle across the Florida Peninsula this aftn as it gets shunted southward in response to an amplifying trough over the East. This will impinge into a pool of anomalous moisture (PWs +1.5 sigma) with SBCAPE above 2000 J/kg fueling thermodynamics that will support heavy rain rates within thunderstorms. As convection develops along the cold front this aftn, storms will track east on 0-6km mean winds of around 5-10 kts, so generally slow moving,=20 with aligned Corfidi vectors (and aligned to the front itself)=20 suggesting training/backbuilding cells across portions of South=20 Florida. The greatest risk for heavy rain will be near the Gold=20 Coast where the eastward advancing cells will interact with the=20 westward moving sea breeze to cause at least periods of nearly=20 stationary motion and redevelopment, enhancing both the rates to=20 above 3"/hr and duration of these rates. This will result in more=20 than 3" of rain in some areas as suggested by HREF probabilities=20 exceeding 90%, and locally as much as 5" of rain is possible. This=20 portion of Florida has been quite dry so FFG is elevated, but where any storms stall over urban areas, instances of flash flooding are possible. ....Southern Oklahoma/North Texas... A trough amplifying over the Great Lakes/Northeast will yield lowering heights over the Central Plains, with additional ascent being forced through modest jet streak development and spokes of energy via vort maxes rotating SE through the mean flow. At the surface, this will leave a wavering boundary in place in the vicinity of the Red River Valley of the South, which will serve as a focus for convective development this aftn/eve, aided by increasing return flow from the Gulf pushing PWs up to 1.5" and forcing pronounced convergence of moisture transport vectors into the boundary. The CAMs have become more robust with activity later today, but there still exists considerable temporal and latitudinal spread with development. What is more certain, however, is that convection should grow upscale into clusters, especially this evening, when moisture flux maximizes in the presence of 40-50 kts of bulk shear. Storms that develop will feature rainfall rates for which the HREF suggests have a 50% chance of exceeding 1"/hr, which could produce 2-3" of rainfall where any short-term training can occur. While this event does not appear to be exceptionally significant, the highest probabilities for heavy rainfall effectively overlap relatively wetter soils via NASA SPoRT, suggesting at least isolated flash flooding instances are possible. The inherited MRGL risk was expanded just slightly to account for the continued spread in model guidance. Weiss Day 2 Valid 12Z Fri May 23 2025 - 12Z Sat May 24 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN KANSAS and NORTHEASTERN OKLAHOMA INTO THE OZARKS... A multi-day heavy rainfall will start to unfold across the region, with the threat for flash flooding likely increasing by the end of the period. A warm front lifting into the region will become the focus for deepening moisture and storm development as it interacts with weak energy moving off of the top of an upstream ridge into the region. While difference in the details continue, there is increasing agreement that rainfall rates will increase as storms that are forecast to develop further west across the Plains earlier in the day move into the region during the evening and overnight and begin to interact with the deepening pool of moisture (PWs 1.50-1.75 inches). While uncertainty in the details remain, increasing agreement amongst the 00Z guidance, including the GFS, ECMWF, UKMET, along with pre-existing wet soils, provided enough reason to upgrade to a Slight Risk for parts of the region with this issuance. Pereira Day 3 Valid 12Z Sat May 24 2025 - 12Z Sun May 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN KANSAS and NORTHEASTERN OKLAHOMA INTO THE OZARKS... With the previously noted front and coinciding pool of deep moisture remaining in place, additional rounds of heavy rainfall are expected, with the threat for heavy accumulations and flash flooding likely to expand across the region. Guidance shows the boundary remaining quasi-stationary through the period, with 30-40 kt southwesterly low level inflow supporting PWs of 1.5-1.75 inches. Models generally agree that the magnitude and expanse of heavy amounts will increase this period as this moisture interacts with a series of weak impulses. Overall, the 00Z models trended further south, with is reflected in the adjusted Marginal and Slight Risk areas. Given the uncertainty, opted not to upgrade beyond a Slight with this package. However, given the potential for consecutive days of heavy rainfall across the same area, an upgrade to a Moderate Risk in this or subsequent periods may be forthcoming. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6bmrpXk6HOd3n_0yUb_8iMT-iRL30hv2wr4PRxlSRUDc= fK0Cs5CbgSP0HO6_twl-ep6WIjlbagK6CG9dK8utj9GplNM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6bmrpXk6HOd3n_0yUb_8iMT-iRL30hv2wr4PRxlSRUDc= fK0Cs5CbgSP0HO6_twl-ep6WIjlbagK6CG9dK8ut9fTON84$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6bmrpXk6HOd3n_0yUb_8iMT-iRL30hv2wr4PRxlSRUDc= fK0Cs5CbgSP0HO6_twl-ep6WIjlbagK6CG9dK8utVAo97yU$=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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