Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 04 2025 23:46:42 FOUS30 KWBC 042346 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 746 PM EDT Sun May 4 2025 Day 1 Valid 01Z Mon May 05 2025 - 12Z Mon May 05 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN NEW MEXICO... Eastern New Mexico... Heavy rainfall is expected through the evening hours near and north of a slowly moving warm/quasi-stationary front. Precipitable water values of 0.8-0.9", which is fairly saturated for that region, combined with 500-1500 J/kg of MU CAPE and 25-50 kts of effective bulk shear have been sufficient to develop organized thunderstorms. Hourly rain amounts up to 1.5" with local totals up to 3" are=20 possible where cells merge, mesocyclones form, or storms backbuild. The 18z HREF indicates heavy rain concerns are possible well into=20 the overnight hours, so the main adjustments were to expand the=20 Marginal Risk eastward and shift the Slight Risk southward to=20 account for the current instability pattern, which isn't expected=20 to extend any farther northward with time per the 22z RAP guidance. Southwest... In the vicinity of a deep layered low, thunderstorms have shifted southeast with time from southeast CA and southern NV into AZ where they are showing some cyclonic banding due to 500-1500 J/kg MU CAPE, precipitable water values of 0.75" or so, and ~25 kts of effective bulk shear. The 18z HREF guidance suggests a very slow erosion of MU CAPE overnight, which should lead to continued=20 convective potential, though it should shrink in scale with time.=20 Hourly rain totals as high as 1.25" could lead to issues where=20 there are burn scars and dry washes/arroyos. The Marginal Risk=20 hoisted on the previous special issuance has been extended=20 southeast into AZ given ongoing radar trends and the signal seen in the 18z HREF probabilities of 0.5"+. Local totals of 2" are=20 possible where cells train, merge, or backbuild/stall. Northeast... A deep layered low across the Ohio Valley combined with daytime heating has led to convective development downwind/north of WV as well as other sections of the northern Mid-Atlantic and southern New England States. Precipitable water values up to 1", MU CAPE of 500-1500 J/kg, and effective bulk shear of 25-45 kts have led to=20 some convective banding across western PA with embedded=20 mesocyclones. Given the amount of moisture available, hourly rain=20 amounts to 1.5" and local totals to 3" are possible. Like the=20 Southwest, instability will be slow to erode so the convective=20 potential should slowly wane after sunset, with the guidance=20 indicating some new development in the early morning hours=20 downstream of where instability attempts to develop in and near the Washington D.C. and New York City metropolitan area, with the=20 stronger signal near New York City. Some of the northern portion of the previous Marginal Risk was removed given the decline of=20 instability across NY State and New England. Roth Day 2 Valid 12Z Mon May 05 2025 - 12Z Tue May 06 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN HIGH PLAINS... ....2030Z Update... ....Southern High Plains... The Slight Risk area was expanded northwestward to include more of the Texas Panhandle and far eastern New Mexico with this update and in coordination with ABQ/Albuquerque, NM and AMA/Amarillo, TX forecast offices. Showers and thunderstorms from the overnight period will likely be ongoing along the NM/TX border at the start of the period Monday morning, with periodic flareups of additional convection expected throughout the day in this same area. Following occasional daytime convection, a new MCS is likely to form during the evening across west Texas and eastern NM and track northward through the night. Thus, portions of the area, likely centered around Lubbock/southern Texas Panhandle, will likely see multiple rounds of heavy rain through Monday night. Soils in this area are well above normal for moisture, especially for the eastern Panhandle closer to Oklahoma. Therefore lesser amounts of rain than normal will be needed to result in flash flooding. ....Northeast... The Marginal Risk across portions of the Northeast was maintained with few changes with this update. With coordination from OKX/Upton, NY; ALY/Albany, NY; BOX/Taunton, MA; and PHI/Mt. Holly, NJ forecast offices, a Slight Risk upgrade was considered for portions of the area, with numerous models forecasting a swath of 1-2 inches of rain from NYC north and east through Long Island, much of Connecticut, and the lower Hudson Valley and Catskills. Despite that, very nominal instability and dry antecedent soils conditions and low river levels were determined to likely prevent much flash flooding outside of the most flood prone areas, so the Marginal was maintained. Upslope will be a contributing factor to locally heavier and steadier rain across Connecticut and the lower Hudson Valley into the Catskills, but here too flash flooding should be confined to only isolated areas. Should instability increase or total rainfall increase a targeted Slight Risk may still be needed with future updates. Wegman ....Previous Discussion... Texas into New Mexico... A closed upper low over the Desert Southwest will continue to meander eastward on Monday. Confidence remains in a focused area for repeating heavy rainfall south from the TX Panhandle to the NM border where a Slight Risk remains without many changes...with main activity occurring once again around the time of maximum daytime heating and then persisting into or through the night. The suite of 04/00Z numerical guidance still shows the magnitude of precipitable water values to exceed 2 sigma by 00Z Tuesday with the 1.25" precipitable water line reaching the Caprock. The 12Z QPF consensus is for 2-4" over a sizable area just south of the TX Panhandle...with locally higher amounts embedded in the broader QPF area. Mojave Desert through Southern Utah... Numerous showers with embedded daytime and evening thunderstorms is still expected as upper low continues to meander over Arizona as it begins to fill. Moisture is still anomalous for this time of year...being some 2 to 3 standardized anomalies above climatology for early May...but concern would be greater for excessive rainfall if the actual values of precipitable water values were greater and intense rainfall rates would be sustained for a longer duration. As a result...the risk of excessive rainfall appears to be non-zero but not enough to warrant a Marginal risk area based on coordination with local offices. Northern Mid-Atlantic into southern New England... Closed low will continue to meanders across portions of the Upper OH Valley through Monday night. Continued onshore flow brings the threat for repeating locally heavy rain over the northern Mid- Atlantic into southern New England which will have seen some heavy rain between now and then. A Marginal Risk is maintained. Bann Day 3 Valid 12Z Tue May 06 2025 - 12Z Wed May 07 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN MISSISSIPPI VALLEY/CENTRAL GULF COAST REGION... ....2030Z Update... ....Lower Mississippi Valley to the Colorado Front Range... The inherited Moderate Risk area was greatly expanded with this update, in keeping with numerous pieces of guidance, ensembles, and means all favoring a significant thunderstorm training and heavy rain event across the Arklatex and into central Mississippi. The RRFS suggests multiple lines of storms will cross west to east across the Moderate Risk area, starting in the north from northeast Texas, along the Louisiana/Arkansas border, and then continuing into Mississippi. The trailing (southwestern) portion of the line will hang back as it is joined by additional storms over southeast Texas that progress eastward. The first line will start out at roughly a southwest to northeast orientation, however, into Tuesday night, the lines will get progressively more west-east oriented, meaning greater chance that repeating lines of storms will move across Louisiana. Meanwhile the outflow on the northern side of the complex of storms will allow for continued lighter rains in northern areas that were hard hit earlier in the evening, which will continue or worsen ongoing flash flooding as a result of the earlier storms. While the RRFS is far from a perfect solution as to who will get heavy rain and when, given the footprint of heavy rain in the RRFS is at least similar and in a roughly the same location as the heavy rain footprint in the global models, it appeared to offer a good first guess as to how the storm evolution will play out through the period. The Moderate Risk area remains a lower-end Moderate for now until there is better consensus (and likely southward shifts) in the area of heaviest rainfall. By then it's possible portions of the area may be considered in a higher-end Moderate Risk. For now...rainfall amounts in this area should not reach High Risk levels, since there is at least some southward movement for most of the storms with time, so no one area sees heavy rains for an extended duration. Further, it's possible that the training storms end up offshore of the Louisiana coast should there be more dramatic southward shifts, so the Moderate Risk was determined to be the appropriate risk level for flash flooding. Should there be significant increases with future runs, that are expected to be more concentrated, then it's not completely out of the question that a targeted High Risk may be needed with future updates. While the heaviest rains will be associated with the moderate Risk area in portions of the lower Mississippi Valley, a good amount of that moisture and instability will be drawn northwestward ahead of a strong shortwave trough associated with the slow-moving upper level low over the 4 Corners region. This will extend an area of heavy rain from the MCS/Moderate Risk region northwest across much of Oklahoma and into southern Colorado. This in turn will lead to a shearing front, but the heavy rainfall will be moving over areas hard hit with prior flooding rains, particularly over Oklahoma and the Panhandles. Thus, the Slight Risk extends well west into southeastern Colorado, where upslope may play a role in producing locally heavy rains along the Front Range, while the higher elevations pick up heavy snow. ....Northeast... A slow moving upper level low over the Midwest will continue to pump Atlantic moisture into New England on Tuesday...a continuation of periods of rain from the same system both today and on Monday. By Tuesday, however, instability into and south of New England will increase to up to 500 J/kg. While this amount of instability is far from impressive, it will still be enough to allow for convective showers and maybe some embedded thunderstorms to move north up the coast and into New England. Antecedent soil moisture conditions are likely to be significantly more favorable for the development of flash flooding in this region considering the rain expected there today through Monday. Thus, a Slight Risk upgrade is possible in this area assuming there are increases in the forecast rainfall once this period moves into the short-term and the CAMs can analyze the system. Wegman ....Previous Discussion... Showers and thunderstorms capable of producing intense rainfall rates and heavy/excessive rainfall will become more numerous on Tuesday as height falls make their way out across Texas and as the parent upper low ejects out over the Southern Plains. With increasing flow off the Gulf drawing deeper moisture on- shore....the convection should become increasingly efficient rainfall makers.especially over portions of Louisiana and Mississippi. There is still some differences in where models place the heaviest rainfall...with the GFS and Canadian favoring a faster solution while the EC/UK favoring the axis from eastern Texas into northern Louisiana. This forecast cycle nudged the Moderate risk area a bit eastward but not as far east as the GFS/Canadian. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8dldfN7hTUzwil4TeDjnY_oyM5SDS0-V_iBWl8aQuGkh= JRTHDk0feO5qhrjXK3a-vWg_0-39Bqe0W9TltdbuGkUjfOo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8dldfN7hTUzwil4TeDjnY_oyM5SDS0-V_iBWl8aQuGkh= JRTHDk0feO5qhrjXK3a-vWg_0-39Bqe0W9TltdbuTSaoDJM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8dldfN7hTUzwil4TeDjnY_oyM5SDS0-V_iBWl8aQuGkh= JRTHDk0feO5qhrjXK3a-vWg_0-39Bqe0W9Tltdbu8E4-bPw$=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) .