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Issued by NWS
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413 FOUS30 KWBC 072331 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 731 PM EDT Mon Jul 7 2025 Day 1 Valid 01Z Tue Jul 08 2025 - 12Z Tue Jul 08 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR PORTIONS OF OHIO, WEST VIRGINIA, & SOUTHWEST PENNSYLVANIA... OH/WV/PA/MD... Convection is trying to align across portions of the Upper Ohio Valley at the present time. Shifted the Slight Risk previously in this area somewhat to account for current conditions. See Mesoscale Precipitation Discussion #615 for more details on the threat there. Elsewhere & Otherwise... Activity across the Ohio Valley, Lower Mississippi Valley, and Northeast is expected to fade overnight into the early morning hours. Effective bulk shear supports organization across the northern half of the Mississippi Valley, northern and central Plains, and Northeast, but systems in these regions will be mobile, limiting excessive rainfall/flash flood potential. Post-Tropical Cyclone Chantal may cause a brief skirmish of heavy rainfall near Nantucket, Martha`s Vineyard, and Cape Cod during the early morning hours on Tuesday, but the system will be moving quickly. The most persistent convection should be with a front moving across MN & WI. Where convection manages hold on and train/merge in any of these areas, hourly rain amounts to 2" with local amounts to 4" will be possible, as sufficient moisture and instability exists to allow for this possibility. Roth Day 2 Valid 12Z Tue Jul 08 2025 - 12Z Wed Jul 09 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE EASTERN MID-ATLANTIC STATES AND SOUTHERN NEW ENGLAND, AS WELL AS PARTS OF THE SOUTHERN PLAINS... ...Mid-Atlantic and Northeast... Post-tropical cyclone Chantal will be exiting east of Cape Cod to start the period as it continues to get caught up in increasing southwesterlies downstream of a trough approaching from the Great Lakes. This trough will continue to translate eastward through the day, but will be slow to advect due to downstream Bermuda-type ridging. Between these two features, SW flow will remain prevalent over the area, with 850-500mb winds being nearly unidirectional from the W/SW at 15-20 kts. This will result in a continued extremely moist environment with PWs likely eclipsing 2" from eastern VA through southern New England Tuesday afternoon, coincident with a plume of SBCAPE exceeding 2000 J/kg. Into these impressive thermodynamics, a shortwave embedded within the mean flow will traverse northeast ahead of a cold front and along a surface trough, providing additional ascent atop the already impressive convergence on these boundaries. This suggests showers and thunderstorms will become widespread, which is additionally supported by high-res CAM simulated reflectivity. Storms that develop will move very slowly, and Corfidi vectors collapsed to just around 5 kts will support backbuilding and training echoes along the surface trough and front. With warm cloud depths potentially eclipsing 14,000 ft, this will support efficient warm-rain processes and rain rates above 2"/hr at times. Where training occurs, this could result in total rainfall in excess of 3", with instances of flash flooding possible across much of the I-95 corridor from Richmond, VA to Portland, ME, and a slight risk has been added for this area. ...Southern Plains through the Ohio Valley... A wavering front will press eastward from Oklahoma through Ohio during Tuesday, providing forcing for ascent through low-level convergence. This frontal convergence will work in tandem with subtle height falls as the driving trough pushes south and east, and modest PVA as spokes of vorticity rotate eastward through the flow. While in general this will result in scattered thunderstorms developing along the front with rainfall rates of 1+"/hr, there may be a localized maxima of heavy rain potential from eastern OK into northern AR. In this portion of the Southern Plains and into the Ozarks, an impressive shortwave is progged to push southward Tuesday evening/night and suppress the ridge to the west. This will drive locally enhanced ascent. At the same time, lingering boundaries from Monday night`s convection and scattered storms earlier on Tuesday will provide additional focus, and the CAMs, while differing spatially and temporally, suggest locally more organized convection Tuesday night. As this shortwave drops south and interacts with surface boundaries, it will impinge into increasingly robust thermodynamics as the 20 kt LLJ pushes PWs to around 2 inches coincident with MUCAPE surging to 2000 J/kg. The HREF hourly rain rate probabilities for 2"/hr peak around 20% during this time, suggesting intense rainfall that could overwhelm soils and produce runoff, especially where backbuilding/training occurs due to Corfidi vectors aligning against the mean wind. Additionally some enhanced shear during this time could organize convection into an MCS, leading to even more impressive rain rates. Although confidence is modest in the exact timing and placement, both the HREF and REFS have increased probabilities for 3" (60%) and 5" (30%) of rainfall, leading to the upgraded SLGT risk area. ...Upper Midwest... A wave of low pressure and accompanying surface trough will move progressively eastward from Minnesota into Michigan the first half of Tuesday. While this will be generally fast moving, it will impinge into a favorable environment to support heavy rain rates which will likely eclipse 1"/hr, especially where it overlaps a ribbon of MUCAPE reaching 1000-2000 J/kg. The fast motion will limit total rainfall, but in some places that receive multiple rounds of heavy rain, event totals could reach 2-3". This falling atop sensitive soils (FFG as low as 1-1.5"/3hrs) could produce instances of flash flooding. ......Southern New Mexico... Another daily round of showers and thunderstorms is expected to develop in the high terrain and then drop southeast into the High Plains and southern portions of New Mexico. Although storms are expected to be generally progressive as they come off the terrain (0-6km mean winds from the north around 15 kts), they will move into an axis of more favorable thermodynamics characterized by PWs of above 1" and MUCAPE approaching 1000 J/kg. This could support briefly heavy rain rates above 1"/hr, supporting an isolated risk for flash flood impacts across urban areas or sensitive terrain and burn scars. Weiss Day 3 Valid 12Z Wed Jul 09 2025 - 12Z Thu Jul 10 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE EASTERN MID-ATLANTIC STATES... ...Mid-Atlantic... Quasi-stationary front from Tuesday will linger in a general east- to-west fashion on Wednesday across the Mid-Atlantic States. Along this boundary, a shortwave will press east within the approaching longwave trough, leading to weak low pressure development moving east along this front. The accompanying ascent will act favorably into continued robust thermodynamics with PWs around 2 inches (above the 90th percentile according to NAEFS ensemble tables) and 1000+ J/kg SBCAPE during the afternoon. Increasing 0-6km bulk shear and aligned 0-6km mean winds to the front suggests storms will repeatedly develop to the SW and then train ENE across the Mid- Atlantic states. Not only will this result in heavy rainfall which may exceed 3 inches in some areas, but it will occur across primed soils from heavy rainfall on D1 and D2, enhancing the flash flood risk, and a SLGT risk has been added from central NJ through northern NC where both GEFS and ECENS 24-hr rainfall probabilities peak. ...Ozarks through the Southern Ohio Valley... Stalled frontal boundary will gradually begin to lift northeast on Wednesday, but persistent ascent along this front as weak shortwaves cross it will maintain a convective risk through the day. These storms will fire in response to thermodynamics characterized by PWs around 2 inches collocated with SBCAPE during peak heating that will approach 2000 J/kg. While convection is likely to be scattered, any storm which trains along the boundary with intense 1-2"/hr rain rates may result in rapid runoff capable of producing instances of flash flooding. Weiss Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt