FOUS30 KWBC 270039 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 839 PM EDT Sun Jul 26 2026 Day 1 Valid 01Z Mon Jul 27 2026 - 12Z Mon Jul 27 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN & NEAR THE LOWER SAVANNAH RIVER BASIN, WESTERN KENTUCKY, & MIDDLE TENNESSEE... ...South Carolina through the Atlantic coast of Florida... Above average moisture remains over South Carolina into GA with low level flow near a lingering quasi- stationary front continuing to feed moisture into the area while poorly defined waves of low pressure stretching across the base of an eastern U.S. trough should focus/enhance the convection near coastal South Carolina/Georgia early on. Maintained/trimmed Slight Risk based on recent convective evolution and mesoscale guidance. A Marginal Risk area extends southward through portions of the FL Peninsula, with some uptick expected overnight into the early morning. There is abundant deep- layer moisture in place as shown by the 90+ percentile values of precipitable water which suggests cells should be efficient rainfall producers with localized rainfall rates in excess of 3 inches possible and rainfall totals generally 2 to 4 inches. ...Four Corners States... Monsoonal showers and thunderstorms appeared to have peaked for the day and should be on decline this evening into tonight. Have maintained the Marginal Risk to accommodate the ongoing activity. ...Portions of the Midwest-Upper Great Lakes-Lower Ohio Valley... Progressive, organized activity has begun to develop within and downwind of a highly unstable airmass (3000-6000 J/kg of ML CAPE) which should continue overnight, particularly from the Lower Ohio Valley through Middle TN into northernmost AL. The instability gradient thunderstorms ride down should shift westward with time as convection and related outflow boundaries slowly stabilize the atmosphere. The concern is for embedded mesocyclones and cell training bringing hourly rain amounts to 2.5" within an area of modest flash flood guidance. Raising a Slight Risk on this update to account for the developing scenario. Local totals could reach 6" where cells repeat/training the longest. The upgrade to Slight Risk was coordinated with the Memphis TN, Nashville TN, and Huntsville AL forecast offices. ...North-central Virginia into central North Carolina... Strong agreement among 18Z HREF/12z REFS guidance on lee-side trough activity developing this evening over north-central VA under a reinforcing impulse on the main trough axis over the Eastern Seaboard. This activity shifts/propagates south ahead of the mid- level impulse reaching central NC late tonight. PWs around 1.75", ML CAPE of 1000+ J/kg, and effective bulk shear could allow for hourly rain amounts to 2.5" where cells train or mesocyclones form. Since coverage is not expected to be significant, left the risk level as Marginal here. Roth Day 2 Valid 12Z Mon Jul 27 2026 - 12Z Tue Jul 28 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS THE FOUR CORNERS STATES ONTO THE CENTRAL PLAINS AND GREAT LAKES/OHIO VALLEY INTO THE NORTHEAST... ...Four Corners States onto the Central Plains... Center of upper high shifts south from CO to NM through tonight which will direct more ridge topping diurnally-driven activity from the central Rockies onto the central Plains on Monday compared to today. Meanwhile continued monsoonal activity is expected across the Four Corners states again. Favored focus areas for QPF are again along the Mogollon Rim in AZ and in CO, though this time it's east from the Continental Divide instead of west like today. Only a minor expansion to the Marginal Risk is noted with the HREF favoring central Plains activity into southwestern South Dakota. ...Great Lakes and Ohio Valley into the Northeast US... A shortwave trough shifts southeast over northern Minnesota Monday morning and through the Upper Great Lakes Monday afternoon/evening. This is in northwest flow downstream of the upper level ridge axis stalled over the western Dakota border. Increasingly difluent flow ahead of the shortwave will lead to the consolidation/ strengthening of a low pressure system Monday night which increases forcing for heavier rainfall over the Great Lakes/Ohio Valley. The Marginal Risk was expanded west along the WI/UP of MI border for morning activity, down through eastern IL for downstream effects from tonight, and farther downstream into New Jersey for Monday night activity beginning before it becomes more significant on Tuesday. ...Tampa Bay... Troughing currently on the east side of the FL Peninsula shifts to the Gulf side tomorrow. Excellent agreement among 12Z HREF/REFS guidance for repeating heavy thunderstorms across greater Tampa Bay Monday morning into the afternoon. Consensus average QPF is 2-4" with potential for 6" from both the HREF and RRFS. A Marginal Risk for flash flooding is warranted in this scenario, especially for urbanized drainage concerns. Jackson Day 3 Valid 12Z Tue Jul 28 2026 - 12Z Wed Jul 29 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE NORTHERN MID-ATLANTIC INTO SOUTHERN NEW ENGLAND... ...Southern Appalachians into New England including northern Mid- Atlantic... Shortwave trough develops into a low pressure system over the eastern Great Lakes Monday night that then stalls over interior NY/PA Tuesday. There is greater certainly on the interior position of this low which then maintains the warm sector over the northern Mid-Atlantic coast into southern New England. 2 inch PW is highlighted through this warm sector which should also have sufficient instability for higher rainfall rates along with upper divergence to promote growth and repeating activity. A Slight Risk remains over the northern Mid-Atlantic into southern New England with additional focus now for northern NJ, southern NY (inland of NYC as of now) into western CT as a higher end slight (aka "enhanced"). Areal average rainfall of 2-4" with locally higher amounts is currently forecast for this "enhanced" area. Elsewhere, warm sector moisture and instability is present down the Mid-Atlantic through North Carolina with potential for south moving activity to repeat from reinforcing shortwaves rounding the new low ot the north. Targeted Slight Risks may be warranted in the southern Mid-Atlantic eventually. Otherwise, the 12Z QPF consensus favored the southern Appalachians more than before, so they were added to the Marginal Risk. ...Four Corners States across the Central Plains... Further suppression of the upper high center south through New Mexico opens further downstream activity from the Four Corners states across the central Plains. The Marginal Risk was expanded east to the KC metro. As of now the RRFS and EC-AIFS highlight the High Plains of both northeast and southeast CO with more typical Monsoonal activity over terrain in the Four Corners states. Jackson Day 4 and Day 5 Valid 12Z Wed Jul 29 2026 - 12Z Fri Jul 31 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST WEDNESDAY... ...Northeast... A dynamic coastal system takes shape Tue-Thu along/near the New England Coast per model guidance. Strong upper level jet energy upstream diving toward the trough base will amplify the upper trough -- to the tune of 2+ standard deviations below late July means at 500 mb-- which enhances the downstream QG forcing (upper diffluence/divergence and deep-layer ascent). Strengthening east to south low-level flow will draw in moisture-rich air off the Atlantic Gulf stream, with a swath of ~1.75" PWs within the deepening TROWAL pivoting around the slow- moving comma head. Given the persistence of favorable cool season dynamics with warm season thermodynamics (high deep- layer TPW/theta-e values), have maintained the Day 4 Slight Risk area across parts of the Northeast, which fits the most recent global guidance rather well. Coordination with GYX/Gray ME and BTV/Burlington VT forecast offices led to some restriction on its north and northeast side, based on the 3-6 hourly FFGs. Operational guidance shows 2-4" across the Slight Risk area on day 4/Wednesday into Wednesday night. Soundings will be moist, and frontogenesis of the comma head could on its own lead to efficient heavy rainfall; not much MU/ML CAPE will be necessary to lead to flash flooding concerns. There has been a decided broadening trend in the heavy rain signal since this time yesterday, and depending on the degree of heavy rainfall that occurs on Tuesday/Tuesday night, an upgrade to a Moderate somewhere in/near the Sight Risk area can't be ruled out. Added a Marginal Risk for Thursday/Thursday night due to some signal for continued heavy rainfall on day 5. ...Four Corners States into the Central/Northern Plains... Monsoonal flow, likely to be enhanced by Genevieve moving offshore Baja California, brings elevated moisture over the Desert Southwest through the central Rockies and into the central and northern Plains. The more suppressed ridge will allow multiple mesoscale/convectively aided shortwaves to push over the top/across the Rockies into the Plains. The Marginal Risk area here only needed some cosmetic changes to better fit the most recent QPF guidance. ...Lower Missouri Valley into the Upper Mississippi Valley... On Day 5 (Thursday-Thursday night), convectively-aided shortwave energy progressing east from the central-northern Plains will amplify a bit (dig a bit farther south) upon reaching the Lower MO-Mid MS Valleys) during this time as the ~600DM 500mb high becomes centered farther west toward the Four Corners. There's near to above average discrepancy amongst the global guidance (GFS most vigorous with the deep-layer forcing and low-level moisture transport) on where the heavy rain falls, and how much. A wet solution with the likelihood of an MCS/upscale convective growth within an area of expanding PWs of 2-2.25+ inches is within the realm of possibility, given the ingredients expected. Could certainly see a bump up to Slight Risk at some point, but for now given the spread in the guidance, have opted to keep the inherited Marginal Risk with cosmetic changes to better fit the most recent QPF guidance. ...Central Gulf Coast Region into parts of the Southeast... Heavy rainfall is expected along the New England cyclone's trailing front near the FL/GA border. Moisture should be ample and there could be enough effective bulk shear for convective organization. The inherited Marginal Risk areas for Wednesday into Thursday night saw only a few cosmetic changes to better fit the most recent QPF guidance. Roth/Hurley 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 $$