031 FOUS30 KWBC 260828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Sun Jul 26 2026 Day 1 Valid 12Z Sun Jul 26 2026 - 12Z Mon Jul 27 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER COASTAL PORTIONS OF THE SOUTHEAST US... ...Southeast into Florida... Above average moisture will remain over the southeastern U.S. with low level flow near a lingering quasi-stationary front with continuing to feed moisture into the area while poorly defined waves of low pressure stretching across the base of an eastern U.S. trough will focus/enhance the convection near coastal South Carolina/Georgia. Given how slow the front has been to depart the area...opted to extend yesterday's Slight Risk area into the new Day 1 period. A Marginal Risk area extends southward along much of the east coast of Florida. 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. Urban corridors can increase sensitivity to high rainfall rates but overall a Marginal Risk covers the threat of isolated flash flooding. ...Four Corners States onto the Central Plains... Monsoonal flow from the Desert Southwest to the central Rockies continues into early next week. Above average precipitable water values and available instability will again support isolated to widely scattered thunderstorms capable of producing intense rain rates capable of producing flash flooding and a multiple weak shortwaves to help focus the daily convection. Some signals remain in the convective allowing models to focus the greatest coverage of heavy rain into western/central Colorado. Held off on any Slight Risk upgrade again given the uncertainties in the impacts. ...Upper Midwest-Upper Great Lakes-Ohio Valley... Global, CAM, and AI guidance continue to show convectively-aided shortwave activity riding down the top side of the upper ridge axis in northwest flow. A northwest to southeast oriented stationary front could also provide a convective focus as slightly above normal PWs but very strong instability come in. Models continue to show considerable spread in terms of where these ridge riders form and track. The location of convection will be affected by placement of upper level convergence associated with a broad arching anticyclonic flow at the jet level. The potential for flash flooding will be mitigated by the progressive nature of individual storms. Bann/Tate Day 2 Valid 12Z Mon Jul 27 2026 - 12Z Tue Jul 28 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL MONDAY ACROSS THE FOUR CORNERS STATES TO CENTRAL PLAINS AND GREAT LAKES/OHIO VALLEY... ...Four Corners States into the Central Plains... The outlook changed little this cycle given the amount of run to run continuity shown by the guidance. Elevated amounts of moisture will linger over the Four Corners region given a persistent monsoon pattern, although guidance still points to a gradual southward shift in the placement of the upper high center. This should result in a subtle shift in the location of the heaviest rainfall potential but not enough to make more than nudges to the Marginal Risk area in the Four Corners states. Circulation around the northern periphery of the high will transport the moisture and chance of showers and thunderstorms eastward into the central High Plains. The ECMWF precipitable water plots have been pointing to this for several runs so the possibility was already well covered. ...Great Lakes and Ohio Valley into the Northeast US... By Monday a more potent shortwave will move southeastward through the Upper Great Lakes region rounding the Rockies/High Plains high. Increasingly difluent flow ahead of the shortwave will eventually lead to the consolidation/strengthening of a low pressure system during the period which in turn help increasing the forcing for heavier rainfall over the Great Lakes/Ohio Valley. A Marginal Risk remained in place. Bann/Tate Day 3 Valid 12Z Tue Jul 28 2026 - 12Z Wed Jul 29 2026 ...Central Appalachians-Mid Atlantic through New England... Introduced a Slight Risk area over portions of New York southward into a portion of the Mid-Atlantic on Tuesday. Precipitable water values over the region begin close to 2 inches as an area of low pressure begins to take shape in response to the amplification of an upper trough. Divergence aloft increases notably as the winds at jet-stream level become increasingly difluent. Whenever favorable (typical) warm-season thermodynamics are present ahead of a more robust, cooler-season trough with strong deep-layer shear, chances for more widespread convection increase. Although a Slight Risk was introduced..a realignment or expansion is possible as the evolution becomes more clear. The 26/00Z guidance still has a few pieces of guidance that advertise 2-4" on Wednesday into Wednesday night. ...Four Corners States into the Central/Northern Plains... Monsoonal flow of moisture, likely to be enhanced by Genevieve moving offshore Baja California,looks to elevate moisture over the Desert Southwest through the central Rockies and into the central and northern Plains with time. The more suppressed ridge will allow multiple mesoscale/convectively aided shortwaves to push over the top/across the Rockies into the Plains. ...Southeast... A continued threat for heavy rainfall lingers 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. Since to some signal seen in the model guidance in this region, maintained a Marginal Risk area. Bann 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... Quite a dynamic coastal system taking shape Wed-Thu along the New England Coast per the global models and ensembles. Strong upper level jet energy upstream diving toward the trough base will amplify the upper trough -- to the tune of 2+ standard deviations at 500 mb per the latest GEFS -- which in-turn will enhance the downstream QG forcing (upper diffluence/divergence and deep-layer ascent). Strengthening southerly low-level flow will draw in moisture-rich air off the Atlantic Gulf stream, with a swath of 1.8-2.0+ inch 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), feel confident in hoisting a Day 4 Slight Risk area across parts of the Northeast to include the Tri- State area and southern New England. Even without the benefit of high-res CAM runs, the operational global runs (and several ensembles to boot) show areas of 3-4+ inches within 24hrs across the Slight Risk areas, while their perspective ensemble probabilities of these QPF thresholds have come up considerably with the 00Z 07/26 runs. Instability will likely be capped at 1000 J/Kg if not lower ("tall/skinny" CAPE profile); however considering the persistent deep-layer forcing and moisture/TPW anomaly, the amount of CAPE or lack thereof should not inhibit a Slight Risk for excessive rainfall within this area, which is supported by the 00Z 7/26 CSU First-Guess ERO fields (UFVS verified version). 12Z 7/26 ECMWF EFI noted a broadening area over 0.7 with a shift-of-tails to 1. Will be keeping an eye on these trends over the next few days prior to this event, as an upgrade to Moderate somewhere in/near the Sight Risk area will be possible. ...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. ...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. Still some discrepancy amongst the global guidance (GFS most vigorous with the deep-layer forcing and low-level moisture transport), however all the models are trending towards a wetter solution with the likelihood of an MCS/upscale convective growth within an area of expanding PWs of 2-2.25+ inches. Could certainly see a bump up to Slight Risk at some point, but for now given the latitudinal spread in the guidance, have opted to go with a Marginal Risk in the D5 ERO for now. ...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. Since to some signal seen in the model guidance in this region, added a Marginal Risk area. Hurley/Roth 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 $$