FOUS30 KWBC 250828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Sat Jul 25 2026 Day 1 Valid 12Z Sat Jul 25 2026 - 12Z Sun Jul 26 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE EASTERN CAROLINAS...PORTIONS OF ARKANSAS...AND THE SOUTHWEST... ...Southeast through the Tennessee Valley... Deep moisture will continue to pool along and near a stalled frontal boundary, which leads to precipitable water values reaching nearly 3 standardized anomalies above climatology for late July and in excess of the 95th percentile of climatology extending eastward from the Tennessee Valley to the Carolinas. Guidance from the 25/00Z model cycle continued to point to heavy rainfall potential. Localized 2-4 inch totals with higher instantaneous rainfall rates appear in this portion of the Slight Risk. Changes made to the outlook areas were largely confined to the Carolinas and to ensure that they capture the effects of shortwave energy crossing the area later this afternoon and the lowering of mid- and upper-level heights as the larger scale trough amplifies along the coast. Over portions of Arkansas, the 25/00 UTC guidance backed away as strongly and quickly as the previous run strongly and quickly showed agreement concerning the potential for a heavy rainfall- producing thunderstorm complex developing today. The HREF neighborhood probabilities for 3-5 inches in 24 hours fell back to 35 percent or less ending at 12Z Sunday. The decrease was comparable for hourly rainfall accumulations exceeding 2-3 inches in an hour. Given the large magnitude of run to run differences...the confidence to upgrade was gone. But the ingredients for heavy rainfall rates and amounts remain present so a Slight risk area was kept and realigned a bit to fit the HREF and RRFS guidance. ...Southwest... The monsoon will get re-invigorated across the Four Corners today with an uptick in coverage and intensity of rainfall expected in the region. QPF amounts did trend up overall and precipitable water values still being forecast at or above the 90th percentile. Convection developed in the overnight hours and guidance such as the HRRR have picked up on an MCV in the vicinity that was anticipated on Friday. It still appears that this becomes a key player in enhancing the rainfall which leads to greater/higher rain rates. Concern is heightened given the possibility that the higher rainfall rates/amounts overlaps with areas which have recently dealt with heavy rainfall and where soils will be less capable of handling the influx of rain water. The Slight Risk was maintained with some adjustments to the latest QPF and across west- central NM, the risk remains skewed toward the higher end of the Slight Risk probability range. Bann/Taylor Day 2 Valid 12Z Sun Jul 26 2026 - 12Z Mon Jul 27 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS THE FOUR CORNERS REGION...UPPER MIDWEST...AND PORTIONS OF FLORIDA... ...Four Corners States onto the Central Plains... Monsoonal flow from the Desert Southwest to the central Rockies continues into early next week. 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. While future outlooks may need to embed a Slight Risk, there was little in the way of consistency in the details of the 25/00Z model cycle that helped define the location of a Slight risk area. ...Upper Midwest-Great Lakes-Ohio Valley-Interior Northeast... Global and AI guidance continue to show convectively-aided shortwave activity riding down the top side of the upper ridge axis over the western Dakota border in northwest flow. Any kind of a mean QPF had too little QPF spread over a large area due to the spread in QPF placement. A nudge towards Wisconsin made last outlook still has some merit given where the axis of highest precipitable water appears in the ECMWF and GFS. This can sometimes signal a preferred track for storms that produce the heaviest rainfall but concern is that a upper level convergence associated with a broad arching anticyclonic flow at the jet level might alter the location of convection. However...a Marginal risk category was appropriate for the uncertainty and the ingredients for heavy rainfall were present in the lower levels so only minor modifications were made. Bann/Taylor Day 3 Valid 12Z Mon Jul 27 2026 - 12Z Tue Jul 28 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL MONDAY ACROSS THE FOUR CORNERS STATES EXTENDING INTO THE CENTRAL PLAINS... ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE CENTRAL AND EASTERN GREAT LAKES INTO THE OHIO VALLEY... ...Four Corners States into the Central Plains... 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. That high will be directing activity from the central plains back toward the Intermountain region. Maintained a Marginal risk area in/the typical terrain of western/northwest Arizona and Southwest Utah and extending eastward as far as the far western High Plains. Plots of ECMWF PW anomaly plots continue to prove helpful in highlighting areas of increased concern. ...Great Lakes and Ohio Valley into the Northeast US... Shortwave energy rounding the top of the High Plains high is forecast to trek from Minnesota into parts of the Great Lakes by the end of the day. Difluent flow ahead of the shortwave will eventually lead to the development of a low pressure system late in the period. This low is expected to help forcing for heavier rainfall over the eastern Lakes/Ohio Valley. Given the progressive convective activity...maintained the Marginal risk area and not particularly close to upgrading with this outlook cycle. Bann Day 4 and Day 5 Valid 12Z Tue Jul 28 2026 - 12Z Thu Jul 30 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS THE FOUR CORNERS STATES EXTENDING INTO THE CENTRAL PLAINS, AND ACROSS THE CENTRAL APPALACHIANS AND MID ATLANTIC TO THE NORTHEAST... ...Four Corners States into the Central Plains... Monsoonal flow bringing elevated moisture over the Desert Southwest through the central Rockies continues with a southward trend in the upper high center directing activity back onto the central Plains early next week. Areal extent appears to be more limited on Day 5 per the guidance, including the latest EC-AIFS and GFS and AIGFS, as the PW anomaly decreases a bit. The more suppressed ridge will allow multiple mesoscale/convectively aided shortwaves to push over the top/across the Rockies into the Plains. ...Central Appalachians-Mid Atlantic to the Northeast... Upper level jet streak riding down the ridge on Tuesday (D4) will lead to amplification of the longwave trough across the Eastern U.S. by the middle-later part of next week. Increasing deep-layer forcing ahead of this amplifying trough and strengthening surface front will coincide with the uptick in pre-trough/pre-frontal low-level theta-e advection, as the PW anomaly expands across the Eastern Seaboard. Whenever favorable (typical) warm-season thermodynamics (deep-layer instability and moisture) are present ahead of a more robust, cooler-season trough with strong deep- layer shear, chances for more widespread convection are much higher. For now, will maintain a Marginal Risk in the Days 4-5 ERO because the guidance continues to move the front through fairly quickly, with a fairly quick transition to strong negative theta-e advection behind it. Over the weekend and early next week we will reassess the need for a targeted Slight Risk, especially once the event moves into the high res (CAM) windows. 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 $$