FOUS30 KWBC 050101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 901 PM EDT Fri Sep 4 2026 Day 1 Valid 01Z Sat Sep 05 2026 - 12Z Sat Sep 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST AND THE OHIO VALLEY INTO THE CENTRAL APPALACHIANS... 01Z Update... ...Great Lakes to the Central Appalachians and Mid Atlantic... Shortwave energy originating over the upper Midwest is expected to support convective development, with storms propagating southeast along an axis of deeper moisture and instability centered along the outlook areas. Both the Marginal and Slight Risk areas were adjusted further south, reflecting recent runs of the HRRR, mesoanalysis, and HREF guidance. While individual cells are generally expected to propagate steadily to the southeast, training storms could contribute to heavier totals and a localized threat for flash flooding. The 18Z HREF indicates totals of 1-2 inches are likely with some potential for heavier amounts, especially from northeastern Indiana to southeastern Ohio. While much of the Mid Atlantic was removed from the outlook, did maintain a narrow Marginal Risk from central Pennsylvania southeastward to southern New Jersey and Delaware. Storms now forming over northwestern to central Pennsylvania are expected to further develop through the remainder of the evening, with some of the guidance indicating heavy amounts accumulating around southern New Jersey and northern Delaware. ...Southwest... Trimmed the western extent of the previous outlook areas, but maintained a Slight Risk from far southeastern Arizona into west- central New Mexico, surrounded by a broader Marginal Risk where storms that developed earlier in the day continue to fire. However, expect the threat for additional heavy rain and flash flooding to be short-lived, with storm coverage likely to decrease with the loss of daytime heating. ...Southern Plains to the Southeast... Maintained a Marginal Risk from portions of the Concho Valley westward to the Texas Big Bend, where ongoing north-south propagating storms are expected to continue for at least a couple moisture hours. These storms will be fueled by deepening moisture, supported by low level southeasterly flow into the region. Some north-south training may accentuate the threat for localized heavy amounts and isolated flash flooding. Elsewhere, across the Southern Plains into the Southeast, the Marginal Risk was dropped. While an axis of very deep moisture remains, the combination of diminishing storm coverage with the loss of daytime heating, fairly steady westerly storm motions, and high FFGs, suggest a limited threat for flash flooding through the remainder of the evening into the overnight. Pereira Day 2 Valid 12Z Sat Sep 05 2026 - 12Z Sun Sep 06 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN NORTH CAROLINA INTO THE VIRGINIA TIDEWATER... ...Ohio Valley, Appalachians, and the southern Mid-Atlantic... 20z Update: The synoptic weather pattern will feature a "back- door" cold front sliding southward across the southern Mid-Atlantic and upper-level northwesterly flow that will provide some bulk shear for storm organization and also guide complexes of storms through the region. At the surface, PWATS around 2 inches and CAPE around 3000-4000 J/kg will support 1-2 inch hourly rainfall rates that could support flash flooding. The main change to this forecast package was to extend the Slight Risk northward from North Carolina to include more of southern Virginia. The expanded area matches well with 24 hr QPF probabilities of exceeding 3 inches, as well as 15-20% 3hr and 6hr flash flood guidance exceedance and closer to 30% in the more flood susceptible Tidewater region. The Slight Risk across eastern North Carolina remains on track with model guidance showing a broad area of 2-3 inches of QPF, some locally higher, though it is worth noting the 3 hour flash flood guidance exceedance is a rather high 3-4 inches along the coastal plain. There was some model guidance to suggest a Slight Risk across eastern Kentucky into West Virginia for Saturday with a heavier QPF signature in this area. Ample instability and moisture, combined with an approaching cold front, should spark thunderstorm development capable of producing hourly rainfall rates of 1-2 inches a hour. After collaboration with JKL, a Marginal ERO was maintained for both model uncertainty and dry antecedent conditions in the area. Wilder ..Previous Discussion.. A lower-end Slight was introduced with this update for eastern/coastal North Carolina. A back-door cold front will make steady progress southward down the Delmarva Peninsula today. Ahead of the front in eastern North Carolina, multiple rounds of storms are possible as instability maximizes in the area ahead of the front. Sea breeze fronts are likely to be the predominant forcing for any storms during the early to mid-afternoon. Then as the front approaches in the late afternoon and evening, a second much more widespread area of storms will impact the area. While soils there are dry currently, expect a round of storms today to help saturate the soils ahead of this Day 2/Saturday round. This nudges the area into the low-end Slight category. Further west into the Ohio Valley, storms along the western extension of the front will also push southward with time. The front will be much slower-moving to the west of the Appalachians. Any associated storms impacting the Ohio Valley may produce localized flash flooding in urban areas and other low-lying and flood prone locales. ...Gulf Coast... 20z Update: The forecast remains on track for localized flooding along much of the upper Gulf Coast and the Florida Peninsula courtesy of a 20-40% chance of seeing 2+ inch hourly rainfall rates in thunderstorms, according to the newest HREF guidance. The mid- level low should continue to progress towards the western Gulf of Mexico and enhance storm coverage across Louisiana and eastern Texas. Sea breeze collisions and daytime heating of a tropical air mass should be another source of localized heavier rainfall totals, especially in South Florida. Wilder ..Previous Discussion.. Another round of typical afternoon convection may cause localized flash flooding in urban and flood prone areas. Once again, most forcings for the storms will be localized, such as sea breeze or old outflow boundaries. An abundance of moisture, with PWATs over 2 inches over a large areas will support any storms being capable of very heavy rainfall. Generally flat terrain with sandy soils should largely hinder any flooding to any urbanized areas. ...Southwest... 20z Update: The ingredients for flash flooding are not as favorable across Arizona and New Mexico compared to yesterday with slightly lesser moisture and instability available for afternoon storms in the Southwest Monsoon. However, PWATS around an inch and surface CAPE around 1000 J/Kg should still support chances for hourly rainfall rates to near or exceed 1 inch per hour. These rates still support a flash flood risk over burn scars and dry washes. A Flood Watch has been issued by the Albuquerque WFO for the Ruidoso area burn scars for the potential of slow moving storms with high rainfall rates. In general, southern Arizona and New Mexico will be favored for flash flooding. Wilder ..Previous Discussion.. A Marginal Risk covers from southeastern Arizona across much of New Mexico and into far southern Colorado. Storms Saturday afternoon should largely be confined to the slopes of mountain ranges. This should keep any resultant flash flooding localized. Wegman Day 3 Valid 12Z Sun Sep 06 2026 - 12Z Mon Sep 07 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE COLORADO RIVER VALLEY FROM THE ARIZONA/NEVADA BORDER NORTHEAST INTO CENTRAL UTAH AND ACROSS MOST OF SOUTH CAROLINA... ...West... 20z Update: Only cosmetic changes to the forecast were made based on the latest model QPF guidance. Northern Arizona, southeast Nevada,and southern Utah have the best chances for flash flooding,especially in sensitive terrain such as slot canyons. Greater moisture will be advected into the region from the tropical Pacific and there will be greater synoptic lift as a shortwave ejects into the region from the West Coast. Instability does look to remain below 1000 J/Kg, which could help reduce rainfall rates some. Across the remainder of the West, a broad Marginal extends across the interior Pacific Northwest, the northern Rockies, and the northern Plains. An early fall upper-trough and plentiful moisture will support half an inch to an inch of rainfall over burn scars across the Inter-Mountain West that may lead to flooding. However, instability will be minimal and will likely not support hourly rainfall rates over an inch. The northern Rockies and Plains will have more instability and upper-level divergence ahead of the trough to support thunderstorms capable of producing 1-2 inch hourly rainfall rates and some flash flood risk. Wilder ..Previous Discussion.. Sunday looks to be a particularly active day over much of the West. Some moisture from the large plume associated with Hurricane Marie well out into the Pacific will be drawn into the Southwest. Meanwhile the large longwave trough over the West will capture and fling a few disturbances around the base and across the West. The result will be much more widespread convection, especially over the Slight Risk area from Las Vegas northeast over much of Utah. The Slight Risk was expanded southwestward to include more flood prone areas near Las Vegas, the Grand Canyon, and some of the western National Parks in Utah, including Zion and Bryce Canyon. Slot Canyons may be a particularly dangerous place to be on Sunday afternoon with the more widespread convection expected. Additional expansions and a potential inclusion of a higher end Slight are probable with future updates given the very favorable pattern. A slow-moving front along the Canadian border will make for a wet go over much of northern Montana and into North Dakota, then spreading south over much of Minnesota. A Marginal Risk remains in place, but here too it's probable a Slight may be needed with future updates as the convective evolution in this area becomes more clear. ...Southeast... 20z Update: A Slight Risk was introduced in South Carolina to include the Columbia and Charleston CWAs. Afternoon instability will likely lead to surface CAPE around 3000-4000 J/Kg and PWATS near 2 inches will interact with a cold front moving into the area. Abundant, Locally heavy thunderstorms are likely to form and may be capable of producing 1-2 inch hourly rainfall rates. This could lead to flash flooding over the Carolina Piedmont, as well as urban areas such as Charleston and Columbia. A Marginal remains across the rest of the Gulf Coast and was unchanged. Wilder ..Previous Discussion.. The Marginal Risk was expanded with this update to include some areas where TS Edouardo just hit, another afternoon with widespread convection over the Florida Peninsula, and some chance that convection could spread over North Carolina's southern coast. The area of heaviest rain could be over much of South Carolina's coast. A Marginal was left for Charleston and adjacent areas in coordination with the forecast office there. Better consistency with convective behavior at the end of a front there may allow for future upgrades, but for now given the difficulty for rain to cause flash flooding in portions of the Low Country, the Marginal is sufficient. Wegman Day 4 and Day 5 Valid 12Z Mon Sep 07 2026 - 12Z Wed Sep 09 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHWEST INTO THE SOUTHERN GREAT BASIN, ACROSS THE UPPER LAKES INTO THE UPPER MISSISSIPPI VALLEY AND OVER THE SOUTHEAST/NORTH FLORIDA... Daytime update... With many of the models now showing tropical moisture from Marie remaining largely offshore, PW anomalies and QPF have dropped off notably in much the latest guidance across California into the lower Colorado Basin on Day 5. Therefore, the Marginal Risk was reduced to include only portions of eastern Arizona, New Mexico, and southern Colorado with the daytime update. Elsewhere, made only minor adjustments to the previous forecast. Pereira Previous discussion... ...Southwest, Southern-Central California, Southern Great Basin... A mid to upper level trof in the mean will persist across the western U.S days 4 and 5. A lead closed low will be ejecting northeastward from the northern Great Basin into the Northern Plains, while additional height falls reform the mid to upper level trof along the West coast. The net effect will be to maintain an axis of much above average PW values from Southern CA into the Southwest day 4 and expanding northward into Central CA and the Southern Great Basin by day 5 in response to the re- amplifying West coast upper trof. A marginal risk is maintained day 4 over eastern AZ into western to central NM/far southern CO in the model consensus qpf axis. Scattered convection across this region will support locally heavy rains and isolated runoff issues. With the expansion of the anomalous PW farther to the north and west day 5 into Central CA and the Southern Great Basin, the marginal risk was also expanded from day 4, stretching from southwest CO, western NM into all of AZ, far southern NV and southeast CA. Increasingly favorable right entrance region jet dynamics on the east side of the re-amplifying upper trof will support locally heavy rains in the widespread scattered convection, supporting isolated runoff issues. ...Northern Plains into the Upper Mississippi Valley/Upper Lakes... The above mentioned northeast ejection of the western trof from the Northern Great Basin/Northern Rockies into the Northern Plains at the beginning of day 4 will support potential for organized convection along and to the northeast of the northwest to southeast oriented frontal zone across the Northern Plains. Strengthening low level southerly flow into this front in an axis of anomalous PW values, 2-3+ standard deviations above the mean, expected across this region, supporting potential for heavy rains and isolated runoff issues day 4. By day 5, the associated surface frontal boundary is expected to become more southwest to northeast oriented along the southern edge of the westerlies. At this time the low level inflow into this front is also expected to become more parallel to the boundary, supporting potential for a period of training along the front Tuesday night into early Wednesday. ...Southeast/North Florida... Widespread scattered convection likely day 4 in the vicinity of the slowly southward moving frontal boundary across the Southeast/North Florida. Low confidence in qpf details in the vicinity of this front, but with PW values 1.75-2.00", locally heavy rains and isolated runoff issues possible. Oravec 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 $$