FOUS30 KWBC 041536 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1136 AM EDT Fri Sep 4 2026 Day 1 Valid 16Z Fri Sep 04 2026 - 12Z Sat Sep 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE SOUTHWEST, PORTIONS OF THE GREAT LAKES INTO THE UPPER OHIO VALLEY, AND CENTRAL APPALACHIANS... ...Southwest... 16z Update:The forecast remains on track for scattered instances of flash flooding possible this afternoon across portions of Arizona and New Mexico. Moisture transport from the tropical Pacific and Gulf of California is supporting PWATS of 1.0-1.5 inches and surface CAPE around 1000-2000 J/Kg that will increase in the afternoon with daytime heating. Model guidance remains consistent in showing widely scattered thunderstorms forming with QPF on the order of a 0.25 of an inch to locally 1-2 inches. The Slight Risk covers much of the areal model signal for the localized higher QPF output of 1-2 inches and encompasses much of the areal 15-30% 3hr and 6 hr flash flood exceedance guidance. Favored terrain such as burn scars, dry washes, and desert soils will be the most susceptible to localized flash flooding. Wilder ..Previous Discussion.. A longwave trough over the Pacific Northwest will keep the jet stream out ahead of it over the monsoon-affected areas of the Southwest. A Slight Risk of excessive rainfall will therefore remain in effect for the flood prone areas of southeastern Arizona and western New Mexico. Convective coverage is unlikely to be anything noteworthy...rather remaining widely scattered to scattered. The storms will propagate northeastward, which should lower the threat for metro Tucson today. Prior days' thunderstorms in this area are keeping the soil conditions wet enough to promote any storms' runoff causing localized flash flooding, especially in low-lying areas, slot canyons, and over and downstream of burn scars. Wegman ....Great Lakes to Mid-Atlantic... 16z Update:A Slight Risk of Excessive Rainfall was added across southern Michigan, northern Ohio, western Pennsylvania, and northern West Virginia. Rounds of storms may bring as much as 2-3 inches of QPF over areas that have received heavy rainfall this week. 3 hour flash flood guidance values have been lowered from 1-2 inches across central Ohio to half an inch across eastern Ohio and western Pennsylvania. Therefore,the corridor of northeast Ohio and western Pennsylvania are the most susceptible to flash flooding. Additionally, urban areas such as Detroit, Toledo, and Pittsburgh will be vulnerable to flash flooding. The broader Marginal across the southern Mid- Atlantic remains on track this afternoon as progressive rounds of storms will be capable of dropping 1-2 inches of rainfall and isolated instances of flash flooding in mostly urbanized areas. Wilder ..Previous Discussion.. A large ridge of high pressure over the middle of the country will steer "ridge-riding" disturbances southeastward from the Great Lakes to portions of the Mid-Atlantic. Many areas expected to see storms again today were hit by storms, sometimes multiple times, yesterday. A Marginal Risk remains over the area since the storms are likely to be very fast moving, as they have been in recent days. Due to abundant moisture still in place, however, any storms will be capable of locally very heavy rainfall. This could cause flash flooding in urban, low-lying, and other flood-prone areas today. Wegman ...Gulf Coast/Central Texas... 16z Update: No changes were made to the ERO outlook as the forecast remains on track. Satellite shows a mid-level low rotating across the Gulf of America, streaming locally heavy thunderstorms across the Central Gulf Coast as of this morning. 1-2 inch hourly rainfall rates have prompted isolated Flood Advisories for coastal LA/MS, including a Flash Flood Warning for urban areas around New Orleans. The potential for isolated flash flooding and water ponding will continue across much of the Gulf Coast as thunderstorm coverage increases in the afternoon with daytime heating and sea-breeze boundary collisions. For central Texas into the Hill Country, the Marginal Risk was held on for continuity. Currently, radar shows light showers west of the I-35 corridor and hourly rainfall rates remain a quarter to half an inch. Going into the afternoon, there is only a 20-40% chance of observing hourly rainfall rates that exceed an inch; meanwhile, hourly flash flood exceedance is rather high at 2-3 inches due to the drier antecedent conditions. Bottom line, any excessive rainfall, if any, will be on a very isolated basis. Wilder ..Previous Discussion.. Abundant Gulf moisture remains along all of the Gulf Coast. A stalled front a bit inland from the Gulf could provide some focus for organized convection in localized areas. However, the storms are largely expected to remain disorganized, likely following any sea breeze fronts. High FFGs due to sandy soils and around average moisture levels in the top soils should keep any flash flooding localized to urbanized areas. Wegman 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... ...Ohio Valley and the Mid-Atlantic... 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... 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... 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... ...West... 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... 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, SOUTHERN TO CENTRAL CALIFORNIA INTO THE SOUTHERN GREAT BASIN, ACROSS THE UPPER LAKES INTO THE UPPER MISSISSIPPI VALLEY AND OVER THE SOUTHEAST/NORTH FLORIDA... ...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 abnd 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 $$