879 FOUS30 KWBC 040107 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 907 PM EDT Thu Sep 3 2026 Day 1 Valid 01Z Fri Sep 04 2026 - 12Z Fri Sep 04 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER OHIO VALLEY TO THE CENTRAL APPALACHIANS AND OVER SOUTHEASTERN ARIZONA... 01Z Update... ...Southern Plains... The Moderate and Slight Risks for excessive rainfall associated with the remnants of Edouard over eastern Texas were removed. While the hi-res models, including recent runs of the HRRR, indicate some isolated activity may develop overnight -- any heavy amounts are expected to be very isolated and centered west of where the heaviest amounts previously occurred. ...Upper Ohio Valley, Central Appalachians, Mid Atlantic, and Northeast... Following radar trends and recent runs of the HRRR, shifted the Slight Risk centered over the upper Ohio Valley and central Appalachians farther south to extend from eastern Ohio to western Maryland and the eastern West Virginia Panhandle. Repeating convection centered over the western extent of the Slight Risk area is expected to continue, with some guidance indicating storms may propagate east all the way to the Delmarva overnight. Refer to WPC MPD 1190 for additional details regarding the near- term heavy rainfall and flash flooding threat across this region. Farther north, extended the Marginal Risk slightly north to include a greater portion of New York. There is some indication that a narrow training band that has developed over western New York may extend eastward into central New York and perhaps into northeastern Pennsylvania, and southeastern New York. ...Upper Midwest and Great Lakes... Adjusted the previous Marginal Risk farther north across Minnesota into the upper Great Lakes. Storms moving across northern Minnesota and out of eastern North Dakota and expected to track east -- remaining mostly over northern Minnesota. These storms are expected to remain fairly progressive -- limiting the flash flooding threat. There has been indication in several of the hi-res model runs that storms currently to the south over northern Wisconsin will become better organized with a period of training storms producing an axis of heavy amounts from near Green Bay southeastward across Lake Michigan. However, the latest HRRR (23Z) has backed away some from this signal. ...Southwest U.S.... Made mostly minor adjustments to the outlook areas over the Southwest, maintaining the Slight Risk over Southeastern Arizona where convection is expected to continue to produce areas of flash flooding into the evening. Refer to WPC MPD 1188 for additional details. ...Northwest... Rain is expected to continue across portions of eastern Oregon into Idaho this evening. While limited instability is expected to hamper rainfall rates, they may be sufficient enough to cause runoff concerns across burn scar areas. Pereira Day 2 Valid 12Z Fri Sep 04 2026 - 12Z Sat Sep 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF ARIZONA AND NEW MEXICO... ...Southwest... 20z Update:Anomalous moisture will continue to stream in across the Southwest due to broad southerly flow ahead of a trough in the West, as well as a tongue of moisture from the tropical Pacific. Around 1000 J/Kg will support enough instability for the development of afternoon thunderstorms across Arizona and New Mexico. These ingredients will support Isolated to scattered flash flooding, particularly over complex terrain and desert soils. The Slight Risk was expanded farther north into central New Mexico to match a higher QPF signal in model guidance.The Slight Risk matches well with the 50-70% contours of 24hr HREF QPF exceeding an inch and 15-30% 3 hourly flash flood guidance exceedance. Wilder ..Previous Discussion:PW anomalies raise back to 2-2.5 standard deviations above normal during this period. Still remains to be seen how much influence the eventual tropical system that ultimately pulls west of Baja will have in terms of the upper level moisture (potential uptick in the PW anomaly). A Slight Risk was maintained for Arizona while a broad Marginal covers southeast California the Four Corner as this is where the signal for better moisture/instability axis aligns. Isolated to scattered flash flooding is expected. Campbell/Chenard ...Gulf states... 20z Update: The Marginal Risk was maintained across eastern Texas and even extended a little more northward in the Piney Woods region south of Dallas.This was to account for the possibility of thunderstorm regeneration in the afternoon from the remnants of Edouard and the antecedent wet soils. For the rest of the Gulf Coast from Louisiana to Florida, the Marginal Risk remains on track. A mid-level disturbance moving across the Gulf, combined with high PWATS and daytime heating, should foster plentiful thunderstorms that could drop 1-2 inches of rainfall per hour. Southern Louisiana in particular had higher probabilities, 20-40%, of hourly rainfall rates exceeding 2 inches per hour and 24 hour QPF greater than 3 inches. Urban areas, such as New Orleans, could see water ponding and localized flash flooding in this weather pattern. Wilder ..Previous Discussion:Lingering moisture and instability along this corridor could result in a continued localized flash flood risk. A Marginal Risk was maintained from eastern Texas to southern Georgia and Florida. Campbell/Chenard ...Upper Midwest/Ohio Valley/Central Appalachians... 20z Update: The Marginal Risk across the Great Lakes, Ohio Valley, and Mid- Atlantic remains on track. Complexes of storms should continue to propagate northwest to southeast along a stationary front. Model guidance is in good agreement for storms capable of dropping 1-2 inches of rain with local maxima upwards to 3 inches. The progressive storm motions and the relatively high hourly flash flood exceedance guidance should limit flash flooding to an isolated basis. The one exception may be eastern Ohio and Pennsylvania where rainfall this week has saturated soils and increased susceptibility to flash flooding. The flash flood signal was not strong enough to increase confidence for a Slight Risk introduction for this region, but there remains greater potential to see flash flooding. The forecast will continue to be monitored. Wilder ..Previous Discussion:Additional organized/widespread convection to develop downwind of the upper ridge axis today. Precipitable water values are high enough with sufficient instability that embedded cells could produce local downpours and isolated flooding. This will likely start over eastern portions of the Upper Midwest, where the upper jet streak to the northeast is expected to strengthen as the pivoting northern stream trough impinges on the broad-scale ridge to the south. Have essentially traced the Marginal Risk area southeast along the mean 850-300 mb thickness fields, where the upper level forcing along/ahead of the surface front will be most supportive of more organized activity. A Marginal Risk is in effect from Wisconsin to Virginia and eastern North Carolina. Campbell/Chenard Day 3 Valid 12Z Sat Sep 05 2026 - 12Z Sun Sep 06 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHWEST,APPALACHIANS AND MID ATLANTIC, AND THE GULF COAST... ...SOUTHWEST... 20Z Update: The main changes to the forecast were to remove the Marginal across Southern California and much of Arizona due to the expected lag of moisture advection into the area that will limit the convective coverage for the central and western half of the previous risk issuance. The Marginal risk was also extended northward into southern Colorado where trends in QPF show an increase from the previous forecast issuance. Synoptically, the forecast rationale remains unchanged as a moisture plume partly related to Hurricane Marie in the East Pacific moves into the Southwest and may support localized flash flooding across the terrain. Wilder ..Previous Discussion: Moisture southwesterly flow will be directed into the Southwest between a deep upper low/trough over the Pacific Northwest and a strong upper high centered over the Southern Plains. A suppressed moisture and instability axis will likely be positioned from southern California through central New Mexico. There is connection to a tropical moisture plume from Marie so this area will continue to be closely monitored. For now, a Marginal Risk covers the level of threat for excessive rainfall and spans from southern California across southern Nevada, Arizona and New Mexico. Campbell/Jackson ...APPALACHIANS/MID-ATLANTIC... A Marginal ERO was introduced from eastern Kentucky to the southern Mid-Atlantic as rounds of storms, perhaps an MCS, move northwest to southeast. The storms will be fueled by anomalous moisture and ample surface instability. Model QPF of 1-2 inches, locally higher, was in general agreement with some uncertainty in the heaviest rainfall placement. The addition of an approaching cold front the north will lead to a sharp northern fringe of instability leading to a majority of thunderstorm activity along and ahead of the approaching front where instability maxima is most prevalent. Any convective activtity should remain relatively progressive leading to more of a Marginal Risk of excessive rainfall for the period. Wilder ...GULF COAST... A Marginal risk was introduced across the Florida Peninsula and along the Gulf Coast extending into eastern Texas. The mid-level feature should remain across the region on Saturday and enhance storm coverage. Additionally, a tropical Gulf atmosphere will support 1-3 inch hourly rainfall rates capable of producing localized flooding. Wilder Day 4 and Day 5 Valid 12Z Sun Sep 06 2026 - 12Z Tue Sep 08 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ON DAY 4 ACROSS PORTIONS OF SOUTHERN AND CENTRAL UTAH... Daytime update... The biggest adjustment from the previous issuance was the addition of a Slight Risk extending from southwestern into central Utah on Day 4. The daytime guidance remained consistent -- indicating widespread convection fueled by tropical moisture extending from portions of Southern California northeastward. It also showed a growing signal for some of the heavier Day 4 totals centering across southwestern to central Utah. This along with the natural vulnerability of the area to rapid runoffs spurred the upgrade to a Slight Risk. Additional expansions of this area may be warranted if model signals remain consistent. Elsewhere, minor adjustments were made to the previous outlook areas based on the 12Z guidance. Pereira Previous Discussion... ...Southern California/Southwest, Great Basin into the Northern Rockies... The anomalously strong western mid to upper level trof will continue to be very slow moving during the upcoming day 4 and 5 period. There is consensus that this trof will be able to transport anomalous PW values in the vicinity of Marie northeastward from Southern CA/Southwest, into portions of the Great Basin, and Northern Rockies. Widespread scattered convection, locally heavy rains and localized flash flooding possible across this area. A more widespread qpf and excessive rainfall footprint expected day 4, with a decrease in threat area day 5, confined to eastern AZ/western to central NM into southwest CO. A slight risk area is possible in future updates when confidence in details of potential heavy rain axes increases. ...Northern Plains into the Upper Mississippi Valley... Shortwave energy moving northeastward in the broadly diffluent upper flow pattern across the Northern Plains/Upper MS Valley region will also support potential for organized convection along and to the northeast of the northwest to southeast oriented frontal zone across the Northern Plains. Anomalous PW values, 2-3+ standard deviations above the mean, expected across this region, supporting potential for heavy rains and isolated runoff issues both day 4 and day 5. ...Southeast/North Florida... Widespread scattered convection likely both day 4 and 5 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 $$