498 FOUS30 KWBC 071547 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1147 AM EDT Fri Aug 7 2026 Day 1 Valid 16Z Fri Aug 07 2026 - 12Z Sat Aug 08 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE NORTHEAST AND THE NORTHERN MID-ATLANTIC... 16Z Update.. A Slight Risk was introduced in this update for eastern Pennsylvania, southeast New York, and northern New Jersey. A pulse of upper-level forcing by way of a shortwave will quickly move through the northern Mid-Atlantic in the afternoon hours. Anomalously high moisture availability, with PWATs exceeding 2", is also expected. A line of short- lived thunderstorms will bring heavy rainfall, in the afternoon and early evening, with hourly rain rates potentially reaching and/or exceeding 2"/hr. Previous rainfall over the past few days have also lowered FFGs, to where areas in eastern PA, southern NY, and northern NJ only need around 0.5-0.75" per hour to exceed them. Latest HREF and RRFS runs also support the potential, with medium-to-high chances (~65%) in the FFGs exceedance probabilities in both models. As such, and in coordination with the OKX, LWK, PHI, and BGM, a Slight Risk was introduced. Additionally, MPD #962 in association with this event has been issued through 21Z. Elsewhere, no further changes were needed in terms of the MRGL Risk throughout much of the country. Thought was given toward an upgrade in northern AR, where an influx of moisture intersecting with the Ozarks could result in heavier rain throughout the afternoon. Ultimately, an upgrade was not needed mostly due to the very high FFGs, which rainfall will struggle to meet. Isolated flash flooding will remain a concern, however. Ongoing convection in Minnesota is set to continue over the next several hours, and has already brought spots of 1"/hr rates. 7-8/C/km lapse rates, solid instability (~2000J/kg), and PWATs around 1.5" will continue to support heavy rainfall. Storm motion will be quite progressive, however, and 1hr FFGs remain above 1.5". Given that, keeping a MRGL area was sufficient. Isolated instances of flash flooding remain possible however, and as such, MPD #963 has been issued through 2130Z. Blanco-Alcala Previous Discussion... Marginal Risks for excessive rainfall cover large portions of the country from the Southwest through much of the eastern half of the country. The Marginal was expanded to include much of Oklahoma and north Texas, as well as expanded through Louisiana and portions of the central Gulf Coast, as well as much of Florida. Abundant deep tropical moisture is in place across much of the Southeast and up the Eastern Seaboard. Forcings remain mesoscale, as the synoptic pattern with a trough generally along the northern tier of states generally is not overlapping most of the deepest moisture. Thus, where there is enough forcing, there's a lack of moisture, and in large swaths of the country where there's plenty of moisture, a lack of a defining synoptic feature to provide forcing will likely mean any storms will be reliant on smaller mesoscale features such as local terrain, cold pools, sea breezes etc. to get them going. Due to a lack of defining features overall, large Marginals remain the most prudent forecast. However, given the aforementioned deep tropical moisture over much of the country, it's quite feasible that there will be Slight Risk levels of impacts wherever the storms can interact with each other in a positive way. This appears most likely on Friday across portions of Arkansas, where the deep tropical moisture will run into the terrain of the Ozarks, and the base of the distant upper level trough could provide a bit of organization to the convection. That said, the very dry soils from the lack of rainfall across the state in recent days will make Slight level impacts difficult to attain. From eastern Ohio through much of Pennsylvania and southern New York, there too will have some minimal amount of forcing from weak upper level shortwaves running over the abundant summertime moisture in place across much of the east. Here too there is a higher risk of Slight Risk level impacts, in part due to low FFGs. Convection across this region looks to be quite progressive though, so it will likely take topographic enhancement in localized areas to produce enough rain to produce widely scattered instances of flash flooding. These should be few and far between enough to not need a Slight there for now. Wegman Day 2 Valid 12Z Sat Aug 08 2026 - 12Z Sun Aug 09 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR THE SOUTHWEST, PORTIONS OF THE MIDWEST, AND MUCH OF THE EASTERN SEABOARD... Monsoonal moisture across the Southwest will continue on Saturday, especially across Arizona. As usual, the greatest rainfall will be along the Mexican border where the Gulf of California's influence is greatest, as well as along the Mogollon Rim. There appears to be little rain in between in the populated areas. Lack of signal for more organized heavy rain should preclude a Slight Risk upgrade. Across the Midwest, specifically Iowa, an overnight Saturday night MCS is expected to impact much of Iowa and small portions of the neighboring states. While guidance has come into better agreement on the areas likely to be affected, there remains a broad variety of solutions as to how much rain will fall across much of central and eastern Iowa. Thus, for now the area remains in a Marginal, but will continue to be monitored for a Slight risk upgrade with future updates. Signals for heavy rain across much of the Eastern Seaboard remain very diffuse again on Saturday. Similar to Friday, there remains plentiful moisture all up and down the Atlantic coast. Somewhat better forcing from a negatively tilted trough north of the Great Lakes should allow for more widespread storms, but they will remain quite progressive. Further south, there will be more moisture, but a lack of forcing. Most of the heaviest rains look to be from north Florida through coastal South Carolina. Wegman Day 3 Valid 12Z Sun Aug 09 2026 - 12Z Mon Aug 10 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST AND UPPER MISSISSIPPI VALLEY... ...Upper Midwest... Multiple rounds of storms are expected on Sunday across portions of the upper Midwest, specifically from the southeast corner of Minnesota southeast through northern Indiana. With plentiful moisture from an LLJ originating from the Gulf being advected northward into the storms, there is little reason to see why the storms won't be able to backbuild and train both with the morning convection with an MCS, and then secondarily with late afternoon through the Sunday night convection across these same areas. Since guidance has come into better agreement, even better than Days 1 and 2, a Slight Risk upgrade was introduced with this update. ...Elsewhere... Elsewhere, the Southwest monsoon will continue to promote afternoon and evening convective development into the 4 Corners region, and southwesterly moisture advection into the southern Appalachians could also provide a focus (due to terrain) for afternoon and evening storms. As on prior days, widespread widely scattered convection is likely all across the Southeast Sunday afternoon and evening. Should storms merge over urban areas, then flash flooding is quite possible outside of the drawn Marginal Risk area. Thus, it's probable additional expansions and changes will be needed over the coming days into the Southeast. Wegman Day 4 and Day 5 Valid 12Z Mon Aug 10 2026 - 12Z Wed Aug 12 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR NORTHERN INDIANA... ...Great Lakes/Midwest... Early to mid next week, broad cyclonic flow expected to persist across the northern tier of the Lower 48 from the Northern Plains/Upper MS Valley, eastward through the Midwest/Great Lakes and into the Northeast, to the south of a closed mid-level low in Hudson Bay. A frontal boundary will lie along the southern periphery of this broad trough. Embedded shortwaves in this broad cyclonic flow will accentuate lift in the vicinity of this front where PW values are expected to rise to ~1.75" ahead of each set of shortwaves and low-level confluence is expected. When looking at the ingredients, organized convection looks most likely south of the Great Lakes within eastern portions of the Corn Belt, where hourly rain amounts to 2.5" are possible. Due to a bit better agreement in the guidance, and the southward trends towards where conceptual models would have placed a Slight Risk 24 hours ago, went ahead and added a Slight Risk for areas in and around northern IN both days. While there is still some possibility of further southward trends, this trend may materialize more as a later southward expansion as we get closer to the event. ..Southwest... Broad Marginal Risk areas exist both days early to mid next week across portions of the Southwest, Rockies, and Great Basin. While there continues to be low confidence with the details, locally heavy rains with the potential of 2" an hour amounts and isolated runoff issues will continue in the axis of above average PW values expected across this area. As usual, urban areas, arroyos/dry washes, box canyons, and burn scars would be most at risk for flash flooding. 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 $$