FOUS30 KWBC 221600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Sat Aug 22 2026 Day 1 Valid 16Z Sat Aug 22 2026 - 12Z Sun Aug 23 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE COASTAL MID ATLANTIC REGION... ....16Z Update... The inherited Slight Risk was adjusted to limit coverage to just portions of the Maryland Eastern Shore and the DelMarVa Peninsula based on the latest guidance which has continued to show a rather narrow focus for storm coverage. However, while the ultimate coverage of heavier rainfall may be relatively isolated, over portions of the Bay, and throughout coastal regions, the HREF and RRFS both continue to show an impressive signal with high probabilities (70-90%) of rainfall exceeding 3 inches and moderate probabilities (40-60%) of rainfall exceeding 5 inches. This would likely still be enough to pose a risk for scattered flash flooding especially for any more urban areas. Otherwise for the eastern U.S., there was a modest expansion of the Moderate Risk to combine the regions over the Mid-Atlantic northwest through the Upper Ohio Valley/Appalachians and into the Great Lakes region. An embedded upper-level shortwave as well as increasing moist, southerly flow ahead of the approaching cold front will support broader coverage of storms and the chance for locally heavy rainfall throughout this region. This includes areas that remain more sensitive to flooding due to recent heavy rainfall as well as typical greater concern due to the more complex terrain through the mountains. There was also an expansion of the Marginal Risk to include portions of eastern Michigan due to ongoing storms (see WPC MPD #1110 for more info). In the western U.S., the forecast remains on track as a Monsoonal pattern persists with broad upper-ridging overhead. Only minor adjustments were made to the ongoing Marginal Risk, mostly to adjust the coverage over the central to northern High Plains based on the latest 12Z CAM guidance. There will likely be a focus for greater storm coverage through this region with a higher chance for some locally heavy rainfall. However, based on the expected rain rates (~ 0.5" or so per hour) and at least modest eastward movement away from the Front Range to limit duration, still expect that any flash flood risk will remain isolated. Putnam ...Previous Discussion... ....Eastern United States... The large scale pattern will continue amplify over the eastern United States with a deepening mid- latitude trough extending from Canada all the way into the eastern CONUS. A moisture-rich airmass in place early in the outlook period from the coastal Carolinas into the Mid-Atlantic region should start to get shunted eastward in response to the lowering mid-level heights of the amplifying upper trough. The larger scale models have been trending drier in this part of the country compared with the past few runs but the 22/00Z HREF/RREF and machine learning guidance continue to focus the potential for rainfall amounts in excess of 2 inches in 6-hours into this afternoon across parts of southeast Virginia and the southern Delmarva. Based on short term terms in satellite and radar...thinking is that rainfall rates could exceed an inch per hour but that the risk of flash flooding will be mitigated somewhat by the progressive nature of the storms afforded by the strengthening mid-leve flow. Confidence is growing that the focus for heaviest rainfall is south of areas hard-hit by flash flooding on Thursday. Elsewhere in the eastern US...maintained the Marginal Risk area over portions of the Ohio Valley and eastern Great Lakes given the proximity of a strengthening low pressure system at the surface and aloft. This pattern may allow for some lake-enhanced precipitation across portions of Ohio, Pennsylvania, and New York. Given recent heavy rains in the Ohio Valley. So there was little reason to make make more than minor changes to previously issued Marginal area given the potential for lingering sensitivities to flooding from additional precipitation on Saturday. ...West... Persistent ridging across the Western U.S. will finally begin to shift back over the Plains as a robust trough over the Pacific Ocean makes headway into the Pacific Northwest and begins pushing the pattern downstream and flattening the northern portion of the ridge in addition to increasing the moisture transport into the region by the end of the period. Pronounced low and mid level dry air in the subcloud layer due to the upper ridge will initially result in little rainfall based on forecast soundings for parts of Washington and northern Idaho this afternoon and early evening...although the influx of moisture with the robust trough should overcome that by late in the period. But elsewhere...the precipitable water anomalies suggest the monsoonal pattern will persist over the Desert Southwest and Great Basin into the Central Rockies. A broad Marginal risk over much of the Western U.S, east of the Pacific coast states will suffice. Bann Day 2 Valid 12Z Sun Aug 23 2026 - 12Z Mon Aug 24 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE CENTRAL US AND THE SOUTHWEST... ...Midwest... Energy approaching from the west begins to flatten the northern portion of the ridge...allowing for showers and thunderstorms to develop across the northern Plains which then builds southeastward towards the Ozarks. Even though flow aloft is from the northwest...the 850 mb flow maintains a plume of precipitable water values in excess of 1.75 inches from the Gulf coast northward into the eastern Dakota which roughly defines the ridge in the thetae field to help fuel locally heavy rainfall rates. And given the potential for repeat convection...maintained the Marginal risk area even though individual cells should remain progressive. ...Western US... The overall pattern is expected to remain relatively unchanged with the upper ridge remained anchored over the Southwest and southern Plains. Moisture and energy moving along the western and northern periphery of the ridge will contribute to daily shower and thunderstorm activity and the potential for isolated flash flooding extending from the Southwest to the northern Intermountain West and Rockies. Bann Day 3 Valid 12Z Mon Aug 24 2026 - 12Z Tue Aug 25 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE WESTERN US AND THE LOWER MISSISSIPPI VALLEY... ...Western US... An upper ridge will remain anchored over the Southwest-southern Plains ON Tuesday...with moisture and shortwave energy on the backside of a high contributing to diurnal showers and storms that will raise the potential for isolated flash flooding from parts of the Southwestern US into the Rockies. Only changes to the Marginal Risk area were minor and largely based on the precipitable water field. ...Lower Mississippi Valley... Maintained the Marginal Risk area which was introduced on Friday as a corridor of increasing instability persists due to a persistent confluent flow of Gulf moisture. At this point...model guidance would tend to suggest more of an urban concern. Bann Day 4 and Day 5 Valid 12Z Tue Aug 25 2026 - 12Z Thu Aug 27 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE SOUTHWEST AND INTERMOUNTAIN WEST TO THE CENTRAL AND SOUTHERN PLAINS, AND OVER PORTIONS OF THE SOUTHEAST TO THE TENNESSEE AND LOWER MISSISSIPPI VALLEYS... ...Southwest and Intermountain West to Central and Southern Plains... Maintained a Marginal Risk both days covering portions of Arizona and New Mexico northward through the Four Corners into parts of the central and northern Rockies as monsoonal moisture is expected to continue to bring scattered, diurnally-driven showers and thunderstorms, with isolated flash flooding possible. Showers and thunderstorms are likely to extend east of the Rockies into the Plains midweek as mid-level energy emanating from the West moves off of the top of the Southwest ridge and interacts with a pool of deepening moisture associated with a cold front dropping through the central U.S. With the guidance continuing to present some signal for potentially heavy amounts, a Marginal Risk was maintained over much of Kansas and Nebraska south of the Sandhills on Day 4. Additional rounds of storms are expected on Day 5 from the Rockies into the Plains as the series of mid-level shortwaves continue to move out of the West. As the front continues to press farther south, storms developing along the high terrain are expected to propagate southeast into southern High Plains into the axis of deeper moisture and greater instability. ...Southeast to the Tennessee and Lower Mississippi Valleys... Maintained a Marginal Risk covering portions of the Tennessee Valley and Southeast on Day 4 as mid-level energy sliding southeast into the region interacts with a pool of deepening moisture across the region. As moisture continues to deepen ahead of a cold front approaching from the north while northwesterly flow directs a series of mid-level shortwaves into the region, the footprint for heavy rainfall may expand and the potential for flash flooding may increase across the region and extend back into parts of the lower Mississippi Valley on Day 5. Pereira 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 $$