353 FOUS30 KWBC 230051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Sat Aug 22 2026 Day 1 Valid 01Z Sun Aug 23 2026 - 12Z Sun Aug 23 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE ROCKIES INTO PORTIONS OF THE SOUTHWEST AND GREAT BASIN, ALONG WITH THE MID ATLANTIC TO THE EASTERN GREAT LAKES... 01Z Update: Notable changes were made to the Eastern CONUS outlook areas. Weak mid-level lapse rates and thus negative dCAPE/dT following sunset will limit the overall intensity of convection later this evening and overnight, though favorable deep-layer forcing (increasing upper difluence) ahead of the longwave upper trough as it becomes more neutrally-negatively tilted will likely maintain organized convection via multiple quasi-linear segments, especially east of the Chesapeake Bay given the warm water temperatures (>27C). We did remove the Slight across the Lower Eastern Shore given the sandy soils and more limited risk for cell training after 01-02Z. 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. Hurley/Putnam 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... ...2030Z Update... A Marginal Risk was added along the shores of Lake Erie from northeastern Ohio into western New York due to expected lake- enhanced showers after an initial round of locally heavy rainfall today. The 12Z CAM guidance now available through the full day 2 period shows the potential for isolated occurrences of 2-4" of additional rainfall, especially for northwestern Pennsylvania and into western New York. Elsewhere, some modest areal modifications were needed across portions of the Plains to the Lower Mississippi Valley for northwesterly flow storms and with Monsoonal activity over the western U.S. Two main genesis locations were noted in the Plains portions of the outlook: the first to the north over the Dakotas and the second to the south over Kansas/Oklahoma southeast through the Ozarks and into the Lower Mississippi Valley. The 12Z CAMs were more aggressive with storm coverage/rainfall amounts with the storms developing over the Dakotas and then progressing southeast into the Missouri Valley, likely due to support from a more prominent shortwave and developing low level jet into the overnight hours. Some of the CAMs show the potential for locally heavy totals in the 2-4"+ range which could result in more than just a few isolated instances of flash flooding. However, due to the expected progressive nature of the storms as well as uncertainty with the degree to which they organize/grow upscale and eventually track, have maintained a Marginal Risk for now. Putnam ...Previous Discussion... ...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... ...2030Z Update... Only very minimal areal adjustments were made to the inherited Marginal Risk over the western U.S. due to persistant Monsoon activity and continued risk of some isolated flash flooding. As was noted below, there is lower confidence in the Marginal Risk over the Lower Mississippi Valley. The latest 12Z model guidance trended downward on QPF amounts/coverage. The rain that falls during the day 3 period currently looks to be mostly tied to ongoing, potentially organized convection from the day 2 period moving southeastward through the risk area. Given the uncertainty on storm coverage/organization at this time frame, have decided to maintain the Marginal Risk for now at least until more guidance from the CAMs come in. Putnam ...Previous Discussion... ...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... 2000Z Update: Minor changes were made to the Days 4-5 EROs based on the latest guidance trends (including the latest AI GFS and ECMWF runs). Still a great deal of spread in the QPF guidance, including the GEFS, ECENS, and CMC ensemble QPF exceedance probabilities, and as such did not make any drastic changes to the Marginal Risk areas. Hurley ...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 $$