FOUS30 KWBC 210847 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 447 AM EDT Tue Jul 21 2026 Day 1 Valid 12Z Tue Jul 21 2026 - 12Z Wed Jul 22 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST AND GREAT BASIN AS WELL AS FOR PORTIONS OF THE OHIO AND TENNESSEE VALLEYS AND INTO THE NORTHEAST... ...Southwest and Great Basin... The combination of anomalous monsoonal moisture streaming northward out of the tropical Pacific and a retrograding upper level shortwave will create a very active day for thunderstorm activity this afternoon across much of the Pacific Northwest. Moisture amounts will be anywhere from 2-4 sigma above normal, with PWATs above 1.5 inches in southern Arizona, and above 1 inch across much of the rest of the Southwest. The upper level disturbance tracking west out of New Mexico will force numerous thunderstorms to form across Arizona. The storms will slowly track westward. Greater concentrations of storms are expected along the Mogollon Rim where topography will help allow the storms to congeal along the Rim. Due to the above normal amounts of moisture, the storms will be capable of heavy rainfall with the strongest cores possibly producing rainfall rates to 2 inches per hour. Given the very favorable hydrology of the Desert Southwest for flash flooding, given generally very hydrophobic soils and bare rocks, the Slight Risk remains in place across much of Arizona, with a higher-end Slight stretching from the southeast corner of the state northwestward along the Rim to the Grand Canyon. As usual, slot canyons, dry washes, and other low-lying areas will be more susceptible to impactful flash flooding downstream of the heaviest rains. Further north across the Great Basin, the area will be on the western periphery of the large ridge over Colorado and a nearly stationary longwave trough off the coast of northern California. A separate shortwave trough will track across Nevada and into Idaho this afternoon. Given similarly anomalous amounts of atmospheric moisture in place as well across this largely flash-flood prone area from the Sierras east through Utah, clusters of storms will form in this area associated with the upper level shortwave. The inherited Slight was expanded west to include the Sierras, where upslope will greatly contribute to afternoon and evening thunderstorm activity there, while the steep slopes will support fast-forming flash flooding downstream of any heavy rain producing thunderstorms. Elsewhere, few changes were needed, as many of the areas in the Slight that will be impacted by storms again today were impacted on previous days, worsening the flash flooding risk. ...Ohio and Tennessee Valleys into the Northeast... Much of the flooding risk across the eastern half of the country today will be in association with a strong cold front at the leading edge of a strong longwave trough over the Great Lakes pushing south and east across the Ohio and Tennessee Valleys as well as into the Northeast. Anomalous moisture originating in part from T.S. Bertha in the Gulf will ride northeastward ahead of the front, providing plentiful moisture for numerous thunderstorms to form, some of which will produce multiple-inch-per-hour rainfall rates. A few areas within the broader Slight Risk are likely to see more rainfall. In central Kentucky and central Tennessee, a southwest-to-northeast oriented band of storms is expected to form on the leading edge of the cold front, with mesoscale "slow-downs" in the front's progression allowing for a second round of activity possibly forming over these same areas where FFGs are depressed from heavy rainfall in recent days. The greatest rainfall totals are expected to be across portions of southeastern Virginia, generally from the Richmond metro east across the Bay and impacting the lower Maryland and Virginia sections of the Delmarva. Here, a plume of greater moisture will interact with the front which will be slowed by its passage across the mountains. An early to mid afternoon round of storms with the initial approach of the upper level trough and associated shortwave energy will move across southern Virginia first. Much more numerous storms will impact the area in the evening hours as the main front approaches the area. Despite the loss of daytime heating, the front is expected to stall over southern Virginia, which could very well allow for a prolonged period of training storms to set up in the aforementioned area, which will persist through the overnight. A short-fused Moderate Risk cannot be ruled out should this evolution play out as expected by several of the CAMs. Finally, across the area form northern New Jersey northeast into the Hudson Valley, morning thunderstorms caught along the Catskills and Poconos associated with a warm front may start out the day with a period of training thunderstorms as moisture and instability levels increase south of the front. This will likely be the feature that causes the most rainfall in the area. Once the fast-moving cold front sweeps eastward across the area, resulting in a much shorter period of widespread thunderstorm activity, its passage will end the threat for heavy rain in this area for the night. ...Gulf Coast/Tropical Storm Bertha... As has been the case for several days, the convective pattern with Bertha in the guidance is little changed. Northeasterly shear will continue to keep any persistent heavy rainfall well south of the Gulf Coast for the next several days. Only small and isolated bands of rain trying to wrap around the center will impact the coast. The inherited Marginal Risk remains little changed from Louisiana through the Florida Panhandle. Wegman Day 2 Valid 12Z Wed Jul 22 2026 - 12Z Thu Jul 23 2026 ...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE INTERMOUNTAIN WEST AS WELL AS FROM NORTHERN GEORGIA THROUGH THE TIDEWATER OF VIRGINIA AND NORTH CAROLINA... ...Intermountain West... Highly anomalous amounts of atmospheric moisture associated with the monsoon over the Intermountain West will be activated by rapid height falls as an upper level and highly negatively tilted trough pushes east from off the coast of Washington and Oregon. Meanwhile, separate upper level energy undercutting the ridge will also provide additional forcing further east across eastern Idaho and Wyoming. Once again, numerous thunderstorms will form in the anomalous moisture and for many areas, will be tied to the local topography. Few changes were needed with this update. The greatest amounts of rain look to concentrate in western Wyoming, where any storms south of Yellowstone could necessitate a higher-end Slight for some of those areas, as well as for the Uintas of northern Utah. With nearly all of the forcing further north, the Marginal Risk into southern Arizona and southern New Mexico was trimmed significantly with this update, as ridging quickly overtakes this area and limits convective potential south/west of the Mogollon Rim as a result. ...Carolinas, Southern Virginia, and Northern Georgia... A strong cold front running into highly anomalous moisture being siphoned from the circulation of Tropical Storm Bertha will exacerbate the gradient of moisture along the frontal interface through the day across the Carolinas and into southeastern Virginia. This will work to keep any forward progress of the front minimal. Storms will train along the front, especially across northeastern North Carolina and into southeastern Virginia. This area remains in a higher-end Slight Risk and with the stalled out front remaining to guide the frequent development of additional storms, a Moderate Risk upgrade in areas where the storms focused in the Day 1 period remains a distinct possibility. Guidance has been laser focused in this area and so forecast changes have been minimal. Further south and west, however, guidance has become increasingly optimistic on storms developing further west into the southern Appalachians. As such, the Slight Risk was expanded west to include this region from north of Atlanta through the western Carolinas. ...Tropical Storm Bertha/Gulf Coast... Strengthening shear across the circulation will continue to keep any organized convection and meaningful rainfall out over the open waters of the Gulf. The immediate coastline could still see amounts greater than an inch, but the rainfall footprint continues to shift off the coast. In coordination with LIX/Slidell, LA forecast office, the inherited Slight Risk for this region was downgraded with this update, with the overall area of the surrounding Marginal remaining little changed. What little rainfall does impact land areas remains with the potential to be locally heavy, so unfortunately no changes to the Marginal Risks can be made so long as Bertha remains a tropical entity. Wegman Day 3 Valid 12Z Thu Jul 23 2026 - 12Z Fri Jul 24 2026 ...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL FOR A PORTION OF THE SOUTHEAST FROM GEORGIA THROUGH THE CAROLINAS AS WELL AS FOR NORTHEAST KANSAS AND SOUTHEAST NEBRASKA... ...Southeast... The stalled out front that has remained across the Southeast will continue to plague the area with very heavy rainfall, as the continuous stream of Gulf moisture from the Gulf funnels northeastward along the front. Yet another day of heavy rain is expected to impact the Carolinas as a result. The front may drift ever so slightly further south and east which should clear all but the Hampton Roads area of far southeast Virginia from the heavy rainfall threat by Thursday, but for much of eastern North Carolina, much of South Carolina, and portions of northern Georgia, generally from Atlanta east, the heavy rainfall threat will continue through Thursday. Once again, potential for very heavy rainfall totals in many areas as are expected to see heavy rain from Wednesday could mean further upgrades to the risks may be needed, especially across eastern North Carolina. Ultimately, where the heaviest rain falls both today and Wednesday should dictate if further upgrades will be needed. ...Central Plains... An MCS is expected to impact a portion of northeast Kansas, southeast Nebraska and the Kansas City and St. Joseph metros on Thursday. The heavy rainfall footprint has drifted south and west, so the Slight Risk has been adjusted accordingly. As with many MCS's the location of the heaviest rain is likely to continue shifting with future updates, most likely towards the south, so stay tuned for further updates. Periods of heavy rain may continue to impact Wyoming into Colorado as well, where upgrades to a Slight are also probable, depending on where the heavy rains from Wednesday overlap with Thursday's forecast. ...Bertha... Bertha, who may no longer be a tropical cyclone by Thursday, and likely to be in a weakening state, should continue to keep any associated rainfall off the coast. The immediate coast remains with the best chance for seeing anything over an inch of associated rainfall. FFGs are significantly higher than that in all areas. Given this, and with very good consistency in this scenario in the guidance, the inherited Slight and Marginal for a portion of the Texas and Louisiana Gulf coasts was removed with this update. The Marginal Risk has been downgraded to under a 5% risk for the upper Texas Gulf Coast, as well as the Slight for coastal Louisiana downgraded to a Marginal with this update. Wegman Day 4 and Day 5 Valid 12Z Fri Jul 24 2026 - 12Z Sun Jul 26 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSOURI VALLEY AND MID MISSISSIPPI VALLEYS ON DAY 4 AND FOR PARTS OF SOUTHEAST COAST DAYS 4 AND 5... ...Southeast... A multi-day rainfall event is expected to continue into the weekend, with additional heavy rains likely, especially for parts of southeastern Virginia and the Carolinas. The focus for these rains will be a slow-moving, moisture-laden front that is forecast to linger across the region through at least the first half of the weekend. Southwesterly flow ahead of a shortwave trough moving through the Ohio/Tennessee valleys will further fortify the already moisture-rich airmass, with models showing PWs increasing in excess of 2.25 inches (+2 to 3 sigma). This moisture, along with pockets of enhanced ascent afforded by mid-level energy, will support periods of heavy, highly-efficient rainfall. While areas of heavy rainfall are likely, forecast confidence regarding their placement remains limited due to continuing model spread with regards to the placement of the front, which will have impacts on where the heaviest amounts occur. ...Gulf Coast... Northeasterly shear and dry air north of the center should keep much of the heavy rainfall threat associated with T.S. Bertha out over the open waters. Also, there is some question regarding the intensity of the system and if it will retain a closed circulation as it moves from the Louisiana onto the upper Texas coasts early in the period. However, cannot not rule out an isolated threat for heavy rainfall and therefore maintained a Marginal Risk for now from the southwestern Louisiana to the upper Texas coasts on Day 4. ...Lower Missouri-Mid Mississippi to the Tennessee Valleys/Southern Appalachians... Ongoing storms associated with pooling moisture along a low-level boundary and ahead a mid-level shortwave dropping southeast may continue to support storms and the threat for heavy rains Friday morning (Day 4) across parts of the lower Missouri Valley. Storms are expected to wane by late in the morning, but there remains a model signal for new storms developing farther to the south and east as additional energy interacts with deepening moisture (PWs increasing to 2-2.25 inches) that will be supported by southwesterly low level inflow. As this energy spills farther south, it may help to organize heavy rainfall across portions of the Tennessee Valley into the southern Appalachians, where a Marginal Risk was introduced for Day 5. Forecast confidence was limited by model spread, however this may be an area where a targeted upgrade is required in a future update if confidence increases. ...Southwest/Great Basin/Central and Southern Rockies... After what is expected to be a downward trend in convective coverage and potential flash flooding late in the week (Day 4), moisture anomalies are forecast to increase again on Saturday. This increasing moisture along with mid level energy moving around the southwestern periphery of a broad high will help expand afternoon and evening storm coverage, including the southern Arizona deserts. 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 $$