126 FOUS30 KWBC 030809 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 409 AM EDT Mon Aug 3 2026 Day 1 Valid 12Z Mon Aug 03 2026 - 12Z Tue Aug 04 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NEW JERSEY TO MASSACHUSETTS, FOR THE COASTAL CAROLINAS, THE BIG BEND OF FLORIDA, AND FAR SOUTHERN ARIZONA... ...East Coast... A shortwave trough rotating through the longwave feature will help close off a mid-level low center over northern GA, driving subtle height rises to its north, but with subtle impulses continuing to rotate cyclonically around it. Meanwhile, a jet streak pivoting into New England will intensify as it arcs poleward, leaving its favorable RRQ for ascent over the Mid- Atlantic/Northeast. This setup supports broad deep layer ascent that will help trigger widespread convection capable of heavy rainfall intensities. The slow nature of the front will help organize some convection along it. This convergence combined with bulk shear of 20-25 kts will lead to clusters/multi- cells that will train thanks to boundary- parallel mean flow to lengthen the duration of heavy rain rates. With both HREF and REFS probabilities indicating a 30-60% of 1"/hr or more rates, where any convection can organize or train, this could produce 3-5" of rain. The most likely locations for this heavier rainfall will be from southern New Jersey to central Massachusetts, which is where the Slight Risk was maintained. ...Southwest... A shortwave tracking beneath the monsoon ridge will squeeze the ridge causing briefing lowered heights over southern Arizona, while the ridge itself expands a bit farther east into the Four Corners and Central High Plains. Although instability will be somewhat limited across the region, the areal coverage of convection will be increased for this period. Higher concentration of heavy rainfall is expected to focus over southern Arizona where a plume of PWs exceeding +1.5 sigma will work together with MUCAPE as high as 2000 J/kg and a pocket of enhanced bulk shear of 25-35 kts to organize thunderstorms with hourly rainfall reaching or exceeding 1.5" (HREF/REFS/UA WRF). A Slight Risk was maintained for southern Arizona with a Marginal Risk covers southern and central portions of Arizona and New Mexico. ...Northern Plains... Showers and thunderstorms are expected to fire up across the Northern Plains as a potent shortwave tracking from Montana to Minnesota drags a wave of low pressure and its associated cold front across the region. A steady stream of moisture will be drawn northward ahead of the approaching system and will aid in the enhance of rainfall. Hourly rain rates have the potential (10-15% chance) to reach 2"/hr, this training/repeating could produce as much as 3-4" of rain, leading to FFG exceedance reaching 5-25% from the HREF/REFS. A Marginal Risk covers the eastern Dakotas and northern half of Minnesota. Campbell/Weiss Day 2 Valid 12Z Tue Aug 04 2026 - 12Z Wed Aug 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN ARIZONA... ...Southwest/Southern High Plains... Shortwave energy rotating around the base of the ridge will briefly suppress it and cause lowered heights. The combination of this locally intense ascent into favorable thermodynamics will again result in scattered to widespread thunderstorms, with some organization possible through 25-35 kts of bulk shear. Slow storm motions and heavy rain rates above 1"/hr will produce at least a local flash flood risk over vulnerable terrain features and burn scars from southern Colorado through southern Arizona, with the greatest concentration across south-southeast Arizona with a Slight Risk area was maintained for this period. ...Southern/Central Appalachians into the Mid-Atlantic States... Cool air aloft along with ample moisture and instability could lead to hourly rain amounts up to 2.5 inches, which would be problematic in rugged terrain, sensitive soils, and urban areas. Recent rain will keep soils sensitive for potential flooding issues. The Marginal Risk area was maintained across the eastern Tennessee Valley/Southern Appalachians to the Mid-Atlantic coastline. ...Central Plains into the Upper Midwest... Showers and thunderstorms will fire up along and ahead of a slow moving cold front sinking southward into the Central Plains and extending into the Upper Midwest. PW values up to 1.75 inches will be drawn into the system. Mean winds parallel to this front and a shortwave tracking along it will help to spawn training thunderstorms with rainfall rates of 1+"/hr, leading to a narrow corridor of as much as 3" of rain which may result with instances of flash flooding. Campbell/Weiss Day 3 Valid 12Z Wed Aug 05 2026 - 12Z Thu Aug 06 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID TO UPPER MISSISSIPPI VALLEY/MIDWEST... ...Mid to Upper Mississippi Valley/Mid West... Heavy rainfall will continue to spread along and ahead of the cold front sinking southeastward from the Upper MS Valley/Upper Lakes region. The presence of a strong upper level jet within the right entrance region will foster local enhancement of rainfall rates potentially up to 2.5 inches/hour in proximity to the boundary. The latest guidance continue to focus the highest axis of QPF southeast Nebraska to southern Wisconsin/ eastward into northern IL including the Chicago metro area. ...Southern to Central Appalachians into the Northern Mid- Atlantic... Whatever is left of the upper level trough across the eastern U.S. mid to late week should devolve into a shear line. An axis of high PW values...1.5-1.75"+, will remain across much of the eastern U.S. High PW values will continue to transport northward into the region which will support locally heavy rain, with potential for hourly rates reaching 2.5 inches/hour. A Marginal Risk area was maintained across the Southern to Central Appalachians into the northern Mid-Atlantic for this period. Much of this region will be sensitive given multiple days of rainfall and lowered FFG values. ...Southwest/Southern High Plains... A Marginal Risk was maintained to cover persistent monsoonal activity across the Southwest and Southern Rockies where heavy rain and isolated runoff issues in the above average PW values are expected. Campbell/Roth Day 4 and Day 5 Valid 12Z Thu Aug 06 2026 - 12Z Sat Aug 08 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHWEST, EAST NORTHEASTWARD THROUGH THE CENTRAL/SOUTHERN PLAINS, MID MS VALLEY, OH VALLEY, NORTHERN MID ATLANTIC AND ACROSS THE CENTRAL TO SOUTHERN APPALACHIANS... ...Central Plains into the Lower Missouri Valley... By Thursday...additional shortwave energy moving along the southern edge of the Westerlies into the Central High Plains may ignite another round of organized convection along the southward moving cold front from KS into MO. With low level southerly flow expected to strengthen Thursday night into early Friday morning into this front, locally heavy rains are possible. Given the moisture available, hourly rain amounts to 2.5" should be possible. The risk level near the front over the Central Plains toward the Lower MO Valley was kept at Marginal for Thursday night/early Friday morning. ...Lower Lakes, OH Valley into the Northern Mid Atlantic... A slow moving frontal boundary expected to persist across the Lower Lakes into the Northeast both day 4 and 5. Shortwave energy expected to push east both day 4 and 5 along the southern edge of the westerlies across these regions, helping to accentuate lift in the high PW axis in the vicinity of this front. There is fairly good model qpf agreement on moderate to locally heavy precip totals across these areas, supporting isolated runoff issues. ...Southern to Central Appalachians... For both day 4 and day 5, a broad marginal risk is depicted through the Central to Southern Appalachians for widespread scattered convection. Not a lot of confidence in qpf details, but with PW values expected to remain in the 1.5-1.75"+ range, locally heavy rains and runoff issues possible. ...Southwest/Southern High Plains... For mid to late week, a Marginal Risk was depicted across much of the Southwest and Southern Rockies for monsoonal convection. While there is continued low confidence with qpf details, locally heavy rains and isolated runoff issues possible in the above average PW values expected across this area. 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 $$