FOUS30 KWBC 031936 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 336 PM EDT Mon Aug 3 2026 Day 1 Valid 16Z Mon Aug 03 2026 - 12Z Tue Aug 04 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NEW JERSEY TO MASSACHUSETTS, BIG BEND OF FLORIDA, AND FAR SOUTHERN ARIZONA... ...16Z Update... The primary change to this ERO issuance was to remove the Slight Risk from the coastal Carolinas. Current activity is mainly offshore, and potential upcoming afternoon activity and another round tonight seem modest enough to be considered Marginal. Probabilities for QPF greater than 2" have been dropping. The Slight Risk over portions of Florida seems reasonable with ongoing convection over the next few hours before ending by this evening, as does the Slight in the Northeast (see MPD 933 for more). The monsoonal convection with a particular focus over southeast Arizona still looks good in the new CAMs, while the north-central U.S. Marginal was adjusted somewhat to account for new guidance. Rain is ongoing in the north-central U.S., which could wet soils before a possibly stronger convective round this evening and tonight. See the previous discussion below for more on the meteorological setup. Tate ...Previous Discussion... ...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... Weak energy aloft could undercut the broad Southwest upper ridge to the south and provide support for enhanced convection Tuesday. The peak instability of 1000-2500 J/kg MUCAPE will be in southern Arizona. Above average moisture transport (4+ standardized anomalies for 850-700 mb moisture flux from the GFS, 2+ from the ECMWF) will be in place as well, allowing for heavy rain rates above 1"/hour. CAMs including the Arizona WRF indicate slow moving/westward moving convection potentially yielding increased amounts. The Slight Risk still looks reasonable across south- southeast Arizona within a broader Marginal. Typical terrain features and burn scars will be particularly vulnerable to flash flooding concerns. ...Southeast to Southern/Central Appalachians into the Mid- Atlantic States... An upper low will meander over the Southeast/southern Appalachians Tuesday, which will be somewhat weak but still provide lower heights than normal for early August and broadly support ascent. Large areas of 1000-2000 J/kg MUCAPE (locally higher) are expected in the southeastern U.S., while above average PWs are expected at least in eastern areas. Scattered convection is likely with high hourly rain rates, so a Marginal Risk with limited edits remains in effect for portions of the Southeast, Appalachians, and southern Mid-Atlantic. Pockets of wet antecedent conditions could exacerbate flooding concerns, while rugged terrain and urban areas are also vulnerable. Florida should also see enhanced instability and moisture, but the greatest rain totals look to be across the Everglades, not a sensitive region for flooding. ...Central Plains into the Upper Midwest... A reasonably strong trough coming through the north-central U.S. will support a cold front slowly moving through the Midwest and Plains. Moisture and instability are forecast to pool along this cold front while the right entrance region of the jet provides ascent for rain/thunderstorms. The front should lay out a bit west- east by Tuesday evening/night and lead to a corridor of training convection. CAMs show axes of 3" totals with some minor spread in placement. Antecedent conditions are on the dry side in most areas, so this still looks like only a Marginal threat for flash flooding through Tuesday, but will continue to monitor trends for higher totals and agreement on the axis of heaviest totals for any potential Slight upgrade. Trimmed out the western area of the Marginal from the previous forecast due to less rain focusing there and falling over the Nebraska Sandhills. Tate 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 CENTRAL PLAINS TO MIDDLE MISSISSIPPI VALLEY... ...Central Plains to Mid/Upper Mississippi Valley... By Wednesday, the upper trough and front described in the Day 2 discussion will further slow down and provide an axis for moisture to pool and thunderstorms to be persistent. Particularly given some potential overlap with rain the previous day affecting antecedent conditions, will continue to indicate a Slight Risk. There has been more model spread in the position of heaviest rain by Wednesday. While thunderstorms should be active all along the front through the Midwest back toward the central Plains, some guidance like the EC-AIFS and AIGFS are indicating the central Plains to Mid- Mississippi Valley should see the greatest rainfall totals. Expanded the Slight Risk to account for this southwestward potential, but maintained the Slight in the Chicago area too given some models (GFS/UKMET/NAM etc.) still show heavy rain over that area. Further shifts may happen in future cycles before models hopefully converge. ...Southern to Central Appalachians into the Northern Mid- Atlantic... Whatever is left of the upper level trough across the eastern U.S. should devolve into a shear line by midweek. However, a moist and unstable environment (PWs greater than the 75th percentile in an already moist season of the year) will continue and expand northwestward as the upper energy shifts. This should keep supporting locally heavy rain in scattered thunderstorms, with high rain rates, so a Marginal Risk still looks good. Any areas seeing repeated rounds of rain from the previous couple of wet days would be particularly sensitive to flooding concerns. Convection may once again be enhanced over portions of the Florida Peninsula, but given a likely rainfall focus over the less sensitive areas of the Everglades westward, will not introduce a Marginal at this point. ...Southwest/Southern High Plains... On Wednesday, moist inflow on the southern part of the Southwest upper high will persist, though moisture transport anomalies seem to be a touch lower than the Monday/Tuesday values and with less of a focus for heavy rain placement. Therefore a Marginal Risk seems reasonable for isolated flooding and runoff issues for much of Arizona and New Mexico. Tate 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 THROUGH THE CENTRAL/SOUTHERN PLAINS, MIDWEST, & EAST... ...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 late this week. Shortwave energy expected to push east along the southern edge of the Westerlies, 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, with hourly rain amounts up to 2.5" possible, supporting at least isolated runoff issues in urban areas and places with some degree of soil saturation. ...Southern to Central Appalachians... For late this week, a broad marginal risk is depicted through the Central to Southern Appalachians for widespread scattered convection. Not a lot of confidence in details, but with PW values expected to remain in the 1.5-1.75"+ range, hourly rain amounts to 2.5" could lead to runoff issues, particularly in urban areas and within sharp relief. ...Southwest/Southern High Plains... For late this week, a Marginal Risk was depicted across much of the Southwest and Southern Rockies for monsoonal convection. The ridge placement across the southern Great Basin during the time frame along with some backing off in the moisture return on the guidance led to some reduction in the Marginal Risk area across southern NV. This led to less reliance on the 00z/12z ECMWF for the QPF breadth, which was collaborated with VEF/the Las Vegas NV forecast office. While there is continued low confidence with details, locally heavy rains with hourly amounts up to 2" could lead to isolated runoff issues. Roth/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 $$