FOUS30 KWBC 020025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 825 PM EDT Sat Aug 1 2026 Day 1 Valid 01Z Sun Aug 02 2026 - 12Z Sun Aug 02 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDWEST, NEW MEXICO, AND THE GULF COAST... ...Portions of the Midwest... An anomalously strong upper low will continue to pivot slowly eastward, reaching central Indiana by Sunday morning. This closed low fosters divergence aloft to its east, while the accompanying occluded surface low and trailing cold front push into/through portions of the Ohio and Tennessee Valleys. There will be moisture to spare across the region, with precipitable water values approaching 2" and sufficient instability for heavy rain concerns. Across IL & IN an impressive deformation axis will exist N/NW of the mid- level low as it pivots E/SE, and will drive ascent into an impressive (especially for August 1) TROWAL as the accompanying theta-e ridge pivots cyclonically around the system. This will result not only as a region of increased instability and moisture, but also a greater training risk as propagation vectors and mean winds become increasingly anti- parallel. This suggests a high threat for backbuilding and repeating cells, through which the 1-2"/hr rain amounts could cause locally more than 5" of rain. This segment of the Slight Risk from continuity was maintained, but the previous portion to the south near the Cumberland Plateau was downgraded to a Marginal as any additional thunderstorm activity should be progressive. ...North-Central/Northeast Gulf coast... An approaching front backs the low-level flow to west to west- southwest. A deep layer cyclone to the north aids divergence aloft. With precipitable water values approaching 2.5" and ample instability moving ashore from the Gulf, hourly rain amounts to 3" are possible in this region. Both the 18z HREF/12z REFS indicate that heavy rain could fall quickly during the 06z-12z period, with local totals of 5-6" within the realm of possibility. Kept the Gulf Coast portions of the Slight Risk though extended it westward through Southeast LA in concert with the HREF/REFS guidance. The higher end Slight Risk area was maintained near Cedar Key. ...Southern Rockies/High Plains to the Texas Hill Country... Convection moving southward through New Mexico could cause heavy rainfall particularly in and near southeast portions of the state per 18z HREF/12z REFS guidance in the very short term, with activity waning after 05z. Both the Marginal and Slight Risks across the region were shrunk a bit from continuity due to convective propagation and the 18z HREF/12z REFS guidance. Until activity fades, hourly amounts to 2" should be possible locally. Roth Day 2 Valid 12Z Sun Aug 02 2026 - 12Z Mon Aug 03 2026 ..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FLORIDA BIG BEND REGION AND THE EASTERN GREAT LAKES... ...East Coast States... Widespread showers and thunderstorms are expected from the Gulf Coast of Florida through the Mid-Atlantic States, and into the interior Northeast on Sunday. An anomalously strong upper closed low (NAEFS 500mb heights down to the 1st climatological percentile and 850mb heights below the climatological minimum) will be positioned over IN Sunday morning. This feature will begin to open during D2 as a piece of vorticity energy swings through the base, but this will result in a still highly amplified/short wavelength trough digging all the way to the Gulf Coast. This evolution will help strengthen a meridionally arcing jet streak into the Northeast and Canada such that the favorable diffluent RRQ will overlap the broad height falls downstream of the opening trough. Together, this will create a large expanse of impressive synoptic ascent from the Gulf Coast all the way into New England. This ascent will act upon impressive thermodynamics as tropical moisture gets drawn northward ahead of the system. PWs as high as 2.25" are progged to flood northward as far as Massachusetts, with still impressive PWs above 1.75" as far west as the St. Lawrence River Valley. Together with a collocated plume of 1000+ J/kg MUCAPE, this will provide an environment for which thunderstorms will be widespread, and create numerous 1-2"/hr rain rates. Generally fast 0-6km mean winds of 20-30 kts should limit the duration of any individual cell in most areas, but this flow will additionally be aligned to an approaching cold front beneath the upper trough, suggesting the potential for repeating/training of cells. This has prompted a large MRGL risk expansion to encompass most of the eastern states, with isolated flash flood potential beneath any of these heavier rain rates. However, there appear to be two area of a more focused threat. The first is across the Florida Gulf Coast/Big Bend and just inland where heavy rain will occur again after heavy rain that is currently falling during D1. An additional 3-5" of rain is possible (30-50% chance of at least 5") due to backbuilding of cells across the Gulf repeating onshore with 2-3"/hr rates. Within this region, an enhanced slight risk was added for Taylor, Dixie, and Lafayette counties which is where the 2-day rainfall is expected to be heaviest. Another area of locally higher excessive rainfall risk exists along the eastern Great Lakes from around Cleveland, OH through Buffalo, NY and into the Northern Adirondacks. While instability may be a bit less here, this is also the region where a pivoting deformation axis and accompanying mid-level TROWAL may exist north of the closed low. This will produce some higher thermodynamics into which storms, which will be slow moving and training, can repeat with 1-2"/hr rain rates. The inherited SLGT risk was cosmetically adjusted for new guidance, and the urban areas will be most susceptible to impacts on Sunday. ..Southwest... The monsoon ridge begins to expand back to the east once again on Sunday, but activity is still expected to be suppressed to just southern AZ/NM. This will be partially due to a shortwave rotating anticyclonically around the base of the ridge to enhance ascent, impinging into areas with favorable thermodynamics characterized by PWs of 1,25 to 1.5 inches, and MUCAPE above 1000 J/kg. The challenge Sunday is that it appears there will be some drying that occurs from NE to SW as a decaying front drops across Texas, followed by a weak surface trough as well. This suggests that during peak heating, there will actually be somewhat of a deterioration of the environment, further limiting convective coverage. Still, scattered thunderstorms are likely for the southeast portion of AZ, with some more isolated activity as far east as the Sacramento Mountains of NM. With sufficient bulk shear of 20-25 kts for modest organization, any of these storms could produce locally 1"/hr rain rates and instances of flash flooding. Weiss Day 3 Valid 12Z Mon Aug 03 2026 - 12Z Tue Aug 04 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE TRI-STATE AREA OF NEW YORK, NEW JERSEY, AND CONNECTICUT.. ...United States East Coast... The impressive upper trough/low moving across the Ohio Valley is progged to cutoff once again as it digs into the Southeast by Tuesday morning. Downstream of this anomalous feature, robust PWs of more than +2 sigma will flood the Atlantic Seaboard, combining with a ribbon of elevated instability within the deep layer WAA to produce an environment supportive of heavy rainfall for nearly the entire coast. While specific locations for heavier rainfall and a greater excessive rainfall risk are difficult to discern at this time range, it appears nearly any location that received repeating showers and thunderstorms with 1-2"/hr rain rates could experienced impacts, so the inherited MRGL risk was adjusted for newer guidance. The narrow SLGT risk for the Tri-State area of CT/NJ/NY was curtailed a bit on the SW side (closer to Philadelphia, PA) due to a slightly more progressive/eastward signal in the QPF from the guidance. Did not remove the SLGT risk all together due to extremely vulnerable soils from recent heavy rainfall in this area, but if the trends continue for lesser/faster QPF, this SLGT may also be removed with later updates. Additionally, there may be the need for a small SLGT across the Big Bend of Florida where additional heavy rain may pivot onshore Monday after 2-days of heavy rain this weekend. However, there is just enough uncertainty as to the amount of rain lingering D3 that, after coordination with WFO TAE, opted to maintain just the MRGL risk for now and re-evaluate the need for an upgrade with later forecasts. ...Southwest... Convection associated with the monsoon will become even less suppressed Monday as the ridge expands even farther east, and weak vorticity impulses rotate around it. Moisture and instability will gradually lift northward during this time, allowing for an expansion north of an environment supportive of heavy rain. The limiting factor Monday appears to be instability moreso than moisture, so the inherited MRGL risk was curtailed a bit from the north, but at least isolated instances of excessive rainfall leading to flash flood related impacts are expected for parts of AZ/NM. ...Northern Plains... A potent shortwave diving along the Canadian/United States border will race eastward Monday bringing height falls into the Dakotas and Minnesota. Downstream of this feature, brief but pinched SW flow will draw moisture and instability northward to create an environment supportive of heavy rainfall, into which a cold front beneath the shortwave will create enhanced ascent for convective development. Strong 0-6km mean winds suggests storms will move rapidly northeast ahead of the front, but with flow aligned parallel to the boundary, at least short duration training could occur. As rainfall rates climb to 1+"/hr, any training could create locally excessive rainfall as reflected by the ECMWF EFI reaching above 0.7 coincident with low-end probabilities for 3"/24 hr rainfall. Weiss Day 4 and Day 5 Valid 12Z Tue Aug 04 2026 - 12Z Thu Aug 06 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, PLAINS, MIDWEST, & EAST... ...Upper OH Valley/Cumberland Plateau/East... There has been a substantial change to most of the model guidance at 12z, with the UKMET holding serve with a more northern solution to the mid- to upper-level low while the other guidance has trended southwest. Conceptually, the non-UKMET guidance makes the most sense based on the position/strength of the ridge forecast to move over the top of the feature. However, the concern is sensitivity over portions of NY from recent heavy rains. Taking into account continuity, NY's sensitivity, and the southwest trend of the guidance led to a bit of southwest broadening of the Marginal Risk area. No matter where the system is, some cool air aloft along with ample moisture and resultant instability could lead to hourly rain amounts up to 2.5", which would be problematic in rugged terrain, sensitive soils, and urban areas. Should the UKMET shift southwest towards the remainder of the guidance later, we can shave off the northeast side of the Marginal Risk area at that time. ...Southwest/Southern High Plains... For both Tuesday and Wednesday, a marginal risk was depicted for monsoonal convection across southern Arizona into New Mexico. Not a lot of confidence with qpf details here, but locally heavy rains and isolated runoff issues possible in the above average PW values expected across this area. ...Central Plains into the Mid to Upper MS Valley... A surface frontal boundary is expected to be slow-moving Tuesday and Wednesday from the Central Plains, east- northeastward into the Mid to Upper MS Valley into the Great Lakes. This front will be situated along the southern edge of the westerlies where shortwave energy pushing east northeastward near the front will support the potential for organized convection both days. PW values expected to be slightly above average...1.5 to 1.75"+ in the vicinity of this front, supporting the potential of hourly rain amounts to 2.5" where convection does organize and isolated runoff issues. For the moment, coverage/amounts appear shy of a Slight Risk, so let the Marginal Risk continue from continuity. Roth 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 $$