FOUS30 KWBC 021919 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 319 PM EDT Sun Aug 2 2026 Day 1 Valid 16Z Sun Aug 02 2026 - 12Z Mon Aug 03 2026 ..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FLORIDA BIG BEND REGION, COASTAL NORTH CAROLINA, AND THE EASTERN GREAT LAKES INTO THE NORTHERN/CENTRAL APPALACHIANS... ...East Coast States... Anomalous upper low will continue to plague the eastern portions of the CONUS with broad and impressive synoptic ascent impinging into extreme thermodynamics. Morning 12Z U/A soundings from a variety of sites in the east measured PWs from 1.8 to 2.2 inches, well above the 90th percentile, and with additional moistening expected through today, a broad plume of +2-+2.5 sigma PWs are anticipated to stretch from Florida to New England. This will provide an environment favorable for heavy rainfall within convection almost anywhere from Florida to Upstate NY. The high-res CAMs support this idea also, with widespread convection continuing through the day, and isolated extreme rainfall events with small spatial coverage of 4-6 inches of rain, similar to what occurred near Mt Airy, NC yesterday, are possible within the inherited broad MRGL risk, and if this occurs along the I-95 corridor between Washington, D.C. and New York, more substantial impacts could occur. While the forcing will be generally broad and diffuse, a few focused areas of heavier rainfall and greater flash flood potential exist. The first is along the Florida Panhandle and into the Big Bend. Not only is this area saturated from heavy rainfall on Saturday, but pronounced onshore flow from the Gulf drawing impressive PWs/CAPE onshore ahead of a cold front will again result in repeating rounds of thunderstorms with 2-3"/hr rain rates which has a 40-50% chance of creating more than 5" of rain (HREF and REFS probabilities). This heavy rain falling atop saturated soils will likely lead to rapid runoff and instances of flash flooding, so the inherited SLGT (and enhanced SLGT) was only cosmetically adjusted. Farther north into the eastern Great Lakes and Central Appalachians, the combination of a TROWAL/deformation axis and vulnerable soils also create a more robust excessive rainfall risk today. While SBCAPE into the Great Lakes may be somewhat more limited, the theta-e ridge rotating into a TROWAL will sufficiently enhanced instability to support 1+"/hr rain rates with slow/backbuilding motions. This will support a stripe of 3-5" of rain from near Cleveland, OH through Buffalo, NY and into far northern Upstate NY, where the SLGT risk continues. A bit farther south into the Appalachians any periods of heavy rain atop vulnerable terrains/soils will also enhance the risk today. Finally, A SLGT risk exists for extreme coastal NC where a coastal band of heavy rain may develop. The guidance varies with how far onshore this heavy rain may pivot, but if this band does move onshore (generally Cape Fear to Cape Hatteras) it would suggest pronounced training of 2"/hr rain rates. Most of this area has been dry outside of far eastern NC and the Outer Banks, and 3-hr FFG is generally 3-4" which has only a very minor threat of exceedance from the HREF. Due to the potential for training if this band comes onshore, left the SLGT in place but made some minor cosmetic adjustments. ...South Texas... A cold front dropping into South Texas is the focus for slow moving thunderstorms this morning. The environment downstream of this front is favorable for heavy rain rates with PWs of +2 sigma overlapping 2000-3000 J/kg MUCAPE. The CAMs are not extremely excited with this event, but may be under-representing the potential as the ingredients remain in place as the front stalls before weakening. Rainfall rates of 2"/hr are likely at times, which despite a slow southern advance, could produce 2-4" of rain and isolated flash flooding. ..Southwest... As the monsoon ridge expands a bit northeast today, convection will again develop across the Southwest. While the past few days have featured generally scattered convection focused in AZ/NM, a surge of instability peeking into CO may allow for a northward expansion of thunderstorm activity today. For this reason, the inherited MRGL risk was expanded a bit northeast for rainfall rates that may exceed 1"/hr at times, especially in the presence of enhanced 25-35 kts shear which could organize cells into clusters across the area. Weiss Day 2 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, AS WELL AS FOR THE COASTAL CAROLINAS, THE BIG BEND OF FLORIDA, AND FAR SOUTHERN ARIZONA... ...East Coast... The anomalous upper trough plaguing the eastern 1/3 of the CONUS will persist on Monday, but with a different evolution. 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. At the same time, a jet streak pivoting into New England will intensify as it arcs poleward, leaving its favorable RRQ for ascent over the Mid- Atlantic/Northeast. Together, this again creates broad deep layer ascent along most of the East Coast, acting upon robust thermodynamics to create widespread thunderstorms with heavy rain rates. While once again the forcing will be somewhat diffuse and broad, leading to widespread scattered convection as reflected by most of the available high-res CAMs. Locally enhanced ascent along a slow eastward-advancing cold 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 are across the Tri-State area of CT/NY/NJ, and down towards Philadelphia, extreme eastern North Carolina, and the Florida Big Bend region. Inherited SLGT risks across these focused areas remain both due to the potential organized convection on Monday, but also since all three have already vulnerable soils due to recent heavy rain leading to more compromised FFG. ...Southwest... A SLGT risk has been added for the Tucson area of Arizona after reviewing the latest computer guidance. A shortwave tracking beneath the monsoon ridge will squeeze the ridge causing briefing lowered heights over southern AZ, while the ridge itself expands a bit farther east into the Four Corners and Central High Plains. This will allow for some increased coverage of convection on Monday, despite somewhat limited coverage of instability exceeding 500 J/kg. However, for southern AZ, 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). This rain falling top low FFG will be sufficient to cause scattered flash flooding in southern Arizona despite modest storm motions, and both the HREF and REFS FFG exceedance reach 25-40%. The inherited MRGL risk surrounding this SLGT was also adjusted slightly and expanded a bit to the northeast. ...Northern Plains... Potent shortwave tracking from Montana to Minnesota Monday will drag a wave of low pressure and its associated cold front across the region. Downstream of this front, 850mb inflow will back to become more southerly within the warm sector and surge to above 30 kts, drawing robust PWs and MUCAPE northward to supply thermodynamics favorable for heavy rainfall. As thunderstorms develop along the front, and within the locally enhanced ascent in the vicinity of the low pressure, they will likely organize into clusters or supercells thanks to impressive bulk shear exceeding 60 kts, and these cells will then repeat along the front. With rain rates possibly (10-15% chance) reaching 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. Weiss Day 3 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... A SLGT risk was added for Tuesday for parts of southeast Arizona near Tucson, AZ. This is due to another day with shortwave energy rotating around the base of the ridge to 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. Storm coverage may not be as significant Tuesday as expected Monday (possibly due to lingering CINH/clouds), but 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 CO through southern AZ, with the greatest concentration across the TWC CWA prompting the new SLGT risk area. ...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. There is some overlap with areas that had rainfall during the Day 1 and 2 period, thus maintaining soil sensitivity for potential flooding issues, and the updated ERO features just some small cosmetic adjustments from the inherited MRGL risk area. ...Central Plains into the Upper Midwest.. A slow moving cold front dropping into the Central Plains and extending into the Upper Midwest will be a focus for convective development on Tuesday. As this front drops southeast, it will become embedded in flattened and diffluent mid- level flow on the eastern edge of the mid-level ridge, causing it to stall as 850mb inflow increases to 20-30 kts pushing PWs to as high as 1.75 inches along it. 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. Weiss/Campbell Day 4 and Day 5 Valid 12Z Wed Aug 05 2026 - 12Z Fri Aug 07 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... There is fairly good model qpf agreement on heavy precip potential along and ahead of the cold front sinking southeastward Wednesday from the Upper MS Valley/Upper Lakes region. Favorable right entrance region jet dynamics along the southern edge of the Westerlies should enhance divergence aloft/upward vertical motion in an axis of 1.75"+ PW values in the vicinity of this front. Hourly rain amounts to 2.5" should be possible. There is a good chance of organized convection along and ahead of the front Wednesday night into the early hours of Thursday, with potential for localized runoff issues. The slight risk area was drawn to encompass the model qpf axis consensus from southern IA/far northern MO, eastward into northern IL/far southern WI, including the Chicago metro area. Only small tweaks were made based on the 12z guidance. ...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/Thursday night, maintaining reasonable continuity. ...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. This will support additional widespread scattered showers in this high PW axis, with locally heavy rains and runoff issues possible. Hourly rain amounts to 2.5" should be possible within this air mass. A Marginal Risk was continued mid to late week across the Southern to Central Appalachians into the northern Mid- Atlantic, with some northeast extension made based on broader model spread in the heavy rainfall pattern introduced by the 12z GFS guidance. After several days of locally heavy rains during the short range period, FFG values will likely be lower/soils should see some level of saturation through the Southern to Central Appalachians into the Northern Mid Atlantic. This may warrant a higher risk level across these regions with future updates, whenever model spread narrows. ...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 details, but locally heavy rains and isolated runoff issues possible in the above average PW values expected across this area. 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 $$