Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 02 2025 00:51:49 FOUS30 KWBC 020050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 850 PM EDT Fri Aug 1 2025 Day 1 Valid 01Z Sat Aug 02 2025 - 12Z Sat Aug 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CAROLINAS, NORTH/NORTHEAST TEXAS, PARTS OF THE CENTRAL AND SOUTHERN HIGH PLAINS, AND THE DAKOTAS... ....Southeast... Scattered to widespread thunderstorms are expected today with daytime heating broadly across the Southeast along and ahead of a relatively strong cold front for mid-Summmer. Plentiful Gulf moisture (mid-70s dewpoints) and instability (1000-2000 J/KG ML CAPE with long, skinny profiles) will promote efficient rain rates of 2"+ per hour. The lack of bulk shear will lead to mostly shorter- lived outflow dominant storms, but given the high rain rates a quick 1-3" could lead to some isolated flash flooding issues. However, a focused corridor of more widespread, better organized thunderstorms is forecast across portions of the coastal Carolinas west into the southern Appalachians in vicinity of a frontal wave and in the presence of slightly stronger bulk shear. Additionally, a bit more anomalous PWATS (2-2.5 standard deviations above the mean) will make it more likely that downpours with rain rates higher that 2" per hour will be realized. Mean flow parallel to the frontal boundary will also bring the potential for a few repeated rounds/training of thunderstorms, encouraging locally heavier rainfall totals. HREF probabilities of greater than 3" are in the moderate to high range (40-90%) with probabilities of greater than 5" between 40-60% along the coast with a secondary maxima of low end probabilities (15-30%) along the southern Appalachians. While FFGs along the Coastal Plain are expectedly higher, the likelihood of the highest rainfall totals will still bring at least a scattered flooding risk into urban areas. ....Southern Plains... Further west, another frontal wave expected in northern Texas as an upper-level shortwave passes overhead will promote a second area of more concentrated thunderstorm development across northeastern Texas. Similar to areas further east, plentiful moisture with dewpoints hanging around the low 70s and similar instability and CAPE profiles will promote heavy downpours, with updated HREF probabilities of greater than 3" of rainfall between 40-60%. More scattered convection could lead to a few isolated instances of flash flooding more broadly across the Southern Plains. ....Rockies and Plains... 0100Z Update...expanded the Slight to include a more elongated stretch across the central High Plains in eastern CO to the southern Rockies-High Plains in northeast NM. This based on the observational/mesoanalysis trends with the broken line of convection, along with the heightened 18Z HREF/12Z RRFS 1/3/6 hour QPF exceedance probabilities. Mixed layer CAPEs of 1500-2500 J/Kg early this evening along with PW values of 1.1-1.3" will be capable of producing 1.5 to 2+ inch/hr rainfall rates underneath the strongest cells.=20 Previous discussion... Scattered convection is expected across much of the Rockies and High Plains in the presence of very moist, post- frontal upslope flow. Regionally high dewpoints into the low 60s=20 have been observed this morning across the region which will help=20 promote locally heavy rain rates of 1-1.5" per hour, with the risk=20 of isolated flash flooding expected especially along some of the=20 more terrain sensitive areas. A few areas of more focused corridors of convective coverage have prompted Slight Risks. The first is over portions of central South and North Dakota as a shortwave rounds upper-level ridging overhead. Hi-res guidance has been consistently maintaining the development of a more organized convective complex/mesoscale convective vortex (MCV) into the evening/overnight hours forecast to dive southeastward following the mean upper-level flow. The more widespread, organized nature of the convection should lead to locally heavier rainfall, with the HREF showing moderate to high probabilities (40-80%) of greater than 3" of rain and at least low- end probabilities of totals greater than 5" (15-40%). Additionally, further west, a Slight Risk has been introduced over portions of the northern Rockies as the attendant upper-wave will help encourage another area of more widespread thunderstorm development this afternoon. This region is also is experiencing some of the highest anomalous moisture values over the CONUS currently, approaching the 90-95th percentile for some locations. Another area of focus will be along and ahead of the Sangre De Cristo Mountains in northeastern New Mexico where more widespread storms will lead to heavier rainfall totals. Hi-res guidance supports areal average of 1-2" and the risk for more scattered instances of flash flooding. Putnam Day 2 Valid 12Z Sat Aug 02 2025 - 12Z Sun Aug 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST AND THE CENTRAL/SOUTHERN PLAINS... ....2030Z Update... ....Rockies and Plains... Similar to Friday (day 1), scattered convection is forecast across the Rockies and adjacent High Plains as post-frontal very moist, upslope flow remains in place. At least one more focused corridor of heavy rainfall and locally greater risk for flash flooding is expected with the development of a mesoscale convective system as shortwave(s) in north-northwesterly flow pass over the central/southern High Plains. Newly available hi-res model guidance showed a significant southward shift in the location of the most likely corridor of heavy rainfall tied to this development, previously centered on western Kansas/the Oklahoma Panhandle and vicinity and now more solidly southward into the Oklahoma and Texas Panhandles. Unfortunately, the typical uncertainty behind convective development of this nature could include additional areal changes, but feel the signal for heavy rainfall and potential for flash flooding is still worthy of a Slight Risk even if further adjustments will be necessary. There is also solid overlap of QPF in this location across the hi- res models, including the RRFS, as well as the ECMWF-AIFS AI model guidance. Deterministic guidance also suggests locally heavy rainfall totals of 2-4" will be possible, supported by at least low-end probabilities (20-30%) of 3"+ in the HREF. A small Slight Risk was also added for portions of the southern Appalachians. Despite the passage of the cold front to the south, now available deterministic guidance and HREF probabilities show the potential for several inches of rainfall. Supportive upslope flow along the terrain as well as increased sensitivity may lead to some more scattered instances of flash flooding compared to adjacent areas of central to northern Alabama and Georgia. Putnam ....Previous Discussion... ....South and Southeast... During this period the strong cold front will sink southward through the Southeast, Deep South and westward in to Texas. A deep pool of moisture will remain readily available enhance local rainfall, especially with the presence of instability along and ahead of the front interacting with the right entrance region of the upper jet. Very heavy rainfall rates are expected and should drive an isolated to scattered flash flood risk. The Slight Risk are was maintained for portions of the Low Country South Carolina, southern Georgia and far northern/northeast Florida and was expanded western into Alabama with this issuance. The front stretching west across the Gulf Coast states in a moist and unstable environment could promote scattered diurnal thunderstorms into Saturday thus maintaining the Marginal Risk. Snell/Campbell Day 3 Valid 12Z Sun Aug 03 2025 - 12Z Mon Aug 04 2025 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR SOUTHEAST GEORGIA AND FOR THE CENTRAL/SOUTHERN PLAINS... ....2030Z Update... There remains a large amount of uncertainty with respect to the potential for heavy rainfall/flash flooding over portions of the central/southern Plains on Sunday. As previously noted, this potential will be tied both to the presence of one or more MCSs from the day 2 period continuing into the day 3 period, as well as the chance for additional development associated with any remnant outflow boundaries and supported by continual shortwave(s) in north-northwesterly flow as well as plentiful moisture. For now, have adjusted the existing Slight to cover the overlap of QPF in the updated deterministic as well as ensemble guidance, with the caveat that there may be one or more focused corridors of heavy rainfall within the Slight Risk area that will require adjustments to the areal coverage. The guidance suggests this would currently be most likely in vicinity of the Red River Valley, and also across areas of central Kansas. Putnam ....Previous Discussion... ....Southeast... Once again the cold front advances further south, shifting the QPF footprint along with it. The heaviest rainfall will likely focus along the far southern portions of South Carolin and coastal Georgia, therefore kept the Slight Risk for this period. A broader Marginal Risk area covers the potential for isolated instances of excessive rainfall as far west as the Florida Panhandle and southeast Alabama. ....Plains... MCSs mentioned in the Day 2 period will persist during this period although shifted east/south further into the plains. Deep influx of PW near 2 inches will continue to fuel convection and enhance rainfall rates within these complexes. The exact location of the highest totals are not certain but the higher potential for excessive rainfall and flash flooding concerns will likely be focused over south-central Kansas and northern Oklahoma. The Slight Risk area was maintained for this period. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-Z6SSFKmYIifTEDlyOgd-ASAyJ2Inw-28yum7MSjVclk= 3gS8XEYgePIx-T9IiSLR74Dh_Q3zd7_wrAn-CMEw1p6Kk6s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-Z6SSFKmYIifTEDlyOgd-ASAyJ2Inw-28yum7MSjVclk= 3gS8XEYgePIx-T9IiSLR74Dh_Q3zd7_wrAn-CMEwqtAI9Q8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-Z6SSFKmYIifTEDlyOgd-ASAyJ2Inw-28yum7MSjVclk= 3gS8XEYgePIx-T9IiSLR74Dh_Q3zd7_wrAn-CMEwoKc9LZQ$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .