Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 03 2025 00:51:32 FOUS30 KWBC 030050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 850 PM EDT Sat Aug 2 2025 Day 1 Valid 01Z Sun Aug 03 2025 - 12Z Sun Aug 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST, SOUTHERN PLAINS, AND NORTHERN PLAINS... ....Southern Appalachians through the Gulf Coast... A wavering hybrid cold/stationary front will remain draped from eastern Texas through South Carolina today, with a slow southern translation of this feature expected into Sunday morning. This front will waver (with the gradual southern progression) in a region of broad troughing extending down from the Northeast, with weak flow across the Gulf Coast leading to the slow movement. However, the front will be entrenched within a plume of elevated tropical PWs (measured via 12Z U/A soundings of 2.2 to 2.4 inches, nearing daily records across the area.) This will support widespread showers and thunderstorms along the low-level convergent boundary of the front, with generally weak west-to-east 0-6km mean winds of just 5-10 kts supporting slow storm motions, and training thanks to this boundary parallel wind. Weak impulses within the flow will enhance ascent leading to locally clustered convection with rainfall rates above 2"/hr (briefly 4"/hr likely as reflected by HRRR 15-min rainfall above 1"), and there are likely to be two primary areas of higher excessive rain risk. The first area is across the Southern Appalachians from northern Georgia through eastern TN and far southwest NC. Here, a narrow ribbon of instability 750-1000 J/kg (MUCAPE) will extend northward and interact with increasing moist upslope flow as 925-850mb winds veer more E/SE late this aftn into the evening. Although PW anomalies are somewhat lower here compared to points south, increased bulk shear and weak mean winds will support heavy rainfall rates of 2-3"/hr within what could be nearly stationary cells at times. 0-10cm soils in this region are saturated above the 95th percentile according to NASA SPoRT, leading to compromised FFG and a 60-80% chance of exceedance from the HREF. Farther south from eastern AL though GA and onto the SC coast, training of cells which develop along the front and within the greatest plume of PW will support an increased excessive rainfall risk, especially through peak heating. Although soils here are a bit drier due to a lack of recent rainfall (7 day rainfall generally 25-50% of normal), a 60-80% chance of 2"/hr rain rates from the HREF and REFS could lead to stripes of rainfall exceeding 5 inches (40-70% chance from both HREF and REFS.) After consideration of the new probabilities and 12Z CAMs, the two slight risk areas were merged into one larger SLGT risk which encompasses more of GA/eastern AL as signals from both the HREF and REFS were similar. ....Northern Rockies through the Southern Plains... An elongated north-south stationary front will drape from Montana through Texas today, while multiple mid-level impulses embedded within generally zonal flow from the Rockies into the Plains interact with this surface feature as they emerge from the west. At the surface, flow downstream (east) of this stationary front will become enhanced as return flow persists out of the Gulf, driving an 850mb LLJ to 30 kts across the Central Plains. This surging LLJ will help draw impressive thermodynamics northward, characterized by PWs of 1 to 1.25 inches (locally higher) overlapped with 1000-2000 J/kg MUCAPE. This will support widespread convection developing beneath any of these shortwaves, with development most likely along and just east of the wavering stationary front into the more impressive thermodynamics. Although convection is expected to move briskly eastward within any clusters that develop thanks to 30-40 kts of bulk shear, rainfall rates of 1-2"/hr are anticipated (30-50% chance from the HREF) which could cause at least isolated excessive rainfall impacts, especially across any urban areas or more sensitive soils with lower FFG. The greatest risk appears to be across the TX panhandle where an MCS may form tonight on the periphery of the greater instability plume and then dive into the moisture beneath one of these shortwaves, with a subtly higher training risk along its SW quadrant, and both HREF and REFS 3"/24 hr probabilities peak above 40%, and the inherited SLGT risk was adjusted cosmetically. There may be a secondary area of heavier rainfall across eastern SD and into ND where some repeating or backbuilding convection may occur later tonight as westerly flow in the vicinity of an MCV/shortwave results in repeating development into the higher instability west, and convection moves into some more sensitive soils in eastern SD tonight. This has prompted a targeted SLGT risk for tonight as well, which is supported somewhat by the CSU UFVS first guess field which has a SLGT risk in the same general area, although displaced from the HREF/REFS probabilities which drove this new excessive area. Weiss Day 2 Valid 12Z Sun Aug 03 2025 - 12Z Mon Aug 04 2025 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST/GULF COAST AND SOUTHERN PLAINS... ....Southeast/Gulf Coast... The cold front from Saturday will continue to waver along the Gulf Coast as a stationary front Sunday in response to subtle additional height falls as the trough extending from the Northeast continues to subtly amplify. The trough axis may reach the Gulf Coast Sunday evening, but flatten at the same time in response to an elongated ridge draped across the Gulf. Moisture pooled along this front will remain impressive as PWs remain above 2.25 inches with deep column saturation noted via moist-adiabatic lapse rates through the depth of the column indicating tall-skinny CAPE which is progged to reach above 1000 J/kg during peak heating. Convergence along the front into these thermodynamics will support another day of slow moving convection noted by 0-6km mean winds of just 5-10 kts pointed to the east, with rainfall rates likely eclipsing 2"/hr at times (HREF 50-60% chance) supported by warm cloud depths nearing 15,000 ft to produce efficient warm-rain collision processes. Late in the aftn and into the evening, a shortwave is progged to rotate beneath the trough and pivot E/NE from the FL Panhandle into eastern GA, coincident with a surge in bulk shear up to 35 kts. This could support some more widespread convection with organization into clusters/multi-cells. With weak storm motions in place, any collisions/organization during this time could briefly pulse up rain rates even more significantly, while also leading to longer temporal duration or repeating rounds of these rain rates. Where this occurs, the HREF and REFS both indicate a 70-90% chance of 3"/24hrs and locally a 40-70% chance of more than 5"/24hrs. This rain will be falling atop areas that will likely receive significant rainfall on D1 as well, further enhancing the threat for flash flooding. Although this region generally takes a lot of rainfall to flood, the setup appears to support a higher than typical SLGT risk especially from the FL Panhandle through coastal GA. Otherwise, scattered thunderstorms with intense rain rates could produce instances of flash flooding on Sunday. The guidance has a secondary area of maximum rainfall potential along a westward advancing surface trough through northern AL into central TN which may require an upgrade in the ERO with later issuances since 3"/24hr probabilities peak at 40-50%, but this correlated with 3-hr FFG exceedance that is just 10-15%. At this time opted to maintain the MRGL risk and re-evaluate with new guidance for a possible upgrade overnight or on the D1 Sunday. ....Plains... The elongated stationary front aligned north to south from MT through TX will weaken Sunday, but leave at least a weak convergence boundary as it decays. Mid-level flow upstream of this feature will gradually become more W/NW as it gets squeezed between an amplifying ridge over the Southwest and a trough across Canada. Within this pinched flow, multiple shortwaves/vorticity impulses will shift eastward, interacting with the front to cause another day of convective development across the High Plains and into the Plains, with several MCSs possible beneath any of the stronger impulses. While there is considerable spread and thus uncertainty in the placement of any of these MCSs, each of them will likely contain intense rain rates of 1-2"/hr (locally higher) in response to meridional 850mb flow drawing elevated thermodynamics northward. =46rom TX to MN a ribbon of PWs of 1.25 to 1.75 inches will overlap a plume of MUCAPE above 2000 J/kg and bulk shear of 25-35 kts to support the intense rainfall rates. While the CAMs are generally suggestive of forward propagating features that will limit the duration of heavy rainfall, locally any of these could produce some short-term training, especially where any MCVs and lingering boundaries can track. Uncertainty is high in where this is most likely, but the REFS and HREF suggest a slightly higher potential from north Texas into central KS, as well as across portions of eastern SD/ND into Minnesota. The large inherited SLGT risk for TX/OK/KS was cosmetically adjusted, but further refinements are likely (with a reduction of category also possible) as confidence in MCS placement becomes more clear. Across MN, a targeted SLGT risk was considered where HREF and REFS probabilities for 5"/24 hrs peak above 15% as storms slow/regenerate beneath a deformation axis downstream of a mid- level closed low. Confidence is modest across this area, but 24-hr LPMM from the HREF is 3-4", so FFG exceedance potential appears higher than surrounding areas. However, there is still a lot of placement uncertainty among the available models, and some concern that instability won't be sufficient to support multiple rounds of heavy rain. While a SLGT risk is still possible with later issuances, after coordination with WFO DLH opted to maintain the MRGL risk at this time. Weiss Day 3 Valid 12Z Mon Aug 04 2025 - 12Z Tue Aug 05 2025 ....THERE ARE MARGINAL RISK AREAS FOR THE NORTHERN ROCKIES, NORTHERN PLAINS/MINNESOTA, AND THE SOUTHEAST... ....Southeast... Moisture streaming northeast ahead of the longwave trough axis will maintain unsettled conditions and widespread showers/thunderstorms from southern AL through GA and coastal SC. There is quite a bit of uncertainty in the placement of the heaviest axis of QPF on D3 owing to uncertainty in accompanying mid-level impulses progged to lift out of the Gulf on Monday, and this limits confidence from upgrading the inherited MRGL risk to a SLGT risk at this time. However, with PWs likely continuing at 2-2.25 inches overlapping MUCAPE of more than 1000 J/kg, rainfall rates of 1-2"/hr or more are likely within any convection that develops and then tracks northeast. Some repeating rounds are possible, but SREF/GEFS/ECENS probabilities for 3"/24hrs are all quite modest at this time, and the EC/EC-AIFS is north of the other guidance which could allow the heaviest rain D3 to not overlap the heaviest rainfall footprint from D1-2. For these reasons the MRGL risk was maintained and adjusted, but a SLGT risk may be needed with later issuances. ....Northern Plains... The closed mid-level low responsible for areas of heavy rainfall Sunday will continue to lift northeast and weaken on Monday. The weakening of this feature will result in less ascent, but a lingering boundary in its wake could still produce enhanced low- level convergence, primarily across Minnesota. The threat for heavy rainfall appears lower D3 than D2, as both ascent and thermodynamics are reduced, but locally rainfall rates in excess of 1"/hr are possible, with a secondary surge in convection possible late in response to 850mb winds reaching 20-30 kts converging into the region. With FFG likely to be lowered from antecedent rainfall D1 and D2, isolated excessive rainfall impacts will again be possible on Monday. ....Northern Rockies... A shortwave moving northeast across ID/MT will help amplify a trough shifting into the Northern High Plains on Monday. This will drive enhanced ascent into the already broad synoptically forced lift, helping to spawn a weak wave of low pressure tracking into the High Plains the latter half of D3. Low-level southeasterly flow will draw elevated PWs and MUCAPE into the region aiding a moistening column from high-level SW flow emerging from the Pacific. The result of this will likely be showers and thunderstorms moving progressively eastward from the Northern Rockies through the northern High Plains. With PW anomalies +1 to +1.5 sigma above the climo mean, rainfall rates may reach 1"/hr at times, which despite the anticipated progressive nature of these cells, could produce isolated impacts due to runoff. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!79WYmQeKiRxUvl_rQ0eb5Yq8XIR4-Nb2YHcnEjCUz3UE= dhsfRCjwGACgVcOuSs6Sy2wQJxeGEK7O4K85lZrcIyCsOZk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!79WYmQeKiRxUvl_rQ0eb5Yq8XIR4-Nb2YHcnEjCUz3UE= dhsfRCjwGACgVcOuSs6Sy2wQJxeGEK7O4K85lZrcuypO0MA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!79WYmQeKiRxUvl_rQ0eb5Yq8XIR4-Nb2YHcnEjCUz3UE= dhsfRCjwGACgVcOuSs6Sy2wQJxeGEK7O4K85lZrccmXtoeo$=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) .