Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 13 2025 00:40:06 FOUS30 KWBC 130039 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 839 PM EDT Tue Aug 12 2025 Day 1 Valid 01Z Wed Aug 13 2025 - 12Z Wed Aug 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN APPALACHIANS AND THE CENTRAL GULF COAST... ....SOUTHERN APPALACHIANS... Ongoing heavy rainfall in the Southern Appalachians is expected to shift northward with time through the Cumberland Plateau. PW=20 values remain sufficient for heavy rain concerns. Convection will=20 be enhanced through this anomalous PW axis by a stream of mid to=20 upper level vorts forecasts to rotate across this region, around=20 the peripheries of a weak mid to upper level ridge centered just=20 off the northeast Florida coast. Unlike early this morning, the 18z HREF is more aggressive on the risk in the Southern Appalachians than the 12z REFS guidance. Hourly amounts to 2.5" remain possible overnight within the Slight Risk. ....Central Gulf Coast... A slight risk was continued along portions of the Central Gulf Coast from from southernmost AL through southernmost MS and southeast LA. Consensus from the latest hi res guidance is for the potential for an overnight period of heavy rain affecting the=20 immediate coastal region in association with with disturbed weather in the northern Gulf, mainly after 0600 UTC and continuing to just after 1200 UTC. Isolated runoff issues possible, especially in=20 more urbanized regions. ....Great Lakes, Ohio Valley, Mid Mississippi Valley into the Southern Plains... A broad marginal risk area was maintained across a large region from the Great Lakes, south and southwestward into the OH Valley, Mid MS Valley and Southern Plains. PW values will remain high across these regions, 1.5 to 2"+, with the latest suite of models showing at least localized heavy rain potential in this high PW axis into the overnight hours. Any lingering issues would be isolated, hence the maintenance of a Marginal Risk. ....Southwest...=20 Widely scattered to scattered convection across this area will=20 support potential for localized heavy rainfall amounts with=20 isolated runoff issues which should minimize towards midnight local time, especially across more vulnerable terrain features, dry=20 washes/arroyos, or burn scars. Roth Day 2 Valid 12Z Wed Aug 13 2025 - 12Z Thu Aug 14 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE EASTERN AND SOUTHERN UNITED STATES, AS WELL AS FOR PORTIONS OF THE NORTHERN PLAINS AND SOUTHWEST... 2030Z Excessive Rainfall Discussion Update... Introduced a Slight Risk across portions of the Appalachians where the latest HREF guidance depicted an increasing chance of 1- and 2-hour inch per hour rainfall accumulation probabilities as well as a signal for the probability of rainfall exceeding flash flood guidance. Model deterministic guidance was generally light with little run-to-run agreement and a fairly large spread in the placement of the QPF. Given the anomalous amount of moisture in place and the best forcing remaining west of the mountains through most of the day...hoisted a Slight Risk area that is largely an extension of the Day 1 Slight Risk area. No change of the outlook or the forecast reasoning in the Upper Midwest or Southwest US. Bann Previous Excessive Rainfall Discussion... No changes of note made to the very broad marginal risk area day 2 stretching northeastward from the Central Gulf coast, into the TN/OH Valleys, Southern to Central Appalachians, Mid-Atlantic into NY State and northern New England. A broad axis of above average PW values, 1.5 to 2+ standard deviations above the mean will stretch along and ahead of the primary surface front moving west to east across the northern tier from the Lower Lakes/OH Valley region, east into the Mid-Atlantic/Northeast. The above average PW values across the Gulf coast into the Southeast/Southern Mid-Atlantic during day 1, will also continue into day 2. There is general agreement in the models for the potential for widespread scattered convection in this high PW axis, with areal average moderate totals depicted and likelihood of locally heavy totals. Confidence is low with respect to where the heavier, more concentrated convection may be. Subsequently, we have not changed the risk level from previous issuances, maintaining it at marginal for the time being. A few areas of concern for potential higher threat level day 2 in subsequent issuances. Southern Appalachians: Early morning heavy rains late day 1 across the Southern Appalachians may linger into the post 1200 UTC Wednesday period across eastern TN/western NC. The HREF 12 hour neighborhood probabilities ending 0000 UTC Thu Aug 14 are high for 1 and 2"+ amounts. Mid-Atlantic: Convection likely to begin to become more organized ahead of the above mentioned frontal boundary pushing eastward. HREF 12 hour neighborhood probabilities ending 0000 UTC Thu Aug 14 are high for 1 and 2"+ amounts from northern VA, northward across much of PA into the Southern Tier of NY State. Central Gulf Coast: HREF hourly precip probabilities for .50-1"+ amounts remain high early day 2 from the expected second round of heavy rains to affect the Central Gulf coastal region during the day 1 period, that may extend into the day 2 period. ....Northern Plains... There continues to be a lot of model spread day 2 with potential convective development ahead of the next front pushing into the Northern High Plains 0000-1200 UTC Thursday. The low level southerly flow expected to strengthen after 0000 UTC Thursday across the Northern Plains ahead this front, supporting increasing convective potential. Several of the global models do show potential for heavy amounts late day 2, but continue to show large amounts of spread with respect to placement, leading to continued low confidence. Given this, the risk level was maintained at marginal. ....Southwest... No significant changes to the overall flow pattern for the Southwest day 2. Additional weak vorts rotating around the Great Basin/Southwest mid to upper high will support another round of scattered convection. Low confidence on any qpf details, but potential will continue for localized heavy rains and isolated runoff issues, especially across more vulnerable terrain features or burn scars. Day 3 Valid 12Z Thu Aug 14 2025 - 12Z Fri Aug 15 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE LOWER MISSISSIPPI VALLEY, TENNESSEE VALLEY, SOUTHERN APPALACHIANS INTO THE SOUTHEAST AND ACROSS THE SOUTHWEST AND THE UPPER MISSISSIPPI VALLEY... 2030Z Excessive Rainfall Discussion Update... The on-going outlook remained in good shape...resulting in only a few subtle modifications. The forecast reasoning changed little. Previous Excessive Rainfall Discussion... ....Southeastern portion of the CONUS... The cold front moving across the northeast tier of the CONUS day 2 will keep the axis of above average PW values, 1.5 to 2+ standard deviations above the mean, concentrated across the southeast quarter of the CONUS from the Lower MS Valley, east across the TN Valley, Southern Appalachians and into the Southeast. There is general model consensus for widespread scattered convection across these areas day 3, with locally heavy rains likely where convection becomes more organized. Large scale uvvs will continue to be enhanced by shortwaves rotating around the mid to upper level ridge that is expected to build westward day 3 across North Florida into the northeast Gulf. Models are showing a lot of spread with qpf details day 3, keeping the risk level, at the moment to marginal across these areas. ....Southwest... Similar to the day 2 period, no significant changes to the overall flow pattern for the Southwest expected on day 3. Additional weak vorts rotating around the Great Basin/Southwest mid to upper high will support another round of scattered convection. Low confidence on any qpf details, but potential will continue for localized heavy rains and isolated runoff issues, especially across more vulnerable terrain features or burn scars. ...Upper Mississippi Valley... The frontal boundary moving into the Northern Plains day 2 will continue to push eastward into the Upper MS Valley day 3. There may be two rounds of precip affecting this area day 3. An initial surge of warm advection rains early day 3, followed by potential frontal convection late day 3. Continued low confidence in any qpf details at this range, warranting only a marginal risk. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_-9XVTBLCLDxOL42-KrGh2jA-8_sTD4cNJk1570XqNRn= ywfCDAXT1Rw6nmRS2HUlp-EYq8xc1IouSwN5G7AMsaL66wE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_-9XVTBLCLDxOL42-KrGh2jA-8_sTD4cNJk1570XqNRn= ywfCDAXT1Rw6nmRS2HUlp-EYq8xc1IouSwN5G7AMhSYXySc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_-9XVTBLCLDxOL42-KrGh2jA-8_sTD4cNJk1570XqNRn= ywfCDAXT1Rw6nmRS2HUlp-EYq8xc1IouSwN5G7AMZ1kLWZ4$=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) .