Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 12 2025 00:45:35 FOUS30 KWBC 120044 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 844 PM EDT Mon Aug 11 2025 Day 1 Valid 01Z Tue Aug 12 2025 - 12Z Tue Aug 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND CENTRAL GULF COAST... 01Z Update: The main changes this forecast were the removal of the SLGT risk over the Carolina's, expansion of the SLGT on the western and southern edge for the threat in the Southern Plains, and maintaining the western periphery of the SLGT risk over the central Gulf coast around the FL Panhandle into southern AL.=20 SLGT risk across the Southern Plains was maintained but expanded on the southern and western periphery of the inherited risk. Highest confidence across central and southwest OK down through North TX, mainly north of I-20. Theta_E analysis is pretty robust within that zone and will likely have the benefit of best nocturnal instability along with proxy to the front. Hi-res suggesting 2-4" with locally upwards of 5" possible in that corridor with extension on guidance a little further south in the potential as of the latest runs. Northern edge of the SLGT was trimmed back, but still lies along and south of I-44 from OKC to Tulsa. Complex of thunderstorms across central and northeast OK will slowly decay, but outflow progression to the east-southeast will likely cause supplemental development downstream over eastern OK over the next several hrs. Environment is suitable for those 2-4" maxima in that particular zone, enough for the risk to expand a touch eastward, but less of a change compared to the aforementioned areas to the west.=20 SLGT was maintained for a small area along the central Gulf coast for prevailing southerlies to keep onshore flow prevalent through the overnight with most guidance indicating a weak impulse propagating inland after 06z leading to an uptick in convection near the coast. Environment is ripe for locally heavy rainfall capable of 2-4"/hr at peak intensity, enough to favor some flash flood concerns in those urbanized zones along and south of I-10. Wanted to cover bases and maintain general continuity with the western extent of the previous SLGT risk issuance. The previous SLGT over the Carolina's was removed as general sat/radar trends favor cold pool dominance leading to decaying convective outputs and warming cloud tops on IR. There's still some guidance indicating perhaps a few zones could see redevelopment of heavy rain this evening with the immediate SC coast near Myrtle Beach up through Hatteras potentially seeing enough onshore component to bring in another round of heavy convective cores off the Atlantic. This signal is scattered amongst the CAMs members=20 with the HRRR/RRFS combo indicating maybe a cell or two getting=20 very close to the shore, but not a probable scenario. Unlike the=20 last few nights, the setup is degrading for the prospect which was=20 enough to remove the SLGT and just settle with a MRGL risk. The=20 interior Carolina's will be interesting to watch as a weak mid-=20 level perturbation currently analyzed over GA swings north and=20 enters the Piedmont area late overnight. Some guidance wants to=20 fire more convection in-of Upstate SC into portions of southwest NC during the time frame of 06-12z. The consensus isn't all there=20 with 18z HREF probabilities for >2" lacking much favor for any=20 higher risk to be implemented. This will likely be more of an=20 isolated scenario, but hourly trends will be monitored for a short=20 term upgrade if they materialize more aggressively than currently=20 shown.=20 Kept a MRGL for the Midwest with another round of convection moving through the area, but forward propagation speeds look solid enough to favor a lower profile flash flood concern. Most concerning areas were the Quad Cities up into Southern and Southeast WI, but didn't see enough of a broad threat to warrant any upgrades. Look for locally 1-2" of rainfall in a quick progression with a local max ~3" possible, but less likely when assessing neighborhood probabilities <10% for the 3" threshold.=20 Kleebauer Day 2 Valid 12Z Tue Aug 12 2025 - 12Z Wed Aug 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE SOUTHERN APPALACHIANS INTO LOWER ELEVATIONS OF NORTH-CENTRAL GEORGIA AND WESTERN SOUTH CAROLINA... ....20z Update... A SLGT has been introduced for much of Southern Appalachia, as the 12z guidance suite has come into remarkably good agreement with a strong signal for localized 3"+ totals (per 12z HREF and 06z REFS neighborhood exceedance probabilities of 40-60%, co-located with rather good agreement in the QPF maxima between the global models). Highly anomalous tropospheric moisture (2.0"+ PW, near max moving average) is already in place across the region with persistent low-level moisture transport over the past 24-hr (with PWs increasing by as much as 0.4-0.5" since 18z yesterday across the region). While the current environment is weakly forced with little shear, this may change rather dramatically tomorrow as two distinct synoptic features interact later in the day tomorrow. The first is a tropical upper-tropospheric trough (TUTT) cell (mid-upper low situated in the sub-tropical easterlies) centered near the FL/GA border, and the second is a shortwave trough (increasingly positively tilted over time due to cyclonic wave breaking with the parent upper-trough lifting northeast near the Great Lakes) advancing from the west. While the interaction of these features is complex, the occurrence near the terrain of the southern slopes of the Appalachians looks to focus convection into this sensitive region (with continued southerly advection and forced ascent via terrain of very moist air from the south). Trends in the guidance will need to be monitored closely for future upgrades, as there is potential for sustained locally heavy rainfall for the entire day (and particularly so after 00z with the diurnal peak in the low- level jet oriented directly into the terrain). Elsewhere, forecast remains largely on track. Went ahead and combined the MRGLs (over both West TX and the OH/TN Valley) with a strong enough signal from the new hi-res guidance to support at least low-end probs for excessive rainfall. Churchill ....Previous Discussion... ....Lower Lakes into the Mid MS Valley and Southern Plains... An axis of high PW values, 1.5-1.75"+ will continue along and ahead of the elongated frontal zone stretching from the Lower Lakes, southwestward into the Mid MS Valley and into the Southern Plains. Scattered convection likely in this high PW axis, supporting locally heavy precip totals. The previous marginal risk area was extended farther southwestward from southern MO, across northwest AR, much of OK and into northern TX to cover model qpf spread. Also expanded the previous marginal risk area farther northwestward into eastern WI and the northern portion of the L.P. of MI to cover the model qpf spread for potentially locally heavy rainfall totals along the northern portion of the frontal zone across these areas. ....Central Gulf Coast into the Carolinas... The axis of high PW values stretching from the Central Gulf Coast into the Carolinas on day 1 will persist across these regions day 2. Shortwave energy embedded in this high PW axis will continue to support potential for additional heavy rains day 2. The previous marginal risk area was extended east northeastward from northern GA/Upstate of SC/far western NC to cover much of the remainder of NC and SC where several models show heavy rain potential. ....Southwest... No changes made to the previous marginal risk area from southeast AZ into much of NM. Weak shortwaves progged to rotate around the mid to upper level ridge building eastward into the Great Basin day 2, enhancing large scale uvvs in an axis of seasonable PW values. Scattered convection across this area will support potential for localized heavy rainfall amounts and isolated runoff issues, especially across more vulnerable terrain features or burn scars. Oravec Day 3 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... ....20z Update... Little change in the overall forecast and below reasoning. Either maintained or expanded the MRGL risks based on the new guidance (with the most significant expansion across the Southern and Eastern CONUS). Uncertainty in the forecast is higher than usual, as global models are not giving any particularly strong signals at this range (despite what will likely eventually need some targeted upgrades from mesoscale details, given the prevalence of anomalous moisture). Churchill ....Previous Discussion... Similar to the day 2 period, an axis of high PW values will continue to stretch from the Central Gulf coast, east northeastward into the Southeast and Southern Mid Atlantic. Widespread scattered convection likely in this anomalous PW axis, values 1.5-2 standard deviations above the mean. Mid to upper level dynamics will have weakened by day 3, with less defined areas of upper diffluence to accentuate lift in this high PW axis. Still, model consensus is for continued potential for continued locally heavy rains across these areas. A marginal risk was added for the upper coast of SC and the lower coast of NC where models do show potential for additional locally heavy rains day 3. ....Lower Lakes/Northeast into the OH/TN Valley, Central to Southern Appalachians... An axis of high PW values, 1.5-1.75"+ also likely to continue along the elongated frontal zone pushing eastward through the Lower Lakes into NY State and northern New England and stretching southwestward into the OH/TN Valley regions and Central to Southern Appalachians. Widespread scattered convection likely along this front in this high PW axis, supporting locally heavy rains and isolated runoff issues. No changes made to the previous marginal risk area along and ahead of this front. ....Southwest... No significant changes to the overall flow pattern for the Southwest 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. ....Northern Plains... The low level southerly flow expected to strengthen after 0000 UTC Thursday across the Northern Plains ahead of a surface front emerging into the Northern Plains. An area of organized convection possible ahead of this front, with locally heavy precip totals possible. There is a lot of model spread with qpf details at the moment, leading to low confidence. The previous marginal risk area was decreased in size to better fit the overlap of potentially heavy rains with the lower FFG values. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!57BRGmO_1o6pfR4UC9rnFS0EZ3KQAYC0dPTS1r2PVftP= tIa1uM2ja59-JY15FmOKqIocWsg6tfDj-xnBxS506Bvw_Ls$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!57BRGmO_1o6pfR4UC9rnFS0EZ3KQAYC0dPTS1r2PVftP= tIa1uM2ja59-JY15FmOKqIocWsg6tfDj-xnBxS50oTDu9UY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!57BRGmO_1o6pfR4UC9rnFS0EZ3KQAYC0dPTS1r2PVftP= tIa1uM2ja59-JY15FmOKqIocWsg6tfDj-xnBxS50hQZAWGI$=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. 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