Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 24 2025 00:12:39 FOUS30 KWBC 240012 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 812 PM EDT Tue Sep 23 2025 Day 1 Valid 01Z Wed Sep 24 2025 - 12Z Wed Sep 24 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO AND MID TO LOWER MISSISSIPPI VALLEYS INTO THE ARKLATEX... ....Ohio and Mississippi Valley's into ArkLaTex... 01z Update: Surface and upper level progression over the Southern Plains will continue through the overnight hours with large scale=20 ascent still primed to impact the Mid to Lower Mississippi Valley=20 tonight into the ArkLaTex early Wednesday morning. Thunderstorm genesis will occur over southeast OK into western AR and move east-southeast with the mean flow as a cold front propagates out of OK and shoves the pattern downstream. Heavy rainfall within a corridor of modest low to mid-level lapse rates and surface based instability will aid in convective schema leading to locally heavy rates between 1-2"/hr at peak intensity with the strongest cores capable of ~3"/hr given the PWATs near 2" in place over the aforementioned area. Copious amounts of rain and flash flood signals were prevalent earlier in the D1 period, and that threat will continue its migration south along and ahead of the cold front. This was more than enough to favor a general maintenance of the SLGT risk from prior forecast, but did scale back on the northern and western flank of the risk due to the heavy rainfall prospects likely done with the frontal approach imminent, and/or loss of favored ascent.=20 The threat for scattered heavy rainfall expands eastward into the=20 Southern Ohio, and Tennessee Valley's as a quasi- stationary front=20 bisects the area from the Ozarks into the western reaches of the=20 Ohio Valley. Expectation is for smaller mid- level perturbations to ripple eastward along the front enacting focused ascent within the boundary confines which could spell for periods of locally heavy=20 rain with some training potential in-of the front. Signal is not=20 nearly as prolific as upstream over the Mississippi Valley, mainly=20 due to the lower instability forecast over the area. That said,=20 lower FFG's add favor to a threat overnight as any cell >1"/hr will have the capability to induce some flash flood prospects. As a=20 result, have maintained the previous SLGT risk forecast over the=20 area with some minor expansion to the east given recent hi-res trends.=20 ....Coastal Central California... 01z Update: Primary threat for flash flooding this evening across CA will be situated within any remnant burn scars that exhibit those isolated pockets of much lower FFG's that would favor a better threat for flash flooding. The setup was forecast to occur this evening, so there was no reason to deviate from continuity. The previous MRGL was maintained with no changes.=20 ....Kansas...=20 01z Update: Shortwave ejection out of the Front Range will slowly migrate eastward with a threat of heavy rain from convection situated in-of the axis of a weak nocturnal LLJ converging with a residual quasi-stationary front over northwest KS. The threat is very isolated in nature, but hi-res continues to depict a small axis of heavy rain potential from the CO/KS border near Goodland down into Dodge City during the evening. The threat remains within the lower bounds of the MRGL risk threshold, but still has merit, so maintained continuity over the region and removed the risk downstream over central and eastern KS as the threat there has ended.=20 Kleebauer Day 2 Valid 12Z Wed Sep 24 2025 - 12Z Thu Sep 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE TENNESSEE AND OHIO VALLEY REGIONS... 20Z Update: No significant adjustments made to this update with the greatest threat of intense rain rates and repeated rounds of thunderstorms over the TN and OH Valleys into portions of the central Appalachians. Other areas with locally higher rainfall and rain rates are across portions of northern NJ, eastern PA, and southern NY late in the period and then across portions of eastern TX as the convective complexes shift southward. ---previous discussion--- Anomalous PW and 850-700 moisture flux values will stretch across a large area from the Lower MS Valley, and into the TN & OH Valleys, Central Appalachians and the Mid- Atlantic, ahead of the amplifying longwave trof stretching from the Great Lakes, southward into the Lower MS Valley. There still is a fair amount of qpf spread with the max axis day 2. WPC is leaning more toward the multi model ensemble qpf mean/NBM to drive our qpf and excessive rainfall axes. This will support a widespread axis of moderate to heavy precip amounts from eastern TX into the Lower MS Valley, TN and OH Valleys. While a large portion of the day 2 QPF and ERO axes are currently in moderate to severe drought, there may be overlap in qpf areas from day 2 and what has fallen over the past 24 hours from eastern TN into central to eastern KY, and the Upper OH Valley region. The previous slight risk was narrowed to better fit this potential overlap areas. The marginal risk was also trimmed significantly from southern L.P. of MI, IL, southeast MO and AR to better fit the latest qpf update. ....Central California... The slow moving closed low approaching the central CA coast late day 1 will continue to push slowly eastward day 2 and push inland by early Thursday morning. Similar to the day 1 period, an axis of anomalous PW values, 2 to 3 standard deviations above the mean, will persist to the east northeast of the closed low across Central CA. Scattered convection in this anomalous PW axis will support locally heavy rains across Central CA and isolated runoff issues, especially across burn scar regions. ....Far southeast Arizona into New Mexico... No changes made to the previous marginal risk area across this region. Scattered convection possible, with localized heavy rain and isolated runoff issues, especially in burn scar regions. Oravec Day 3 Valid 12Z Thu Sep 25 2025 - 12Z Fri Sep 26 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST ARIZONA AND FAR SOUTHWEST NEW MEXICO... The slow moving closed low moving inland into Central CA day 2 will drop back to the south southeast toward Southern CA day 3. This system will continue to have an area of anomalous PW values in the vicinity of the upper center across Central CA into the Great Basin. This closed low will also begin to tap anomalous PW values over northwestern Mexico, transporting them into southeast Arizona day 3. Model consensus is for a max precip area over Southeast Arizona and far southwest New Mexico. The previous slight risk across this area was only modified slightly, expanding it slightly to the north and east to cover the model max qpf spread. Only some minor changes to the marginal risk to the northwest of this across northern AZ, southern NV, far southwest UT and into the central Sierra. Locally heavy rains and isolated runoff issues possible across these areas, especially over burn scars. ....Southern to Central Appalachians... There will continue to be a broad axis of above average PW values pushing eastward day 3 ahead of the elongated upper trof stretching from the Great Lakes into the OH and TN Valleys. There is a lot of qpf differences in this high PW axis, leading to low confidence in where to put risk areas. Much of the eastern U.S. is in drought conditions with low relative soil moisture levels. One exception is over portions of the Southern to Central Appalachians from eastern TN/far western NC/far southwest VA and eastern WV. Kept the day 3 marginal confined to these areas and removed the broader marginal from the previous issuance that extended north and east into the Mid-Atlantic and southwestward to the Gulf Coast. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4p_bm-eNh2_g7NQ8dqT5VqOJxwnnOneinigyAjdiEhEF= 8Vp84CGLuHD8UTziNOgK9SJGyrhcRLuNlIMz9JDU5Sc5mvY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4p_bm-eNh2_g7NQ8dqT5VqOJxwnnOneinigyAjdiEhEF= 8Vp84CGLuHD8UTziNOgK9SJGyrhcRLuNlIMz9JDUFyNPMwI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4p_bm-eNh2_g7NQ8dqT5VqOJxwnnOneinigyAjdiEhEF= 8Vp84CGLuHD8UTziNOgK9SJGyrhcRLuNlIMz9JDUFxuMuKM$=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) .