Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 19 2025 20:02:56 FOUS30 KWBC 192002 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 402 PM EDT Fri Sep 19 2025 Day 1 Valid 16Z Fri Sep 19 2025 - 12Z Sat Sep 20 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST, UPPER MIDWEST, CENTRAL TO SOUTHEAST KANSAS, AND MUCH OF SOUTHERN ARKANSAS AND NORTHERN MISSISSIPPI... ....1815Z Special Update... The Marginal Risk over the Mississippi Valley was expanded along the slow moving frontal boundary to include much of southern Arkansas and northern Mississippi with this update. This update is in response to increasing coverage of convection in those areas. Please see MPD 1107 at wpc.ncep.noaa.gov/metwatch/metwatch_mpd_multi.php?md=3D1107&yr=3D2025 for more details. ....Previous Discussion... ....16Z Update... ....Southwest... The Slight Risk across a small portion of the Sierras has been removed with this update. While the northern area of the previous Slight is still picking up rainfall, rates have been a half inch per hour or less, and should weaken over the next few hours, reducing the flooding threat in that area. A renewed round of showers and thunderstorms across Nevada, Arizona, and New Mexico may cause additional instances of flash flooding again this afternoon and evening with peak heating. Considering the rainfall is over a much larger area as compared to yesterday, the forcings should be more diffuse and likely more tied to terrain. Further, the wave of precipitation is continuing to shear apart as it pushes north and east. A local maximum of rainfall is noted in the guidance across portions of western and southern New Mexico, but the signals remain too diffuse to pinpoint any one area for a Slight Risk upgrade. The area will continue to be monitored for a possible special upgrade should precipitation coverage and rates in that area overperform expectations. ....Midwest... Rainfall around a slow moving low centered near the Minnesota and Dakotas tripoint has largely been light so far this morning. Expect stronger convection to break out in some areas this afternoon, with the low level jet taking over as the primary source of instability and moisture overnight tonight. Once again there isn't good agreement on exactly where the heaviest rainfall will occur, but there's some consensus on the trimmed Marginal area from near Chicago northwest through the MN/ND/SD tripoint. ....Plains... A pair of MCS's are expected to impact NE and KS into tonight. The one in NE has already formed and is moving across the middle of the state, and is expected to hold together into southeastern Nebraska. Meanwhile a second one is expected to form in southeast Kansas late tonight. Considering the flow of moisture and instability from south to north, the storms to the south should prevent some of the wetter solutions from materializing with the northern one in Nebraska. While there is some training potential with both, and embedded convection may briefly cause higher rain rates with both MCSs, the higher threat should be with the southern one in Kansas, so that Marginal remains. ....Mississippi/Tennessee/Arkansas... Ongoing convection along the Mississippi River into northwestern Mississippi has been backbuilding and rain cores have been locally producing 3 inch per hour rainfall rates based on radar data. Guidance is very poor on the handling of these storms, with most not recognizing the storms even exist. However, given the high rainfall rates and the likelihood the storms will persist for at least a couple more hours based on past behavior, a Marginal Risk was drawn to cover the flooding potential in this area through the midday period. Wegman Day 2 Valid 12Z Sat Sep 20 2025 - 12Z Sun Sep 21 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF OKLAHOMA THROUGH SOUTHERN MISSOURI, AS WELL AS FOR PORTIONS OF=20 WISCONSIN AND WEST VIRGINIA... ....2030Z Update... ....Oklahoma through southern Missouri... The inherited Marginal across this portion of the country was adjusted southward with this update. A complex of storms across southeast Kansas will be ongoing at the start of the period Saturday morning. The storms will drift south and east across the Moksarok through the morning, then will likely weaken with daytime heating providing competing forcing, as is typical. New storms are likely to form in the evening and overnight across Oklahoma with a second cluster in southern Missouri. There is a tremendous amount of models spread on how this entire scenario plays out, with the Moksarok storms having the best agreement, so additional changes to the ERO risk area are likely. ....Wisconsin... A negatively tilted trough over the northern Plains will lift northeast into the Upper Midwest/Great Lakes on Saturday. A slow moving surface low will also progress northeastward towards the area during the day. This unfolding weather scenario will likely result in multiple rounds of storms tracking northeastward across Wisconsin both during the day Saturday but also into Saturday night. Lack of instability will be the primary ingredient missing that would support flooding, but the combination of some prior days' rains in this area and the multiple rounds as the aforementioned larger scale forcing remains favorable should still support isolated areas of flash flooding, particularly in an urban areas. ....West Virginia... Considering the lack of agreement in much of the CAMs guidance regarding placement and timing of storms in other areas of the country, there is remarkable agreement on persistent and training storms developing along the Allegheny Front of West Virginia on Saturday afternoon. A strong shortwave tracking northeastward at the base of the same negatively tilted trough moving across the northern Plains and Midwest will provide the forcing for the storms, as instability and some southerly flow of adequate moisture advects north into the area. There will likely be some topographic influence as well as to where the heaviest rains set up. Since the area, like much of the Midwest and Northeast, has been very dry in recent weeks, it's likely most of the expected rainfall in this area will be beneficial. However, where heavier rates are the most persistent, isolated flash flooding of smaller streams and creeks could cause a few impacts. Wegman Day 3 Valid 12Z Sun Sep 21 2025 - 12Z Mon Sep 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF ARIZONA AS WELL AS FOR PORTIONS OF THE SOUTHERN PLAINS THROUGH MUCH OF ILLINOIS... ....2030Z Update... ....Southern Plains through Illinois... The Marginal Risk inherited was expanded in most directions with this update. A stalled out front will continue to draw increasing amounts of Gulf moisture northeastward around a large high pressure area centered along the East Coast. Training showers and storms will track along the front from Oklahoma north and east. Oklahoma will have "first dibs" for the best instability and moisture, so it's likely the heaviest rainfall in the area will occur in this region. However, the storms tracking north and east will move into areas that will likely have had recent heavy rainfall from the Day 2/Saturday period, which could mean less rainfall will be needed to cause flooding. Due to diffuse signals, there is not yet the confidence to pinpoint any area within the larger Marginal for a Slight, but the potential for a future Slight Risk upgrade is present and will need to continue to be monitored. ....Arizona... No significant changes were made, as monsoonal moisture moves into the state ahead of a longwave positively tilted trough. Should signals increase in this area, then here too may need to be considered for a possible future Slight Risk, especially in areas that get heavy rain in the Day 1/today period. Wegman ....Previous Discussion... ....The Southwest... An upper low off of the California coast will drift southeast, supporting the return of deeper moisture spreading north across Arizona and western New Mexico on Sunday. With PW anomalies increasing to 2-3 standard deviations above normal, numerous showers and storms, producing locally heavy rainfall amounts and isolated flooding concerns, can be expected. ....Ozarks through the Mid Mississippi Valley... A broad upper trough will drift slowly east from the Plains into the Mississippi Valley this period. A series of mid level shortwaves lifting along the leading edge of the trough, interacting an axis of deeper moisture is expected to produce showers and storms with periods of heavy rainfall possible. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_4I9kcK6u2ECDFxKfPPdg2gSPKnoOxbT111gMuNwt3aW= Wdu-w6jvSuJ0bcDu0U-X6icTkIApa8JI9_d99V78vvZZtmc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_4I9kcK6u2ECDFxKfPPdg2gSPKnoOxbT111gMuNwt3aW= Wdu-w6jvSuJ0bcDu0U-X6icTkIApa8JI9_d99V78YVIPsRk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_4I9kcK6u2ECDFxKfPPdg2gSPKnoOxbT111gMuNwt3aW= Wdu-w6jvSuJ0bcDu0U-X6icTkIApa8JI9_d99V78iPVm4zQ$=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) .