Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 21 2025 00:40:01 FOUS30 KWBC 210039 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 839 PM EDT Sat Sep 20 2025 Day 1 Valid 01Z Sun Sep 21 2025 - 12Z Sun Sep 21 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS, OZARKS, & CENTRAL APPALACHIANS... ....Southern Plains and Ozarks... A festering area of convection across OK & near the MO/AR border continues to be sufficient for a Marginal flooding threat. Earlier rainfall should have partially saturated soils, and with the topography across some locales, at least isolated instances of flash flooding are anticipated. A later complex will likely form very late=20 tonight near the Red River of the South. These storms should also=20 pose an isolated overnight flash flooding risk. Regionally, there is a pool of 1000-3500 J/kg of MU CAPE to feed thunderstorms and about 25 kts of effective bulk shear which should lead to organization at times. With precipitable water values ~1.5", hourly amounts to 2" with local amounts to 4" should be possible. ....Central Appalachians... Ongoing pulse convection should continue for several more hours=20 near the Virginia/West Virginia border before MU CAPE fades and/or=20 CIN wins out. Precipitable water values above 1.25" along with MU=20 CAPE of up to 1000 J/kg are feeding them currently. Until they=20 dissipate, hourly amounts to 2" are possible as they move slowly=20 near the rough topography, continuing the Marginal Risk of=20 excessive rainfall. Roth Day 2 Valid 12Z Sun Sep 21 2025 - 12Z Mon Sep 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS, MID MISSISSIPPI VALLEY, AND THE SOUTHWEST... ....19Z Update... ....Plains to Mid-Mississippi Valley... Overall across the middle of the country, the signal for heavy rain on Sunday remains quite diffuse. There will be a corridor where heavier rain will be more likely, which includes the Marginal Risk region from Oklahoma through Illinois. However, the guidance is in otherwise poor agreement on where any clusters of storms will develop and how much rain they will produce. Thus, taking antecedent dry soil conditions into account across nearly all of the Marginal Risk area, it remains a rather low-end Marginal. ....Southwest... Very few changes needed across the Southwest as monsoonal moisture in place across much of Arizona and portions of the California Deserts may allow for some limited shower and thunderstorm development. A positively tilted longwave trough approaching the area from the northwest will increase the large scale forcing in the area, which should allow for some "slopover" of any storms into the normally drier desert areas. Couple that with the western portion of the area (far southern California and far western Arizona) having recently picked up locally flooding rains, the Marginal Risk remains in place largely unchanged. As usual, burn scar areas will be particularly susceptible to renewed flooding should heavy rains occur over or near them. Wegman ....Previous Discussion... ....Eastern Kansas and northeastern Oklahoma to southern Illinois... Deepening moisture (PWs above 1.5 inches) along an axis of strengthening southerly to southwesterly flow ahead an amplifying upper trough will fuel showers and storms, possibly producing heavy amounts. While models differ significantly on the details, the general consensus indicates widespread precipitation is likely, with locally heavy amounts of 1-2 inches possible. ....Southwest... An upper low dropping to the south along the California coast will slow as it becomes cutoff west of Southern California. Southerly flow ahead of the system will support increasing moisture and the chances for precipitation across parts of Southern California and Arizona. Both the GFS and ECMWF show PW anomalies increasing to 2.5 to 3 standard deviations above normal across far Southern California through the lower Colorado Basin into Arizona. Pereira Day 3 Valid 12Z Mon Sep 22 2025 - 12Z Tue Sep 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST AND FROM KANSAS AND OKLAHOMA TO THE OHIO VALLEY... ....19Z Update... ....Kansas/Oklahoma to the Ohio Valley... A plume of moisture streaming off the Gulf will collide with a stalled frontal boundary from Kansas through the Ohio Valley on Monday. Expect multiple clusters of storms riding up that boundary from southwest to northeast to douse the region with periodically heavy rain. The details of where any such clusters of storms are likely to form remain highly uncertain. Thus, while the signal for heavy rain across the region is significantly higher than the Day 2/Sunday period, there remains little agreement on where and whether any persistent heavy rain will significantly impact any areas hit with heavy rain on previous days. Thus, once again taking into account antecedent conditions, which only get drier the further north and east you go, the large Marginal Risk area across the area remains valid. ....Southwest... Very little has changed in the forecast with another day of afternoon showers and storms likely to impact much of eastern Arizona into portions of western New Mexico Monday afternoon. With ridging aloft and therefore no larger scale areas of forcing, the storms will likely be tied to topography, such as the Mogollon Rim, limiting the flash flood threat to isolated instances. Wegman ....Previous Discussion... ....Kansas and Oklahoma to the Ohio Valley... Unsettled weather will continue, with the footprint for potentially heavy rain expanding as energy moving through the base of a broad upper trough interacts with deep moisture that will be supported persistent southwesterly flow. While models generally agree on a large footprint of moderate to heavy amounts, they lack agreement on the details. At least locally heavy amounts are expected, especially within the western portions of the highlighted area as a well-defined shortwave dives southeast and is expected to interact with a pool of deepening moisture along a strong southwesterly low level jet. Given the indication of 2-3 inches in some of the models, which is forecast to fall in addition to the previous days' amounts, an upgrade to a Slight Risk may be forthcoming if the signal persists and the guidance moves into better agreement. ....Southwest... An upper low is forecast to remain cutoff and drift slowly east along the Southern California coast. Anomalous moisture in place across Arizona is expected to spread east into parts of New Mexico, extending the chances for showers and storms, with potentially heavy amounts into the region. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5dYUTd9YHCCo5BqlE_nbMUgURPa5l2yb1Rem_wl9VvKb= FhcyE-LhEl4C9yiWyMCBZUNQFvF60fl3YI89yJAdkURTWtk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5dYUTd9YHCCo5BqlE_nbMUgURPa5l2yb1Rem_wl9VvKb= FhcyE-LhEl4C9yiWyMCBZUNQFvF60fl3YI89yJAdOOsCxlw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5dYUTd9YHCCo5BqlE_nbMUgURPa5l2yb1Rem_wl9VvKb= FhcyE-LhEl4C9yiWyMCBZUNQFvF60fl3YI89yJAd8G0duIM$=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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