Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 20 2025 15:57:43 FOUS30 KWBC 201557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Sat Sep 20 2025 Day 1 Valid 16Z Sat Sep 20 2025 - 12Z Sun Sep 21 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDWEST, THE SOUTHERN PLAINS AND OZARKS, AND THE CENTRAL=20 APPALACHIANS... ....16Z Update... Overall, the 3 inherited Marginal Risk areas were only tweaked a little bit each.=20 ....Southern Plains and Ozarks... An ongoing area of convection near the Moksarok continues to be a Marginal flooding threat as the storms either linger or reform over this same area later this afternoon. Most of the Moksarok has seen about 1-2 inches of rain areally, but with locally higher amounts. Thus, soils in this small region have become saturated enough that additional rainfall from stronger storms later this afternoon could cause isolated instances of flash flooding. Meanwhile, further southwest, a secondary area of convection that is more aligned with the core of the low level jet, will likely form very late tonight across the Texas Panhandle, southwest Oklahoma and far north Texas. These storms will pose an isolated overnight flash flooding risk, and should be entirely separate from the storms ongoing near the Moksarok. Since they will both be along the same boundary, the Marginal remains in place across central Oklahoma, though it appears likely the I-44 corridor from the Oklahoma City metro=20 through the Tulsa metro will see less in the way of total rainfall. ....Midwest... The inherited Marginal Risk across Wisconsin and Illinois was expanded just a bit to include the Green Bay, Milwaukee, and Chicagoland regions with this update. A slow moving occluded front attached to a low currently along the ND/MN border will push eastward towards the western shores of Lake Michigan this afternoon. Scattered showers and thunderstorms are likely to sweep across the metros this afternoon and evening as a result, which could themselves pose an urban and small stream flooding threat. Behind the line, however, guidance suggests there could be additional rainfall, most likely in the form of just showers, but that will train over some of these same areas. Since guidance has shifted a bit to the east with the greatest instability and moisture advection this afternoon to along the Lake Michigan shoreline, the urban I-94 & I-43 corridors were added to the Marginal, which now generally spans from I-90/I-39 east to the=20 lakeshore. ....Central Appalachians... The inherited Marginal Risk across portions of West Virginia was nudged north just a bit in keeping with the latest guidance. Generally areas east of the Appalachians should not be under much flooding threat, as the heaviest rainfall remains in the portion of eastern West Virginia from the southern tip of Maryland's western Panhandle south to the Highland County, VA's northernmost point. Guidance has been very consistent highlighting this area for the most rainfall. Given the ongoing moderate to severe drought conditions, it's likely there will be storms that may have embedded heavy rain cores outside of the Marginal Risk area, but it will take the persistence of heavy rain highlighted in this region in the guidance to result in isolated instances of flash flooding. Wegman ....Previous Discussion... ....Wisconsin and northern Illinois... A negatively-tilted upper trough will lift slowly across the mid to upper Mississippi Valley today. The associated ascent interacting with a ribbon of relatively deeper moisture (PWs 1.25-1.5 inches) along a modest low level jet will support showers and storms redeveloping this afternoon, which may potentially continue into the evening. With differences in the details, the CAMs generally indicate two areas of locally heavier amounts -- a north-south axis associated with storms lifting across eastern Wisconsin during the afternoon/evening and a west-east stripe produced by storms developing across Iowa and then training across northern Illinois later during the overnight. HREF neighborhood probabilities indicate localized amounts in excess of 2 inches are possible in these areas. ....Ozarks to southwestern Oklahoma.... CAMs indicate that ongoing convection firing north of a stationary front over southern Kansas will continue through the late morning, potentially producing heavy amounts as they train further east into the Ozark Region. The HREF shows high neighborhood probabilities for accumulations over 2 inches centered over this region. Then, as upstream energy sliding southeast across the central Plains interacts with deepening moisture that will be supported by increasing southerly low level flow, some guidance indicate that storms will redevelop over this same area later today. However, other models show storms developing further to the west before moving back into areas impacted by this morning's storms. While there is fair amount of uncertainty on how these storms across central into eastern Oklahoma will evolve, there is greater model consensus that storms developing over the Texas Panhandle and western Oklahoma by this evening will merge across southwestern Oklahoma to produce locally heavy amounts across the region during the overnight. ....West Virginia... CAMs continue to show isolated heavy amounts associated with slow- moving storms that are expected to develop over eastern West Virginia this afternoon. A weak impulse lifting northeast out of the Ohio Valley, interacting with pooling moisture along a slowing front, will support storm development. HREF neighborhood probabilities indicate that amounts of 2-3 inches are likely within the highlighted area. Pereira 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... ....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... ....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!4vorv7wPyzXg_TocNka6JuSEQZ0wkXx5Y4krahy1mdsO= ywtRd3H2PXjh4-Szwa30XMiQiWMhZ7SGqi52-TRdv12RGKw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4vorv7wPyzXg_TocNka6JuSEQZ0wkXx5Y4krahy1mdsO= ywtRd3H2PXjh4-Szwa30XMiQiWMhZ7SGqi52-TRdBqbSoDk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4vorv7wPyzXg_TocNka6JuSEQZ0wkXx5Y4krahy1mdsO= ywtRd3H2PXjh4-Szwa30XMiQiWMhZ7SGqi52-TRd9MJxIWo$=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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