Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 19 2025 15:57:45 FOUS30 KWBC 191557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM 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=20 OF THE SOUTHWEST, UPPER MIDWEST, CENTRAL TO SOUTHEAST KANSAS, AND A SMALL PORTION OF NORTHWESTERN MISSISSIPPI AND ADJACENT AREAS... ....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 ....Previous Discussion... ....California/Great Basin... Remnant energy from Mario will continue to lift north across California and become sheared by an upstream low over the Pacific, with some energy wrapping back offshore, while the remainder moves east into the Great Basin. This energy, along with highly anomalous moisture (+4 std dev above normal), is expected to continue to produce showers and storms early in the period across parts of California, including the Sierra Nevada. While forecast amounts have trended down, there is still some signal for locally heavy rain to continue early in the period across portions of the central Sierra Nevada. A small Slight Risk was maintained, mostly for the initial 6-hours, where the HREF shows higher probabilities for additional amounts exceeding an inch. By the afternoon, as the mid level energy and greater moisture anomalies move east, expect the greater chance for heavier rain to begin to focus over central Nevada, where the HREF is showing a broader footprint of high probabilities for amounts over an inch. ....Southwest... Expect widespread coverage of showers and storms today, as moisture from Mario bolsters PWs across the region. The general consensus of the CAMs show greater coverage across eastern Arizona and western New Mexico during the afternoon, before storms move southeast into southeastern New Mexico and West Texas by this evening. Storm motions are expected to generally progressive, limiting the threat for widespread heavy amounts and flooding. However, there is some signal for backbuilding and training, which could raise at least isolated runoff concerns. HREF neighborhood probabilities for amounts exceeding an inch are above 50 percent within much of the Marginal Risk area. ....Upper Mississippi Valley/Midwest to the Lower Missouri Valley... A mid-to-upper level low/negatively-tilted upper trough will continue to move slowly east from the northern Plains into the upper Mississippi Valley this period. While not expected to be a widespread heavy rainfall event, south-to-north training storms ahead of the associated occluded front, may produce locally heavy amounts and isolated flooding from northern Illinois back across the upper Mississippi Valley. The HREF shows widespread high probabilities for amounts exceeding an inch across this region, with some localized high probabilities for amounts over two inches. Meanwhile, shortwave energy diving southeast out of the northern High Plains, is expected to produce a secondary area of organized amounts, with some locally heavy amounts possible from southeastern Nebraska into southwestern Iowa and northwestern Missouri this evening into the overnight. Here also, the HREF is showing high probabilities for amounts over an inch, with heavier amounts possible. ....Central Plains... Energy moving through the base of the previously noted upper trough, interacting with deepening moisture along a strengthening low level jet is expected produce showers and storms developing and dropping southeast across Kansas, with locally heavy amounts possible by this evening. HREF probabilities indicate localized amounts of an inch or more are likely, with heavier amounts possible. Pereira Day 2 Valid 12Z Sat Sep 20 2025 - 12Z Sun Sep 21 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEASTERN KANSAS AND NORTHEASTERN OKLAHOMA THROUGH THE OZARKS... Storms developing late in the previous period across Kansas may continue, producing locally heavy amounts across portions of the Ozark Region Sunday morning. Redevelopment is possible back to the west later in the day as shortwave energy emanating from the west moves into the region and interacts with deepening moisture afforded by strengthening southerly flow. Pereira Day 3 Valid 12Z Sun Sep 21 2025 - 12Z Mon Sep 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST AND FROM THE OZARKS THROUGH THE MID MISSISSIPPI VALLEY... ....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!5aaCviy4I5IFWrGEYPTpv0hxy6bLFUxtYpcRX0IRWQV8= sMKmL9SsAbLGC0hn4Yw3mLa2j1NQuxXJQNwxyYg8fjXkX64$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5aaCviy4I5IFWrGEYPTpv0hxy6bLFUxtYpcRX0IRWQV8= sMKmL9SsAbLGC0hn4Yw3mLa2j1NQuxXJQNwxyYg8Vvnp5dg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5aaCviy4I5IFWrGEYPTpv0hxy6bLFUxtYpcRX0IRWQV8= sMKmL9SsAbLGC0hn4Yw3mLa2j1NQuxXJQNwxyYg8hKS0UcM$=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) .