Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 17 2025 19:47:29 FOUS30 KWBC 171947 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 347 PM EDT Wed Sep 17 2025 Day 1 Valid 16Z Wed Sep 17 2025 - 12Z Thu Sep 18 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR WESTERN AND CENTRAL SOUTH DAKOTA... ....16Z Update... ....Central Plains... A slow-moving surface low with at least some tap into some Gulf moisture is moving across central Nebraska this midday. Wraparound bands of rain are presently north to west of the circulation center, over central and western South Dakota. With daytime heating and advection, some instability is expected to work into the circulation, especially southern portions of the rainfall shield. Any convection will be capable of locally heavier rainfall rates. Since the overall circulation is nearly stationary, so too is the comma-head band of rainfall. Thus, expect a long-duration light to moderate rain to continue across this region well into tonight. Embedded convection locally enhancing rainfall rates should result in widely scattered instances of flash flooding. Further east however into Minnesota and Iowa, there will be more convection, but that convection will be faster moving, resulting in less total rainfall. Thus, the Marginal Risk remains in place. From central Minnesota into Wisconsin, a similarly slow-moving attendant warm front may also cause isolated flash flooding, where slow- moving convection fed by the southerly flow of moisture and instability could also result in highly localized areas of much higher rainfall totals and isolated flash flooding. ....Mid-Atlantic... A second vertically stacked low over the Mid-Atlantic continues to weaken this afternoon and will likely dissipate entirely by opening up into a surface trough by this evening. Until such time however, there remains a band or two of a bit heavier rain across the Maryland and West Virginia Panhandles into far northwestern Virginia. Additional daytime heating today may allow for some limited elevated convection to form within this band before it dissipates, so a small Marginal Risk was left in place, but it is very low-end. ....South Florida... Very slow-moving but highly efficient areas of convection are ongoing in and around much of South Florida, including the Keys. There is a distinct possibility that one or more of those very slow moving cells could stall over one or more of Keys today, which could cause localized ponding. Thus, a Marginal Risk was added. For mainland Florida, the main flooding hazard would be confined to the urban I-95 corridor from Melbourne south. It will also be for the possibility of slow moving convection, perhaps along any localized cold pool boundaries or sea breezes. Wegman ....Previous Discussion... ....Northern and Central Plains... A broad mid-latitude cyclone will continue to impact a large portion of the Plains over the course of Wednesday into Thursday with multiple areas of heavy rain plausible given a variety of dynamical and thermodynamic processes. The area with the greatest concern is likely within the well-defined axis of deformation that is forecast to materialize across the western half of SD leading to scattered flash flood concerns thanks to wet antecedent conditions in wake of a previous disturbance that distributed copious amounts of rain to areas along and west of the Missouri River. Guidance is in agreement on the threat for 1-3" over the area extending from the river over to the SD/WY state line, delineating the deformation structure forecast to materialize within the northern and western half of the cyclone situated over NE. FFG's on the order of 0.5-2" between 1/3/6 hr. intervals are running much lower than climo leading to a lower threshold to breach that could very well cause issues across that broad area of western SD. 5 and 10-year ARI exceedance probs are running between 50-80% and 30-60%, respectively between the two intervals, a solid representation of at least some scattered flash flood concerns with perhaps a broader scope of impact considering the areal footprint of the probabilities in question. Rates will be ~1"/hr at peak intensity, but could very well last for multiple hours due to the anticipated synoptic scale evolution of the surface lows slower forward propagation speed leading to training over a large area of SD down into northwest NE. In this case, the previous SLGT risk was maintained with some minor expansion on the edges to account for the latest probabilities and HREF blended mean QPF output. By the afternoon and evening, defined warm conveyor belt (WCB) will be situated across much of the Missouri River basin extending from OK/AR up into eastern SD. A warm front will lift north and transition to a quasi-stationary boundary across east-central SD through south-central MN leading to a focused frontal alignment for storms to materialize later in the period. This has been a trend within the recent CAMs as convective initiation will likely be spurred by the arrival of a shortwave pivoting around the eastern flank of the primary ULL, generating a better large scale ascent pattern focused downstream of the center of circulation. With dew points expected to be in the 60s for locations within the defined WCB, there will be room for scattered to widespread convective development during afternoon and evening time frame as the environment ripens with the typical diurnal heat flux. The frontal positioning over the Northern Plains and Upper Midwest will be a target for training convection as cells can become anchored to the front and lead to localized areas of heavy rainfall that could drop a quick 2-4" over a span of a few hrs. Rates between 1-2"/hr in convection are likely in this setup for anywhere within the WCB leading to a general maintenance of the MRGL risk across the Plains, but have now expanded the MRGL risk eastward into MN to account for the frontal alignment and growing threat of convection spawning near and within the boundaries forecast location. ....Southern High Plains... Multi-round convective episode anticipated across the southeast CO Front Range down into the OK/TX Panhandles and adjacent northeast NM beginning later this afternoon, carrying through the evening hours. Lee side low over eastern CO/western KS will materialize within the broad synoptic scale evolution occurring over the center of the CONUS. A trailing cold front will drag south and southwest through the above areas with an initial flare up of convective activity this afternoon along the front creating a moment of priming before a larger shortwave ejecting out of the Central Rockies becomes the focus for later. Models are in agreement on quite a strong mid-level vorticity ejection into the Front Range late-afternoon and early evening today leading to a blossoming convective initiation in-of the terrain just west of I-25 in southeast CO. Storms will grow upscale and migrate eastward into the Front Range, continuing to slide east-southeast as they deliver locally heavy rainfall in their wake leading to an isolated to scattered flash flood concern through the second half of the forecast. This setup has been documented in some way over the past several forecast iterations, but the threat is now clear in the main show focused within the strong mid-level ascent pattern generated by the shortwave rounding the trough base. Expect localized totals of 2-3" in the hardest hit locations with a max of up to 4" across the far southeast corner of CO down into the northern TX Panhandle. This is reflected by the modest >3" neighborhood probabilities in the latest 00z HREF suite, a solid indicator for a MRGL risk maintenance from previous forecasts. ....Southwest... Little change from the previous forecast as guidance maintains a signal for another period of isolated to scattered convection over the terrain encompassing southern AZ up through the central and eastern portions of NM with the primary threat being heavy rainfall situated over more complex terrain and burn scar remnants. Assessment of forecast soundings and hodographs indicate relatively slower cell motions, a classic characteristic for flash flood prospects over the desert southwest. Guidance reflects local 1-2" totals scattered across the region extending from southeast AZ into NM with some of the better signals focused within those mountain ranges like the Huachucas and Sacramento's. The previous MRGL risk was relatively unchanged considering the favorable run to run continuity. ....Southeast Florida... Broad cyclonic pattern across the eastern CONUS will aid in advecting a relatively stout mid-level shortwave currently centered along the northern coast of Cuba. IR satellite this evening shows the disturbance is very well-defined with a centroid of cooler cloud tops indicating scattered to widespread convective coverage in-of the western Florida Straits at this hour. Considering the south to southwesterly flow between the 850-300mb layer, the expectation is for convection to move generally northeast with the mean flow, ushering in a more focused area of forcing capable of impacting all of South FL, including the Keys with heavy rainfall at any point. PWATs are on the rise according to the latest 00z RAOB's out of KMFL and KEYW this evening with a broad expanse of 1.9-2.2" PWATs over the region. Forecast is for PWATs to continue to increase with an expectation of indices to settle between 2-2.3", a solid 1-1.5 deviations above normal when assessing the latest NAEFS output. Guidance is keying in on the FL Keys to be the primary focus for heaviest QPF during the forecast period, however the environment is favored for any area situated south of I-4, especially over southeast FL where sea breeze components and deep moisture presence will lend to pockets of very heavy rainfall from convection that develops over the area. These cells will be scattered in nature, but the prospects for 2-3"/hr rates with higher intra-hour rates across the urban corridor from Melbourne down to Miami will make for at least a modest MRGL risk for excessive rainfall over the next 24+ hours. The previous risk was expanded north to include KMLB as hi-res trends are keying on a convergent area north of where the previous risk forecast(s) have been made. Kleebauer Day 2 Valid 12Z Thu Sep 18 2025 - 12Z Fri Sep 19 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTHERN CALIFORNIA... ....2030Z Update... ....Southwest... In coordination with SGX/San Diego, CA; LOX/Oxnard, CA; VEF/Las Vegas, NV; PSR/Phoenix, AZ; and HNX/Hanford, CA forecast offices, a Slight Risk upgrade has been issued for much of Southern California and the Arizona side of the Colorado River. The mid- level remnants of Mario are making their way north up the coast. They are advecting abundant tropical moisture along with them, with PWATs well over 1.5 inches at and along the coast. This is at least 4 sigma above normal for this time of year. Thus, this amount of moisture is highly anomalous for this part of the country. In addition to the moisture, significant instability will also be present, with a rough average of the CAMs suggesting there will be over 1,000 J/kg of CAPE in much of southern California Thursday afternoon. Finally, the mid-level circulation center, while fully detached from the now dissipated low level circulation of Mario, will itself remain strong enough to provide adequate forcing for shower and thunderstorm activity across much of southern California. Expect any thunderstorm activity to be capable of 1 inch per hour rainfall rates, locally higher in favorable terrain areas. Working against any flooding will be the fast storm motion and potential for cloud cover to stabilize the lower atmosphere. Clouds will increase along and ahead of the low center, and where clouds are, the sun will be blocked, which will limit the daytime heating necessary to locally increase instability values. There is high uncertainty as to how much cloud cover there will be in between the storms. More cloud cover will mean lower chances for flash flooding. Further, with the circulation of Mario continuing to track north, expect any storms over California to be moving along Thursday afternoon. This should in most cases limit the potential for flooding. However, conditions are also favorable for training of storms, so while no one storm is likely to stall out over a given area, the combination of multiple storms appears likely. This event appears to be a low-confidence but high impact event. This means while the chance of flooding in any one area remains low, there will likely be multiple instances where heavy rain from training storms impacts a given area, resulting in flooding. ....Plains... No significant changes were made to the Marginal Risk area across the Plains. A slow-moving front may provide the forcing for more widespread storms from eastern Oklahoma north into the Dakotas, but the signal for heavy rain is low given the potential for the storms are moving quickly or that there is less coverage of storms. Meanwhile into eastern ND, the maximum of QPF is likely from a long-duration stratiform rain, which should only cause isolated flooding impacts. Wegman ....Previous Discussion... ....Central U.S... Broad occlusion will be taking shape across the Central CONUS with locally heavy rainfall potential focused within the residual axis of deformation across the Dakotas, as well as along the progression of the cold front from MN down into the Southern Plains. Guidance is all over the place with the exact placement of relevant QPF maxima, however the ensemble means still emphasize the potential within these zones referenced above. Expectation is for multiple storm clusters to aid in the threat with some places likely to see 2-4" inches thanks to the +1 to +2 deviation PWATs situated from OK up through the Upper Midwest and Northern Plains. Environment favors rate potential of 1-2"/hr in any cell within the WCB as general buoyancy should spur several thunderstorms along and ahead of the front as it migrates into the Mississippi Valley. As we get closer in time, it's plausible to have a targeted upgrade, or two, so it will be a period to monitor. ....Southwest U.S... Remnant moisture from post-tropical cyclone Mario will advect poleward with California generally in the cross-hairs for elevated moisture to move into the southern half of the state during the end of the D1 into D2 time frame. Energy from Mario will lag the initial low to mid- level moisture surge, but eventually will enter into the area by the late-morning hrs. Thursday. Models are pretty consistent in the meridional push of elevated moisture leading to scattered/widespread convection forming across much of SoCal, especially the lower deserts and terrain focused areas from Big Sur down into the Transverse and Peninsular ranges. It will be important to monitor the progression of the remnant mid-level energy as a more consolidated vorticity maxima would likely cause a period of enhanced, focused rainfall in-of the coastal terrain north of Los Angeles. As of now, the setup favors the energy becoming increasingly sheared with a more scattered convective depiction in the precip field. 00z ECMWF shows what could transpire with a more consolidated vorticity maxima with a stripe of heavy rainfall aligned south to north which would easily necessitate a risk area higher than the current MRGL in place. With PWATs likely to surge to 1.7-2.0", rainfall rates >1"/hr are certainly possible, a threshold that historically has caused localized flooding to some degree, leading to totals of 1-3" with room for higher if everything breaks unfavorably for the region. This is something to monitor as we step closer in time, but for now the threat is deemed a MRGL with a chance at an upgrade as we move further into the CAMs window for a better assessment on potential precip maxima. ....Southeast Florida... General persistence in the pattern across southern FL with elevated PWATs and streaming mid-level vorticity plaguing the region leading to scattered bouts of heavy rainfall within any convective development. The area of interest remains centered on the urban corridor from Melbourne to Miami just due to the repeated nature of convection and the lower FFG's aligned within the urbanization footprint. Models are still everywhere in the placement of the heaviest precip, some just offshore, and others hitting the Keys and southern peninsula pretty hard over the course of D1 and D2. Considering the environmental factors and continuity in the pattern, maintaining the previous MRGL risk is more than sufficient for the setup. Little to no change was necessary given the 00z NWP output. Kleebauer Day 3 Valid 12Z Fri Sep 19 2025 - 12Z Sat Sep 20 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS, MISSISSIPPI RIVER VALLEY, SOUTHWEST U.S., CENTRAL SIERRA NEVADAS, AND SOUTHEAST FLORIDA... ....2030Z Update... No significant changes were made anywhere in the country. Much of the flooding risk in each of the Marginal Risk areas is tied to=20 where and how numerous any clusters/complexes of storms develop,=20 which remain highly uncertain. It appears probable with improving signal that a future Slight Risk may be needed in some areas, such as along the Mogollon Rim in AZ and NM. This will depending on how much instability and forcing remains associated with the remnants=20 of Mario. Otherwise, the previous discussion remains valid. Wegman ....Previous Discussion... ....Northern Plains to Mississippi Valley... Our occluded low will continue to churn across the northern CONUS with another round of convection forecast on the eastern flank of the remnant cyclone. The threat is forecast to wane compared to the previous periods, but there is still a general MRGL risk for flash flooding in any cells that develop from the Mid to Upper- Mississippi Valley. Models are still all over the place in location and magnitude, but the premise of the environment remaining favorable for the threat warranted a continuation of the MRGL risk from previous forecasts. ....Western U.S... Elevated moisture from remnants of Mario will linger and allow for scattered to widespread convection to materialize over the Southwestern U.S. and over portions of the Sierra's of CA. Pending the mid-level evolution of the energy from Mario bleeding into D3, a heavier precip threat is possible, but not anticipated at this time. The setup should be able to adequately shear any mid-level vorticity leading to a less pronounced setup regionally. Still, the moisture anomalies and expected instability presence will allow for at least widely scattered flash flood concerns heading into the end of the week and early weekend. A MRGL risk remains for the Southwest and the California Sierra's. ....Southeast Florida... A chance for lingering heavy rainfall across southeast FL near Miami remains within the means, enough to warrant a maintenance of the inherited MRGL risk. It will really come down to finer mesoscale details and timing of when the incessant shortwave energy progression vacates the area. For now, there's enough to keep what was forecast prior, but will assess as we get closer in time. The area is small in spatial coverage, so there's a chance it is removed, or expanded once more CAMs windows shed light on the setup. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_1cvpfKDgweNcL4lCCgkNIXBc85QnQoWhj_12tS-TpQ6= -Yb2CGPgbHr6BhHvQNIZnw9Fdsk2-JPvuYLJ4__xik8tJEY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_1cvpfKDgweNcL4lCCgkNIXBc85QnQoWhj_12tS-TpQ6= -Yb2CGPgbHr6BhHvQNIZnw9Fdsk2-JPvuYLJ4__xGGoyozQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_1cvpfKDgweNcL4lCCgkNIXBc85QnQoWhj_12tS-TpQ6= -Yb2CGPgbHr6BhHvQNIZnw9Fdsk2-JPvuYLJ4__xcz4waJQ$=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) .