Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 17 2025 00:59:54 FOUS30 KWBC 170059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Tue Sep 16 2025 Day 1 Valid 01Z Wed Sep 17 2025 - 12Z Wed Sep 17 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHEASTERN VIRGINIA ALONG WITH SOUTHERN NEBRASKA INTO FAR NORTHWEST KANSAS... ....Mid Atlantic... ....01Z Update... Very little change to the Slight and Marginal areas based on the latest observational trends, mesoanalysis, recent HRRR runs, and HREF/RRFS rainfall exceedance probabilities. We were able to nudge the southern boundary of the Slight Risk area to the NC-VA border, while also pulling the Marginal Risk north as well to far northeast NC. This given the sharp drying (TPW gradient) south of the main precipitation shield.=20 Hurley ....16Z Update... The marginal and slight risk areas were shifted north slightly=20 over portions of the Virginia Tidewater and northeastern North=20 Carolina based on model trends. A small marginal risk area was=20 introduced, in coordination with the local Billings office, over=20 portions of Yellowstone and the Absaroka Range below 10,000 feet=20 where easterly moisture advection into the terrain could generate=20 upslope enhanced precipitation over vulnerable soils. PWATs between 0.5-1 inch, low flash Flood guidance and HREF 24hr 1-3 inch=20 exceedance probabilities over 5% support some kind of isolated=20 flash potential. Kebede ....Southwest... Scattered convective pattern will initiate later this afternoon and the early evening hours before fading with the loss of diurnal heating. Current progs are for convection to be tied closer to the terrain across southeast AZ into southern NM with isolated signatures of >1"/hr plausible in the stronger cell cores. These areas generally prone to runoff due to terrain complexities, as well as remnant burn scars present in some locations. There was very little change from the inherited MRGL risk with the forecast running close to general continuity. ....Central High Plains... ....01Z Update... Based on the latest trends with the observations (radar/satellite), mesoanalysis (strong deep-layer CAPE and low-level moisture transport), along with the most recent HRRR runs and HREF/RRFS rainfall exceedance probabilities, have included a targeted Slight Risk area across parts of southwest to central NE. Based on these recent trends, expect the highest risk to persist until 03-05Z. Hurley=20 Previous discussion... Strong shortwave trough ejection out of the interior Northwest=20 will lead to convective development and regional enhancement across the Central High Plains. Greatest ascent is aligned with the=20 diffluent region of the mid-level shortwave with left-exit region=20 dynamics likely to maintain a prevalent heavy rain signature across the western third of NE down into neighboring northeast CO and=20 northwest KS. Guidance has come into alignment with the axis of=20 heaviest precip centered near and just north of North Platte, NE=20 with the corridor extending south into southwestern NE and=20 northwest KS by the second half of the forecast period. 00z HREF=20 neighborhood probs for >3" are running between 20-40% across the=20 referenced area near North Platte and points south. This still=20 aligns well with the previous forecast MRGL risk leading to little=20 deviation in the current thinking. The areas of highest risk will=20 likely be smaller towns and urbanized areas, like North Platte that are more susceptible to runoff. Kleebauer Day 2 Valid 12Z Wed Sep 17 2025 - 12Z Thu Sep 18 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE NORTHERN PLAINS... ....2030z update... A slight risk was introduced over portions of western South Dakota based on increasing qpf trends and better agreement on location of deformation axis. FFGs are relatively low (0.75-2") over parts of west and northwestern South Dakota with ongoing rainfall likely to bring those values down by Wednesday. A marginal risk was introduced over much of the Mid-Atlantic coastal area due to recent qpf trends. Deterministic guidance have increased qpf over an area extending from the Virginia Tidewater up to Long Island New York where a plume of subtropical moisture is likely to continue interacting with a slow moving surface front propagating northward along the coast. ....Previous Discussion... ....Northern and Central Plains... A strong mid-level shortwave trough will eject into the WY Front Range and eventually close off across NE generating a maturing lee cyclone across the Central High Plains. The setup will likely cause quite a robust deformation signature on the northwest flank of the cyclone with ensemble mean QPF placement situated over western NE up through western SD into southern ND, arcing back into the nose of the warm- conveyor belt (WCB) on the northeast side of the system. This is a classic mid- latitude lee cyclogenesis setup with a strong jet coupling leading to broad large scale forcing and widespread precip across the Plains. Eastern side of the cyclone will exhibit the best convective potential given the modest theta_E advection pattern poleward within the WCB leading to scattered thunderstorm potential from the Southern Plains to as far north as the Northern Plains and adjacent Upper Midwest (Western MN). Meanwhile, the deformation axis across NE/SD/ND will provide a solid precip field with moderate to locally heavy rates for several hrs. located in that northwest side of the low. Totals >2" are forecast within the means across western NE into the Dakotas with scattered elevated QPF across the Missouri River basin located in eastern SD and NE down through KS and far western OK. This setup is conducive for at least some isolated to scattered flash flood prospects considering the broad synoptic scale evolution. A very broad MRGL exists across much of the Northern and Central Plains with a cull located over NE and northwest KS where the low center will be present and drier air entrains into the region Wednesday evening and beyond as the low intensifies and approaches occlusion phase by Thursday AM. ....Southern High Plains... Very little change in the previous forecast as run to run continuity has allowed for negligible adjustments within the previous MRGL risk. A maturing lee side low to the north will help drag a trailing cold front through the Central and Southern High Plains on Wednesday. The front will slip south and southwest once down near the latitude of the OK Panhandle, leading to a more textbook backdoor frontal progression as it enters into the northern TX Panhandle and northeast NM. Flow begins to run more parallel to the boundary in these zones when assessing forecast soundings from relevant global deterministic. The key in the setup is a relatively solid isentropic ascent pattern in a small zone bordering CO/OK/NM/TX with cross-section views over the area displaying a classic sloped low-level surface FGEN with strong omega correlating to rapid ascent as the front progresses through the area. Locally enhanced corridor of precip will transpire along and in wake of the front with local maxima of 1.5" to as much as 3" plausible during the event. This could be enough for an isolated flash flood threat, especially in those smaller urban areas situated across the four state intersection. ....Southwest... Another period of isolated to scattered convection is anticipated 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. This signal garnered enough attention to warrant an addition of a MRGL to account for the localized threat. ....Southeast Florida... Stalled front with an enhanced moisture return across South FL will lead to a heightened threat for flash flooding over the urban centers of southeast FL. The setup is historically favorable for flash flood concerns over the urban zones of southeast FL with PWATs hovering between 2-2.3", a solid 1/1.5 deviations above normal. CAMs guidance has recently come into range for at least the first half of the period with a mixture of results ranging from very little precip to multiple heavy convective cores impacting the zone between PBI down to MFL. Neighborhood probs for >2" are pretty high (>60%) across a large chunk of that above corridor, but there's still a precipitous drop off in the probs for >3" with a very low EAS signal accompanying. At this juncture, the prospects for heavy rainfall seem to be more isolated in nature, but the environment is leaning favorable for any cells to potentially drop appreciable amounts of rain in a short period of time. The heaviest rainfall totals among guidance seem relegated to the FL Keys, an area that historically is difficult for flash flood prospects just due to the sandier soils and much smaller land footprint available. The previous MRGL risk was relatively maintained given the setup and the chance for the signal to grow, but for now remains more within the lower end of the risk threshold. Kleebauer Day 3 Valid 12Z Thu Sep 18 2025 - 12Z Fri Sep 19 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS, MISSISSIPPI RIVER VALLEY, SOUTHWEST U.S., AND SOUTHEAST FLORIDA... ....2030z update... The marginal risk in the Southwest expanded across southern Arizona and into far southwestern New Mexico, due to an increased signal for convection. The marginal over the Plains was also expanded a bit into the Texas/Oklahoma panhandle area. There's uncertainty on where the maximum qpf axis will set up over the Northern Plains on Thursday. No changes were made to the southeast Florida marginal. Kebede ....Previous Discussion... ....Central U.S... Broad, occluding cyclone over the center of the CONUS will lead to a continuation of scattered convection on the eastern flank of the disturbance with a continued axis of deformation over parts of the Dakotas. Flash flood potential will be relatively modest considering the cyclone undergoing a deteriorating phase by the D3 juncture. Ensemble means are generally between 0.5-1" within the convective zone in the decaying warm-conveyor belt (WCB) with 1-2" located within the axis of deformation. The best flash flood risks will likely coincide in the convective elements just due to the nature of enhanced rates, so overall we're looking at a mid-grade MRGL risk for the period, but will assess if there needs to be any targeted upgrades as we get closer to the period. For now, the broad MRGL inherited will suffice. ....Southwest... Moisture advection from the northeast edge of tropical cyclone Mario off the California Baja will make its presence known across Southern CA into adjacent NV/AZ by Thursday morning leading to an increasing threat of scattered to widespread convective initiation during peak diurnal instability. As PWATs surge to 2-3 deviations above normal by Thursday afternoon and beyond across SoCal, the environment will be conducive for several convective cores to produce fairly heavy rainfall potential, especially over the terrain focused from Big Sur, south into the Transverse and Peninsular Ranges. The threat for thunderstorms will exist over the SoCal deserts, an area very prone to flash flooding due to the high runoff capabilities and multitude of dry arroyos littered across the region. The northern periphery will likely make its presence into southern NV, another area prone to flash flooding due to similar issues and added complex topography scattered about the area. Totals of 1+" will be common with a likelihood of a few 2+" totals considering the environment likely in place. A MRGL risk is currently forecast, but would not be surprised to see an upgrade in future forecasts, especially over portions of SoCal. ....Southeast Florida... A rinse and repeat setup carrying over from D2 will lead to a continued threat of heavy convection across much of South Florida. Frontal alignment still indicates a scattered to widespread convective posture across southeast FL where the urban corridor will be under threat for more heavy rainfall and flash flood prospects. The greatest threat will lie within that zone from Melbourne down to Miami with some of the ensemble means greatest near West Palm Beach and Port St. Lucie. A MRGL risk exists once again for the defined area above between MLB to MFL. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5fbjs7HIzYzZKKSCFYCu6NKR_d0rb25k-vPgadY_yBYh= -fPN1e5AkgTdrWmudWfaaGFI3Wc1jPQDWE3M13jiakCRirU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5fbjs7HIzYzZKKSCFYCu6NKR_d0rb25k-vPgadY_yBYh= -fPN1e5AkgTdrWmudWfaaGFI3Wc1jPQDWE3M13jimtkpDow$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5fbjs7HIzYzZKKSCFYCu6NKR_d0rb25k-vPgadY_yBYh= -fPN1e5AkgTdrWmudWfaaGFI3Wc1jPQDWE3M13ji24_bLZI$=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) .