Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 29 2025 00:35:55 FOUS30 KWBC 290035 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 835 PM EDT Sun Sep 28 2025 Day 1 Valid 01Z Mon Sep 29 2025 - 12Z Mon Sep 29 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS=20 OF THE SOUTHWEST AND COASTAL SOUTHEAST... ....Southwest... We will maintain a Marginal risk area over NM into far west TX=20 with this update. A few more hours of locally intense convection=20 can be expected, with increasing low level easterly flow helping=20 support some continued development through the evening hours. The=20 expectation is that instability will erode overnight resulting in a decrease in convective coverage and intensity. Generally think the coverage of any additional flash flood risk has decreased enough=20 to remove the Slight and let the Marginal cover the threat. ....Southeast... A Marginal risk was maintained from the east coast of FL into eastern North Carolina. Across FL, onshore flow north of Imelda is helping enhance coastal convergence and resulting in localized areas of slow moving convection. This should continue through the overnight hours, and isolated rainfall totals over 3" are possible. Farther up the coast we have a stationary front just offshore, with moisture well north of Imelda interacting with this front. We also have the mid level trough and upper jet over the Southeast. As the night progresses we will probably see some of this rain offshore=20 move inland across the Carolinas, with the aforementioned trough=20 and jet helping support this inland advancement. Most of this=20 activity will likely be lower rate rainfall given the low level=20 northeasterly flow and lack of instability. However we may be able=20 to get some higher rates into immediate coastal areas closer to=20 the stationary front. 18z/12z HREF and REFS runs had the higher 3"=20 probabilities along the SC coast, however more recent HRRR and RRFS runs are shifting towards the NC coast. Recent radar trends also=20 seem to point towards far southeast NC as seeing the best chance of some slow moving low topped convection getting onshore, which=20 could locally drop upwards of 3" of rainfall. Overall the flash=20 flood threat appears pretty localized along these coastal areas=20 from FL into NC, but the risk is non-zero and a Marginal risk will=20 be maintained. Chenard Day 2 Valid 12Z Mon Sep 29 2025 - 12Z Tue Sep 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE IMMEDIATE CAROLINA COASTLINE... 21z update: ....Central New Mexico... Upper-level trough across the Southwest continues to march east- northeast, filling fairly rapidly as it reaches the Four Corners in the early forecast period. This has diminished overall wind field and pushed the deeper warm conveyor/moisture plume into the Plains. However, lingering sufficient moisture (.75"-1"), modest divergence aloft at least initially,clearing for solid insolation and solid lapse rates (500 J/kg) aloft will allow for potential for stronger thunderstorms along the Sacramento Range. While the overall intensity and potential for higher rates is reducing relative to prior days, there is sufficient rainfall rate/total signals with 12z HREF probability of 50% of 2"+ and even a small 15% or 3" totals across the Sacramento Range and ridge lines east of the Rio Grande Valley. As such, will include a small Marginal Risk area across the ridge of south-central to central New Mexico. ....Southeast... Strong easterly onshore flow will continue and in proximity to the coastal front, weak elevated showers and embedded convective elements are expected along/downstream across the western Piedmont into the foothills of the southern Appalachian Range, combined with upslope enhancement localized 1-2" totals are possible and have expanded the Marginal Risk slightly westward into the eastern slopes of the Appalachians. However, the trends for T.S. Imelda remain slow with a sharp eastward turn, further shifting the QPF gradient southeastward and offshore relative to prior cycles. However, strong confluent flow in proximity to the coastal front and Gulf Stream may allow for scattered thunderstorms to remain possible very close to the beaches of southeast North Carolina and the South Carolina coast. Higher probabilities suggest strongest rates/totals will remain offshore. While there was some solid consideration for removing the Slight Risk from this update; collaboration with the local forecast offices and a few remaining solutions that bring the surface rooted frontal convection further north to the coast have maintained a narrow Slight Risk mainly for the coastal cities of Northeast SC/Southeast NC. ....Northern California/Great Basin/Idaho,W Montana Rockies... The sharp, elongated meridional trough axis has trended a bit faster in the northern portion to allow for a slightly more evolved wave along the frontal zone reaching far NW California Coast, increasing duration of moderate onshore rainfall allowing for a slight northward expansion of the Marginal Risk across SW OR. Additionally, the upstream strong shortwave originating from Kamchatka, rapidly advances and deepens the upstream wave and further narrows the spacing between it and the leading wave. This provides increased downstream divergence across the Great Basin into the Idaho/Montana Rockies, while also strengthening the cross Sierra Nevada flow to allow for a favorable back-building regime across western Nevada (just east of the natural rain shadow of the Sierra Nevada Range). Potential repeating convective elements across N NV, SE OR and SW ID show an uptick in rainfall totals with even some HREF probabilities of 1"/hr toward late evening, early overnight period. Instability is a bit better along the upwind edge reaching near 1000 J/kg across the Great Basin enough that 2"/12hr probabilities are nearing 50% in central NV. At this time, confidence is increasing, but not enough to delineate any Slight Risk area at this time. Gallina ~~~~Prior Discussion~~~~ ....Southeast... Guidance continue to depict T.C. Nine propagating north toward the Southeast Coast on Monday, while a surface front draped along the coast acts as the focus for heavy rainfall. QPF has trended downward given slowing trends in the track of T.C. Nine on Monday. The GFS suite in particular diverges most from the Canadian and Euro suites when depicting the system's slower forward motion. Despite this, enough moisture should work its way into the coast and interact with the surface front to generate scattered thunderstorms. There'll be enough moisture (1.5-2.5in PWATs) and instability (1000+ J/KG MUCAPE) present to support rainfall rates in excess of 1 in./hr.. ....Northern California/Great Basin/Southern Cascades... An upper low centered over the East Pacific will send mid-level vortices into the West Coast on Monday. Pacific moisture may collide with the remnant tropical moisture from Narda to produce heavy rainfall from northern California into the Great Basin. Synoptic forcing provided by a strong surface cold front should focus PWATs in the range of 0.75-1.25 in. to produce quarter inch rain rates along the immediate coast. More robust instability over the Great Basin should yield higher rain rates between 0.5-1 in./hr. in some areas. Upslope enhancement may generate higher totals in parts of the terrain. Kebede Day 3 Valid 12Z Tue Sep 30 2025 - 12Z Wed Oct 01 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE GRAND STRAND AND SOUTHEAST NORTH CAROLINA... 21z update: ....Southeast... As noted above in Day 2, the trends for Day 3 QPF are even further offshore as the sharp eastward turn of T.S. Imelda is expected. The coastal front and general confluent flow along the Gulf Stream north and northwest of the developing system will keep some risk of thunderstorm activity in the vicinity of the coastal cites. Much like Day 2, there was a strong consideration for full removal of the Slight Risk, however, in coordination with the local forecast offices felt removal may be premature, with some scattered thunderstorms in the area with potential for training/back- building off the Gulf Stream instability gradient and now lies along the urban centers of the Grand Strand through Wilmington, NC. ....Intermountain West... At the start of the Day 3 period (30.12z), the end of the Hi-Res CAM window, most show some organized showers and embedded convective activity along and east of unseasonably strong Pacific cold front. Initial meridional mid-level trough will be weakening, but upstream stronger trough will be sharpening the flow and maintaining favorable divergent flow aloft to provide larger scale ascent. Highly anomalous moisture with 2.5-4 standard anomaly values in the .75"+ range remain confluent on 15-25kts of 850-700mb southwesterly flow. The remaining Hi-Res CAMs (Nam-Nest and GEM Regional), along with global guidance (GFS/ECMWF) suggest some weak remaining instability/weak lapse rates to support some continued convective activity through the early morning. Some insolation could help recharge the upper reaches of the Snake River Basin and combined with solid upslope flow may reinvigorate convective activity across the terrain of SE ID into W WY. Scattered spots of 1-2" are possible. This atypical moisture regime and solid dynamic forcing suggest introduction (supported by continuation from Day 2 period) of a Marginal Risk across the northern Great Basin into the Snake River Basin for Day 3. Gallina ~~~~Prior Discussion~~~~ ....Southeast... The inherited marginal and slight risk areas along the Southeast Coast were downsized based on trends in the guidance suggesting T.C. Nine's hard turn out to sea on Tuesday. Despite the resulting smaller QPF footprint than previous cycles, the flash flood risk remains. Instability will be negligible (less than 500 J/Kg MUCAPE), which supports the possibility that the flood threat will be due to runoff over saturated soils from Sunday and Monday's rainfall. Exceedance probabilities of over 2 inches is over 10% in the 00z ECENS/CMCE, while below 10% in the GEFS along the immediate Carolina Coast. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9yPOzGL9oyHiBEtiIushDRM9R_t6bGNc0FUruPl0c_5X= vkxXC8CV9jQJNW7EiYKk3oUh7c0Zrl_OVAejQjbu7juiK_0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9yPOzGL9oyHiBEtiIushDRM9R_t6bGNc0FUruPl0c_5X= vkxXC8CV9jQJNW7EiYKk3oUh7c0Zrl_OVAejQjbu5RVs0Eo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9yPOzGL9oyHiBEtiIushDRM9R_t6bGNc0FUruPl0c_5X= vkxXC8CV9jQJNW7EiYKk3oUh7c0Zrl_OVAejQjbu8X3o1Wg$=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) .