Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 13 2025 00:50:45 FOUS30 KWBC 130050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 850 PM EDT Fri Sep 12 2025 Day 1 Valid 01Z Sat Sep 13 2025 - 12Z Sat Sep 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NEW MEXICO & THE NORTHERN PLAINS... ....New Mexico & Colorado...=20 A meridionally-oriented axis of heavy rainfall has been moving=20 slowly eastward this afternoon into this evening within an=20 anomalously moist airmass with sufficient instability. Due to radar reflectivity trends and trends in the 12z REFS/18z HREF, reduced=20 the size of the risk areas, particularly the Slight Risk to cover=20 portions of southern and central NM. Activity should fade with time as CIN sets in and instability erodes further. ....Northern Rockies onto the Northern Plains... The upper low center shifts north to MT by late tonight, aiding in the import and persistence of above average moisture and sufficient effective bulk shear across the region. Isolated to widely=20 scattered showers and thunderstorms should persist across MT, with=20 the greatest threat of organized and persistent convection and=20 heavy rainfall remaining near the MT/SD/ND border overnight. =20 ....Florida...=20 A stationary front persists over South Florida. Moisture remains plentiful and effective bulk shear remains elevated, continuing the possibility of organized convection. The risk areas were=20 restricted to the southernmost peninsula where daytime=20 thunderstorms should linger for a couple more hours and for the=20 possibility of short training bands along the immediate East Coast where some onshore flow from over the unstable Gulf Stream waters could lead to random short, training band development. ....Upper Midwest... Ongoing activity over MN Arrowhead and portions of WI are expected to slowly fade overnight. Maintained the Marginal Risk for the heavy rain possibility over the next few hours, though the guidance does try to flare some overnight convection once more across northern WI. The 12z REFS/18z HREF and radar reflectivity trends were used to reshape the Marginal Risk area in this region. Roth Day 2 Valid 12Z Sat Sep 13 2025 - 12Z Sun Sep 14 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE TEXAS AND OKLAHOMA PANHANDLES INTO EAST NEW MEXICO AND FAR WESTERN TEXAS, AS WELL AS FOR WESTERN NORTH DAKOTA... ....20z Update... Some notable changes for this update include: 1) Trimming the northern end of the inherited SLGT risk, as the most notable signal for convective organization and potential training/repeating exists from eastern NM into the TX/OK Panhandles (as noted in the previous discussion with the enhancement of the low-level jet). The MRGL was still maintained into the Central High Plains, and in fact was generally expanded westward into more of the Central/Southern Rockies and Intermountain West as anomalous moisture may be slower to retreat than prior guidance suggested (based largely on the new 12z CAMs, as well as the character of convection today being more impressive than models indicated). 2) The removal of the SLGT risk over South FL, as the decreasing moisture trend noted below is still evident with a notable lowering of expected QPF based on the new 12z CAMs (which have generally been too hot in this pattern with convection over South FL to begin with). The MRGL risk was maintained for any lingering excessive rainfall threat (and may well be able to be removed early tomorrow). 3) Introduction of a MRGL risk for Chicagoland and surroundings (southeast WI, northeast IL, much of IN, and far southwest MI). While the threat for localized flash flooding is conditional, there is a notable signal from the 12z CAMs suggesting the potential for 3" exceedance (12z HREF 3" exceedance probs of 20-30%) with both the ARW and ARW2 showing potential for training heavy rainfall (as well as the NAM-nest, which is the heaviest outlier). Otherwise, adjusted the SLGT/MRGL borders for the Northern Plains based on the new guidance. Churchill ....Previous Discussion... ....Central High Plains through Southern Rockies... The upper level trough over the Intermountain West becomes negatively tilted on Saturday with the monsoonal moisture axis over NM during the day shifting to the southern High Plains overnight. The position of this trough will enhance the low level jet on the High Plains Saturday night and draw deeper Gulf moisture north over the Panhandles and eastern New Mexico and through western Kansas. This will encourage diurnally developing lines of storms from New Mexico terrain to press eastward onto the High Plains. Given the +2 sigma PW anomalies and overall slow storm motion, the Slight Risk area was expanded a bit over western KS and down into far western Texas. ....South Florida... The front will push south from Florida during the day Saturday with anomalous moisture lingering in the morning over the Miami metro where A Slight Risk persists. The Slight Risk still looks generous given the decreasing moisture profile through the day, but is maintained for now. ....Northern Plains... The upper low center over central MT will allow the focus for heavy convection to shift north a bit from Friday with both morning and diurnal max activity looking to focus on western ND. Given good overlap in heavier precip among available CAMs and the EC-AIFS, a Slight Risk is raised over western ND. Pivoting of the low without much lateral movement is expected, prolonged heavy rainfall is possible with. Jackson Day 3 Valid 12Z Sun Sep 14 2025 - 12Z Mon Sep 15 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE DAKOTAS... ....20z Update... Only minor adjustments were necessary based on the new 12z model suite, as the bulk of the guidance are in good agreement on the axis of highest QPF. Churchill ....Previous Discussion... ....Central and Northern Plains... Negatively-tilted trough axis lifting up the central Plains Sunday should provide a narrow focus to convective development over the western or central Dakotas. This activity will be heavy given PW anomalies of +3 sigma persist over this area. Further development is expected from central KS up through the Dakotas where a generous Marginal Risk remains in effect. Jackson Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_eINTUX3O7B4xwC_MJSDoDetOMYxKEjy7TPto8Ookjdl= vbGrWtjeaP5-plefHnFtNlJFjH9rcJpMix9bZ9I3t-ZjQHc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_eINTUX3O7B4xwC_MJSDoDetOMYxKEjy7TPto8Ookjdl= vbGrWtjeaP5-plefHnFtNlJFjH9rcJpMix9bZ9I3BsP9iZw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_eINTUX3O7B4xwC_MJSDoDetOMYxKEjy7TPto8Ookjdl= vbGrWtjeaP5-plefHnFtNlJFjH9rcJpMix9bZ9I3og1XDbM$=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) .