Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 12 2025 19:25:30 FOUS30 KWBC 121925 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 325 PM EDT Fri Sep 12 2025 Day 1 Valid 16Z Fri Sep 12 2025 - 12Z Sat Sep 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH FLORIDA, SOUTHWESTERN COLORADO THROUGH CENTRAL NEW MEXICO, AND THE NORTHERN PLAINS... ....South Florida... Stationary front persists over far South Florida today with the positive moisture anomaly focused over the Miami metro with the eastern coast likely to get more slow moving heavy thunderstorms in the light mean layer flow. The Slight Risk is shrunk to be closer to the Miami-Palm Beach metro area with the Marginal Risk still extending up the east coast nearly to Jacksonville. ....New Mexico and Colorado... The positively tilted upper level trough from an upper low over northern NV will pivot to neutral tilt this afternoon. Therefore, expect a more south to north oriented axis of heavy rainfall, stretching from south-central New Mexico to across much of western Colorado. Moisture from the Pacific will also be drawn northward into eastern Arizona and western New Mexico with PW anomalies of +1.5 to 2 sigma, resulting in a rather large QPF footprint. Diurnal convection is the main focus at play today, though guidance continues to suggest repeating rains over southeast AZ and southern NM into the overnight. The Slight Risk is maintained over southwest CO and central NM. ....Northern Rockies onto the Northern Plains... 16Z update... A very small westward expansion of the Slight Risk was made across southeast Montana to reflect the latest observations and QPF trends. Campbell The upper low center shifts north to MT by late tonight. Notable activity ahead of the low/trough looks to focus over the western Dakotas into eastern MT per 00Z CAM consensus where a Slight Risk has been raised. PW anomalies of 1 to 2 sigma remain over northern ID/western MT where the Marginal Risk is maintained. ....Upper Midwest... 16Z update... Several inches of rain were reported with the storms overnight/early this morning that are currently tracking across eastern North Dakota and northern Minnesota. The storms are straddling the northern boundary of the Marginal Risk and are increasing the threat for excessive rainfall for areas outside of the outlook. The northern boundary of the Marginal was lifted northward to include areas up to the International border of Minnesota. Campbell Ongoing activity over eastern ND and northern MN should shift southeast by 12Z and track to northern WI. Then diurnally driven convection looks to cross a similar path. Given the advance of moisture over the upper ridge axis extending over eastern IA, it was worth adding a Marginal Risk to northern MN/WI and the western U.P. of MI. Jackson 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!6vltT5ekkXKO5Za2YjghIypKPrsRw8etUrzuCCGsas0i= yw1bzrdUH4e8eDAuzgyHbiR-UimRFasJRFnM_dYX7xzPs-c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6vltT5ekkXKO5Za2YjghIypKPrsRw8etUrzuCCGsas0i= yw1bzrdUH4e8eDAuzgyHbiR-UimRFasJRFnM_dYXpmqOO08$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6vltT5ekkXKO5Za2YjghIypKPrsRw8etUrzuCCGsas0i= yw1bzrdUH4e8eDAuzgyHbiR-UimRFasJRFnM_dYX-BZoqss$=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) .