Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 12 2025 00:45:36 FOUS30 KWBC 120045 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 845 PM EDT Thu Sep 11 2025 Day 1 Valid 01Z Fri Sep 12 2025 - 12Z Fri Sep 12 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 EASTERN FLORIDA, THE NORTHERN ROCKIES, AND THE CENTRAL INTERNATIONAL BORDER WITH CANADA... ....Eastern Florida... Onshore flow within a moist and unstable air mass behind a front=20 sagging through the Florida Peninsula brings a possible threat of=20 heavy rainfall during the overnight hours from the Space Coast=20 south to the Gold Coast. Given the effective bulk shear available regionally, there is some chance that short training bands could develop from time to time and pose an isolated flash flooding risk, primarily after 06z. ....Northwest... An area of well above average moisture on the northwestern side of a nearly stationary longwave trough/cutoff low over the=20 Intermountain West has acted as a focus for shower and=20 thunderstorm activity in the typically dry areas from eastern=20 Oregon northeast through much of Montana. Both the 18z HREF and 12z REFS indicate that heavy rainfall would be possible for another several hours. Areas most sensitive to heavy rainfall would be=20 urban, slot canyons, burn scars, or any dry washes/arroyos. ....Central U.S./Canadian border... Showers and thunderstorms have shown some training character today in far northern ND. Both the 12z HREF and 18z REFS indicate some chance of overnight activity redeveloping and festering between northeast MT and northern MN, so kept the Marginal Risk in place. Roth Day 2 Valid 12Z Fri Sep 12 2025 - 12Z Sat Sep 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH FLORIDA AS WELL AS FOR PORTIONS OF SOUTHWESTERN COLORADO INTO CENTRAL NEW MEXICO... ....South Florida... The front across South Florida on Friday will make additional progress southbound down the Peninsula. The portion moving down the west coast of the peninsula will make better progress southward compared to the portion of the front along the East Coast. This should support the front becoming increasingly aligned southwest to northeast. Since the front will be moving a bit more on Friday, there should be a bit better coverage of storms across south Florida, where the Slight Risk area inherited remains largely unchanged. As the front pushes south, it will shove the bulk of the tropical moisture into the Florida Straits and into the Bahamas. This should work to limit storm coverage on future days, as a much drier and cooler air mass continues to envelop the rest of the state. ....New Mexico and Colorado... 21Z update... The environmental setup continues to look rather favorable for heavy rainfall for the higher terrain of Colorado and New Mexico. The Slight Risk area was reshaped a bit on the eastern and southern sides across New Mexico to account for the latest WPC QPF and model trends. The Marginal Risk remained in good order. Campbell The positively tilted upper level trough from Thursday will begin to pivot to become neutrally tilted on Friday. This means the southern end of the trough will get a boost to advance eastward, while the northern end will largely stay in place. The result will be a more south to north oriented axis of heavy rainfall, stretching from the southern end of New Mexico north across much of western Colorado. Moisture from the Pacific will also be drawn northward along the New Mexico/Arizona border. This should expand the rainfall footprint for Friday across much of the southern portion of New Mexico and southeastern Arizona. Expect Friday to the wettest day this week for New Mexico and Colorado. CAMs guidance suggests the plume of moisture stretching NNE from southeast Arizona through north-central New Mexico and into Colorado will be the focus for additional convection capable of producing flash flooding. With better southward extent, the storms will be able to tap into some deeper moisture and instability from the Pacific on Friday. This will make thunderstorm coverage Friday greater than in previous days. Since the whole plume will also be lifting north and east, expect a much larger area of rain to impact the area Friday. Embedded thunderstorms among the mostly lighter shower activity will be the foci for any potential flash flooding. This will be particularly true in any areas where multiple rounds of storms producing heavy rain occur. ....Northwest into Northern High Plains... 21Z update... Still expecting convection to fire up across the Intermountain West and track eastward into the High Plains during this period. There was a notable uptick in QPF for western portions of North Dakota, as such, the Marginal Risk was expanded further into western North Dakota and northwest South Dakota to capture this trend. Campbell Shortwaves and vorticity maxes rounding the periphery of the longwave trough will be the foci causing additional convection from northern Idaho east across much of Montana. The expected rainfall forecast has decreased the overall rainfall in this area. However, mountain forcing may offset this in some areas. Wegman Day 3 Valid 12Z Sat Sep 13 2025 - 12Z Sun Sep 14 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN NEW MEXICO THROUGH SOUTHWESTERN KANSAS, AS WELL AS FOR A PORTION OF THE URBAN CORRIDOR OF SOUTH FLORIDA... ....New Mexico through Kansas... 21Z update... The best focus for heavy rainfall and associated flash flooding will continue to be across eastern New Mexico, Texas Panhandle and into western Kansas during this period. The Slight Risk was reshaped a little to become more SSW to NNE orientated, rather than SW to NE. Campbell As the upper level trough over the Intermountain West becomes more negatively tilted, a portion of the low level jet out ahead it will draw deeper Gulf moisture north into the Panhandles and eastern New Mexico and into southwestern Kansas. This will support a line of storms that will develop over central New Mexico and then press eastward into the Slight Risk area. Once over eastern New Mexico and the Panhandles, the eastward progress of the storms will diminish greatly, resulting in much slower-moving but still organized storms. This will be the basis for training, resulting in widely scattered instances of flash flooding where the storms are the most persistent. ....South Florida... The front over South Florida will finally push east and clear all of the deepest tropical moisture well out into the Atlantic and Bahamas by Sunday morning. Thus, expect Saturday to be a transition day in the area...where there will still be storms, particularly along the urban I-95 corridor from Miami through Palm Beach, but there will be less coverage over Florida as the moisture is pushed eastward out into the Atlantic. The newly issued Slight is a low- confidence one, as once the front clears the state, that should end the flooding threat. However, given the many days of daily afternoon convection in the I-95 corridor, soils are sensitive enough that even the reduced time and coverage of storms could still cause widely scattered instances of flash flooding. ....Northern Plains... 21Z update... A minor trim was made to the southern bounds across central South Dakota as the latest guidance suggests QPF may not advance that far east during this period, thus lowering the threat for excessive rainfall. Campbell Across the Northern Plains, ahead of the neutrally tilted trough that is becoming negatively tilted, the northern extent of the low level jet ahead of the trough will draw deeper Gulf moisture westward across the Dakotas. This should support persistent convection. However, this area of the Dakotas is fairly difficult to flood, so despite the higher probability for training and heavy rain, the area remains in a higher-end Marginal for now. A Slight Risk upgrade may be needed for portions of the area as guidance begins to resolve this part of the event across the northern Plains. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6qai9V0v2dO4sl8YACTwTp86NCZqL_Sfl-Y1nbUd3iNU= Q3SsGhF3ejQkkYkzInwmpw3K9rKVhWyiknAXiPZ3JSz0MSk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6qai9V0v2dO4sl8YACTwTp86NCZqL_Sfl-Y1nbUd3iNU= Q3SsGhF3ejQkkYkzInwmpw3K9rKVhWyiknAXiPZ3oQiaDRE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6qai9V0v2dO4sl8YACTwTp86NCZqL_Sfl-Y1nbUd3iNU= Q3SsGhF3ejQkkYkzInwmpw3K9rKVhWyiknAXiPZ3NcTPX0g$=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. 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