Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 22 2025 00:46:47 FOUS30 KWBC 220046 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 846 PM EDT Sun Sep 21 2025 Day 1 Valid 01Z Mon Sep 22 2025 - 12Z Mon Sep 22 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR NORTHWEST ARKANSAS... ....Central-Southern Plains into the Mid Mississippi Valley... While an initial shortwave is moving towards the MO/AR/OK border junction currently, another shortwave is upstream across eastern KS. Fairly unidirectional flow out of the west-southwest will exist ahead of the KS shortwave. Along with 25-35 kts of effective bulk shear and a broad area of 500-1500 J/kg of MU CAPE will support=20 continued convection from southeast OK through northwest AR into southern MO. Adding to the flash flood sensitivity is heavy=20 rainfall which occurred across portions of northeast OK, northwest=20 AR, and southwest MO over the past 24 hours. A Slight Risk area=20 remains across portions of this region, which were advised by continuity, radar reflectivity trends, the 18z HREF, and the 12z=20 REFS guidance. ....Southern California into Arizona... An upper level trough across central CA will keep favorable right=20 entrance region jet dynamics over Southern CA into Arizona. While ongoing convection has been mainly constrained to southeast AZ, some of the mesoscale guidance suggests renewed activity across portions of western and central AZ as the upper level trough interacts with lingering 500-3000 J/kg of MU CAPE, with more than=20 sufficient effective bulk shear available to organize convection.=20 As moisture remains above average here as well, locally heavy rains and isolated runoff issues are expected intermittently through=20 12z. ....Southern Minnesota... Convection under the base of an upper level low moving across central MN is expected to persist overnight within an environment of 1.25"+ precipitable water values and 1000-2000 J/kg of MU CAPE, with plentiful effective bulk shear to potentially enhance=20 convective organization. Given the moisture values, believe any heavy rain-related issues would be isolated. Added a Marginal Risk into this area per the ingredients listed above and the 12z=20 REFS/18z HREF probabilities of 2"+ through 12z. Hourly amounts to 1.5" with local totals to 3" are possible where cells manage to train and/or merge. Roth Day 2 Valid 12Z Mon Sep 22 2025 - 12Z Tue Sep 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL TO SOUTHERN PLAINS, MIDDLE TO LOWER MISSISSIPPI VALLEY AND OHIO VALLEY AND ACROSS ARIZONA INTO WESTERN NEW MEXICO... 19z Update: There may be an opportunity to add in a Slight risk or two on future updates, by for now we will maintain the broad Marginal risk from portions of the Plains into the OH and TN Valley. The 12z high res guidance indicates one area of heavier rainfall (upwards of 2-4") across portions of central KY and TN. Considered a targeted Slight risk here, but this is a notable QPF uptick from the previous cycle, and would prefer to wait for another cycle to see if the signal remains consistent. On top of that, FFG is quite high over this area, and so even with the increased QPF signal the HREF probabilities of exceeding 3hr FFG remain low. Another area to watch is western to central KS into northern OK where a convective complex is expected Monday night into early Tuesday. At least some training appears probable, but the extent remains a bit uncertain. Thus the Marginal risk should suffice for now. Chenard ....Previous Discussion... ....Mid to Lower Mississippi Valley into the Ohio Valley... The height falls pushing east southeastward day 1 into the Central to Southern Plains and toward the Mid Mississippi Valley will continue to move eastward through the Ohio Valley day 2. Well defined large scale lift in an axis of above average PW values, 1 to 1.5+ standard deviations above the mean, will support the likelihood of widespread moderate to heavy precipitation in an axis from northern AR, far southeast MO, far southeast IL, western to central KY, much of IN and OH. At the moment the convection across these areas may be fairly progressive. This and very low relative soil moistures, much of the area in moderate to severe drought, will keep the risk level at marginal for the time being. With respect to changes to the previous issuance, the northern edge of the marginal risk was trimmed to the south by about 50-75 miles to account for the latest model consensus. ....Central to Southern Plains... The marginal risk area from the previous issuance was expanded slightly westward into western KS to cover the model spread with the next round of convection forming ahead of the upstream amplifying height falls dropping southeastward late day 1 from the Northern Rockies into the Central Rockies/Lee of the Central Rockies on day 2. The GFS and NAM begin to diverge day 2 with the track of this closing off mid to upper level center, with the NAM taking a more southeastward track into the Central Plains, while the GFS takes a more south southeastward track into the Central Rockies. The NAM solution, at the moment, is an outlier with the EC, UKMET and CMC more in line with the GFS. The differing mid to upper evolutions have more of a qpf impact over south central WY into north central CO, where the more southeast NAM solution is much lighter than the remainder of the guidance. Do not believe there will be any runoff issues farther northwest across CO/WY given lack of instability. While models differ farther to the southeast, there are still non NAM solutions that have potentially heavy rains as far northwest as northwest KS day 2, supporting the west northwest expansion from the previous issuance. ....Southwest... The closed low off the Southern CA coast day 1 is expected to move little on day 2. The right entrance region of the jet streak to the east of this closed low will push eastward from its day 1 position, but continue to support uvvs enhancement in the above average PW axis, 1 to 3 standard deviations above the mean, that will remain in place across Southern CA into the Southwest. Additional scattered convection across these areas will support localized heavy amounts and isolated runoff issues. The previous marginal risk area was expanded westward into southeast CA to cover the model qpf spread. Oravec Day 3 Valid 12Z Tue Sep 23 2025 - 12Z Wed Sep 24 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN MISSOURI, NORTHERN ARKANSAS, NORTHEAST OKLAHOMA AND SOUTHEAST KANSAS... 19z Update: On Tuesday an uptick in the flash flood risk is expected across portions of eastern OK, southeast KS, southern MO and northern AR. Mid level shortwave energy, favorable upper jet dynamics and strong low level moisture transport all support an organized convective risk. Model instability forecasts are sufficient for deep convection capable of heavy rainfall rates, and PWs of at least 1-2 SD above average are expected. The current model consensus is for the max rainfall axis to stretch from west to east near the AR/MO border. This axis also saw heavy rainfall last night, and could very well see additional areas of heavier rain through Monday. When considering these antecedent conditions, an upgrade to a MDT risk may be needed on future updates across portions of the area. However, we are still pretty far out, and there remains some latitudinal spread with the heaviest rainfall axis. This event will soon move into the range of more of the high res models, at which point we will likely see some fine tuning and adjusting of the rainfall axis and magnitudes. Thus we will hold off on any upgrade, and still consider this a higher end Slight risk area. Either way would expect at least scattered instances of flash flooding Tuesday, some of which could be locally considerable. Chenard ....Previous Discussion... There continues to be model differences in the evolution of the mid to upper level trof over the Central to Southern Plains toward the Mid Mississippi Valley. The NAM continues to be an outlier in being more amplified and faster to push the height falls downstream. Despite the differences, models do show re-strengthening of the low level southwesterly flow ahead of surface low pressure moving east across Oklahoma into Arkansas. This will support potential for organized convection, that likely forms late day 2 to the north of this low across the Central Plains, continuing day 3 from southeast KS/northeast OK, across Southern MO and northern AR. Model consensus is for the heaviest totals across these areas with amounts of 2-4" possible. This heavy axis will possibly overlap where heavy rains have fallen over the past 24 hours and where additional heavy rains are possible during the day 1 period. There were no major changes to the slight risk area from the previous issuance. The marginal risk was extended farther westward into northeast CO where several models show heavy rain potential day 3. The eastern portion of the marginal risk was also extended into the Upper OH Valley to cover the model qpf spread. ....Southern to Central California... The nearly stationary closed low off the Southern CA coast day 2 will begin to slowly move back to the north on day 3 toward the Central CA coast. This slow north movement will also allow for a northward expansion of the anomalous PW values over Southern CA into Central CA, with values as high as 3-4 standard deviations above the mean. There are differences with qpf distributions given differences with the northward push of the closed low. Not a lot of confidence in any solution at this range, but continued potential for locally heavy rains and isolated runoff issues, especially over any burn scars. No changes made at the moment to the previous broad marginal risk area over Southern to Central CA. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!89DQGVhHmg8BsgSkY1cstxr58Oofnh1tlb8fZWzWw-CZ= TPjBP-9z2Mh_WmDRg4l9C5ERosBnoUHnVsQ0xzckXP0FuJc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!89DQGVhHmg8BsgSkY1cstxr58Oofnh1tlb8fZWzWw-CZ= TPjBP-9z2Mh_WmDRg4l9C5ERosBnoUHnVsQ0xzckPdPtY_E$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!89DQGVhHmg8BsgSkY1cstxr58Oofnh1tlb8fZWzWw-CZ= TPjBP-9z2Mh_WmDRg4l9C5ERosBnoUHnVsQ0xzckjhFycl8$=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) .