Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 21 2025 19:02:44 FOUS30 KWBC 211902 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 302 PM EDT Sun Sep 21 2025 Day 1 Valid 16Z Sun Sep 21 2025 - 12Z Mon Sep 22 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE VICINITY OF THE KANSAS/OKLAHOMA/ARKANSAS/MISSOURI BORDER AREA... ....16Z Update... ....Central/Southern Plains to the Mid-Mississippi Valley... The inherited Slight Risk for portions of the Missouri/Kansas/Oklahoma/Arkansas region has been expanded southward across much of far eastern Oklahoma with this update. 12Z guidance has some agreement that once the area of showers and storms moving along the OK/TX border moves through eastern Oklahoma and into Arkansas later this afternoon, a renewed area of convection forms along a trailing boundary somewhere in eastern Oklahoma. While the agreement on where that trailing boundary will set up is poor, there is far better agreement that it may result in multiple hours of trailing thunderstorms in a narrow corridor aligned northwest to southeast. Perhaps with some persistence bias given the events of last night where narrow corridors of 4-6 inches of rain fell further north, resulting in localized flash flooding, confidence has increased for such an event to occur again a bit further south tonight. While rainfall amounts are expected to be lesser in this area tonight, any additional rainfall on top of areas hard hit tonight could cause renewed stream, creek, and river rises, so the Slight Risk in that area is maintained. Further north into Missouri, there is less confidence due to lesser instability. While the potential for training is high, how strong those storms get and how long they persist and where those most persistent bands set up across Missouri and into Illinois is of low certainty. Thus, for now the Marginal is maintained, but the Slight may need to be expanded on short-notice should the storms show the ability to produce heavy rains for an extended period of time. ....Southwest... Few changes were needed, as the forecast described below in the previous discussion remains valid. Wegman ....Previous Discussion... ....Central-Southern Plains into the Mid Mississippi Valley... Additional shortwave energy will be diving east southeastward through the base of the mean trof centered through the Central to Southern Plains into the Middle Mississippi Valley. These height falls will help strengthen the low level west south west to southwesterly flow into a west to east northeasterly oriented low level boundary stretching from the Southern Plains into the Middle Mississippi Valley. This will support another round of convection pushing east from central to eastern KS/OK into much of MO, northwest AR and central to Southern IL. A slight risk area was added in the vicinity of the KS/OK/AR/MO border area that has seen heavy rains over the past 24 hours and may see additional heavy totals later in the upcoming day 1 period. Otherwise, there were no significant changes made to the previous marginal risk area, that continues to fit well with the latest axis of high HREF neighborhood probabilities for 1 and 2"+ amounts during the upcoming day 1 period. Across these areas, relative soil moistures remain very low, which should help mitigate runoff issues, at least initially. ....Southern California into Arizona... The closed low initially off the Central California coast will be drifting only slowly southward day 1 to a position off the Southern California coast. A jet streak on the eastern side of this upper trof will keep favorable right entrance region jet dynamics over Southern CA into Arizona. This will help accentuate lift in an axis of anomalous PW values, 1 to 3+ standard deviations above the mean that are expected to persist across these areas day 1. This should support potential for additional widespread scattered convection, locally heavy rains and isolated runoff issues. Oravec 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=20 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=20 that, FFG is quite high over this area, and so even with the=20 increased QPF signal the HREF probabilities of exceeding 3hr FFG=20 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.=20 At least some training appears probable, but the extent remains a=20 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=20 expected across portions of eastern OK, southeast KS, southern MO=20 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,=20 and PWs of at least 1-2 SD above average are expected. The current=20 model consensus is for the max rainfall axis to stretch from west=20 to east near the AR/MO border. This axis also saw heavy rainfall=20 last night, and could very well see additional areas of heavier=20 rain through Monday. When considering these antecedent conditions, an upgrade to a MDT risk may be needed on future updates across=20 portions of the area. However, we are still pretty far out, and=20 there remains some latitudinal spread with the heaviest rainfall=20 axis. This event will soon move into the range of more of the high=20 res models, at which point we will likely see some fine tuning and=20 adjusting of the rainfall axis and magnitudes. Thus we will hold=20 off on any upgrade, and still consider this a higher end Slight=20 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!53vZvBQjfeVOLpokbfnH09UBUnsiIhQycEtcm6bf2K9U= MBmjS7vMUF4hwmskHlj62euYqyxNkTTRnlXgtWTWxmlIigg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!53vZvBQjfeVOLpokbfnH09UBUnsiIhQycEtcm6bf2K9U= MBmjS7vMUF4hwmskHlj62euYqyxNkTTRnlXgtWTWvkAhyQ4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!53vZvBQjfeVOLpokbfnH09UBUnsiIhQycEtcm6bf2K9U= MBmjS7vMUF4hwmskHlj62euYqyxNkTTRnlXgtWTWVyGByEs$=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) .