Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 23 2025 00:45:26 FOUS30 KWBC 230045 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 845 PM EDT Mon Sep 22 2025 Day 1 Valid 01Z Tue Sep 23 2025 - 12Z Tue Sep 23 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR KANSAS... ....Central to Southern Plains... Thunderstorms with heavy rainfall are near the Black Hills of SD,=20 northern IA, and generally near the northern CO/KS border at the=20 time of this discussion's writing. The most organized convection=20 capable of heavy rain is forming near and north of a surface low=20 across northwest KS. The 850-700 mb moisture flux becomes=20 anomalous, upwards to 2 to 3 standard deviations above the mean by=20 Tuesday morning. The neighborhood probabilities in both the HREF=20 and RRFS for 3"+ amounts through 12z remain high enough to keep the Slight Risk in place. With time convection moving through eastern=20 CO should merge with the northwest KS convection and form a cold=20 pool to allow the activity to drop east-southeast as it tries to=20 ride in an near an instability gradient, with a similar expectation for the southwest SD thunderstorm cluster. Wherever cell mergers,=20 mesocyclones, or training elements occur, hourly amounts to 2.5"=20 are possible which would be most problematic in urban areas. ....Mid to Lower Mississippi Valley into the Ohio Valley... A broad marginal risk area was maintained in association with an upper level trough pushing east northeastward from the Mid=20 Mississippi Valley through the Ohio Valley. Well defined large=20 scale lift in an axis of above average PW values, 1 to 1.5+=20 standard deviations above the mean, will support the likelihood of=20 widespread moderate to heavy precipitation in an axis from northern AR, far southeast MO, far southern IL, KY, and the Upper OH Valley. Convection across these areas appears progressive. This and very=20 low relative soil moistures, much of the area in moderate to severe drought, keep the risk level at marginal for the time being. The guidance is unclear whether or not a heavier rain signal occurs near the KY/TN border overnight, which bears watch. Isolated Slight Risk impacts cannot be ruled out in that area. Roth Day 2 Valid 12Z Tue Sep 23 2025 - 12Z Wed Sep 24 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MISSOURI, ARKANSAS, EASTERN OKLAHOMA, WESTERN TENNESSEE, WESTERN KENTUCKY, SOUTHERN ILLINOIS, NORTHWESTERN MISSISSIPPI AND SOUTHEAST KANSAS... ....2030z update... The marginal risk area over central/southern California was removed from the Bay area based on drier trends in the qpf. The risk areas over the Central U.S. were expanded based on high 12z HREF exceedance probabilities (2+ inches) across the Middle Mississippi Valley. Anomalous moisture and instability along a strong surface front should promote heavy rainfall. There's uncertainty on where exactly the axis of maximum rainfall will develop, due to the global and hires models' difficulty resolving potential phasing over the Central U.S. on Tuesday. Kebede The strengthening 850-700 mb moisture flux late day 1 over the Southern Plains will remain anomalous day 2 as surface low pressure continues to push eastward from the Southern Plains toward the Lower Mississippi Valley. 850-700 mb moisture flux anomalies are forecast to remain in the 2 to 3+ standard deviation above the mean level. This will support the likelihood for the continuation of the day 1 organized convection over the Southern Plains, pushing eastward into the Lower Arkansas Valley and toward the Mid to Lower MS Valley. There are differences with the track of the primary surface low, resulting in north to south spread with respect to where the max precip axis will be. WPC is favoring the more southern qpf axes solutions, with the max axis depicted from far eastern OK, northern OK/far southern MO into western TN. This is a slight southward trend from previous forecasts, reflecting a southward trend in qpf guidance. Changes to the previous slight and 25% areas were to follow the slight southward qpf axis trend, pushing them approximately 40-60 miles farther to the south. ....Coastal Central California... There is good model agreement with the nearly stationary closed low off the Southern CA coast day 1 beginning to slowly move back to the north on day 2 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 coastal Central CA, with values as high as 3-4 standard deviations above the mean. With good model agreement on the closed low track day 2, there is also good agreement on potential for areal average moderate precip totals, with potential for locally heavier amount along the Central CA coast. Isolated runoff issues are possible, especially over any burn scars. Oravec Day 3 Valid 12Z Wed Sep 24 2025 - 12Z Thu Sep 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE TENNESSEE AND OHIO VALLEY REGIONS... ....2030z update... The marginal risk area over central/southern California was removed from the Bay area due to recent drier qpf trends. The Marginal risk area over the Southwest was confined to southern New Mexico, where the tail end of the Tennessee/Ohio Valley front may produce localized instances of flash flooding early on Wednesday. The slight and marginal risk areas over the Ohio/Tennessee Valleys were expanded to account for antecedent rainfall from today's storms. There isn't enough of a qpf signal to overcome the severe to extreme drought across the region so an ERO upgrade did not occur with this cycle. Kebede The mid to upper level trof moving east from the Southern Plains day 2 will amplify day 3 as it pushes into the Lower MS Valley. Anomalous PW and 850-700 moisture flux values will stretch across a large area from the Lower MS Valley, and into the TN & OH Valley, Central Appalachians and the Mid-Atlantic. Model consensus is for a broad region of moderate to heavy precip in this anomalous PW and 850-700 mb moisture flux axis. There is a typical amount of qpf spread for a day 3 time period. WPC is leaning more toward the multi model ensemble qpf mean axis/NBM axis to drive our qpf and excessive rainfall axis. While a large portion of the day 3 QPF and ERO axes are currently in moderate to severe drought, there may be overlap in qpf areas from day 3 and day 1 over the TN and OH Valley regions and overlap between the day 2 and day 3 qpf axes across the Lower MS Valley into the TN Valley, which will likely increase soil saturations and increase stream flows. The previous slight risk area was trimmed on the western edge by approximately 75-100 miles across far southeast MO, southern IL and southern IN to better fit the latest QPF. ....Central California... The slow moving closed low approaching the central CA coast late day 2 will continue to push slowly eastward day 3 and push inland by early Thursday morning. Similar to the day 2 period, an axis of anomalous PW values, 2 to 3 standard deviations above the mean, will persist to the east northeast of the closed low across Central CA. Scattered convection in this anomalous PW axis will support locally heavy rains across Central CA and isolated runoff issues, especially across burn scar regions. Only some small changes made to the previous marginal risk area, trimming it westward across western NV. ....Far southeast Arizona into New Mexico... No changes of note made to the previous marginal risk area across this region. Scattered convection possible, with localized heavy rain and isolated runoff issues, especially in burn scar regions. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!59bEQ8SMoGRUIJDRrYci71k3kiwLN5p7dPOklpa_wI6d= Qsa8aotAoU2BPAlv4C5Xx2XxZDMmfKgzepgc4Xqj3j9C1rY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!59bEQ8SMoGRUIJDRrYci71k3kiwLN5p7dPOklpa_wI6d= Qsa8aotAoU2BPAlv4C5Xx2XxZDMmfKgzepgc4XqjrhrCVN8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!59bEQ8SMoGRUIJDRrYci71k3kiwLN5p7dPOklpa_wI6d= Qsa8aotAoU2BPAlv4C5Xx2XxZDMmfKgzepgc4XqjPLgGsIo$=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) .