Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 09 2025 00:51:23 FOUS30 KWBC 090051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Mon Sep 8 2025 Day 1 Valid 01Z Tue Sep 09 2025 - 12Z Tue Sep 09 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHWEST, SOUTHERN PLAINS, THE UPPER MIDWEST, AND FLORIDA... 01Z... While notable moisture remains, the MCV associated with the very heavy rainfall that occurred over portions of central Kansas has=20 moved east and SPC Mesoanalysis indicates the airmass has stabilized. Therefore, the Slight Risk over central Kansas and portions of the surrounding Marginal Risk that extended into the=20 lower Missouri Valley were removed. The southern extent of the=20 Marginal Risk that extended from southern Kansas down into the=20 Panhandle Region was maintained. Storms have been developing=20 through the evening along the increasing low level jet and a=20 deepening moisture (PWs ~1.5 in)/greater instability (MUCAPEs at or above 2000 J/kg) axis. MRMS estimates show localized amounts over=20 2 inches have fallen in the northern Panhandle Region, with the 18Z HREF indicating additional amounts of 1-2 inches likely within the region before storms wane later tonight. For the other areas across the Northwest, the Upper Midwest, and Florida, made smaller changes based on recent observation trends and hi-res guidance.=20 Pereira Previous Discussion... ....Texas Panhandle into the Central Plains... Ongoing convective complex over central Kansas is slow-moving and backbuilding this morning to produce locally very heavy rainfall and radar estimates over 5 inches. The environment over the central Plains isn't overly impressive, with PWs of 1.4-1.5", but MUCAPE of 1,000-2,000 J/kg and strong deep moist convergence on the nose of strong low- level flow amid modestly steep mid- level lapse rates aloft are likely the culprit for maintaining thunderstorms. However, veering and weakening low-level flow is expected to lead to diminishing updrafts through early this afternoon around 18Z and thus ending any additional flash flooding in central Kansas. MPD 1016 is in effect through 17Z for this region. Contemplated a targeted Slight Risk, but opted against considering the event is expected to end shortly and is still fairly localized. Isolated thunderstorm activity is expected from southwestern Kansas through the Texas Panhandle during the afternoon and evening hours. The Marginal Risk is being maintained for consistence with prior forecasts, although newest models depict modest convective coverage. Nevertheless, supercellular wind profiles and steep lapse rates aloft may result in intense convection and local rain rates exceeding FFG for a few hours during the afternoon and early evening. This Marginal is conditional on sufficient convective coverage for localized prolonging of heavier rainfall. Lastly, models depict a resurgence of elevated convection on the nose of strong low-level flow. The best convergence (and resultant convective coverage) should extend from southeastern Nebraska south-southeastward along the Kansas/Missouri border region. Localized backbuilding and training could result in spots of 2-4 inch rainfall totals, and flash flooding is possible on an isolated basis. Soil moistures are relatively dry though, further suggesting only an isolated flash flood threat. ....Oregon, northern California... A substantial mid-level system approaching the region evident on GOES-WEST mid-level WV imagery from the northeastern Pacific will trigger scattered convection this afternoon/evening. Ascent/cooling aloft will be well aligned with peak heating, resulting in sufficient instability for widespread showers and occasional thunderstorms. Recent CAMs suggest an area of focus for convection across higher terrain of southwestern Oregon (near the Medford area) and into terrain- favored areas of northern California. Appreciable mid- level lapse rates and PW values approaching 1 inch suggest potential for several areas of wetting convection, and areas of flash flooding are possible especially near burn scars and low- lying, flood prone areas. Deeper convection may also extend toward northern Oregon along the Columbia River Basin, with isolated flash flood potential expected. ....Upper Mississippi Valley... Low-level warm advection across a warm frontal zone from the Dakotas into Iowa should result in areas of showers and thunderstorms by 00Z along with surging PWs (remaining modest near 1.2-1.4"). The orientation of this convection favors some training/backbuilding in an east- west fashion that could enhance 6-hr rain totals locally. 2-3 inch totals are expected, with locally higher amounts possible particularly across northeastern Minnesota into the Arrowhead. This may lead to localized flash flooding, especially in urban locations. Only minor modifications were made to the previous outlook based on latest model guidance and a subtle northward trend. ....Florida... A nearly stationary front will remain across northern Florida. Abundant moisture (2+ inch PW values) should reside south of this front. A weak low should also reside just west of the state over Gulf of America waters. Convection is expected to develop over much of the Peninsula in tandem with daytime heating and destabilization. Cells should remain relatively slow-moving, with areas of 2+ inch/hr rain rates (and 3 inch totals) expected throughout the forecast period. Isolated flash flooding is possible especially where these heavier rates can materialize over urbanized/sensitive areas where mesoscale convergence exists associated with sea breeze effects and future outflow boundaries. Only change made to the previous outlook was to remove the Florida Keys and far southwest Florida. Snell/Cook Day 2 Valid 12Z Tue Sep 09 2025 - 12Z Wed Sep 10 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE FLORIDA PENINSULA, PARTS OF EASTERN KANSAS, NORTHERN CALIFORNIA AND WESTERN OREGON... ....Florida... Few changes were made from the inherited Marginal Risk area inherited from yesterday's Day 3 ERO. The quasi- stationary surface boundary over north Florida early Tuesday will nudge slowly into central FL by Wednesday morning as southern periphery of the mid- upper level trough remains parked across the central Gulf Coast region. Continued large-scale forcing downstream of the upper trough -- weak upper level difluence and subtle shortwave energy/periodic DPVA -- will again favor more widespread/organized convection during the peak heating hours from late morning into early evening on Tuesday. Low-level convergence over the eastern peninsula will likely get a boost from the sea- breeze component, as the southwesterly low-level flow weakens and backs more southerly near the east coast. As a result, could see more widespread convective clusters and heavier rain farther east toward the I-95 corridor, as supported by the higher HREF and RRFS probabilities of 2-3 inch/hr rainfall rates across this area after 17Z Tuesday. Thermodynamic parameters will be more than supportive for highly efficient warm rain processes, with mixed-layer CAPEs at least 1000-2000 J/Kg, TPWs of 2.2-2.3", and wet bulb zero levels above 14KFT (optimal warm layer depth). Moreover, 0-6km bulk shear values around 20-25 kts should allow for more convective organization, though the degree of downwind propagation (southwesterly Corfidi vectors around 10-15kts) should inhibit the potential for cell training. As a result, any excessive short-term runoff on Tuesday will be localized and likely tied to urban and/or poor drainage areas similar in nature to today (Monday). ....Eastern Kansas.. Continued mid-upper northwest flow will be ongoing across the central Plains Tuesday morning along with embedded shortwaves and the potential for ongoing overnight convection. PWs will remain rather unimpressive from an anomaly standpoint and near 1.5". However, the backbuilding potential will remain as relatively weak mean layer flow within a weak upper trough continues out of the west-northwest and advecting from the greater instability/lapse rates over the High Plains. CAMs remain uncertain and rather muted on amounts (generally under 2"), but there is some potential for an overachieving event similar to central KS today. Thus, a Marginal Risk was added to this region. ....Northern California and western Oregon... The mid-level system approaching the West Coast of northern CA/OR today will move over the region on Tuesday maintaining elevated moisture levels (PWs above the 90th climatological percentile) and sufficient upper lapse rates supportive of scattered convection. This may lead to isolated flash flooding where rainfall rates exceed 0.5"/hr near burn scars and sensitive terrain susceptible to rapid runoff. A Marginal Risk was added with this update over a similar area compared to Day 1. Snell Day 3 Valid 12Z Wed Sep 10 2025 - 12Z Thu Sep 11 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE NORTHERN GREAT BASIN AND MUCH OF THE FLORIDA PENINSULA... ....Northern Great Basin east of the Cascades and Shasta Siskiyous to central and northern Idaho... Deep/closed upper low centered near the OR/CA/NV tri-state area will average around 2 standard deviations below normal for early September. Plenty of deep-layer Qs forcing (upper difluence/divergence and periodic DPVA) will combine with above- normal moisture (TPWs near 1.0" or 2-2.5 standard deviations above normal) and sufficient diurnal instability (MUCAPEs 500-1000 J/Kg) to produce locally excessive rainfall across portions of the northern Great Basin Wednesday. The 00Z global guidance all show a modest rainfall footprint, with the ensemble blends showing areal-average totals between 0.50-1.00" within much of the outlook area. However, largely tied to the degree of deep-layer instability and available moisture, localized heavier totals were noted (2-4+ inches per the RRFS, GFS, and CMC), albeit with considerable areal spread. The GEM in particular was most robust, noting a swath of 3-5+ inches across central OR east of Bend. For now, given the spread in the guidance QPFs, will keep the ERO Risk at Marginal, though this setup synoptically certainly would have the potential for a more enhanced (i.e. Slight) risk as the event draws near and CAMs are available. ....Florida Peninsula... Similar setup to Tuesday, though the guidance shows a bit more robust shortwave activity SW-NE across the Peninsula along and south of the surface front which should remain quasi-stationary across central FL on Wednesday. As a result, the multi-model blend is higher with the QPF in terms of areal-average (ensemble mean) on Wed compared to Tue, particularly over the central and eastern portion of the peninsula where some backing in the low- level flow (from the Atlantic sea breeze) will likely enhance the low-level convergence due. Once again though given the downwind propagation (S-SW Corfidi Vectors ~10+ kts), anticipate progressive cell motions while not expecting much in the way of persistence or training of the convection. Therefore the excessive rainfall risk remains Marginal given the more isolated/localized risk for flash flooding. Snell/Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7q0OEI61LG1-9Y5BOiHv29XPwNa0_myjIjba_nyEoo2j= LDdKS5eqf8-E1g1rktdsicfh3G1NELLA01_Lzaj8-evFRro$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7q0OEI61LG1-9Y5BOiHv29XPwNa0_myjIjba_nyEoo2j= LDdKS5eqf8-E1g1rktdsicfh3G1NELLA01_Lzaj8qjeO-D4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7q0OEI61LG1-9Y5BOiHv29XPwNa0_myjIjba_nyEoo2j= LDdKS5eqf8-E1g1rktdsicfh3G1NELLA01_Lzaj83T_2XUs$=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) .