Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 27 2025 16:00:27 FOUS30 KWBC 271600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Wed Aug 27 2025 Day 1 Valid 16Z Wed Aug 27 2025 - 12Z Thu Aug 28 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER SOUTHEAST KANSAS, FAR SOUTHWEST MISSOURI, AND FAR NORTHEAST OKLAHOMA... 16Z Update: A Moderate Risk of Excessive Rainfall was introduced this forecast cycle for southeastern KS towards the very edges of both southwest MO and northeast OK. Higher end SLGT risk is forecast for the Upper Snake River across eastern ID into far southwest MT. More on these setups below... ....Central Plains... A sheared 500mb vort max exiting the Central Rockies will spawn a surface low over the southern High Plains today. Working in tandem with a dome of high pressure to the east, this will tighten the pressure gradient and prompt a strengthening LLJ to form over the Southern Plains late Wednesday afternoon and into Wednesday night. This enhanced LLJ will not only act to deliver rich Gulf moisture northward, but it will also cause 850mb frontogenesis to ensue over the Central Plains. PWATs will surge to 1.8-2.0" across much of southeastern KS into the adjacent borders of MO/OK between the 00-12z time frame. With MUCAPE ranging between 500-1,000 J/kg and the incoming anomalous moisture advection regime, these parameters would support up to 2"/hr rainfall rates with some intra-hour encroaching 3" within storms that develop more prolific meso- cyclones. This is due to vertical wind profiles that sport 30-40=20 knots of sfc-6km shear and sfc-1 km SRH values in excess of 200 m/2 s/2. These vertical wind shear values support mesocyclone=20 formation, which are efficient rainfall producers within=20 environments containing deep warm cloud layers and highly saturated sfc-500mb environments, all of which apply in this situation. There has been a notable uptick within the 12z HREF prob fields that brought attention to a growing concern for more locally significant flash flood and areal flood concerns during the time frame of interest (00-12z Thu). Neighborhood probabilities for totals rainfall >3" are now above 80% over a large area encompassing much of southeast KS, including near the Salina and Wichita urban corridors. There's even a bullseye of 90+% location to the northeast of Wichita, a testament to a strong signal for repeated rainfall in proxy the maximized 850mb FGEN evolution. The most prominent signal is now incorporated into the higher end precip accums with the same neighborhood prob field indicating between 50-70% for >5" within that aforementioned corridor referenced above, as we as upwards of 25-30% for locally 8+" in the area from I-35 to the northeast of Wichita down to the MO/AR/OK/KS intersection. HREF EAS fields are also very robust for even the 2" (50-80%) and 3" (25-40%) signals, a strong correlation between the various CAMs input in the hi-res ensemble. Individual deterministic output indicates some 5-7" totals within each member, but a few=20 more robust outputs within the ARW/Nam Nest windows indicate upwards of 9-10" across that area north and east of Wichita. The metro corridor is right on the edge of the heaviest when assessing the prob fields, so it will be prudent to monitor near term trends in guidance and general observations to determine if a higher risk is necessary for the population center. For now, a MDT risk is now in place within that region extending from Salina down to the quad-state border with an inclusion of both far northeast OK and southwestern MO.=20 Meanwhile the Slight Risk area in the Central High Plains and along the Front Range of the Rockies remains in place. Many locations now sport overly saturated soils, as shown by the NASA SPoRT-LIS 0-40cm soil percentiles which are above the 90th percentile. PWATs remain above the 90th climatological percentile and anywhere from 500-1,000 J/kg of MUCAPE will provide sufficient instability to support up to 1.5"/hr rainfall rates along the Front Range, and perhaps as high as 2"/hr in the CO High Plains. The Slight Risk remains in place due to the combination of anomalous moisture and modest instability triggering storms over areas that feature sensitive soils. Mullinax/Kleebauer ....The West... 16Z Update: The previous SLGT risk was relatively unchanged with the signal still prominent within the latest 12z CAMs suite for locally heavy rainfall up to 2-3" in spots. 10 year ARI exceedance probabilities are now 80+% across the Upper Snake River basin that includes those locations north of PIH towards the ID/MT state line. Environmental conditions are ripe for locally enhanced rainfall over those more sensitive locations situated across the area in question. KBOI sounding this morning was a resounding 1.53", putting it 5th all time and smashing the daily record by more than 0.3". These elements favor a higher than normal threat for flash flood concerns leading to a more higher end SLGT risk wording necessary for areas of the Snake River Basin over into the Sawtooth Range.=20 Kleebauer ...Previous Discussion.. There remains no shortage of monsoon moisture throughout much of the West, but the focus for flash flooding will become more pronounced in the Northwest. An organized 700mb low will have an abundance of moisture in place from OR/WA on east into the northern Rockies. PWATs from the Blue Mountains and Snake River valley on east across the Bitterroots and into southern MT/western WY are likely to top the 99.5 climatological percentile and the presence of the 700mb low will provide additional synoptic-scale support. In addition, modest instability for the region will be present with the ECMWF-EFI showing a pronounced signal for unusually high CAPE (or instability) values across the northern Great Basin, eastern OR, and much of ID. The same tool (ECMWF-EFI) depicts an impressive QPF footprint over much of eastern OR, southern ID, northern NV, and into much of WY. Sufficient instability (500-1,000 J/kg MUCAPE) is also present for storms to tap into from the northern Great Basin on north into the Northern Rockies to justify rainfall rates that approach 1.5"/hr. The Slight Risk remains on track given the forecast remains largely unchanged, and thus provides additional confidence in the flash flood potential in the Bitterroots, Tetons, and Snake River Valley today and into this evening. Mullinax Day 2 Valid 12Z Thu Aug 28 2025 - 12Z Fri Aug 29 2025 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS, CENTRAL ROCKIES, & LOWER MISSISSIPPI VALLEY... ....Central Plains & Lower Mississippi Valley... A 500mb vorticity maximum tracking towards the Ozarks on Thursday will work with the diffluent right-entrance region of a 250mb jet streak to provide excellent upper-level ascent over the Lower Mississippi Valley. Further supporting the synoptic-scale forcing over the Southern Plains and Lower Mississippi Valley is the GFS/ECMWF showing 700mb Q-vector convergence over the region. This is all occurring in the same area where low-level WAA is occurring amidst WSW flow into a warm front that will meander around the ArkLaTex on Thursday morning. This WAA will also usher in rich Gulf moisture into the region that increases PWATs to 2.0-2.25" throughout the Southern Plains and Lower Mississippi Valley. There is also no shortage of instability, highlighted by a plume of 1,000-2,000 J/kg of MLCAPE for storms to work with. Given these parameters, maximum rainfall rates could range between 2.5-3.0"/hr within the most intense thunderstorms. These storms will also have sufficient vertical wind shear (35-45 kts) and low-level SRH above 200 m2/s2, fostering mesocyclones within some thunderstorms just north and east of the warm front. All these factors support the potential for flash flooding within the Slight Risk area. Aside from minor tweaks based off latest WPC QPF trends, no significant adjustments were made this forecast cycle. ....Intermountain West... A large pool of monsoonal moisture remains anchored over the Rockies and Great Basin, highlighted by PWATs that are routinely above the 90th climatological percentile in these regions. The Northern Rockies will still have the lingering 700mb low over the region that helps provide modest upper-level ascent to support developing thunderstorms. Farther south, the Central Rockies are located at the nose of a 250mb jet streak and low-level convergence along the Front Range will foster widespread thunderstorms. Soils will continue to saturate further in the days leading up to Thursday over the Northern and Central Rockies that will further exacerbate the flash flood threat. A Slight Risk is in place over the Central Rockies that spills over into the the central High Plains given the higher PWATs and instability in the region, coupled with the gradually moistening soils. Given the lingering spread in guidance regarding the placement of the strongest thunderstorms, a Slight Risk was not issued as of this forecast cycle. Should confidence increase in both heavier totals and placement, and should antecedent soil moisture conditions continue to grow more sensitive following Wednesday's rainfall, a categorical risk upgrade may be necessary. Mullinax Day 3 Valid 12Z Fri Aug 29 2025 - 12Z Sat Aug 30 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A LARGE PORTION OF THE GREAT PLAINS, THE FRONT RANGE OF THE ROCKIES, AND ACROSS MUCH OF THE DEEP SOUTH... A pool of anomalous atmospheric moisture will maintain a stronghold over the Northern Rockies and Plains on south through the Southern Plains. A series of 500mb disturbances exiting the Great Basin will provide some upper-level support for thunderstorms from the northern Rockies and Plains on south to the Central Rockies Front Range and Plains. Farther south, a nearby frontal boundary in the southern High Plains will be the primary trigger for thunderstorms. This area will have a greater concentration of PWATs to work with (1.3-1.5" in the TX/OK Panhandles into eastern NM) and greater MLCAPE values (topping 1,000 J/kg). These areas will have seen soils dry up following a couple days of drier weather, and given some global guidance members differ on the location of the storms, there was no categorical risk upgrade this forecast cycle. Farther east, a wave of low pressure will track east along a frontal boundary that stretches from the Front Range of the Rockies to Florida. Numerous showers and thunderstorms are likely to envelope the South, with the Lower Mississippi Valley favored to see the heaviest rainfall at this time. Guidance shows the Lower Mississippi Valley sporting the highest PWATs (2.0-2.25") and instability (MLCAPE 1,000-2,000 J/kg) while also being more favorably located closer to the emerging area of low pressure in eastern TX. The GEFS/GEPS camps favor the Lower Mississippi Valley, but many ECMWF members were still showing storms that could produce locally heavy rainfall as far east as eastern GA. Given the lingering spread in model guidance, and the Lower Mississippi Valley's currently higher 1-3hr FFG exceedance values, there was no categorical risk upgrade hoisted as of this forecast cycle. But the >2.0" PWATs and modest instability does support a localized flash flood threat across much of the South during a time that also just so happens to be the kickoff to Labor Day weekend. Flash flooding could result in some locally hazardous travel conditions, especially along roads that drain poorly. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4oUiuMHMUgvvHBaj8bA-fpp8ACdNAenAcim8nRhRsFpp= NReAx0LGwP99xdJoFPmLaZY6xjSNPkd3xCIbOVCI4U_HBhE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4oUiuMHMUgvvHBaj8bA-fpp8ACdNAenAcim8nRhRsFpp= NReAx0LGwP99xdJoFPmLaZY6xjSNPkd3xCIbOVCI2yRwePY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4oUiuMHMUgvvHBaj8bA-fpp8ACdNAenAcim8nRhRsFpp= NReAx0LGwP99xdJoFPmLaZY6xjSNPkd3xCIbOVCIkaiLa_Q$=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) .