Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 27 2025 00:46:25 FOUS30 KWBC 270046 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 846 PM EDT Tue Aug 26 2025 Day 1 Valid 01Z Wed Aug 27 2025 - 12Z Wed Aug 27 2025 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SIERRA NEVADA INTO SOUTHERN NEVADA, AS WELL AS FOR MUCH OF THE ROCKIES FROM IDAHO TO NORTHERN NEW MEXICO... ....01Z Update... ....Sierra Nevada... Ongoing activity across the northern Sierras as well as into portions of southeastern California are being forced by an upper level wave embedded within a much broader trough whose axis is along the West Coast. This trough and embedded shortwave disturbance will kick eastward through the evening and overnight, as convection across southern California grows upscale and into a bubble of higher instability centered over southern Nevada. Meanwhile the other area of convection across northeastern California and northwestern Nevada will continue southeast and with additional upscale development, may try to link up with the convection to the south before all progressing east across Nevada. Once into Utah, instability greatly drops since ongoing convection there has largely used it up, and therefore, the lack of instability when the next round moves through tonight should result in a weakening and eventual dissipation of the areas of=20 convection. ....Rockies... Numerous areas of convection from New Mexico northwest into Idaho have resulted in widely scattered instances of flash flooding across this area. Since the longwave trough will be pivoting east into the ridging currently in place over this area, the additional forcing will allow convection across this region to continue at least until sunset. After that, much of the rainfall capable of producing flash flooding should wane. However, up across Idaho, strengthening vort maxes tracking north across eastern Idaho will likely couple with local upsloping into the various mountain ranges in the area to keep the more widespread, steady rainfall ongoing well into tonight. Given the continued signal for heavy rainfall in the latest guidance across portions of eastern Idaho, the higher end Slight continues, and is a bit expanded to the north and west=20 to account for ongoing shower and thunderstorm activity across=20 southern Utah. Meanwhile, the area of convection along the I-25 corridor along the Front Range of Colorado will continue to press south and east, merging with a front and a moisture tongue extending northwestward from the Gulf Coast states, which will likely result in renewed rainfall from southeast Colorado into Oklahoma. While much of this area has seen heavy rainfall recently, this time around instability will be an inhibiting factor, which should keep most areas to light and more beneficial rainfall. Thus, the Marginal into Oklahoma remains in place with few changes. Day 2 Valid 12Z Wed Aug 27 2025 - 12Z Thu Aug 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS... 20Z Update: The previous forecast SLGT risk areas over the interior Northwest CONUS, and Central Plains to Ozarks remain favorable for their respective pattern evolutions based off the latest 12z numerical suite. Trends have favored scattered heavy convective signatures for the SLGT risk over the interior Northwest with aid from a significant moisture advection regime inferring PWAT anomalies increasing closer to +3 deviations above climatological norm. Remnant shortwave propagation stemming from previous convective activity on D1 will be one of the main drivers for the potential as regional forcing will be ongoing under the influence of a consolidated MCV moving northward from UT into eastern ID. Guidance is consistent on the threat lingering from D1 to D2 with the shortwave becoming increasingly sheared by the afternoon time frame leading to more of a west to east aligned convective development during peak diurnal instability maxima. Low FFG's across much of ID/Eastern OR/Southwest MT infer more susceptibility for flash flooding concerns in these locations creating a suitable threat for flash flooding during much of the D2 period. Only minor adjustments were necessary, mainly expansion on the southern flank of the SLGT risk into the northern fringes of UT/NV. Across the Central Plains to Ozarks, consensus for the evolution of the next frontal wave maintains a pretty stout signature for heavy rainfall focused over central KS down through southeast KS into the MO/AR Ozarks as we move into the final 6hr window in the forecast cycle. One of the biggest changes in the forecast from the morning update was the convective pattern trending a bit faster and further east in terms of the heavier precip signal as the shortwave progression and evolving LLJ should spur the convective pattern to push downstream a bit quicker as it materializes. 12z HREF probabilities for all relevant intervals have shifted the maximum prob outputs further southeast into southwestern MO and even the northwestern portions of AR. This is a significant addition compared to the overnight iteration of the hi-res ensemble, as the previous forecast was focused more to the northwest comparatively. That said, there is a still a pretty strong signal overall for the heavy rain risk across all of southeast KS with the upstream depiction viable for the potential of >2" focused as far back as Hays, KS. The trend was just an adjustment in the heavier QPF output more into the MO Ozarks with a secondary QPF maxima positioned southwest of Springfield, MO. This is in good agreement with the latest EC AIFS Ensemble depiction which also trended ever so slightly to the southeast with the higher probability fields for the heavier precip notions. This was enough to warrant a small extension into those portions of MO/AR to account for the eastern expansion of heavier convection. Kleebauer ...Previous Discussions.. ....Central Plains... A sheared 500mb vort max exiting the Central Rockies will help to spawn a surface low over the southern High Plains on Wednesday. Working in tandem with the 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" within the Slight Risk area with MUCAPE ranging between 500-1,000 J/kg. These parameters would support up to 2.5"/hr rainfall rates within storms that could contain mesocyclones. This is due to vertical wind profiles that sport 30-45 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 formation, which are efficient rainfall producers within environments containing deep warm cloud layers and highly saturated sfc-500mb environments, all of which apply in this situation. Latest guidance has continued to trend wetter in the central High Plains and even along the foothills of the Central Rockies, leading to an expansion of the Slight Risk into northern CO and southeast WY. But the greatest concern lies over southern Kansas where even 75th percentile 24-hr QPF on the NBM is topping 5". This setup bears watching as locally significant flash flooding cannot be ruled out in parts of southern and eastern KS, northeast OK, and southwest MO. ....The West... 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. There is increasing confidence in enough instability (500-1,000 J/kg MUCAPE) 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 inherited 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 on Wednesday. Mullinax Day 3 Valid 12Z Thu Aug 28 2025 - 12Z Fri Aug 29 2025 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS & LOWER MISSISSIPPI VALLEY... 20Z Update: Main change this period was the removal of the SLGT risk across AL given the current trends maintaining the heaviest QPF signature further west into MS with a more pronounced south- southeastern push of the convective regime anticipated over the D3 time frame. The signal for significant rainfall still exists across much of Arkansas and the threat could very well be upgraded pending the influence of the CAMs and their more defined convective outputs for the threat. At this juncture, there was not enough of a robust QPF signature to warrant an upgrade over portions of the Lower Mississippi Valley, especially when you account for the substantially higher FFG's situated over the region. This period will be one to monitor closely given the areal average QPF at these leads which can infer at least the potential for a targeted upgrade. For the Central Rockies and adjacent Front Range of CO, the signal for heavy rainfall remains consistent in the assessment of run to run variability, so there was little reason to deviate from the previous forecast. Decided to maintain continuity given the consistency from previous forecast(s). Kleebauer ...Previous Discussions.. ....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 a healthy amount of 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 MUCAPE 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, but even as far east as MS/AL and as far west as central OK could also be at-risk for flash flooding on Thursday. ....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. A potential categorical risk upgrade may be necessary in the northern Rockies, but it will come down to how much instability is available and what the state of antecedent soil moisture conditions are following Tuesday and Wednesday's rainfall. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-K-T7_laH81f5fuIoK3HoCUSEJLIc7gH_aEP54Mh2hqT= MfkpfgyUBpPPbWBGv_1uDZAZJM-hqhmzBOGlDfM1tUhYjZ8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-K-T7_laH81f5fuIoK3HoCUSEJLIc7gH_aEP54Mh2hqT= MfkpfgyUBpPPbWBGv_1uDZAZJM-hqhmzBOGlDfM18j84f54$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-K-T7_laH81f5fuIoK3HoCUSEJLIc7gH_aEP54Mh2hqT= MfkpfgyUBpPPbWBGv_1uDZAZJM-hqhmzBOGlDfM1ptO0FHU$=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) .