Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 26 2025 15:56:29 FOUS30 KWBC 261556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Tue Aug 26 2025 Day 1 Valid 16Z Tue Aug 26 2025 - 12Z Wed Aug 27 2025 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN ARIZONA, THE NORTHERN GREAT BASIN, THE SIERRA NEVADA, THE CENTRAL ROCKIES AND THE SOUTHERN PLAINS... ....The West... 16Z Update: SLGT risks inherited were either maintained or=20 expanded with the largest expansion occurring over the Great Basin=20 including much of eastern and south-central NV, as well as southwestern UT. Areal buoyancy during peak diurnal heating along=20 with increasing convergence on the western flank of the MCV located across the NV/UT border will lead to enhanced convective posture=20 across the terrain located north of the valley in southern NV. 12z=20 HREF probs for >1" increased substantially to 50-90% over much of=20 the above area with the >2" interval a solid 30-45% in the same=20 location. FFG's are naturally lower in these regions due to=20 flashier soils and complex terrain that leads to funneling,=20 something that will be greatly important to monitor as we move=20 through the day as the convective risk blossoms.=20 Kleebauer ...Previous Discussion.. The sprawling upper ridge over the Southwest continues to funnel rich monsoonal moisture northward through the Intermountain West. ECMWF guidance shows PWATs in excess of 1" advancing as far north as the Blue Mountains and Columbia River Valley of WA/OR. Some PWATs are also expected to top 1.25" in portions of the Great Basin and Snake River Valley. Helping to add some upper-level support aloft will be the presence of a meandering 700mb low positioned over the Pacific Northwest and a healthy diffluent pattern at 250mb thanks to a longwave trough situated along the West Coast. With such high moisture content, not much in the way of surface based heating will be needed to spark widespread thunderstorms Tuesday afternoon and through Tuesday evening. Combined with MUCAPE between 500-1,000 J/kg, thunderstorms will be capable of generating up to 1.5"/hr rainfall rates, which will pose a flash flood threat in areas of complex terrain, burn scars, and nearby creeks and streams. The Slight Risk over the northern Great Basin and portions of the Snake River Valley remains in place. The Slight Risk area resides within the strongest signal from the ECMWF-EFI with 0.8-0.9 values, which give support to highly unusual QPF totals and potential impacts for this time of year. A Slight Risk was also introduced over the central Sierra Nevada given the lingering threat for flash flooding over the rugged terrain that is growing increasingly more saturated and sensitive. Mullinax ....Arizona... 16Z Update: No major changes reflected in the area of the SLGT risk across AZ today. Another round of convection will spawn in-of the Mogollon Rim and the terrain of southern AZ to create another threat of scattered flash flood occurrences later this afternoon and evening. SLGT was expanded on the northwest edge to account for trends in CAMs signaling higher coverage near and south of Flagstaff later today. Kleebauer ...Previous Discussion.. Fresh off a busy day of thunderstorms on Monday, there is the possibility that the atmosphere is overworked to where converage of thunderstorms is not as expansive as Monday. However, southeast AZ was not as active on Monday and should tap into modest instability (500-1,000 J/kg of MUCAPE) and moisture aloft (PWATs up to 1.5" in some cases). This continues to support rainfall rates up to 2"/hr in the more intense storms. While storms will remain more pulse- like in nature, these potent storms still pose a flash flood threat, especially in dry washes and in spots that drain poorly. Mullinax ....Central & Southern Rockies... 16Z Update: Overall pattern remains favorable for widespread convective initiation over the central and southern Rockies this afternoon and evening with a favorable flash flood threat given the elevated PWATs and focused ascent present within the mid-levels. SLGT risk was expanded to the northwest to include more of western CO and all of southeast UT as guidances elevates the signal for locally heavy rainfall over some fairly sensitive terrain areas in the aforementioned locations. The SLGT was also expanded on the southern edge to include portions of the Sacramento Mtns, mainly in the vicinity of the burn scar near Ruidoso, NM where extreme=20 sensitivity persists. 12z HREF mean QPF is quite robust with great=20 agreement among the CAMs for locally heavy rain to spawn sometime=20 between 18-00z. Burn scar remnants and slot canyons will be the=20 most susceptible for flash flood concerns this period as rates in=20 the strongest convective cores will likely reach 1-1.5"/hr at their peak intensity. This is more than sufficient for flash flood=20 generation in these locations. Kleebauer ...Previous Discussion.. Guidance across the board shows a higher concentration of atmospheric moisture over the region on Tuesday compared to Monday, and that is notable considering PWATs were already above the 97.5 climatological percentile on Monday. PWATs in the tallest peaks will generally hover near 1" at their highest values, but along and just east of the Front Range, PWATs are likely to range between 1.3-1.5". These values are surpassing the 99th climatological percentile while also having 250-500 J/kg of MLCAPE present. With soils steadily growing more and more saturated due to recent heavy rainfall and the favorable atmospheric parameters in place, more instances of flash flooding are expected with complex terrain and burn scars more at-risk. Mullinax ....Southern Plains... 16Z Update: Complex of showers and thunderstorms over OK continue to migrate to the southeast with the main shortwave exiting the western half of the state leading to an end in the convective potential for parts of the state. A more stable environment awaits further east leading to the threat of heavy rainfall sort of petering out as it enters the Lower Mississippi Valley. Expectation is for the disturbance to continue its progression with a general decay in the rates and coverage over the course of the late-morning and afternoon. A brief lull to materialize across the Southern Plains, but more convection firing upstream will migrate over northeast NM into the bordering TX/OK Panhandle border with another period of locally heavy rainfall occurring late tonight, mainly beyond 03z. Signals are mixed on the threat, but regional forcing is sufficient for scattered heavy precip genesis, so wanted to maintain the MRGL to account for the threat. Look for the MRGL across OK to be removed in the next update at 01z, or sooner as the threat across central and eastern OK comes to an end.=20 Kleebauer ....Western Florida Peninsula... 16Z Update: No changes necessary to the previous MRGL risk forecast over the west-central FL Peninsula. Local convergence pattern remains prevalent in the vicinity of Tampa and surrounding locales leading to a low-end probability for heavy rainfall between 3-5" from any cells that migrate off the Gulf and move inland. PWATs are hovering ~1 deviation above normal with the 12z KTBW sounding indicating a PWAT of 2.25", right on track with the previous forecast lending credence to maintaining general continuity in the setup. Kleebauer ...Previous Discussion.. A similar setup to Monday is expected to produce thunderstorms capable of producing torrential downpours over the heavily urbanized western Florida Peninsula today. An approaching cold front will act to trigger thunderstorms in an atmospheric environment that contains up to 2.25" PWATs and MLCAPE around 1,000 J/kg. Surface-500mb mean winds remain fairly uniform out of the WSW, so there is the potential for training thunderstorms around the Tampa-St. Pete metro area. The 00Z HREF shows moderate- to-high chances (50-70%) for 24-hour rainfall totals >3" and low chances (10-30%) for 24-hour rainfall totals >5". The inherited Marginal Risk remains on track with the more urbanized areas containing the greatest concentration of hydrophobic surfaces most at-risk for flash flooding. Mullinax 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... ....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... ....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!9zOLeJlpu28Qx5U5WYPr484iMUEfuJip3D1FFTcaguAs= RtZhGUT3UyhPko_F5zXCzwZ5_foyE8Pd8CImIdCsIveDlco$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9zOLeJlpu28Qx5U5WYPr484iMUEfuJip3D1FFTcaguAs= RtZhGUT3UyhPko_F5zXCzwZ5_foyE8Pd8CImIdCsUGr-xlQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9zOLeJlpu28Qx5U5WYPr484iMUEfuJip3D1FFTcaguAs= RtZhGUT3UyhPko_F5zXCzwZ5_foyE8Pd8CImIdCsr1fAfSo$=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) .