Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 27 2025 15:35:56 FOUS30 KWBC 271535 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1135 AM EDT Sun Apr 27 2025 Day 1 Valid 16Z Sun Apr 27 2025 - 12Z Mon Apr 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL AND SOUTHEASTERN MONTANA... ....16Z Update... Very few changes were needed to the outlook areas with this update. Guidance remains largely on track, with the Slight Risk across south-central and southeastern Montana remaining in place.=20 Moderate rainfall rates up to an inch per hour (heavy in upslope=20 areas) will combine with snowmelt and burn scars to locally increase the coverage of instances of flash flooding to widely scattered. Most of the event will occur tonight, continuing in some areas into Monday morning.=20 Southeastern portions of the Marginal were trimmed over Minnesota, Iowa, and South Dakota with the latest guidance shifting the potential for training thunderstorms north and west.=20 Wegman ....Previous Discussion... ....Summary... Minimal modifications were made to the Slight and Marginal Risk areas from yesterday's Day 2 ERO. ....Northern High Plains to the Mid Mississippi Valley... Mid-Upper low migrating across the central Great Basin-central Rockies during the period will set the stage for robust deep-layer forcing over the outlook areas as the upper ridge axis moves east. Helping matters will be coupled upper level jet streaks, i.e. with the ribbon of enhanced lift within the right-entrance region of the upper jet streak traversing south-central Canada, while also within the left-exit region of the upper jet streak lifting into the Front Range and Central Plains. Robust low-mid level moisture transport turning westward into the maturing Cold Conveyor Belt (CCB) and maturing TROWAL will reach 2-3 standard deviations above normal across the Northern Plains per the 00Z GEFS, while anomalies peak between +3 and +5 across the Upper MS Valley Sunday night. This as the south-southwesterly LLJ peaks by Sunday night (~50 kts at 850 mb), while strengthening ENE-NE low-level flow peaking between 30-40 kts across the backside of the CCB will lead to a considerable upslope component to the lift across the foothills and High Plains of south-central and southeast MT into northeast WY. The Slight Risk area was largely unchanged. Peak rainfall totals of 2-4+" west and southwest of Billings will combine with high snow levels and burn scars to locally enhance the flash flooding potential in this area of Montana. Farther east across the northern Plains and Upper MS Valley, deep-layer instability will become more plentiful (MUCAPEs peaking aoa 1,000 J/Kg), while the low-level moisture transport becomes more robust Sunday night given the aforementioned uptick in the LLJ. Despite the more favorable deep- layer thermodynamical profile, the spread in the model QPFs is much higher over this area, particularly with the heavier rainfall. This is likely to the more transient (less persistent), mainly WAA- driven upper level forcing, along with the low-level fronts remaining upstream of the Upper MS Valley through Sunday night. In addition, the forward-propagating Corfidi vectors would support a rather progressive motion of any organized convection that is able to grow upscale. As a result, the Marginal Risk area was maintained across the lower (Northern) Plains and Upper MS Valley. Hurley Day 2 Valid 12Z Mon Apr 28 2025 - 12Z Tue Apr 29 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS INTO THE UPPER MIDWEST... ....Summary... Based on the favorable deep-layer synoptic forcing and thermodynamic profile and the uptick in areal-average model QPFs (especially the upper-bound totals), have hoisted a fairly large Slight Risk area across parts of the Northern Plains and Upper Midwest. Also expanded the Marginal Risk area across SD, southwest MN, and Iowa from yesterday's Day 2 ERO. Mid-level circulation will open into a shortwave trough across the northern Plains and Upper Midwest on Monday-Monday night. Ample pre-frontal low-mid layer moisture transport and PW values climbing to 1.5-1.75" will make for a favorable thermodynamic profile, along with deep-layer instability as MUCAPEs peak between 2,000-3,000 J/Kg (even ~1,000 J/Kg wrapping westward into the TROWAL zone). The biggest concern during this period will be with cell training, especially late Monday-Monday night across the Upper Midwest with the strengthening LLJ (50-55 kt at 850 mb) and weakening downwind Corfidi Vectors. The 00Z guidance has become a bit more clustered with the heaviest QPF within the Slight Risk area, with pockets of 3-5+ inches per the higher-resolution models. Hurley Day 3 Valid 12Z Tue Apr 29 2025 - 12Z Wed Apr 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS INTO THE OZARKS AND LOWER MISSOURI VALLEY... Another potential significant rainfall event will be taking shape across western portions of the TX-OK Red River Valley northeast into the Ozarks and Lower MO River Valley. This as the upper trough west/ridge east amplify and in-turn exhibit slower W-E progression. This will allow for a more prolonged period of favorable upper level forcing (coupled southern and northern stream jet streaks) along a surface front that will become quasi- stationary across OK- southeast KS into central MO. Robust synoptic-scale forcing, favorable thermodynamic profiles (Mixed Layer CAPEs 1000-2000 J/Kg and TPWs 1.5-1.75") and the the enhanced risk of repetitive convection along the slow moving/stalled frontal boundary will likely lead to impressive 24 hr rainfall totals within the Slight Risk area (3-5+ inches, with higher localized totals per the higher-resolution GEM-Region and RRFS runs). Tuesday night in particular may bring a more enhanced flash flood threat, as the LLJ veers and becomes nearly parallel (and of similar magnitude of) the mean 850-300 mb flow, thereby leading to weaker forward-propagating Corfidi Vectors and allowing for a greater risk of cell training. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!9NOn9KboixnxaNzaKZu4V1mxwpx5W186AQwejzt_8lbxDFRzT295qMlvZn5L9Uy= 1msQjOAJiqk1sOZ_mJf1hUZtH0N8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!9NOn9KboixnxaNzaKZu4V1mxwpx5W186AQwejzt_8lbxDFRzT295qMlvZn5L9Uy= 1msQjOAJiqk1sOZ_mJf1hmr3km_4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!9NOn9KboixnxaNzaKZu4V1mxwpx5W186AQwejzt_8lbxDFRzT295qMlvZn5L9Uy= 1msQjOAJiqk1sOZ_mJf1hzOqfKMQ$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .