Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 28 2025 00:39:22 FOUS30 KWBC 280038 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 838 PM EDT Sun Apr 27 2025 Day 1 Valid 01Z Mon Apr 28 2025 - 12Z Mon Apr 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL AND SOUTHEASTERN MONTANA... ....01Z Update... Few changes needed to the previously issued Excessive Rainfall Discussion. Convection began increasing in coverage during the afternoon across portions of south central/northeast Wyoming in proximity to a quasi-stationary north-south oriented deformation zone. Concern for excessive rainfall is driven by modest amounts of rainfall falling on a warming snowpack at mid-levels across portions of south-central/southeast Montana into northeast Wyoming. Farther to the east in portions of southeast Wyoming into western South Dakota...flash flooding from convection firing along a well- defined dry line with easterly flow north of a warm front providing amply shear. The main concern from this convection looks to be hail but there is ample moisture flux to support increasing rainfall=20 production and risk of flash flooding from slow moving cells given=20 Flash Flood Guidance values ranging from 1 to 1.5 inches per hours=20 or near 2 inches in 3 hours across far southeast Montana into the=20 Black Hills of South Dakota. The activity farther west is expected=20 to take on a broader coverage of stratiform character later,=20 The Marginal Risk area extending across the remainder of the Dakotas into Minnesota remained in place without modification. The expectation remains for thunderstorms to develop later tonight with an increasing chance for heavy rainfall as the low level jet taps deeper moisture and transports the moisture/instability into the region. Bann ....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. Moderate rainfall rates up to an inch per hour (heavy in upslope 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. 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. 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 MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PLAINS AND INTO THE UPPER MIDWEST... ....20Z Update... In coordination with UNR/Rapid City, SD; ABR/Aberdeen, SD; BIS/Bismarck, ND; FGF/Fargo, ND; MPX/Twin Cities, MN; ARX/LaCrosse, WI; and DLH/Duluth, MN forecast offices, the inherited Slight Risk was removed with this forecast update. A 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. There remains some concern during this period with cell training, especially late Monday-Monday night across the Upper Midwest and Central Plains with the strengthening LLJ (50-55 kt at 850 mb) and weakening downwind Corfidi Vectors. The 12Z guidance remains highly variable with where any convection along the cold front will set up, regardless of training potential, with some of the guidance into northern Wisconsin and the UP of Michigan, with more limited potential further west from southeast ND into northern MN. This more limited potential is due to a preponderance of the guidance suggesting the dry slot will move over that area, greatly reducing the time of any rainfall through the period. For much of North Dakota southwest into southeastern Montana and far northern Wyoming, there is a bit higher potential for flash flooding stemming from rainfall and snowmelt from the northeast flow against the Bighorns, but for most of that area, soils remain very dry with an ongoing drought, and it's unlikely with limited instability on the cold side of the low that rates will overcome the otherwise sandy soils of the area to result in anything other than isolated instances of flash flooding. Hurley/Wegman Day 3 Valid 12Z Tue Apr 29 2025 - 12Z Wed Apr 30 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHWESTERN OKLAHOMA AND NORTH TEXAS... ....20Z Update... In coordination with OUN/Norman, OK forecast office, a Moderate Risk upgrade was introduced with this update. 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. Much of the Moderate Risk area picked up several inches of heavy rainfall from a slow-moving and persistent MCS that tracked across southwestern Oklahoma and north Texas yesterday. This has greatly diminished FFGs and swelled local rivers, streams, and creeks. While both today and most of tomorrow will be dry, (with severe thunderstorm potential tomorrow), it appears unlikely that there will be enough recovery to return soil conditions to normal by the time Tuesday's heavy rainfall event gets going. There are therefore heightened concerns for flash flooding. The Moderate Risk highlights the hardest hit areas expected to pick up heavy rain from both yesterday and expected again on Tuesday. Additional expansions may be needed if forecast rainfall increases further south and west in areas that were also hard-hit Wednesday, as well as towards the Northeast from Tuesday's rainfall alone, as the highest rainfall amounts for the day may be closer to the Oklahoma/Kansas/Missouri/Arkansas corners region. As is common for forecast areas of thunderstorms, shifts in the axis of heaviest rainfall are common and therefore adjustments to the outlook areas may be needed in the coming days. Hurley/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!7YddaiAQS0SbyLMtFiksr5THjFrosztOZT4qLu9WxUsp4NUYZzS78eG3B1xMQxJ= EpbymMGzVMnFpeESdJp6RDTbQO6k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!7YddaiAQS0SbyLMtFiksr5THjFrosztOZT4qLu9WxUsp4NUYZzS78eG3B1xMQxJ= EpbymMGzVMnFpeESdJp6RQNrsyRE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!7YddaiAQS0SbyLMtFiksr5THjFrosztOZT4qLu9WxUsp4NUYZzS78eG3B1xMQxJ= EpbymMGzVMnFpeESdJp6R4BdLozE$=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) .