Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 29 2025 00:58:16 FOUS30 KWBC 290056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Mon Apr 28 2025 Day 1 Valid 01Z Tue Apr 29 2025 - 12Z Tue Apr 29 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PLAINS OF THE CENTRAL UNITED STATES... 01z Update... Maintained the inherited Slight Risk area over the South Plains where convection was still growing upscale this evening...but introduced two small but targeted Slight Risk areas. One of the areas was from Minnesota into a small portion of northern Iowa and southwest Wisconsin where the trailing end of a line of convection was beginning to slow forward speed while the northern portion of=20 the line remained progressive. Increasing speeds of the low level=20 jet should result in a period of enhanced low level moisture flux=20 convergence upon encountering the front. The threat appears to be=20 fairly short lived before the low level convergence weakens. Refer=20 to Mesoscale Precipitation Discussion 191 for additional details. A second targeted Slight Risk was introduced mainly across northwest Kansas where convection has grown into a line of storms aligned=20 with the same orientation as the low level flow. This has already=20 led to instances of flash flooding across parts of northwest Kansas and the expectation is for continued training into the early=20 morning hours given the quasi-stationary boundaries that the storms have focused upon. Refer to Mesoscale Convective Precipitation=20 Discussion 192 for additional details here. 16z update...for Southern Plains... A few 12z CAMs suggest a strong cluster/few super-cells that will be capable of intense heavy rainfall this afternoon across portions of KS/OK into Northwest Texas. Recent heavy rainfall in the region, highlighted by NASA SPoRT 0-40cm saturation ratios over 70-80% and well above normal in the 90-95% percentiles suggest little infiltration is expected even in rural locations. There remains sufficient uncertainty to the precise latitude of these individual cells or small clusters to have high confidence. 12z HREF probability have increased across the most saturated areas with greatest 1" and 2" per hour and 3-hour periods and now as high as 30% across the most saturated areas of Southern Oklahoma. In coordination with local forecast offices, this is sufficient highlight a small upgrade to a Slight Risk mainly along the the Red River into central OK where both meteorology and hydrology overlap the most. Gallina ~~~~Prior Discussions~~~~ ....Northern Plains & Upper Midwest... The period in question will remain active on the rainfall front, but a few key pieces will be missing, relegating flash flood prospects closer to the MRGL risk category as inherited from the previous update. Upper level progression from the Inter-Mountain West through the Northern Plains and Upper Midwest will exhibit a decaying mid to upper level structure of a closed ULL out west with a more sheared, progressive pieces ejecting into the High Plains and propagating northeastward through MN and northern WI. The evolution is such that a reasonable upper forcing pattern will materialize with scattered to widespread convective initiation located downstream of a broad diffluent axis situated from the Front Range through the Upper Midwest. Instability on the order of 1500-2500 J/kg of MUCAPE will be present ahead of the mean trough with ample mid-level perturbations ejecting into the northern tier of the CONUS during the morning and afternoon periods. To round off the setup, a budding 140-150kt jet streak will be oriented over south-central Canada providing a solid RER dynamic suitable for maintaining a more organized precipitation element over the northern tier of the CONUS. Despite the above support for convective prospects, the pattern will be in the process of breaking down as the upper low out west becomes a more open wave with the primary bulk of mid-level energy quickly ejecting northeast with a textbook strong PVA to NVA signature as the energy vacates over the northern Lakes. This setup has shifted to a very front loaded convective scheme over the Northern Plains into the Upper Midwest with a majority of the convective pattern quickly ushering east-northeastward through the course of the morning and afternoon periods. By the evening, remnant convective elements will become more scattered in nature, eventually moving out of the primary area of interest after nightfall leading to a diminished threat for the end of the forecast cycle. 00z HREF EAS probs remain pretty elevated (50-80%) for at least 1" across the eastern half of ND through Northern MN, but a sharp decline in probs for >2" leading the case for more isolated to widely scattered flash flood prospects in the given pattern. There are some respectable probabilities in the neighborhood scheme for >2" within the Arrowhead of MN back over the the Dakotas (50-80%), but again a sharp decline once into the >3" fields limiting the upper bounds of the potential. A few areas may see between 3-5" at the peak, but those will likely be few and far between for an area that is actually in a low-end drought overall. Considering the soil type and antecedent dry conditions in the region, the inherited MRGL was maintained with little change in the risk area over the Northern Plains into the Upper Midwest. ....Central and Southern Plains... The Central and Southern Plains will reside within a broad warm- sector environment positioned between a vacated warm front pressing north through the Mississippi Valley and a slow-moving cold front through the High Plains with a secondary cold forecast to plunge southeast through the Northern and Central High Plains by the end of the forecast cycle. A fairly meridional upper pattern will end up being a key factor in the eventual ejection of stronger shortwaves out of Chihuahua, MX, barreling northeastward through the Southern Plains with scattered to widespread thunderstorm development along and east of a dry line situated over West TX. The combination of relevant buoyancy/instability positioned over the Southern and Central U.S. coupled with favored regional ascent and thermodynamics will create a period of late-afternoon and evening convective development capable of localized flash flood prospects. Recent heavy rain episodes have contributed to an elevated soil moisture presence from the Red River Basin up through much of OK into southeast KS, and MO. Rainfall totals up to 2-3" in a short time frame will be capable of causing flash flooding within the bounds of those areas that exhibit those moist top-layer soils since the time frame for recovery will have been short. 00z HREF neighborhood probs for >1" are respectable (50-70%) within portions of the Central and Southern Plains with the focus generally over north-central OK, Concho Valley, and Western Rolling Plains of West TX. Probabilities for >2" are very low leaving the threat for flash flooding fairly conditional and likely more reserved for more sensitive areas within urbanized zones and areas more susceptible due to low-water crossings. Kleebauer Day 2 Valid 12Z Tue Apr 29 2025 - 12Z Wed Apr 30 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS INTO THE MISSOURI VALLEY... Numerous flash floods, some of which may be significant, are becoming more likely for portions of the Southern Plains on D2. The synoptic pattern continues to feature ingredients that will be supportive of an impressive rainfall event across portions of Texas and Oklahoma on Tuesday, and potentially as far northeast as Missouri, Illinois, Indiana, and Kentucky. The driver of this event will be an amplifying trough reflected by a mid-level low closing off over the Four Corners Tuesday afternoon. While there continues to be some uncertainty as to the speed and placement of this development, the deterministic global models have all trended deeper with this 500mb heights over AZ/NM by 00Z Wednesday, lending support to greater amplification of this pattern. As this low closes off, downstream ascent will begin to intensify through 700-500mb divergence, and increasing height falls, especially later D2 as the closed low begins to pivot more rapidly to the east. Overlapping this ascent, secondary jet streak development emerging from Mexico will lift meridionally into the Southern Plains, and as some interaction of the northern and southern streams occur, impressive upper diffluence will focus from Texas into the Mid- Mississippi Valley, leading to even more pronounced ascent on D2. At the same time, a cold front will be dropping southward from the Central High Plains, but it is likely that intensifying southerly return flow out of the Gulf will impinge this movement, leading to a nearly stationary front much of Tuesday aligned from the Rolling Plains of Texas E/NE towards the Bootheel of Missouri. The combined ascent of the low-level convergence along this front and the upper level synoptics will result in impressive and widespread lift, into which spokes of vorticity rotating around the trough over the Four Corners will lift periodically northeast through the period. This pronounced ascent will act upon a region characterized by impressive thermodynamics to support rounds of heavy rainfall. PWs as progged by the NAEFS ensemble may exceed the 97th percentile, which will overlap with MLCAPE of 1000-2500 J/kg to provide fuel for thunderstorms. Storms are expected to be widespread as shown by available simulated reflectivity from 12Z high-res CAMs, with training from WSW to ENE likely on mean winds of 20-30 kts aligned to the impressive instability gradient. While storms may be progressive, multiple rounds are expected, and Corfidi vectors aligned well to the right of the mean flow indicate that short term training of clusters is likely. Rainfall rates within any storms are likely to be impressive, with HREF probabilities for 2"/hr reaching 20-30% near the Red River Valley of the South, and reaching as high as 10-15% as far northeast as Kentucky and Indiana, leading to stripes of significant rainfall accumulations. The ECENS/SREF/GEFS 24-hr probabilities all feature a low to moderate risk (10-40%) for 3+ inches on D2, while the HREF is much more aggressive at over 90%. The ECENS/HREF overlap considerably in their footprint, but the GEFS/SREF are displaced to the west, indicating at least some continued spread in the axis of heavy rain. This axis will likely be modified by both the frontal position and timing of any mid- level shortwave impulses, but also through convective overturning and modifying of the environment through the day, with modest adjustment in response to differential heating boundaries that may develop from clouds and moist soils. This variation is additionally reflected by large spread in the WSE QPF plumes with minimal clustering around any value, but with a trend upward overall in the areal means. As far as those moist soils, the NASA SPoRT 40cm soil moisture anomaly is quite impressive, with widespread saturation above the 95th percentile from the TX Panhandle through much of the Red River Valley of the South and into eastern OK, with percentiles above 80% extending all the way into IL/IN/KY due to recent heavy rain the past few days. This has compromised FFG to as low as 1.5 to 2 inches in 3 hours in some areas, which has a 30-50% chance of exceedance according to HREF probabilities, highest across OK and TX. It is in this area where the inherited MDT risk mostly overlaps, and where the CSU UFVS first guess field suggests a moderate (non UFVS has an enhanced risk area). This is also where the EFI indicates a >70% chance of an extreme event for rainfall, which, while impressive, is lower than what is preferred for an increase in ERO category at this time. However, if confidence in placement can increase, especially if that occurs atop the most sensitive soils, a targeted high risk could be needed. Downstream and to the northeast, the cold front will press more rapidly eastward, but into a still favorable thermodynamic environment to support fast moving storms along the front, with rainfall rates exceeding 1"/hr at times. This has resulted in some minor cosmetic adjustments to the inherited MRGL risk. In general, though, the broad MRGL extending all the way through the Central and Northern Ohio Valley into Northwest PA and far Southwest NY was left unchanged. Weiss Day 3 Valid 12Z Wed Apr 30 2025 - 12Z Thu May 01 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE ARKLATEX, EASTERN OKLAHOMA, AND WESTERN OZARKS... Another active day of convection with numerous flash floods, some of which may be significant, is likely Wednesday across portions of the Southern Plains and Lower to Mid Mississippi Valley. The synoptic pattern continues to evolve in a way that is favorable to widespread convection with training heavy rainfall across portions of the southern and central CONUS. The primary driver for this development is a potent mid-level trough which is progged to move from the Four Corners Wednesday morning to the lower Mississippi Valley Thursday morning. As this trough moves east, it may evolve from a positive to a neutral tilt, increasing downstream divergence and height falls, especially from eastern TX/OK into AR/MO the latter half of D3. This ascent will be enhanced by the RRQ of a strengthening and meridionally advancing jet streak pivoting into the Great Lakes, leaving the strong and diffluent RRQ atop the most impressive mid-level lift. Combined, this will produce strong lift along and ahead of a cold front, into which impressive convergence and moisture confluence will additional help drive ascent. This deep layer lift will impinge upon a moistening column and low-level flow emerging from the Gulf maximizes moisture as reflected by a PW plume above the 90th percentile according to NAEFS translating slowly eastward through the day. As this front and the overtopping synoptic lift move into this moistening column, the result will be widespread showers and thunderstorms, aided by MLCAPE that may eclipse 2000 J/kg in parts of the region. Regional soundings indicate LIs that may climb to -6 to -10 after 00Z Thursday, which suggests wide updrafts to limit dry air entrainment despite the overall modest breadth of the PW plume. This suggests that rainfall rates of 2"/hr are likely as the LLJ backs and ramps up into TX, surging moisture northward, and setting up an environment favorable for training and backbuilding. Where these rain rates train, the major global ensembles all indicate at least a 10-20% chance for 3+ inches of rain, and locally 5+ inches of rain appears possible, although the location of the maxima vary considerably at this time range. Soil moisture across eastern TX, OK, and into southern MO is well above normal, reflected by NASA SPoRT 40cm soil moisture anomalies that are above 90% in many areas, and may be additionally enhanced by rainfall in the 48-hours leading into this D3 period. This will keep FFG marginally lowered, so any training of these intense rates will likely result in scattered to numerous flash flooding, especially from NE TX into far SW MO where the inherited MDT risk was only adjusted cosmetically. There is some potential for the need for an upgrade in parts of TX/OK D3, but at this time confidence is not high enough, and this decision will be deferred to later updates as the D1 and D2 rainfall footprints materialize. Otherwise, D3 changes to the ERO were minor with the SLGT and MRGL risks extending as far northeast as St Louis, MO and Cincinnati, OH, respectively. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!585AmR56bVEdXGJMmdplZRrEYHam7Q6Vk_SfDDLJ7Iityw4EirsWL_FDGqCtnbS= lwNVvUI4TkRWTyfmXvYPG9blKXEo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!585AmR56bVEdXGJMmdplZRrEYHam7Q6Vk_SfDDLJ7Iityw4EirsWL_FDGqCtnbS= lwNVvUI4TkRWTyfmXvYPG2dJnEtY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!585AmR56bVEdXGJMmdplZRrEYHam7Q6Vk_SfDDLJ7Iityw4EirsWL_FDGqCtnbS= lwNVvUI4TkRWTyfmXvYPGo63cdp0$=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. 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