Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 18 2025 00:29:05 FOUS30 KWBC 180028 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 828 PM EDT Sat May 17 2025 Day 1 Valid 01Z Sun May 18 2025 - 12Z Sun May 18 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS=20 OF SOUTHERN PLAINS TO LOWER MISSISSIPPI VALLEY AND NEW ENGLAND... ....Southern Plains and Lower Mississippi Valley... Convection across Northeast TX will continue motioning to the east-northeast over the next several hrs., following an instability tongue aligned south of a quasi-stationary front. Mean flow will keep the storm cluster moving fairly progressively with sights on Northern LA and Southern AR over the course of the overnight. LLJ will initiate shortly providing a mechanism for sustaining current=20 convection, allowing it to trek all the way towards MS by 06-09z.=20 Storms will begin to lose their muster from there as LLJ across the Southeast is much weaker, so the threat is expected to wane prior=20 to the end of the D1. Latest HRRR has had a good handle on the=20 convective setup with the primary axis of heavy precip aligned=20 within 40 miles north/south of the I-20 corridor which is where one portion of the MRGL risk remains.=20 Two strong singular cells, one right-moving supercell and one left-split supercell will enter similar spacing over the next hour with a likely cold pool merger in-of Southeast OK. Guidance is not as keen on maintaining a strong convective structure as the influence of each other will act to cancel out with a weakening pattern as we move further east. Prior, heavy rain will be situated over the area between I-35 and I-30 creating a swath of heavy rain in their respective paths. The positive in the cell motions are the forward propagation speeds are relatively steady, so the flash flood threat so far has been tamed with severe weather the primary impacts from each cell since origin. Further west, a weak mid-level perturbation will eject out of the TX Panhandle with mid-level steering pattern sending energy due east through OK. Cells have already materialized across the High Plains northeast of Amarillo with an expectation of more cells to materialize as energy pivots eastward and interacts with a 30-35kt LLJ structure over Central OK. Pockets of up to 2" of rainfall are possible within a corridor along and north of I-40 leading to isolated flash flood concerns overnight. The previous MRGL risk was generally maintained from last update to account for the setup.=20 ....New England... Slow-moving ULL over Ontario/Quebec will continue providing significant forcing the next few hrs across Northern New England with a line of thunderstorms motioning eastward through NH at this hour. Pockets of 1-1.5"/hr rates could cause localized flash flood concerns the next 2-3 hrs. before the threat fades as cells rotate into Maine and eventually die off. Further south over Western MA, a strong thunderstorm capable of rainfall rates between 1-2"/hr will move east along I-90 with localized flood prospects until the cell dissipates. This is more of a near term trend that has been under-modeled in terms of intensity. The MRGL risk over Northern New England was kept across Eastern VT through NH and Southwest ME with an extension through North-Central MA to encompass the strong thunderstorm occurring at this juncture.=20 Kleebauer Day 2 Valid 12Z Sun May 18 2025 - 12Z Mon May 19 2025 ....THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY... 20Z Update: The overall synoptic pattern remains steady within the run to run variance as the longwave evolution still indicates a broad upper trough migrating east with large scale ascent increasing over much of the Southern Plains to Mississippi Valley. The northern extent of the convective threat and subsequent flash flood concerns are still locked in as secondary surface reflection over the Western High Plains will induce a regional axis of sfc-700mb convergence with the best threat of convection aligned within a defined theta_E ridge positioned over the Front Range up through south-central MT. Slower storm motions and terrain influences from local topography within the Big Horns will lead to general training with heavier convective cores likely within the broad instability and low-level ascent pattern. 12z HREF neighborhood probs for >2" is between 30-60% over a large area between the northwest Sandhills up through Central MT, a signal that is coincident with elevated flash flood concerns due to the local topography and complex drainage in-of those mountains/hills. A targeted SLGT was contemplated, but the coverage of heavy convection and the current FFG exceedance probs for the region above are still on the lower end with the best probabilities aligned within the Big Horns up through the Crazy and Little/Big Belt Mountains. The MRGL was sufficient for this update, but will monitor trends for a potential upgrade in future updates. Across the Southern Plains to Mid-Mississippi Valley, the threat is largely influenced by the broad large scale evolution upstream creating a significant diffluent pattern within a corridor of elevated instability and deep moisture presence. As of the 12z suite, the jury is still out on exactly how the more organized convective scheme across the Central Plains and Mid-Mississippi Valley will transpire with the general deterministic variance positioning the threat from as far north as Northeast KS and Southeast NE to as far south as Southern MO, a solid 200 mile deviation from one member to the next. The CAMs suite provides a little more favor in-between with the ML output sort of edging into that corridor from Northeast Oklahoma up into Central MO. Considering some overlap with the CAMs on the alignment, the threat for flash flooding is likely highest in that general zone with a margin of error within 50 miles in any direction. The SLGT given the above information was relatively unchanged with a high-end SLGT still focused over the region extending from Northeast KS to points southeast with the southern edge over the Ozarks of AR/MO. A targeted MDT risk is possible, but with still some discrepancy in the exact placement of the expected MCS, the SLGT was maintained. The MRGL risk into the Southern Plains seems locked in as well with a potential for a targeted SLGT pending any overlap with convection that occurs tonight. The DFW metro is the main population center of focus in this setup as convection fires within a zone of very unstable air. Rates of 1-2"/hr will be possible with forward propagation speeds another deterrent for an upgrade at this time. The MRGL risk across New England was adjusted a touch south to include some lingering convective concerns over north-central VT/NH. Otherwise, the setup didn't offer any significant run to run discrepancies. Kleebauer ...Previous Discussions.. ....Plains, MS Valley and Southeast... A favorable pattern for widespread convection and heavy rainfall on Sunday across the middle portion of the country. A longwave trough and embedded shortwaves pushing eastward across the Rockies and into the Plains will provide ascent over a broad region. The frontal pattern will likely feature a developing low moving into the High Plains of CO/NE, a warm or stationary front extending east of this low across portions of OK/KS into MO/AR, and a dryline across OK/TX. A large pool of instability is expected east of the dryline and south of the stationary front, with values upwards of 4000-5000 j/kg. Some convection may be ongoing near the warm/stationary front Sunday morning, but more robust development is expected by Sunday afternoon into the overnight hours as stronger forcing ejects into the Plains. The main area of focus for flash flooding appears to be near the aforementioned warm/stationary front where some training of convection is probable. The overall expectation is that robust convective development occurs along/ahead of the dryline over central OK/KS during the afternoon hours. Strong deep layer southwesterly flow will likely quickly push this activity northeastward towards the warm/stationary front. Activity will likely grow upscale Sunday evening into the overnight, potentially into an MCS, at which point it should feel the impact of the strong southwesterly low level jet and begin turning easterly and then southeasterly aligned with the upwind Corfidi Vectors. As this process occurs some training/backbuilding on the south/southwest extent is probable resulting in a scattered flash flood threat. The exact MCS location/track remains a bit uncertain. All but certain that the 00z NAM is too far north, as its QPF evolution is an outlier and does not align with pattern recognition. The GFS and GEFS mean are also probably too far to the northeast, with the aforementioned expected MCS propagation taking it on a farther south track. The 00z Gem Reg and RRFS are very far south, tracking across eastern OK into central/northern AR. This may be too far south, but can not be ruled out as easily as the farther north NAM can. The AIFS has a track record of pinpointing organized convection location quite well, and the 00z AIFS favors northeast OK into southeast KS and southwest MO. This also happens to generally be the middle ground of the other available models, and so does appear to represent the most likely outcome at this time. Thus a higher end Slight risk will remain in place across this corridor, where scattered flash flooding is expected. The Slight risk area was cut back a bit on its northern extent and expanded to the southwest into more of OK and AR to account for the southern trend. Can not rule out additional southward shifts/expansion..but for now think the Slight risk area represents the general region of most concern well. Another area of interest will be farther northwest into central and western NE, closer to the low track. A multi-model signal exists for a second QPF max over this area...which makes sense given what should be impressive synoptic driven ascent and instability along the stationary/occluded front. Considered expanding the Slight risk into this area...but opted against that for now. This area remains in severe to extreme drought and possible the rainfall max ends up over the high FFG sand hills. But we will continue to monitor trends and an additional focused Slight risk may eventually be needed. A localized flash flood risk also exists both along the dryline from central TX into OK...and over the Southeast where a lingering boundary and substantial instability will support heavy rainfall with any storms that area able to develop. ....Northeast... A Marginal risk continues over portions of northern New England on Sunday as a deep layered low remains overhead. This will be the 3rd straight day of localized heavy rainfall, and thus some areas may be more sensitive by this time. Generally thinking both instability and moisture will be lower by Sunday, however cold air aloft under the low will likely still support some heavier convective cells and an isolated flash flood risk. Chenard Day 3 Valid 12Z Mon May 19 2025 - 12Z Tue May 20 2025 ....THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY... 20Z Update: The previous broad SLGT risk was generally maintained with an expansion into portions of the western Ohio Valley given the latest trends in a further east push of convection Monday night into early Tuesday. The theme for D3 is broad convective pattern meeting with wetter antecedent conditions come Monday evening. The overwhelming consensus is the coverage of thunderstorms will be vast with thunderstorm genesis likely from south-central TX all the way up through the Missouri Valley. Two questions remain; where will there be an intersection of heavy rainfall in successive days that would exacerbate flash flood concerns, and what latitude will there be an eventual MCS propagation? The jury is still out on those specifics, but the easy signature to note is the available moisture and instability with a sharpening dryline over the Southern Plains and strong upper forcing coinciding within a very large jet coupling between the occluded cyclone to the north and the southern stream jet displaced just to the south. A large area of heavy QPF is likely within that zone above creating a broad coverage SLGT risk with higher end flash flood threats within the coverage. A targeted MDT is more likely than not (Although not a perfect slam dunk) considering the D2-3 tandem of heavy rainfall in the same areas. The exact placement is subject to variability, so the higher risk was not added on this update. Despite that, the pattern is indicative of widespread flash flood concerns with some locally significant flooding increasingly favored given the evolving pattern. Stay tuned for future updates and potential upgrades as we move closer. Kleebauer ...Previous Discussion.. Another day of widespread heavy rainfall expected across the middle portion of the country on Monday as the longwave trough and embedded shortwave energy slowly moves eastward. It will likely be an active day/night with a broad region where flash flooding will be possible. A Slight risk stretches from northeast TX northward into southern IA. The northern extent of the Slight risk will be close to or just north of the stationary front extending east of the central Plains low pressure. Strong forcing overrunning this boundary will likely result in an expansive area of convection, with some training/backbuilding possible. The southern portion of the Slight risk is driven by initial dryline convective development, likely enhanced by the eventual eastward acceleration of the cold front south of the low. The convergence from the dryline, cold front and lifting warm front will all aid in convective development. The general stagnant nature of the frontal feature through much of the day (both the warm/stationary front and dryline) support a training/backbuilding convective threat. Exactly where the focus of highest QPF ends up remains a bit of a question, but generally anywhere within the Slight risk is in play. There is some signal that the axis from northeast TX into southern MO could actually see the most intense rates and greatest flash flood potential, with both the 00z CMC and AIFS highlighting this corridor. However ingredients will also be quite favorable farther north into MO and southern IA as well (pending exact frontal positioning) and thus both of these areas could end up seeing training/backbuilding at some point Monday and/or Monday night. Some of these areas will also likely see heavy rain on day 2 as well, and so hydrologic conditions could be more sensitive by this day 3 time frame. Thus do consider just about all of the Slight risk as a higher end Slight at this time, and think areas of flash flooding are likely. It seems possible (maybe even probable) that an embedded MDT risk will eventually be needed somewhere within the broad Slight risk area. However uncertainty in both the convective details and hydrologic conditions (pending what rainfall happens on day 1 and 2) leads to low confidence in any MDT risk location at this time. Thus we will hold with a higher end Slight and continue to monitor trends. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9jNgqT1j2amIwwv4gIpjSgtQD5lnwntTde4R9mABAt1s= p6wR7mCIEHV9q-uilrpu66scLBLPdLIloMLfq3bhE1bABQg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9jNgqT1j2amIwwv4gIpjSgtQD5lnwntTde4R9mABAt1s= p6wR7mCIEHV9q-uilrpu66scLBLPdLIloMLfq3bhUzFrCx4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9jNgqT1j2amIwwv4gIpjSgtQD5lnwntTde4R9mABAt1s= p6wR7mCIEHV9q-uilrpu66scLBLPdLIloMLfq3bhL78ltAU$=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) .