Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 19 2025 00:25:56 FOUS30 KWBC 190025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 825 PM EDT Sun May 18 2025 Day 1 Valid 01Z Mon May 19 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... 01z Update: The flash flood threat this evening will be predicated on the behavior of several cold pool mergers among the expanse of supercell/multi-cell convection situated across the Central and Southern Plains. There's really 3 areas of interest in the remainder of D1. The first is a more localized threat across North-Central TX where multiple splitting supercells along the dryline have already dropped some significant rainfall on a southwest to northeast orientation between Abilene and Throckmorton, TX. Additional rainfall will occur so long as the persisting mesocyclones in these storms continue to churn with localized totals reaching 2-4" in those areas where repeated cells occur. For more information on this setup, please see MPD #0284. A MRGL risk remains forecast across this locale. The second area of interest will reside over Eastern KS and MO where a quasi-stationary front will slowly lift northeast, bisecting the two areas above with persistent convective activity within the confines of the front. The issue from this setup is upstream convection will eventual merge due to several cold pools initiating off the plethora of cells forming across KS. Any mergers will likely yield a complex of moderate to heavy rainfall that will situate near proxy of the front as indicated by various CAMs. There is some discrepancy on the exact placement of heavier QPF, but the signal is present, so maintained general continuity from previous forecast, but downscaling the magnitude from a high-end risk to a more modest SLGT risk setup.=20 Further north, cell mergers over NE will likely yield a large MCS somewhere over the western Sandhills, propagating eastward within the confines of a tight theta_E gradient well-defined with the last mesoanalysis assessment. This has been a mainstay within the hourly CAMs, HRRR and RRFS suite, which is likely considering the speed shear component of the fcst hodographs across Central NE when LLJ factors come into play. The good news is the forward propagating complex would likely dump copious amounts of rain within the Sandhills leading to lower probability of flooding due to the sandier soils exhibiting much higher FFG markers. Further south towards the southern edge of the Sandhills, down towards Grand Island and Kearney, FFG's are lower due to the change in soil type, however lean dry due to the recent drought over the area. Some hydrophobic aspects of the soils could increase runoff given the low moisture anomalies, as well as larger population centers providing typical urbanization factors for runoff. Local areas could easily pick up 2-4" with the setup in a short time which will be enough to reach that lower-end of the SLGT risk threshold. Given the 18z HREF probs of 25-55% for at least 3" between North Platte and Hastings, including the two other towns above, this was enough to warrant the SLGT risk into those areas of NE.=20 A broad MRGL extends from the northern Big Horns down through the Central Plains and parts of the Southern Plains and Mississippi Valley on the edges of the SLGT risk in place. Kleebauer Day 2 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 12Z guidance did not support making any significant changes to the previous outlook. For reasons described below, still believe there is good support for a broad Slight Risk area extending from the Central Plains to the Ozarks and Mid Mississippi Valley. With differences in the details, most of the models also continue to show two areas of higher accumulations within this area, attributed in part to training/backbuilding convection. This includes the 12Z HREF, which shows high neighborhood probabilities for totals exceeding 3 inches centered over southern Iowa-northern Missouri and southern Missouri-northern Arkansas. Will continue to monitor the need for an upgrade to a Moderate, especially across this southern area, where the models are suggesting a greater threat for overlapping heavy amounts areas on days 1 and 2. Pereira 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. Not much change was needed to the inherited risk area, with a Slight risk stretching from northeast TX into southern IA. Expect convection to again develop Monday afternoon along the dryline and approaching cold front from TX into NE. The northern extent of the Slight risk will be near the warm/stationary front extending east across northern MO and southern IA. 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 from TX into eastern OK and AR covers initial dryline development and possible upscale growth. While the low pressure driven stronger convergence will be farther north over IA/MO, there are indications that a corridor of enhanced low level moisture transport could evolve from TX into AR aided by shortwave energy at the base of the longwave trough. Several pieces of guidance thus depict a secondary max QPF swath over these areas from possible training convection. 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 are meteorological reasons (described above) for two maximum rainfall swaths, one farther north near the warm front over northern MO and southern IA, and another from northeast TX into AR. However getting these details right at this lead time is difficult. So whether or not we end up seeing two separate areas of training convection or one more consolidated area, the main story is that the ingredients will be in place for training/backbuilding convection leading to excessive rainfall. Some of these areas will also likely see heavy rain on day 1 as well, and so hydrologic conditions could be more sensitive by this day 2 time frame. Thus do consider most of the Slight area as a higher end Slight risk 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 area. However uncertainty in both the convective details and hydrologic conditions (pending what rainfall happens on day 1) 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 3 Valid 12Z Tue May 20 2025 - 12Z Wed May 21 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS FROM THE UPPER MISSISSIPPI VALLEY TO THE OHIO VALLEY AND THE CENTRAL APPALACHIANS... 20Z Update... Following model trends, adjusted the previous Slight Risk area a little further northeast. Otherwise, no significant changes were made to the previous areas. Pereira Previous Discussion... The system described in the day 1 and 2 discussions begins moving east on Tuesday. The overall setup for heavy rainfall amounts is generally not as good by this time as it was on previous days farther west. The large scale forcing is more progressive by Tuesday and the degree of instability is not as robust. Thus not surprising that the global model QPFs are generally lower than what is forecast farther west on day 2. With that said, there is likely still an excessive rainfall risk Tuesday. While the system is becoming more progressive by this time, there will be a stationary front extending from IL into KY through, with enough moisture transport and synoptic ascent to support a broad area of convection along this front. This setup thus could allow for either multiple convective rounds near the front, or even a more prolonged period of repeat/training convection. The main question will be whether we are able to maintain enough instability to support prolonged intense convection and also whether the mid/upper level forcing is enough to allow for a persistence of stronger convection. It is also worth noting that portions of KY and WV are more sensitive to additional rain, with higher soil saturation and streamflows in place. Overall think this is a solid Slight risk, with isolated to scattered flash flooding possible anywhere from IA to KY/TN/WV...generally along and just south of wherever the stationary/warm front ends up by this time. The potential for greater flash flood coverage and higher impacts is uncertain...noting the aforementioned uncertainties with forcing/instability and the general downtick in model QPFs by this time. However the orientation of the forcing and stationary front does suggest a potential for training and more significant rainfall totals, which could open the door for a corridor of more widespread and/or higher impacts. The 00z RRFS depicts a swath of strong convection Tuesday, but generally shows things progressive, keeping most areas in the 1-2" range...likely not enough for widespread flash flooding. However it does show some swaths over 3", especially over portions of KY and TN where a bit more training occurs. So we will continue to monitor trends, and pay close attention to areas that may be more sensitive to additional rainfall, such as KY and WV. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-USmG9jlZ_R0R0pBheV7LvjnhQx3NnLRE6sBhgRbrpO0= R5M4bnFq55zXD2lQ779rIkTBW3K-ed9MBI4ImjPjb3gtXmw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-USmG9jlZ_R0R0pBheV7LvjnhQx3NnLRE6sBhgRbrpO0= R5M4bnFq55zXD2lQ779rIkTBW3K-ed9MBI4ImjPjqpdGwRg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-USmG9jlZ_R0R0pBheV7LvjnhQx3NnLRE6sBhgRbrpO0= R5M4bnFq55zXD2lQ779rIkTBW3K-ed9MBI4ImjPj6dfLGNs$=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) .