Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 13 2023 08:53:25 FOUS30 KWBC 130852 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 451 AM EDT Sat May 13 2023 Day 1 Valid 12Z Sat May 13 2023 - 12Z Sun May 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTH TEXAS... ....Texas and Oklahoma... Tricky excessive rainfall forecast for today into tonight. The inherited Moderate risk looks to be in some trouble, as convection overnight into early this morning has been forward propagating at a greater clip than anticipated. Thus the greatest flash flood risk may be past by the time the new day 1 starts at 12z. It will be close, but still think coastal areas of south TX should still be getting heavy rainfall at 12z, and the eastern portion of the line should still be onshore and producing some very heavy rainfall rates over 2" in an hour. Although even this portion of the line is forecast to begin a weakening trend by mid morning. So while heavy rainfall rates will probably be ongoing at 12z, the forward progression of convection and gradual weakening trend should limit the magnitude/extent of the flash flood risk. With that said, the heavy rates and short duration backbuilding could still pose a locally significant flash flood risk into the morning hours, especially over any urban areas that are impacted. Once this activity clears, the question becomes do we get another round of convection tonight. Initially instability will be eroded, so we'll have to see if the continued southeasterly inflow can advect instability back into the area. Additional weak shortwave energy may move into the area out of Mexico, and so could envision convection forming in Mexico and moving back into the area tonight. Several of the 00z HREF members depict a solution like this, which results in 3"+ neighborhood probabilities of 40+ percent between 00z-12z Sunday. However it is not unanimous, and the 06z HRRR notably does not develop additional convection. This is a plausible outcome as well, and so will just need to monitor observational and model trends through the day. Given some potential lingering convection at 12z this morning, and at least some potential for redevelopment overnight, did not want to completely remove the MDT risk at this time (although certainly some chance this is eventually dropped). Instead opted to just reduce the size and trim back the western portion of the risk area. A second area of convective focus should be across portions of northern TX into OK. This is related to the retrograding shortwave/MCV that is expected to lift north northwest across the region today into tonight. Even here there remains uncertainty with regards to the degree of instability and location of boundaries helping focus convection. The overall setup does appear to favor some swaths of training convection, so just a matter of whether we get enough instability to drive rainfall rates high enough, and exactly where this better convective concentration will end up. For now left a pretty broad Slight risk over the area. A bit broader on the southern extent to cover where some overlap of additional convection and heavy antecedent rainfall may occur, and a bit sharper with the TX Panhandle cutoff as this areas has been drier and instability should generally be lower the further north and west you go. Oklahoma is interesting and could very well be the focus for heaviest rainfall today over the Southern Plains within training banded features east of the MCV/low. However ongoing convection this morning likely complicates boundary locations and instability later today, so even here confidence is low...but still within Slight risk levels. ....Northern Plains into Upper MS Valley... Training convection this morning into the afternoon should pose an isolated to scattered flash flood risk over portions of eastern SD, southern MN and central/eastern IA. Convection is focusing along the 925/850mb boundary east of slow moving deep layered low, and convergence along this boundary should remain pretty persistent through the morning hours. Low level southeasterly flow should maintain instability into the region as well...with 1000-2000 J/KG of CAPE. Expect that we will continue to see periodic backbuilding/training convective segments along this corridor, which should be enough to drive some flash flood risk. HREF guidance supports a broad area of 1-2" and localized amounts over 3". In fact trends support some 3-5" potential within swaths across this corridor. Portions of southern MN are saturated from recent heavy rainfall which would allow for a quicker development of potential flash flooding. The heaviest rain today may end up just south of this saturated area, but it will be close. Other areas of the Slight risk have been drier and so it will likely take longer training for issues to develop...but even here there is enough of a signal to support at least an isolated to scattered flash flood risk. ....OH/TN valley into the Mid Atlantic... Above average moisture and instability will cover portions of this region on Saturday. A weak shortwave currently over OH will move into the Mid Atlantic and help drive a convective threat. Individual cells will be quick moving, but broad forcing is supportive of multiple convective rounds. On top of that these cells should be capable of dropping a quick 1-2" in localized swaths. So do think at least an isolated, mainly urban, flash flood risk could exist. Chenard Day 2 Valid 12Z Sun May 14 2023 - 12Z Mon May 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN TEXAS AND THE CENTRAL HIGH PLAINS... ....Central and Southern Plains... The excessive rainfall forecast remains tricky into Sunday across the area. The main shortwave feature should have retrograded into NM by this time, and this relatively slow moving feature will continue to be a focus for potentially heavy rainfall. Strong and persistent convergence near this feature should result in a more focused area of heavy rainfall, and PWs will be very high for the time of year, probably around mid May max values across portions of NM/CO/KS/OK. Thus the eventual magnitude of the flash flood threat will probably come down to instability. In general we did not change the inherited Slight risk much at all, with it covering portions of southeast CO, northeast NM, the TX/OK Panhandle, and far southwest KS. Not convinced much instability will get into CO/NM, so this portions of the Slight risk may end up more stratiform in nature and could eventually be downgraded to a Marginal risk. However, bot the ECMWF and Gem Regional have been pretty persistent now in showing some 3"+ rainfall here. It is possible the persistent convergence could prolong the steady rainfall enough that even just some embedded heavier convection could be enough to drive a flash flood risk. So for now left it Slight and will continue to monitor. While the magnitude is still uncertain, does seem like at least some instability will work its way into the east and south quadrants of the low/wave over the TX/OK Panhandles and into portions of southwest KS. The slow movement of the low and persistent convergence associated with it could result in some repeat/training convective activity. And with better instability potential, the threat is there for higher rainfall rates. One caveat here is that most of this corridor has been in a relative minimum of rainfall over the past couple days, so it will likely take a a good amount of rain to drive a flash flood threat. If we can get the instability, the forcing and moisture would allow for rates/totals high enough...so think the Slight risk is still warranted. Future updates will need to keep an eye on both the northern and eastward extent of the Slight risk, as some expansion in these directions is possible. Elsewhere over TX confidence in the convective details is low. The overall synoptic setup will remain conducive for heavy rainfall, however exact magnitudes/locations are likely at least partially dependent on boundaries laid down by convection today/tonight, and where instability is sufficiently worked over or not. Thus really hard to pin own specifics at this point. For the ERO really relied on ensemble data for the most part, with the GEFS, ECENS and CMCE all showing highest rainfall probabilities over south TX, where our Slight risk is outlined. Subtle shortwave energy and convection moving out of Mexico and potentially exhibiting some backbuilding characteristics remains the main threat over this region during the period. ....Mid MS Valley into the OH/TN valley and Mid Atlantic... The deep layered low that is over the SD/NE border on 12z Sat will be an open wave along the MN/IA border by 12z Sunday. Stronger shortwave energy to the north will merge with this feature and then accelerate southward across the region. Should be pretty good forcing with this feature and associated cold front and it will interact with a substantial pool of instability and well above average PWs. Thus seems likely that some organized convective development will be possible across portions of MO/IL/IN/KY/TN Sunday. Activity may tend to be quick moving given the progressive nature of the forcing, so this could end up being a limiting factor for flash flooding. Given this and the only modest model QPFs, think a Marginal risk should suffice. However will need to monitor as forcing/moisture/instability do support intense rainfall rates and decent convective coverage potential. Portions of WV and southwest VA have some convection potential Sunday night as the stronger trough axis moves across along with some upper jet support. Main question here will be available instability, however some swaths of locally heavy rain are possible. Chenard Day 3 Valid 12Z Mon May 15 2023 - 12Z Tue May 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN TEXAS... The low confidence forecast continues into Monday across the Southern Plains. Large scale forcing should be weaker by this time...but we will still likely have some embedded shortwave energy lingering as well as an upper jet in the area. Lingering boundaries, outflows and any MCVs from convection Sunday night will likely again play a role in convective locations and magnitudes. The greatest GEFS/ECENS/CMCE QPF probabilities focus over south central TX, where a Slight risk was maintained. Well above average PWs will remain in place, so just a matter of where the more subtle forcing will set up and where instability will be maximized. Areas further north should gradually become more stable in a post frontal environment, but exactly where this airmass difference will be is still uncertain. Bottom line is that the ingredients will remain in place for potential heavy rainfall, with south central TX the most likely location...but low confidence with coverage/magnitude of the event. On top of the meteorological factors going into the flash flood risk...this will be several days into a wet pattern...so hydrologic factors such as streamflows and soil saturation levels will also likely play a role in exactly where the greatest flood risk ends up. All of these factors should gradually become more clear as we get closer in time. There is a non-zero flash flood risk over portions of the Southeast this period as a cold front and strong mid level trough axis moves into a substantial instability/moisture pool. Much of this areas has been relatively dry and thus has high FFG, and there remains considerable model spread with regards to the strength of the forcing and associated convective coverage/organization. For this reason held off on introducing any Marginal risk at this time, and will continue to monitor trends. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7Dp9ZN6QxU4wiGmK0R-EOdz54PDn3_t4K6j3wsfDxqH1= h947hMW3Azz7Y5BzF6xqp37NUlTi2KJ4G468COc0E5LqiiA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7Dp9ZN6QxU4wiGmK0R-EOdz54PDn3_t4K6j3wsfDxqH1= h947hMW3Azz7Y5BzF6xqp37NUlTi2KJ4G468COc0SixWhVM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7Dp9ZN6QxU4wiGmK0R-EOdz54PDn3_t4K6j3wsfDxqH1= h947hMW3Azz7Y5BzF6xqp37NUlTi2KJ4G468COc0U3aLg30$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .