Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 13 2023 18:54:26 FOUS30 KWBC 131854 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Sat May 13 2023 Day 1 Valid 16Z Sat May 13 2023 - 12Z Sun May 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF COASTAL TEXAS, THE TEXAS PANHANDLE, AND PORTIONS OF THE UPPER MS RIVER VALLEY TO THE NORTHERN PLAINS... ....16Z Update... ....Coastal Texas... The evolution of the storms ongoing along coastal Texas has continued to move offshore into the Gulf with the strongest convection in Deep South Texas, and while there is some training and slow moving storms along the middle Texas coast, this area of convection is rather isolated. The storms north and east of there approaching Houston are nearly stationary, but the rainfall rates are either not there or very isolated. To summarize the above, the flash flooding risk is far from widespread across south Texas. Thus, the Moderate Risk has been downgraded to a Slight with this update. Further, as for guidance for the rest of today into tonight, few CAMs show showers and thunderstorms redeveloping at all, and for those that do, that convection moves quickly, and therefore even if the storms are able to produce heavy rain, the rates will be short-lived over any one area. Thus, the Slight Risk was also shifted eastward closer to where the ongoing convection is, with little convection noted along the Rio Grande except well to the north close to Rocksprings. This rather isolated convection is training, but rates for now are 1.5 inches/ hour or less. Isolated flash flooding may be possible, consistent with the Marginal Risk over the area. ....Texas Panhandle to Oklahoma... The Slight Risk over much of central and eastern Oklahoma was downgraded to a Marginal with this update. The convection that was over southeastern OK is dissipating. This afternoon, isolated to widely scattered convection is likely to develop to the east of a mesolow tracking over west TX and the TX/OK border. The storms may be strong enough to produce heavy rainfall, but there's good agreement that such storms will be moving quickly, and will therefore be unlikely to produce anything more than isolated flash flooding. Further west towards the TX Panhandle, stratiform rain is likely to continue for much of the day into tonight, and while rates are unlikely to ever get too high, the long period of steady rain across this region west of the aforementioned mesolow may cause isolated to widely scattered flooding. ....Iowa/Southern Minnesota... Somewhat better agreement exists among the guidance that showers and thunderstorms will redevelop in the Slight Risk area over IA and southwestern MN this afternoon, which is likely separate from the convection ongoing over eastern IA. The front will pivot northwestward, which will allow the storms to train north/northwestward with time. This area has also seen heavy rainfall lately, particularly on the Minnesota side of the border. The Slight Risk was expanded a bit to the north and east to cover areas with lower FFGs that are likely to pick up additional heavy rainfall late this afternoon through tonight. Wegman ....Previous Discussion... ....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... ....19Z Update... ....Southern Texas... Few changes were made to the Slight Risk area across South Texas with this update. A steady supply of Gulf moisture will continue advecting northwestward into South Texas on 20 kt mid-level winds. There doesn't look to be too much indication there will be much upper level forcing to help organize the storms, but given the steady supply of moisture, an evolution into an MCS is possible. Since guidance suggests a better chance of organized convection and given all of the showers and thunderstorms over the past couple days, the Slight remains in effect. ....Central Plains... Much of the guidance on the whole has decreased forecast rainfall across this area enough that confidence was no longer there that the pattern met Slight Risk criteria, so the entire Slight Risk area was downgraded to a Marginal with this update. Isolated to widely scattered showers and thunderstorms are still expected over much of this region, and continuing east across KS, but confidence those storms will be widespread enough to result in more than isolated flash flooding has decreased. Elsewhere, guidance is beginning to hone in on portions of eastern KS and a second area associated with a front along the MO/IL border, but as these areas of concern are relatively new in the guidance and it has been quite changeable, opted to stick with a broad Marginal Risk area until there is consistency in the guidance which will increase confidence. Wegman ....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 CMC 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_i7GJ-2LVCowsJ3hdX8D995hOjP2gmo5vIiKIZBnZOx7= kw9r17_4LmAtURVSWxjsHuJjMDzYu91A7BsSMz9_FFiySuE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_i7GJ-2LVCowsJ3hdX8D995hOjP2gmo5vIiKIZBnZOx7= kw9r17_4LmAtURVSWxjsHuJjMDzYu91A7BsSMz9_75sZC2E$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_i7GJ-2LVCowsJ3hdX8D995hOjP2gmo5vIiKIZBnZOx7= kw9r17_4LmAtURVSWxjsHuJjMDzYu91A7BsSMz9_JHPCGMU$=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) .