Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 14 2023 01:20:56 FOUS30 KWBC 140120 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 920 PM EDT Sat May 13 2023 Day 1 Valid 01Z Sun May 14 2023 - 12Z Sun May 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF COASTAL TEXAS, THE TEXAS PANHANDLE, AND IOWA INTO SOUTHERN MINNESOTA... ....Eastern Texas... Convergent onshore flow has set up repeating heavy thunderstorms from the Gulf, across the Galveston Bay area into east-central Texas this evening. Most CAMs have had this feature farther west, in areas that have received excessive rainfall over the past day. While this area farther east has had less antecedent rainfall the flood prone Houston metro is in for repeating, heavy rainfall overnight with recent HRRRs indicating the risk for 2 to 4" until 12Z. The Slight Risk was trimmed to the mid-upper TX coast into east-central TX where this convergent activity should be best. ....Texas Panhandle into Southwest Oklahoma... Convection continues this evening under/east of a slow northward moving mid-level impulse over the TX Panhandle. tracking over west TX and the TX/OK border. Much of the activity over the eastern TX Panhandle into southwest OK is elevated north of a stalled frontal boundary, but the continued nature of the activity warranted keeping some of the low end Slight Risk. ....Iowa/Southern Minnesota... Ample moisture convergence will continue northeast of a low-level low centered over eastern Neb tonight. 1.5 inch PW are +2 sigma above normal and persistent instability over IA into southern MN will allow heavy rain to continue. 3.5"/hr rates were reported near Laurens, IA in training heavy cells there. The Slight Risk was honed into focus on the greatest threat area overnight which is central and northeastern IA into southern MN. ....Mid-South into Oklahoma and northeast Texas... Broad, rather moist flow from the Gulf up the eastern southern Plains into the Mid-South will continue to allow heavy rain development overnight with persistent threats for excessive rain over central OK through southern MO per recent CAMs. A Marginal Risk is maintained. ....Lower OH/TN valley into the Mid-Atlantic... Ongoing activity along and south of a slowly sagging frontal boundary will allow continued heavy rain threats into the overnight. Individual cells will remain quick moving, but broad forcing is supportive of some training which could continue to drop 1-2" in localized swaths. So the marginal risk was maintained with some northern trimming. Jackson 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 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... ....19Z Update... ....Central Texas... The Slight Risk area across central TX remains largely unchanged with this update. Storms from the last several days by this point will have likely left many remnant cold pools, which given the nonstop advection of Gulf moisture into the region from the southeast are likely to result in additional storms over this relatively hard hit region. The focus in much of the guidance is around the Eagle Pass area along the Rio Grande, but the southerly flow would advect any storms that develop there northward. Soils are already saturated, so the additional rainfall raises the threat into the Slight Risk category. ....Portions of the Southeast... A Marginal Risk was introduced for this time frame from the southern Appalachians south to the eastern FL Panhandle. There is some agreement on a better chance of showers and storms here, that would develop along a southward moving cold front, so the threat moves southward through the day. There is significant uncertainty beyond just a Marginal Risk indicating potential isolated flash flooding. Wegman ....Previous Discussion... 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/CMC 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!9eWUbomVrLZguyUdD8PuAKSY99rHF5hPuV0wb0iehHHT= hI_8ncuVof4TQNz0lXrYqUAfKg5lu_M66DNYeLrup1QdgmU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9eWUbomVrLZguyUdD8PuAKSY99rHF5hPuV0wb0iehHHT= hI_8ncuVof4TQNz0lXrYqUAfKg5lu_M66DNYeLruTvxuJ_4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9eWUbomVrLZguyUdD8PuAKSY99rHF5hPuV0wb0iehHHT= hI_8ncuVof4TQNz0lXrYqUAfKg5lu_M66DNYeLrumpK3ibM$=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) .