Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 13 2023 00:50:39 FOUS30 KWBC 130050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Fri May 12 2023 Day 1 Valid 01Z Sat May 13 2023 - 12Z Sat May 13 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL TEXAS... ....Southern Plains... Current radar imagery shows a fairly large area of deep convection this hour from northern Texas south toward the Rio Grande region with IR imagery continuing to indicate deeper/growing convection. The combination of a slow moving front and a mid-level disturbance is helping focus the areas of convection while southeasterly flow is providing a continuous surge of higher moisture, with the latest blended TPW product indicating values exceeding 1.6-1.7" and increasing. The expectation over the next several hours is for the multiple lines of storms to congeal/merge, progressing eastward through the overnight hours affecting a large area of south-central Texas. The very anomalous moisture and high instability expected to remain in place will support an extensive area of hourly rain totals greater than 1-2" at times with some localized additional rainfall totals through 12Z of 3-5"+. This is likely to lead to scattered to perhaps numerous instances of flash flooding, some of which may become higher-end and locally significant within the Moderate Risk area. ....Southeast... Lingering daytime convection continues mainly across two focus areas, one cluster across portions of Mississippi into western Alabama and another across south-central Georgia into the Florida Peninsula. Both of these areas are in the presence of weak mid/upper forcing, but higher moisture and fairly sufficient instability for now. Some embedded strong clusters have produced intense rain rates over the last few hours, particularly across south-central Georgia where some interactions with a surface trough and land/sea breeze boundaries have focused the convection. In general with the loss of daytime heating and the lack of any real surge of moisture or forcing over the next several hours, there should be a downward trend in the convective intensity such that any further risk of heavy rainfall and flash flooding will become increasingly isolated through the evening and overnight hours, especially after about 04Z-05Z. ....Ohio Valley to Central Appalachians... In the broad area of upper diffluence downstream of the closed mid/upper level low over Nebraska, slightly higher moisture has begun to lift north/northeast today and pool along an existing surface boundary from portions of northern Illinois through southeast Ohio. Regional radar shows a thin line of moderate to locally heavy rainfall and through the rest of this evening into the overnight, a slight increase in moisture with the nocturnal low-level jet (southwesterly flow at 850 mb approaching 20-25 kts), should continue showers and thunderstorms with some uptick in intensity possible later tonight. HREF neighborhood probabilities do show some signal for isolated 1"+ hourly totals, particularly over portions of Ohio and West Virginia where FFGs are slightly lower. ....Northern Plains... Isolated to scattered flash flooding remains possible across portions of the Northern Plains, along a stationary boundary draped across portions of the Dakotas through central Minnesota as well as along the leading edge of deeper convection associated with the occluded low pressure in Nebraska. The first area has lesser instability to work with which may end up keeping rain rates low enough to limit the scope of flash flooding, but does have the low level convergence that remains fairly persistent as well as fairly low FFGs that could be exceeded based on the 18Z HREF neighborhood probabilities. To the south, the best instability and forcing will be found from eastern Nebraska into portions of Iowa as the leading edge of convection lifts north/northeast. Localized 1-2" hourly totals will remain possible for a few more hours into the evening then some waning in the instability will help lower the risk of flash flooding. Taylor Day 2 Valid 12Z Sat May 13 2023 - 12Z Sun May 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTH TEXAS... ....2030Z Update... ....Texas... Very few changes were made to the Moderate Risk area across south Texas. As mentioned below, it's likely that the event will be ongoing at 12Z Saturday for the start of the Day 2 period, but unfortunately there is not great agreement in the CAMs as to that timing. Most of the CAMs suggest there will be multiple rounds of storms that form to the west towards the Rio Grande, then track with some speed off to the east out to the Gulf as the next round of storms does the same, with this pattern continuing into the night. Of course, depending on which CAM you use, the timing of when each storm will be in a given location varies widely. There's also some disagreement on how far west new storms initiate. As such, the core of the heaviest rainfall in the consensus has shifted east close to the Gulf Coast. Should this verify then Corpus Christi would be on the hook for 3+ inches of rain, with 12Z HREF neighborhood probabilities at over 80% for 3 inches of rain in town, 30% for 5 inches of rain, and a 10% chance of 8 inches of rain in the 24 hour period from 12Z Sat through 12Z Sun. Higher probabilities remain west of the city however, with a wide variety of probabilities from the Gulf Coast to the Rio Grande. Meanwhile, the Slight Risk area was expanded north and west to include all of the TX Panhandle, most of western OK, and some of the OK Panhandle. CAMs guidance suggests a Mesolow will move along the TX Panhandle/OK border, with steady stratiform rain over the Panhandles, with rounds of showers and storms over much of OK. With abundant moisture available with PWATs over 1.5 inches in much of TX and OK Saturday afternoon, and instability values between 1,000 and 2,000 J/kg the storms will have plenty of available moisture and energy for producing heavy rains across north TX and OK. ....Northern Plains and Upper MS Valley... Guidance trends continue to increase for this area, as the convective threat along a stationary/warm front comes into better focus. 3 inch plus neighborhood probabilities from the southwest corner of MN across central IA to near the IA/IL/MO junction are over 45% in the 12Z HREF guidance, and deterministic trends increasingly suggest over 4 inches of rain may fall in a few spots within the Slight Risk area. ....Mid-Atlantic... A back door cold front moving in from the north will collide with the plentiful Atlantic moisture in place across southern VA and northern NC to produce locally heavy rains in the Tidewater region and the southern Delmarva. While FFGs are high, the combination of over 2,000 J/kg of CAPE and over 1.5 inches PWAT will be more than enough to result in thunderstorms that produce heavy rains that may cause isolated flash flooding, especially in urbanized areas such as Richmond and Hampton Roads. Wegman ....Previous Discussion... ....Texas... The event discussed in the day 1 discussion should be ongoing at 12z Sat across portions of south TX. The expectation is that the convective complex will be making slow eastward progress by this time. The forecast by this time is already becoming lower confidence, as convection on day 2 will be heavily dependent on the location of cold pool driven outflows and ongoing convective activity. Guidance has trended a bit quicker with convective progression, so we did trim back some on the western extent of the Moderate risk with this update. The overall synoptic environment will be conducive to excessive rainfall over central/southern TX, so really comes down to outflow and instability recovery, which will be tough to pin down at even this lead time. Do think the greatest flash flood risk will be over south TX associated with the likely continuing convective complex, but some renewed development behind this can not be ruled out either. Just like on day 1, extreme rainfall rates of up to 3" in an hour seem probable with this convection over south TX. Still thinking the potential is there for 6"+ rainfall amounts in spots across south TX downstream of the day 1 convection. Think this is a solid MDT risk level event, with locally significant flash flooding possible. Can not rule out a High risk level event, but the trend for a slightly quicker convective progression seems to more solidly put this more in the MDT level for now. A second area of flash flooding is possible across portions of north TX into OK associated with retrograding mid level shortwave and persistent upper level diffluence. Just like over south TX, we have some questions regarding the degree of instability and boundary locations here as well. Seems like a solid Slight risk level event with some backbuilding/training potential, and an embedded more organized threat could exist. Just too early to pin that down at the moment, but will continue to monitor trends. ....Northern Plains into Upper MS Valley... The closed mid level low will finally begin to open up into an open trough on Saturday, however enough forcing will persist to drive a convective threat near the stationary/warm front. It seems likely that we will get upwards of 2000 J/kg of instability into this boundary, which will be enough to support robust convective development near the front where convergence is maximized. Overall the ingredients appear to be in place to drive a heavy rainfall threat, and now model QPFs continue to suggest a narrow heavy rainfall event as well. Even through it only goes through the first half of day 2, the 00z HREF is already showing 3"+ neighborhood probabilities over 40%, and 00z ECMWF and GEM Reg QPFs are locally over 3" as well. This QPF support, combined with the favorable ingredients, increases confidence in a narrow heavy rainfall event. On top of that, it currently appears like there will be decent overlap with the portion of southern MN and northern IA that just recently saw heavy rainfall and some flooding impacts. Given all this, a Slight risk upgrade was warranted...with isolated to scattered flash flooding seeming more likely Sat into Sat night. ....OH/TN valley into the Mid Atlantic... Above average moisture and instability will cover portions of this region on Saturday. Guidance is also indicating a weak shortwave which could help focus convective development. The details remain unclear, and convection may end up fairly quick moving, however felt like the overlapping moisture/instability and convective signal in the 00z CAMs supports the potential for localized flash flooding. Chenard Day 3 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... ....2030Z Update... ....South Texas... Few changes were made to the overall Slight Risk area. While QPF in this region has come down some to account for the inaccuracy of the global models, the area still looks well in line for a Slight Risk flash flooding day as additional storms broadly drop 1 to 3 inches of additional rainfall on what will be an already completely water-logged area. There is still some argument to be made that the cumulative effects of 3 days of very moisture-laden storms traversing the area could keep impacts at a more Moderate level category in this region, despite good consensus that less rain will be seen on Day 3/Sunday verses Day 2/Saturday. Nonetheless, will hold at a Slight at least until there is some high-resolution guidance to make a determination on any upgrades from. ....Southeast CO, Northeast NM, and the OK and TX Panhandles... Guidance trends have increased in this area as regards a second area of potentially heavy rainfall. As mentioned in the previous discussion below, instability will remain a limiting factor for heavy rain, particularly over southeastern CO, but FFGs in southeastern CO have been depressed from recent rains, thus even if it's mostly stratiform, localized flash flooding will be possible due to the moisture-laden soils. Further southeast into the Panhandles, there will be more instability, and likely lower FFGs due to rains expected on Day 2/Saturday. Those favorable antecedent conditions combined with increasing guidance trends increased the confidence to introduce a Slight Risk for this area, in coordination with LUB/Lubbock, TX, AMA/Amarillo, TX, and ABQ/Albuquerque, NM forecast offices. Wegman ....Previous Forecast... Forecast becomes pretty tricky by this time across the Southern Plains into the OH/TN valley. The overall synoptic setup over TX/OK will remain conducive for heavy rainfall, however exact magnitudes/locations are likely at least partially dependent on boundaries laid down by convection on day 2, 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. This area seems to have the greatest potential for excessive rainfall, but also has bust potential depending on the above mentioned factors. A second area of potential heavy rainfall exists from northeast NM into north TX and into OK. This should be closer to the lingering shortwave feature and lower level synoptic boundaries. However the question here is how much instability remains this far to the north and west. Certainly some potential for Slight risk upgrades here, but for now thought the best solution was to place the Slight risk where ensemble QPF and ingredients have the higher end potential, and cover the remaining areas with a broad Marginal risk. The broad eastward Marginal expansion into the MS valley into OH/TN valley is primarily driven off of sufficient CAPE/PW overlap. Forcing will generally be weak and partially related to convective driven vort centers, which will be hard to narrow down at this lead time. So generally followed the overlap of favorable ingredients, and most of this area is within a QPF max in at least one model. A stronger shortwave is forecast to move through the OH valley by Sunday night, and it is possible that this could help drive a more organized convective threat. However at the moment the core of this forcing seems to be positioned north and east of the instability pool, so will just have to wait and see how much overlap we end up between the greater forcing and instability. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!62vs9ukVAxgXK55ve8R_t5lvBJnmMKleA3Nb4DkvY8d9= we0zb_7GKEF1mxuqTYTa2EGpr1qqx-4coz59J-XO15Dhr9E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!62vs9ukVAxgXK55ve8R_t5lvBJnmMKleA3Nb4DkvY8d9= we0zb_7GKEF1mxuqTYTa2EGpr1qqx-4coz59J-XOOhF6J4o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!62vs9ukVAxgXK55ve8R_t5lvBJnmMKleA3Nb4DkvY8d9= we0zb_7GKEF1mxuqTYTa2EGpr1qqx-4coz59J-XOrHZR21k$=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) .