Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 18 2023 20:18:23 FOUS30 KWBC 182018 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 417 PM EDT Thu May 18 2023 Day 1 Valid 16Z Thu May 18 2023 - 12Z Fri May 19 2023 ....THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL/SOUTHERN PLAINS; COASTAL SOUTH CAROLINA & GEORGIA... ....Central/Southern Rockies & High Plains... A frontal boundary approaching from the north, along with a weak 500mb shortwave trough approaching from the heart of the Intermountain West, will provide both a trigger at the surface and sufficient lift aloft to support thunderstorms forming over the Rockies and make their way east into the High Plains. Storms will develop in the afternoon as daytime heating is maximized and will further organize Thursday evening. The organizing clusters of storms transpires as the region becomes ideally placed beneath the diffluent right-entrance region of a 250mb jet streak tonight. At the same time, the low level jet (LLJ) will strengthen, prompting a strengthening 850mb moisture transport to ensue along with causing the low-level shear profiles to increase. PWs after 00Z will grow to above 1.5" in central OK and north TX after 00Z and southeasterly 850mb winds will direct greater moisture content into southeast CO, northwest NM, and over the both southwest KS and the TX/OK Panhandles. In fact, NAEFS shows the available PWs in the southern High Plains range between the 90-99th climatological percentiles for the time of year. The 12Z HREF also depicts about 1,000-1,500 J/kg of MUCAPE at these storms disposal this afternoon, then backing off to 500-1000 J/kg tonight. The two factors that limit the extent of the flash flood threat are storm motions (850-300mb mean wind speeds up to 15 knots), and the ongoing drought conditions. However, there are still some areas in southeast CO, southwest KS, and the TX/OK Panhandles that have received up to 300-400% of normal rainfall over the last 7 days. This is the reason why NASA SPoRT-LIS 0-40cm soils moisture percentiles in some of these locations are >80%. The available PWs, instability, and low-mid level RH values approaching 80-90% do support the potential for these clusters of storms to produce up to 2"/hr rainfall rates. Chose to keep the Slight Risk, but did adjust the area to be confined to areas where soils are more sensitive to excessive rainfall rates. This includes areas as far east as the OKC metro where the MUCAPE gradient just to the south and west tonight could act as an ideal track for the emerging MCS in the south-central High Plains to follow along it and cause excessive rainfall rates between 1.5-2.0"/hr after midnight. Storms forming along the surface trough in west Texas may spawn other thunderstorms as well, but their steady movement east should keep the flash flood threat very localized. Have maintains the Marginal Risk as far south as I-10 and Fort Stockton, TX. ....Southeast... A frontal boundary located between southern SC and southern GA and a broad upper trough over the Southeast are the focus for what will be a stormy day from the TN Valley to the Southeast coast. The atmosphere is already primed to generate showers and thunderstorms containing efficient warm rain processes. The 12Z sounding out of Charleston, SC featured a saturated profile of just under 90%, a warm cloud layer close to 11,000ft deep, mean cloud layer winds of 14 knots, and a classic skinny CAPE profile. This atmospheric profile will largely stay in place throughout the day with storms on occasion being exceptionally slow moving due to the equaling out of easterlies at low levels and westerlies at mid-levels. Some CAM guidance (the HRRR, 3kmNAM, NSSLWRF, and ARW most notably) have also shown the potential for the axis of newly developing convection this afternoon to push south towards Savannah and as far inland as Augusta. Some of these areas also feature 0-40cm soil moisture percentiles above 80%. Hourly rainfall rates could range between 2-3"/hr in some cases as MPD 309 mentions, along with 3-hr localized totals ranging between 2-5" in some spots. Areas most prone to flash flooding are in the more urbanized communities, as well as the locations with more sensitive soils from recent rainfall and in poor drainage areas. Hi-res guidance also remains steadfast in the potential for slow moving cells as far west as AL and the TN Valley. A Marginal Risk remains in place here given the opportunity for hourly rainfall rates >2"/hr and even more sensitive soils according to NASA SPoRT-LIS (0-40cm soil moisture percentiles >90% from central AL to the AL/TN border). ....North Central AZ... For this update, I chose to introduce a Marginal Risk area for the Mogollon Rim of north central AZ. PWs remain anywhere from the 90-99th climatological percentile while satellite also shows strong surface based heating is underway. The 12Z HREF did show up to 500 J/kg of MUCAPE available along with 1-hr QPF > 1-hr FFG probabilities of 10-20% this afternoon. NASA SPoRT-LIS still shows the area with >90% soil moisture percentiles in some cases as well. Given the available moisture and lingering sensitive soils, there could be some very localized flash flooding in poor drainage areas and along complex terrain. Mullinax Day 2 Valid 12Z Fri May 19 2023 - 12Z Sat May 20 2023 ....THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL/SOUTHERN ROCKIES AND CENTRAL ARIZONA, AS WELL AS THE SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY... ....Southern/Central Rockies into Central Arizona... Low level easterly flow upsloping into the northern High Plains and the southern Rockies is set to deliver an increase of 850-700mb moisture into southern CO and northern NM. Meanwhile, the upper low over northern Baja will provide sufficient vertical ascent for developing showers and thunderstorms along the mountain ranges. PWs will reach as high as 0.75" and according to NAEFS, PWS throughout the affected regions will easily top the 80th climatological percentile. Storm motions will also be rather slow as 850-300mb wind speeds generally hover around 5 knots or less. With soils throughout the at-risk region still quite sensitive (0-40cm soil moisture percentiles above the 90th percentile in some cases), hourly rainfall rates over 1"/hr could cause localized flash flooding along complex terrain, in poor drainage areas, and dry washes. ....Southern Plains to the Lower Mississippi Valley... A pre frontal trough setting up over southern OK and the ongoing southwesterly 850-700mb moisture fetch provides thunderstorms with more than enough moisture content to produce hourly rainfall rates up to 2"/hr in some cases Friday morning and into the afternoon. The cold front approaching from the north will act as a more robust trigger with mean 850-300mb winds running parallel to the both the front and surface trough. This could create a scenario where backbuilding thunderstorms briefly train over portions of eastern OK and central AR Friday morning and into the afternoon hours. Storms will also reside in a favorable environment to support efficient warm rain processes This window of training would be brief, however, as the front to the north will remain rather progressive. At this time, hi-res CAM and deterministic guidance do not agree on whether the heaviest axis of precipitation resides farther north closer to the cold front, or farther south where the better instability will reside. Given the lingering uncertainty in where the heaviest rainfall resides, opted to keep the present Marginal Risk and wait another forecast cycle to see if there is better agreement on whether a Slight Risk is necessary in parts of the south-central U.S.. The Marginal Risk extends as far south as south-central Texas where storms forming ahead of the cold front could take advantage of the more sensitive soils there and produce localized flash flooding. ....Coastal North Carolina.. It is possible that some of the North Carolina beaches; from Cape Fear to the Outer Banks, could see rainfall totals of 4-5" in some spots on Friday as low pressure along the front just off the Southeast coast lifts north. However, storm motions are expected to be more progressive and the beaches there will likely be able to handle the rainfall amounts in the forecasts. No Marginal Risk in place, although some localized street ponding and nuisance flooding cannot be fully ruled out. Mullinax 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!5kl2-IIKA253TIkPqrRBr8ddd3n8rpH1nX4AR50TANoQ= JFWXU6gwpAmvRWHPEDkOIqy12suavAaT4BmEcibifZu8qdU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5kl2-IIKA253TIkPqrRBr8ddd3n8rpH1nX4AR50TANoQ= JFWXU6gwpAmvRWHPEDkOIqy12suavAaT4BmEcibig8b8o3o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5kl2-IIKA253TIkPqrRBr8ddd3n8rpH1nX4AR50TANoQ= JFWXU6gwpAmvRWHPEDkOIqy12suavAaT4BmEcibiziLl_II$=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) .