WeatherWheeler35 Posted May 18 Share Posted May 18 Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1152 AM 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 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Quote Link to comment Share on other sites More sharing options...
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