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  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 


...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.


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. 


  Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt 

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