Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 15 2023 06:58:53 AWUS01 KWNH 150658 FFGMPD MSZ000-LAZ000-ARZ000-151200- Mesoscale Precipitation Discussion 0734 NWS Weather Prediction Center College Park MD 257 AM EDT Sat Jul 15 2023 Areas affected...Portions of southern AR...northwest MS...northern LA Concerning...Heavy rainfall...Flash flooding possible Valid 150700Z - 151200Z Summary...Rainfall rates of 1-3"/hr may lead to localized totals of 3-4". Isolated/localized instances of flash flooding are possible. Discussion...A mature mesoscale convective system (MCS) is propagating SSE across portions of the Mid-South late tonight, producing hourly totals 1.0-2.5" and 15-min totals up to 1.25", per recent MRMS estimates. Cooling cloud tops are still evident via GOES-East infrared imagery, and the MCS should continue to propagate towards the SSE (following the upwind propagation vector) at the current speed near 30 kts (notably about double the magnitude of the UP vector, owing to the strength and dominance of the cold pool). As the MCS reaches the later stages of life (once cloud tops begin to warm), there will likely be a reduction in forward speed (which may compensate a bit for the expected decrease in the rainfall rates). In addition, the veering low-level (925-850 mb) jet may allow for some backbuilding along the southwestern flank of the MCS, though the associated moisture transport is already quite modest (and decreasing). The failure of the CAMs to represent this MCS (including the entirety of 00z HREF suite and more recent HRRR runs) has resulted in a more uncertain forecast that usual. The issue with the HREF is that just about every member (except the HRRR rather ironically) is too weak/slow with the progression of the MCS, whereas the more recent HRRR runs fail to properly initialize and/or propogate the MCS. Therefore a persistence forecast is most appropriate, and the continuation of rainfall rates of 1-3"/hr may lead to some localized flash flooding issues going forward (as 1-hr FFGs generally range from 2.0-3.0"). However, the bigger concern is associated with any potential slowing in the forward propagation with the potential for backbuilding along the southwest flank (as recent rainfall in association with the MCS has been handled by infiltration quite well, per the fairly muted MRMS FLASH reponse). Should backbuilding occur and allow for localized repeating of these rates (conceptually most favored across south-central AR and north-central LA), this may lead to localized totals approaching 3-4" over a 2-3 hour period (with 3-hr FFGs generally 2.5-3.0"). Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!_N_XlgR11sWUzEyGDHXtuRGYVwXOclAivAatClXED0ZmuXSuFAfh9FPHF6a1aCWVxrQ8= 77ILgriwyf5zohlLnz5nmxc$ for graphic product... ATTN...WFO...JAN...LZK...MEG...SHV... ATTN...RFC...LMRFC...NWC... LAT...LON 34539073 34438997 33049016 32509128 32599313=20 33499364 34299335 34079177=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) .