Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 04 2024 21:53:51 AWUS01 KWNH 042153 FFGMPD MSZ000-LAZ000-050351- Mesoscale Precipitation Discussion 0077 NWS Weather Prediction Center College Park MD 452 PM EST Mon Mar 04 2024 Areas affected...portions of southeast LA & southwest MS Concerning...Heavy rainfall...Flash flooding possible Valid 042151Z - 050351Z Summary...Showers and thunderstorms are expected to continue to cause heavy rainfall as a cold pool develops and moves somewhat as the rain area as a whole shifts east-northeast with time. Hourly rain totals to 2.5" with local amounts to 4" could lead to flash flooding. Discussion...Heavy rains have fallen across portions of central LA, with radar estimates of 4"+ west of Simmesport. Showers and thunderstorms have been forming within a region of decent 850 hPa confluence across LA ahead of an upper level shortwave moving into northwest LA. Surface observations show a forming cold pool across areas near the southwest MS/LA border, with upper 60s temperatures in the cold pool and mid to upper 70s south of the boundary. CIN is developing; MU CAPE of 1000-2000 J/kg exists in this area per SPC mesoanalyses. Effective bulk shear is near 40 kts, which has supported occasional mesocyclones. Precipitable water values of ~1.5" exist here, per GPS data. The convective area appears to be forming on the cool side of the 5C isotherm at 700 hPa. The mesoscale guidance has a strong signal for 3"+ through 04z over portions of LA and southwest MS, but show a bit of dispersion on location and on areal extent of the 3"+ area anticipated.=20 Recent RAP runs give as suggestion as to the evolution expected.=20 The area of 850 hPa confluence shifts east with time, which should move the heaviest rainfall slowly eastward. Normally the guidance can be too poleward with convective activity, which is probably true with the 12z Canadian Regional guidance. However, RAP runs show warming at 700 hPa moving in from the southwest which implies some northeast shift with time due to a possible mid-level capping inversion nosing in. The movement of the 850 hPa confluence zone eastward implies an east-northeast shift with time. Since CIN has already developed, convection is likely to remain elevated and if it follows the 5C isotherm at 700 hPa northeast, the outflow boundary/possible section of the polar warm front would follow suit/retreat somewhat in that direction. The 18z HREF probabilities of 3"+ seemed to be a reasonable starting point for future expectations; generally followed the 15% outline for the MPD area since the guidance dispersion is likely underplaying the potential. Due to mesocyclone formation, short periods of cell training, and cell mergers, hourly rain totals to 2.5" and additional local amounts to 4" remain possible, which could lead to widely scattered flash flooding concerns, particularly across urban areas. Roth ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!4ksHIHuYp0XD1Gt-F8Mdd-ZiTyOTkqVclIW7XInJb45Bg4SRU46sNeIpzrDvmRi9938y= 8H5R4FoNY1sFM0Yr6Zaucmo$ for graphic product... ATTN...WFO...JAN...LCH...LIX...SHV... ATTN...RFC...LMRFC...NWC... LAT...LON 31749064 31368990 30888946 30468936 29848957=20 29719039 30299142 30749237 31329219 31699132=20 =20=20=20=20=20=20=20=20=20=20=20=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .