Mesoscale Precipitation Discussion 0026
NWS Weather Prediction Center College Park MD
720 AM EST Wed Jan 24 2024

Areas affected...portions of LA and southern MS

Concerning...Heavy rainfall...Flash flooding likely

Valid 241219Z - 241819Z

Summary...A pair of convective lines are producing heavy rainfall.
 Hourly rain totals to 3" with local amounts to 6" are possible.

Discussion...Active lines of showers and thunderstorms are moving
ahead of a cold front quicker than the guidance expected across
portions of LA and MS, with some evidence of line echo wave
pattern noted, implying the presence of mesocyclones.  Inflow at
850 hPa is 50 kts per VAD wind profiles out of the south- 
southwest.  Precipitable water values are near or above 1.75". 
The storms are utilizing ML CAPE where CIN is near zero across
southern LA and MU CAPE where CIN exists farther north.  MU CAPE
of 500-1500 J/kg is present per SPC mesoanalyses, and advancing
northward.  Heavy rain rates are occuring a bit farther north than
indicated by the 06z HREF guidance across MS, which is concerning.

The guidance indicates that progression will be held up at times
both by the near unidirectional flow out of the southwest with
height and by embedded mesocyclones, with a larger scale wave
across southern LA potentially holding up progression for a time
across MS.  The tail end of the activity will tend to slow,
waiting for the incoming cold front/upper level trough to approach
from the west.  The ingredients support the potential for hourly
rain totals to 3" and local amounts to 6", with amounts in the
northern portions of the discussion area expected to outperform
guidance, as seen recently.  High rain rates are expected to be
caused by cell mergers, training, and embedded mesoscyclones
holding up the convective line from time to time.  While the heavy
rain is moving into an area that has been dry this past week, this
degree of heavy rainfall should overwhelm the three hourly flash
flood guidance.  The bigger issues are expected in urban areas. 

Roth

ATTN...WFO...JAN...LCH...LIX...MOB...

ATTN...RFC...LMRFC...SERFC...NWC...

LAT...LON   33038999 32808877 31428865 29978985 29459169
            29679300 31089220 32379105