Mesoscale Precipitation Discussion 0166
NWS Weather Prediction Center College Park MD
742 AM EDT Mon May 10 2021

Areas affected...southern Louisiana

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

Valid 101052Z - 101800Z

Summary...A couple of mesoscale convective complexes are producing
heavy rainfall in much of the discussion area, with precipitation
likely to persist through at least 18Z.  Flash flooding is likely
- especially in areas that received heavy rainfall earlier this
morning.

Discussion...A couple of mature MCSs have materialized over the
discussion area this morning.  The first, more mature MCS extended
from southern Alabama through southeastern Louisiana and produced
3-6 inches of rainfall near/south of the New Orleans Metro this
morning beneath training convection.  The second is currently
organizing/growing upscale near/just west of Fort Polk, LA where
recent radar-derived rainfall rates are upwards of 2-2.5 inches
per hour.  The pre-convective airmass feeding these thunderstorms
is quite moist (PW around 1.8-2.0 inches), moderately unstable,
and weakly capped, allowing for efficient rainfall rates. 

The orientation of low-level boundaries associated with the MCSs
has allowed for (generally perpendicular to 20-30 knot
southwesterly 850mb flow) has allowed for an area of concern to
emerge along an axis from Lafayette to New Orleans Metro.  A
stationary outflow boundary will reside in this region through
much of the morning, allowing for redevelopment and training of
convection in areas that experienced substantial rainfall and
flash flooding impacts already this morning.  This regime will
remain in place until the second, upstream MCS near Fort Polk
migrates east-southeastward through the area.  The upstream MCS
will provide another area of very heavy rainfall, but also
potentially demarcate the end of the heavy rainfall threat. 
Models/CAMs suggest that this MCS will clear the region by around
18-19Z.  Before then, areas of 1-3 inch-per-hour rainfall rates
will exist beneath heavier cells, which could result in another
1-4 inches of precipitation in the discussion area through 18Z.

Cook

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

ATTN...RFC...LMRFC...WGRFC...NWC...

LAT...LON   31309210 31119107 30849035 30628943 30258893
            29988880 29648885 29468904 29098907 29128971
            29249039 29539138 29869212 30129298 30299350
            30509372 30699371 30909355 31159331 31279290