Mesoscale Precipitation Discussion 0987
NWS Weather Prediction Center College Park MD
610 AM EDT Tue Oct 16 2018

Areas affected...SRN LA...FAR SRN MS

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

Valid 161010Z - 161340Z

Summary...Thunderstorms with intense rainfall were ongoing near
the coast, and may evolve inland this morning, carrying a threat
of flash flooding.

Discussion...While synoptic deep layer lift was better defined
over Texas and northern Louisiana, a small MCS had flared up this
morning near coastal southern Louisiana. This region was being
forced by what appeared to be a wave on the low level frontal
boundary, supported also by subtle upper level difluence, all
while precipitable water values were very high, 2.00 to 2.20
inches per the SPC Mesoanalysis. The 00Z KLCH sounding which had
been launched as the front was arriving, measured a remarkable
2.40-plus PW value. Instability in the inflow region was also
plentiful, with the same mesoanalysis depicting the ongoing
convection at 10Z occurring along the gradient near CAPE values
that increase quickly from 500 J/kg to 1500 J/kg with southward
extent.

The only guidance handling this anywhere close to reality was the
NAM CONUS Nest, which fits well with the radar / observational
trends that suggest as deep convection persists near Vermilion
Parish it may eventually shed an MCV and we will see convection
expand in coverage east and northeastward along the CAPE gradient
toward eastern LA and far southern MS.

The greatest rain rates are likely to remain on the southern /
southwestern flank of all this activity. Given the moderate CAPE
and high PW environment, dual-pol rainfall estimates were quite
large, at up to 2.5 inches per hour and with local 6-inch amounts
already estimated between Intracoastal City and Forked Island.
Flash flooding is likely occurring, and a few more instances of
flash flooding may develop elsewhere in the region through 14Z.

Burke

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

ATTN...RFC...LMRFC...

LAT...LON   31329086 31258978 30778957 29719108 29229233
            29479283 30399235