Mesoscale Precipitation Discussion 0735
NWS Weather Prediction Center College Park MD
1235 PM EDT Fri Jul 26 2024
Areas affected...Much of South-Central to Southeast LA
Concerning...Heavy rainfall...Flash flooding likely
Valid 261635Z - 262235Z
SUMMARY...Heavy showers and thunderstorms will be developing and
expanding in coverage this afternoon across south-central to
southeast LA. High rainfall rates and relatively slow cell-motions
will likely favor some areas of flash flooding and especially
around the more urbanized locations including the New Orleans
metropolitan area.
DISCUSSION...The midday GOES-E visible satellite imagery is
showing an expanding CU/TCU field across south-central to
southeast LA to the east of the ongoing convective mass that has
been impacting southwest LA. Some shower and thunderstorm activity
has begun to develop locally and an expansion of convection is
likely over the next several hours as much stronger boundary layer
destabilization occurs with a very moist airmass.
The activity over south-central to southeast LA is expected to be
heavily influenced by multiple mesoscale boundary collisions that
will include convective outflow arriving from the west and
smaller-scale seabreeze/lake-influenced boundaries over southeast
LA from a combination of the Gulf of Mexico and Lake
Pontchartrain. MLCAPE values over much of southern LA are already
on the order of 1500 to 2000 J/kg, with PWs locally near 2.25
inches, and some very modest mid-level shear overhead.
Additionally, the low-level flow off the Gulf of Mexico is rather
convergent and this will complement the regional development and
expansion of convection.
Very heavy rainfall rates are expected given the environment and
the 12Z RAOB from LIX is extremely supportive of highly efficient
rainfall processes given the depth of the warm cloud layer. Some
rainfall rates may reach 3 inches/hour, and the 12Z HREF guidance
does support this going through the afternoon hours as stronger
convective cores materialize.
Some localized 4 to 6 inch rainfall amounts will be possible given
the expectation of rather slow cell-motions overall and the high
rainfall rates. Despite rather high FFG values, these rainfall
rates and totals should tend to favor at least some areas of flash
flooding and especially around the more urbanized locations
including the New Orleans metropolitan area.
Orrison
ATTN...WFO...LCH...LIX...
ATTN...RFC...LMRFC...NWC...
LAT...LON 30979041 30678976 30028956 29578982 29379047
29629145 29619225 29829296 30269289 30619197
30949127
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