Flash Flood in Louisiana, August 11, 2022

Area Forecast Discussion
National Weather Service New Orleans LA
344 AM CDT Thu Aug 11 2022

.SYNOPSIS...

Already seeing a significant surge of convection northward from
the Gulf of Mexico to the south of Interstate 10, primarily over
southeast Louisiana. This is several hours earlier than what
we've seen the past couple of mornings. With precipitable water
values in excess of 2 inches, will be continuing to monitor for
the potential of brief street flooding. At this point, convection
is moving, so as long as training doesn't develop, most areas will
be able to deal with one surge.

&&

.SHORT TERM (through Saturday)...

The upper flow continues to shift to a more northwesterly flow
over the next several days as the upper ridge over Colorado
extends southeastward toward the area. This will combine with
upper troughing over the Great Lakes and New England to push a
frontal boundary toward the area. By Saturday, the weakening
surface boundary will be approaching the northern portion of the
area, which could provide a bit of drying by that point.

Until then, the threat of daily thunderstorms will remain high
across the area, with most areas seeing at least some rain each
day. Little change in precipitable water values is noted today and
tomorrow, so with deep tropical moisture (PWATs above 2 inches)
remaining in place, some thunderstorms could drop 2 inches or more
of rain in less than an hours time. If that occurs over an urban
or poor drainage area, brief street flooding is likely. Overall
threat doesn't rise to a large scale Flash Flood Watch, but the
threat is certainly non-zero. Daily convective timing has
generally been late morning through late afternoon, with not much
occurring beyond sunset. However, coastal areas could see
convection as early as sunrise or even before, on Friday morning,
similar to what's occurring this morning.

With widespread clouds/precipitation the next couple days, most
areas will see high temperatures remain in the middle and upper
80s. As convective coverage lowers a little on Saturday, a few
more locations may sneak into the lower 90s. There wasn't much
need to adjust NBM numbers as guidance is in pretty good
agreement. 35

&&

.LONG TERM (Saturday night and beyond)...

As the ridge to the northwest builds southeastward a little bit,
the surface boundary to the north will sag southward a bit.
Precipitable water values as noted in forecast soundings should
lower into the 1.6 to 1.8 range by late Saturday or Sunday. While
this won't cut off convective development completely, it should
lower the areal coverage of the daily thunderstorms on Sunday,
Monday and perhaps Tuesday before moisture levels creep back up
toward 2 inches. The threat of very heavy rainfall should also
diminish accordingly. Haven't departed from the NBM PoPs much at
this point, but wouldn't be surprised to see those numbers drop a
bit in the next day or two. With the diminished areal coverage of
storms, that'll probably tend to allow high temperatures to creep
back into the lower 90s over a larger area, especially Monday and
Tuesday.

&&

.AVIATION (valid through 12z Friday)...

Very localized LIFR or lower conditions likely at KMCB through
about 13z this morning. Other concern will be convection, which
will be in progress at New Orleans terminals and KASD prior to
sunrise, producing IFR or lower visibilities and MVFR ceilings.
Beyond the early morning convection, there will be a continued
threat of TSRA for most or all of the daytime hours before
diminishing near sunset. VFR conditions during the evening hours,
with some potential of a repeat of low conditions at KMCB between
06z and 12z Friday. Low conditions at other terminals beyond 06z
will be more dependent on where significant rainfall occurs during
the day today. 35

&&

.MARINE...
Outside of showers and thunderstorms, mostly benign marine
conditions through the rest of the week and into the upcoming
weekend. The region will sit on the western periphery of the
persistent subtropical high over the western Atlantic, which will
only provide light to moderate southerly winds. Ridging to the
northwest of the area will try to nudge into the area toward the
end of the weekend, but won't have a significant impact over the
next few days.  Otherwise, in and around convection expected
brief heavy rainfall, cloud-to-water lightning, and perhaps a
waterspout or two. With the elevated tides and persistent (albeit
relatively light) southerly winds, expect some minor coastal
flooding during high tide cycles for at least one more day today.
Extended the Coastal Flood Advisory through 00z this evening, as
the high tide cycle will occur at midday today in some areas, and
the resulting high water will take several hours to drop off.
Tidal ranges start to diminish on Friday as we head toward neap
tides, so we may not need to go beyond today with the advisory. 35

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  87  70  86  70 /  80  40  80  20
BTR  89  73  88  73 /  90  30  80  20
ASD  88  73  88  73 /  90  40  80  30
MSY  87  75  86  75 /  90  50  90  30
GPT  86  75  86  75 /  90  60  80  30
PQL  85  73  86  74 / 100  60  80  40

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Coastal Flood Advisory until 7 PM CDT this evening for
     LAZ066>070-076-078.

GM...None.
MS...Coastal Flood Advisory until 7 PM CDT this evening for
     MSZ080>082.

GM...None.
&&