Flash Flooding in Louisiana, July 5, 2021

AREA FORECAST DISCUSSION
National Weather Service New Orleans LA
405 AM CDT Mon Jul 5 2021

.SHORT TERM (Today through Wednesday)...
A broad area of upper level low pressure exists across the central
Gulf Coast. The 06Z LIX upper air flight showed ample moisture in
place throughout the column with a PW around 2.1". This is about
where its been the last few days. WV imagery does show some mid
level drier air pushing southeastward across southwest LA and into
the northern GOM. East of there to the FL coast, the imagery shows
moisture slightly moving northwestward. The resultant flow is
broadly and weakly cyclonic. As daytime heating gets going later
this morning, should see rapidly expanding convection. Storm motions
probably won't be particularly fast outside of some outflow driven
storms. Flash flooding concern will be greatest along the LA and MS
coastal areas from around Donaldsonville to New Orleans to
Pascagoula. 1-3" per hour rainfall rates easily attainable.

Going into Tuesday and Wednesday, any resemblance of ridging aloft
from high pressure over the Desert Southwest will be gone. At the
same time, deeper tropical moisture will be creeping in with PW's
across the entire CWA running the 2.2 to 2.3" range. That's quite a
bit above 90th percentile for reference. Looking at CAM max qpf
potential, which can sometimes give a good idea of upper bound
rainfall amounts, indicates upwards of 5-10". If this does occur,
it'll be the typical summertime events where 1 to 2 areas about the
size of half a parish/county see those types of rainfall amounts.
Precisely where it falls will determine if any flash flooding
occurs.

MEFFER
&&

.LONG TERM (Thursday through this weekend)...
Active forecast continues as models and ensembles continue to show
rain chances will remain high through the forecast. With the latest
forecast from NHC Elsa should be well out of the picture by Thursday
so that should not be a concern. If the track shifts any west then
possibly some lingering issues in the outer waters could still be in
play. Overall the latest NBM looks pretty good and see no real
reason to deviate from it right now.

As we have been mentioning for the last few days the pattern that
will be in place suggest scattered to numerous showers and
thunderstorms will continue each day. Set up still has an amplified
ridge over the 4 corners and the other ridge in the western
Atlantic. So of course in between we have a weak trough. This
weakness over the area will be reinforced late Wednesday and into
Thursday and this should lead to a very good chance of numerous to
possibly widespread showers and thunderstorms. Combine that with a
possible jet streak lifting out across the area increasing the
diffluence aloft, unstable conditions, and very deep moisture still
in place and the recipe is there for additional locally heavy rain.

There may be a slight break in the numerous to widespread rain
Friday and Saturday as weak ridging tries to build in by Thursdays
system. There will likely also be a slight drop in moisture so
scattered diurnal convection should be expected Friday and Saturday.
The next much stronger s/w drops down into the Great Lakes late next
weekend. This could once again reinforce the trough across the
region with rain chances increasing again.

/CAB/
&&

.AVIATION...
Still looks like some low cigs and even lowered vsbys through the
early morning hours. This will cause a few terminals to drop into
MVFR and possibly IFR status temporarily but all terminals should be
back in VFR around 14z. Current scattered convection is expected to
again substantially increase late in the morning across the entire
area through the day with VCTS on just about all TAFs.

/CAB/
&&

.MARINE...
The wind field has begun to weaken but will be prevailing onshore
flow of 5 to 10 knots remaining in place through Thursday. However,
a swell train is expected to emanate from Tropical Storm Elsa
beginning Tuesday and continuing through Wednesday. This swell will
produce higher seas of 4 to 6 feet in the open Gulf waters.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  89  72  86  72 /  60  50  80  50
BTR  89  74  87  73 /  80  60  90  50
ASD  87  74  87  73 /  90  70  90  50
MSY  88  77  87  77 /  90  70  90  50
GPT  86  74  86  75 /  80  70  90  60
PQL  85  74  86  73 /  90  60  80  60

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...None.
MS...None.
GM...None.
&&