Flash Flood Warning 48, NWS New Orleans, LA

Area Forecast Discussion...UPDATED
National Weather Service New Orleans LA
537 PM CST Mon Nov 20 2023

...NEW AVIATION...

.SHORT TERM...
(This evening through Tuesday night)
Issued at 423 PM CST Mon Nov 20 2023

So what can we expect tonight. We are currently getting a balloon up
to see how much the environment has changed from the 12z flight this
morning. We will have another balloon up for 00z and that should
give us an even better idea of things. Biggest question marks are
how much have we recovered and how much of the strong cap around
h8 is still around.

There are a bunch of pros and cons with respect to severe weather
tonight but at a quick glance the main area of concern appears to be
southwest MS and adjacent LA parishes which fits the Enhanced area
but we can not rule out a few possible.

Southwest flow aloft will slowly but steadily increase through the
evening and it appears from GOES16 that there may be a little more
forcing coming across the western Gulf than what some models were
indicating and possibly why now the CAMs have been favoring coastal
convection and thus would cut off much of the land area from the
significant activity overnight which would be associated with the
front. Prior to that storms will likely begin to move into the area
around and shortly after 00z and this would be associated with the
development of a LL jet which the core may move right across
southwest MS during the evening hours. This is the location that
could be touch and go this evening. It would be before any
convection tries to fire to the south and could maximize the shear
and helicity values. This area should be far enough north and
removed from any possible marine layer with these storms most likely
sfc based. Farther to the south the marine layer does become a
concern, especially immediately along the north side of the lake
and across much of the SELA coast.

Also for areas along and south of I-12/10 there seem to be more
negative than positive with respect to severe weather. First this
location may have to contend with some minor marine layer issues but
the bigger concern is the LL jet which the tail end of it will move
across this area initially and then lift to the northeast and this
will lead to LL divergence which will hurt convection potential. In
addition the sfc pressure is expected to rise even with a weak front
approaching and the hghs aloft look like they may slightly rise
overnight as the main impulse pulls off to the northeast. The one
aspect that does seem to hint at possibly better potential is what
was mentioned earlier and that is GOES16 showing some slightly
better lift coming northeast out of the western Gulf but if this is
the case then there is a really good chance that robust convection
could develop overnight in the more unstable areas over the coastal
waters.

Front should move through most of the area around or shortly after
sunrise. Rain will come to an end after the front moves through and
much drier air will push back into the area. We see a reinforcing
shot of dry air early Wednesday as a strong disturbance drops out of
Canada and closes off over the Ozarks Tuesday night. That will help
keep your travel day Wednesday on the dry end with a low closing off
over the Baja Tuesday night. /CAB/

&&

.LONG TERM...
(Wednesday through Sunday night)
Issued at 423 PM CST Mon Nov 20 2023

Focus is mainly on the threat for tonight but Thanksgiving is
only a few days away and with multiple people on the road
Wednesday and out and about Friday through the weekend can't
completely gloss over the extended portion of the forecast.
Overall medium range guidance is in agreement however, there are
some differences with strength and specific features Thursday
night into Friday and on the back end of the weekend into next
week. Luckily even with the differences it is not anything of
significant impact and is more a light rain or no rain scenario so
with not strong lean one way or the other we will stick closely
to the NBM for now and try to get a better grasp on these features
tomorrow and Wednesday.

Very active pattern is already in place and likely to remain through
the forecast. The problem is trying to time specific
impulses/disturbances and how they strong they may be. After
tonight's weather the focus will shift towards how the strong
disturbance coming out of Canada reacts with the base of the highly
positively tilted trough across the CONUS. All indications are the
back end of our trough will break off and cut off over the Baja but
how fast does it start to kick out and how much latitude can it
climb. This will have a lot to say about where rain develops and if
it can move into the area Thursday evening and overnight. That said
even if the low starts to lift across seTX and through the Lower MS
Valley we would likely just see light rain with a few embedded spots
of moderate rain. Not expecting any lightning with that system. If
the disturbance opens up faster or is a little further south as it
lifts out then the rain would most likely be confined to the Gulf.

After it passes Friday morning we remain in WSW flow through
Saturday night as the next system tries to drop back down the
backside of the L/W. It is that system that models have the biggest
disagreement with as a much stronger deeper and closed low would be
slower and likely begin the work week dry where a much weaker and
more progressive system would possibly bring rain back into the area
Monday. /CAB/

&&

.AVIATION...
(00Z TAFS)
Issued at 532 PM CST Mon Nov 20 2023

VIS/CIG reductions anticipated through much of this cycle.
Convection tonight will develop generally from west to east with
the strongest activity having strong gusty winds and lightning.
This activity should clear the western terminals early morning and
the MS Gulf Coast an hour or two after sunrise. Behind the front
southerly winds will transition to a northwesterly direction with
some gusts in excess of 20kts. Some VIS/CIG improvements are
anticipated by the end of the cycle with the cloud deck lifting to
around 5kft. (Frye)

&&

.MARINE...
Issued at 423 PM CST Mon Nov 20 2023

Winds have been overachieving and have placed all of the waters
in a SCY through 12z. This is mostly for frequent gusts but even
at 21z most platforms and buoys were hitting 20kts sustained.
Likely to need some sort of headlines for most of the waters into
at least Wednesday, although there may be brief lulls in hazardous
conditions. Don't anticipate many, if any, periods of calm or
near calm conditions this week. /CAB/

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  58  64  43  56 / 100  10  10  10
BTR  60  66  46  60 /  90  10  10  10
ASD  63  71  47  60 / 100  20  10  10
MSY  66  69  52  60 /  90  10  20  10
GPT  64  73  49  60 / 100  40  20  20
PQL  64  75  47  61 /  90  60  20  10

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...Small Craft Advisory until 6 AM CST Tuesday for GMZ530-532-534-
     536-538-550-552-555-557-570-572-575-577.

MS...None.
GM...Small Craft Advisory until 6 AM CST Tuesday for GMZ532-534-536-
     538-550-552-555-557-570-572-575-577.

&&

SHORT TERM...CAB
LONG TERM....CAB
AVIATION...RDF
MARINE...CAB