Severe Thunderstorm Warning 95, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
133 PM CDT Sat Oct 29 2022

...New UPDATE, AVIATION...

.UPDATE...
Issued at 1249 PM CDT Sat Oct 29 2022

Our 17z sounding shows a much better thermodynamic profile than the
12z sounding this morning. SBCAPE is up to 1700 j/kg, which lines up
with SPC mesoanalysis for Slidell. Mesoanalysis also has pockets of
higher instability in the range of 2000-25000 j/kg in southeast
Louisiana and coastal Mississippi. Meanwhile, while adequate for
rotating storms, shear is slightly weaker than the 12z sounding with
0-3 km SRH around 200 m2/s2. The best wind shear mostly exists in
the Florida parishes north of Lake Pontchartrain. Due to this
environment, a tornado watch is issued for almost all of the area
except the far western portions until 7pm. The two main areas of
convection are two north-south oriented convergent lines in central
southeast Louisiana and coastal Mississippi. Many cells along these
lines have rotation, but it remains broad for the time being. We
will continue to monitor these areas of convection as they progress
eastward.

&&

.SHORT TERM...
(Today through Sunday)
Issued at 404 AM CDT Sat Oct 29 2022

Upper low over northeast Texas early this morning, with an
occluding surface low near Lake Charles. The occluded/warm front
appears to be just off the Louisiana coast at 3 AM CDT. Radar
shows rain and embedded thunderstorms associated with the upper
low, mainly north of Interstate 20, with stronger thunderstorms
well offshore of Terrebonne/Lafourche Parishes in the Gulf of
Mexico, deep into the warmer air. Very little in the way of
precipitation noted on radar over extreme south Mississippi or
southeast Louisiana at this time. Temperatures at 3 AM ranged
from the lower 60s far north to the lower 70s at Boothville and
Galliano.

Best instability remains either well offshore or well to the west
(near the TX-LA border) at this time, as noted by CAPE/lifted
indices/lapse rates. Upper low should begin lifting north/northeast
over the next 3-6 hours as it slowly weakens. Main forecast issue
is the eventual location of the occluded/warm front. As the upper
and surface lows move northeast this morning, the front should
follow suit. The remaining question is the disposition of the
convection over the Gulf. As long as it maintains itself, the
moisture influx into southeast Louisiana will be disrupted,
limiting convective development over land. Most of the CAMs show
convection over the Gulf diminishing around mid to late morning,
with a corresponding increase over land shortly thereafter, which
would fit usual land/marine convective development trends. Where
does this occur? Most of the CAMs develop the convection somewhere
between the Atchafalaya Basin and Interstate 55, with the severe
threat increased to the east, per SPC's Day 1 Slight Risk area.
Main threat would be damaging winds, but all modes possible. The
other concern of course, would be heavy rainfall. While the 00z
LIX sounding had precipitable water values below 1 inch (and
essentially dry between 900-600 mb), the LCH sounding closer to
the low had a value of 1.55 inches. That airmass will eventually
advect into the area this morning, if the Gulf convection gets out
of the way. As the surface low lifts away from the area, the dry
line/cold front will see a slowing of it's eastward progress,
potentially setting up a training scenario. If this occurs, the
likely target area would be the Mississippi coastal counties late
this afternoon and evening. Rainfall rates with the strongest
storms could exceed 2 inches/hour. A couple hours of this over one
location, especially an urban one, could cause problems, even
though we've had next to no rain for the last 6 weeks in most of
the area. WPC will place a Slight Risk of excessive rainfall over
the eastern portion of the Mississippi coast for today, with a
marginal risk elsewhere.

Precipitation will end from west to east with the advancing dry
line/cold front late this afternoon and evening, with all
significant precipitation likely having moved out of the area by
midnight. No significant precipitation expected Sunday, although
there may be some wrap around clouds, especially north of the
Interstate 10/12 corridors. Clouds would hold highs close to
70 tomorrow, with sunshine allowing mid to upper 70s.

Would note that a spring tide cycle and onshore flow could push
tide levels close to levels where problems could being in the
normally prone areas, but current reasonable worst case guidance
keeps levels just below action stages.

&&

.LONG TERM...
(Sunday night through Friday)
Issued at 404 AM CDT Sat Oct 29 2022

Next southern stream shortwave forecast to move eastward along the
northern Gulf Coast Tuesday or Tuesday night. Moisture
availability is somewhat in question, with the ECMWF solutions
being quite a bit drier than the GFS. The NBM numbers are more
supportive of the ECMWF solution for now, but can't rule out rain
chances increasing with that system in later forecasts. Beyond
Tuesday night, global models build ridging into the western Gulf
of Mexico, meaning dry weather for the second half of the week.
That'll bring well above normal temperatures to the area for much
of the first week of November.

&&

.AVIATION...
(18Z TAFS)
Issued at 1249 PM CDT Sat Oct 29 2022

The showers and thunderstorms across the area are currently
affecting all terminals except BTR and HUM. Overall, the convection
is progressing eastward, so the timing for each terminal should be
reflected. When the convection passes by each terminal, expect gusty
winds and lower visibilities into at least the MVFR range. The
convection should pass through all of the terminals by 10pm tonight.
Expect improved conditions with the exception of MVFR ceilings in
the early morning at BTR and MCB. This should clear up by mid-
afternoon.

&&

.MARINE...
Issued at 404 AM CDT Sat Oct 29 2022

While winds aren't currently supportive of the Small Craft
Advisory, expect that to change over the next 3-6 hours, so will
continue the advisory, and headline the tidal lakes and sounds
with Small Craft Exercise Caution. As pressure fields are expected
to relax this evening, won't extend advisory beyond the current
expiration time of 00z. May need headlines again around Tuesday or
Wednesday, but probably Exercise Caution and not an Advisory.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  55  73  53  75 /  10  10   0   0
BTR  58  76  55  78 /   0  10   0   0
ASD  57  76  53  78 /  30   0   0   0
MSY  60  76  59  77 /  10   0   0   0
GPT  61  76  57  77 /  70  10   0   0
PQL  62  75  55  77 /  80  20   0   0

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...Small Craft Advisory until 7 PM CDT this evening for GMZ550-552-
     555-557-570-572-575-577.

MS...None.
GM...Small Craft Advisory until 7 PM CDT this evening for GMZ552-555-
     557-570-572-575-577.

&&

UPDATE...JZ
SHORT TERM...RW
LONG TERM....RW
AVIATION...JZ

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