Special Marine Warning 52, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
1100 PM CDT Wed Apr 7 2021

.AVIATION...

.06Z TAF FORECAST...

For tonight, the main focus will be an approaching cold front and
strong/severe storms expected to press east across the area
through the early morning hours. Have timed approximate arrival
times of impacts to area TAF sites per latest short-range guidance
and observed trends which brings this line across the region
generally from after midnight for far western areas, racing east
to the MS coastline by 10-11Z. Expect a broken line of
thunderstorms to temporarily reduce flight categories with gusty,
erratic downdraft winds the main threat, as well as periods of
reduced VIS from heavy rain. This activity will exit to the east
after 12Z with only a few isolated SHRA remaining across coastal
SE LA and coastal MS, with improving conditions/VFR flight
categories expected thereafter into the day on Thursday. KLG

&&

.EVENING UPDATE... /issued 812 PM CDT Wed Apr 7 2021/

Main focus with the forecast update this evening is with the risk
of strong to severe storms later tonight. Going through a large
suite of short-range CAM guidance confidently illustrates this
activity to persist east across our area later tonight. Analyzing
the pre-storm environment ahead of this line reveals a few
interesting points to monitor for this evening...

1) Observing recent radar trends from KPOE reveals a tail-end
convective plume identifiable also from GOES-16 w/ some nearing -70C
cloud tops over Ft. Polk, LA. This area of convection resides
right at the tail end of a 30-35kt jet which is forecast to press
east tonight. A few WoFS members identify this quite well and
evolves it into a messy line or bow segment entering Pointe Coupee
Parish/Wilkinson Co, MS between 10PM and 11PM. This may be a
localized area to monitor for the potential of a few strong wind
gusts, which could even continue to progress east across our SW MS
counties or along/north of I-10/12 tonight. As far as severity
goes by analyzing vertical wind profile trends, we do see some
uptick of 925mb winds collocated within this messy, broken line
which may locally enhance this line segment, especially if any
mesoscale cold pooling can develop behind this cluster to help
accelerate this line east. Can't rule out a few severe wind gusts
tonight, especially in the latest D1 00Z SLGT risk area
(along/north of I-10/12), with primarily sub-severe wind gusts
(30-40mph) potential to the south towards the SE LA coastline.

2) The 00Z KLIX observed sounding this evening indicates a few
noticeable differences in the pre-storm environment when visually
comparing a few CAM soundings, mostly in the moisture profile
below 500mb. The magnitude of deep moist return flow in the
mid/lower troposphere is being slightly underplayed by CAM's, with
perhaps more compressional warming/sinking in the model guidance
owing to a stronger 700mb subsidence/cap inversion. Observed
soundings are much more moist, with this cap steadily eroding due
deep moist ascent in progress. This leads me to believe that the
line to the west may likely fill in towards the LA coastline with
such deep moisture along and ahead of the line (similar to what
HRRR simulated reflectivity trends have shown) by what could be a
blow up of some weak coastal cells, eventually merging into a
broken line as it races east beyond I-55 after midnight. But
again, the severe wind risk will be more limited for far
southern/coastal SE LA areas, with the better risk along/north of
I-10/12.

3) Even though damaging wind gusts will be the main risk tonight,
a few isolated weak tornadoes cannot be ruled out. Model soundings
indicate largely curved low-level hodographs owing to rather
modest SRH (ranging upwards of ~300m2/s2 0-1km SRH to ~400m2/s2
0-3km SRH). If surface winds can remain backed (S to SSE) enough
nearest to the deepest convection, a quick weak spin up can not be
ruled out generally along/north of I-10/12. Will have to watch
observations south of the interstate to determine if a tornado
risk will extend to the coast. If winds shift more SSW to SW ahead
of the line, this will likely cause the line to become more cold-
pool dominant reducing a tornado risk. However, supportive deep-
layer shear exists well to the coast so a spin up or two even
south across SE LA could still be possible.

We will be closely watching it. Overall, we are not anticipating
a signifficant severe weather event, rather more of a sub-severe
line with a few severe wind gusts or a few quick spin ups
possible. This activity will exit to the east by around 11-12Z
with improving conditions/clearing skies Thursday morning with no
significant adjustments necessary for afternoon highs (just
leaning slightly on a warmer bias). Look for more details about
the extended with the AM package early Thursday morning. KLG

PREV DISCUSSION... /issued 321 PM CDT Wed Apr 7 2021/

SHORT TERM (Tonight through Friday)...

Tonight into Thursday morning, an upper level trough is expected
to move through the area, enhancing rain chances. Southerly
surface winds ahead of the system will help to enhance the warm
air advection and moisture advection into the area. Upper level
divergence ahead of the system will help to enhance lifting in the
environment as well. As the system moves through, the dynamic
forcing will be enough when combined with other parameters to
provide a favorable environment for strong to severe storms. A
Slight Risk of Severe Weather is in effect for our northern most
area for tonight and overnight. And a Marginal Risk of Severe
Weather is out for the rest of our area. The main threats from
this system will be gusty winds (up to 60mph or greater) and frequent
lightning. A few tornadoes and hail cannot be ruled out either.
Localized heavy rainfall will be possible as well inside of
thunderstorm development. The main timing of these threats will
be from 10pm through 6am for most of our area.

Thursday and Friday, after the system moves through the area
Thursday morning, zonal flow will dominate the upper level
pattern. Southerly surface winds will help to enhance warm air
advection and moisture advection for the area. Upper level
divergence will help to enhance lifting for the atmosphere as well
these days. As a result, showers and storms will be possible
throughout the area, primarily during peak daytime heating hours
both of these days. Frequent lightning will be possible with these
showers along with locally heavy rainfall. MSW

LONG TERM (Saturday through Wednesday)...

Saturday into Sunday, another upper level low pressure system will
make its way through the area. Southerly surface winds ahead of
the system will continue to enhance warm air advection and
moisture advection into the area. Upper level divergence will act
to enhance the lifting in the environment as the system moves
through the area. Looking at the shear, the speed shear looks like
it will be favorable for severe weather development, but the
directional shear may be limited, looking at the models. The best
directional shear / helicity will be in the Southwest Mississippi
counties and northern Southeast LA Parishes. Another interesting
note and potential limiting factor for severe weather development
is the timing of the parameters, however. Looking at some of the
model consensus, the timing of the best helicity and directional
shear may not line up with the best lifting and upper level
divergence. This will be something important to monitor as we get
closer to the event. Overall, the main threats from this system
based on the parameters will be gusty winds, frequent lightning,
and the potential for a few tornadoes. Locally heavy rainfall
inside thunderstorm development will be possible as well.

Monday through Wednesday, zonal flow will dominate the upper
level pattern. Easterly surface winds will allow for some
enhancement (or maintaining status quo) of moisture advection and
warm air advection into the area. Upper level divergence will act
to enhance the lifting in the environment primarily Tuesday and
Wednesday. As a result of these conditions, showers and storms
will be possible across the area Monday through Wednesday, mainly
during daytime heating hours during these days. The main impacts
from these storms would be frequent lightning and locally heavy
rainfall inside thunderstorm development. Overall, these storms
will be scattered in coverage and overall rainfall totals are
expected to be low in general. In addition to the scattered
showers, maximum temperatures are expected be higher than normal
for this time of year with most locations forecast to be in the
upper 70s to low 80s for this time period. MSW

MARINE...

Winds will be moderate (<15 knots) and southerly tonight through
Saturday morning. Saturday morning through Sunday morning, winds
will be southerly and strong (15-20 knots). Sunday, winds will be
shifting from southerly to northerly to easterly and will be
moderate (<15 knots). Winds will be easterly and moderate (<15
knots) Monday through Thursday morning. Wave heights will
correspond to the wind speeds. MSW

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  61  83  64  82 /  90  10  10  40
BTR  62  85  66  85 /  90  10  10  30
ASD  65  83  65  82 /  90  30  10  50
MSY  67  83  69  83 /  90  40  10  40
GPT  66  81  66  79 /  80  50  30  50
PQL  65  80  64  79 /  70  60  30  60

&&

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

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE NEW ORLEANS LA
155 AM CDT THU APR 8 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0118 AM     TSTM WND DMG     10 NE DENHAM SPRINGS    30.60N 90.85W
04/08/2021                   LIVINGSTON         LA   LAW ENFORCEMENT

            LIVINGSTING PARISH SHERIFF OFFICE RELAYED A REPORT OF
            ROOF DAMAGE TO A HOME ON THE WEST SIDE OF HIGHWAY 1019
            JUST SOUTH OF RAFE BALLARD ROAD. WATER WAS COMING INSIDE
            THE HOME DUE TO THE ROOF DAMAGE. TIME ESTIMATED BASED ON
            RADAR.

&&

MEFFERSO