Storm Warning 1, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
507 PM CST Mon Jan 23 2023

...New SHORT TERM, LONG TERM, AVIATION, MARINE...

.SHORT TERM...
(This evening through Tuesday night)
Issued at 506 PM CST Mon Jan 23 2023

The calm before the storm, yes I said it but you walk outside and
you would have zero indication of the potential for highly
impactful weather tomorrow and tomorrow night. Clear skies, dry
and cool conditions, and light winds made for a very pleasant day.
The evening and probably most if not all of the night will be
quite nice as well. By tomorrow morning, around sunrise, things
may start to finally change but still the greatest impacts for our
area probably won't be till after 21z and generally through the
evening and the early morning hours.

So our main antagonist is spinning over southern Arizona and with
the mid lvl jet still upstream and not expected to round the the s/w
till possibly 12z tomorrow. Because of that our system will not
start to lift to the northeast until tomorrow morning or midday. At
12z it will be entering the TX Big Bend area but by 21z the s/w will
begin to take on a negative tilt as it moves through central TX. The
potent s/w will begin to quickly lift to eh northeast across the
Lower Ms Valley and Ozarks. The mid lvl jet will respond with the
core approaching 115kt! This will enter MS overnight and that along
with falling hghts will provide very strong forcing across the
region. The s/w will continue to lift to the northeast and will move
into the western fringes of the OH Valley with the very strong
mid lvl jet still draped from seTX and now entering PA.

Aloft we already have the subtropical jet stretching from the Pac
across Mexico and the entire northern half of the GOM. At the same
time the Polar jet is trying to round our system. As our main
disturbance start to turn and lift northeast tomorrow the polar jet
will star to race off to the NNE while the subtropical jet remains
over the entirety of the Gulf coast. This ends up creating a coupled
jet structure over the Lower MS Valley with some of the most
impressive upper lvl diffluence we have seen in some time. By 00z
Wed h25 winds over the LA coast will be moving just north of due
west while winds over northeast TX and northwest LA will be moving
almost due north (basically the upper lvl winds just make a big L).
The diffluence aloft remains in place until early Wednesday morning
when the subtropical jet starts to move over the area.

At the sfc is where the more interesting and difficult questions
start to take shape. Right now as we mentioned earlier the sfc cold
front was still slowly working south and had gotten all the way into
the Bay of Campeche and approaching the northern Yucatan. This
should stall soon and begin to slowly retreat north but likely will
be very slow until our system over Orison gets into TX and start to
lift northeast. As that happens we will begin to see cyclogenesis
over southern TX. This developing low will move northeast and should
approaching southeast TX around early afternoon. At that time the s/w
taking a negative tilt and increasing diffluence aloft will begin to
lead to rapid deepening of the sfc low as it moves into southwest
and central LA. As the low approaches southwest/west MS around 00/3z
it could be sub 1000mb (some models even want to bring it down to
around 996mb). This is going to lead to very concerning wind issues
tomorrow but more on that later. The deepening sfc low, strong
dynamics in the mid lvls and strong diffluence aloft will cause the
LL wind field to respond. By 21z tomorrow h85 winds will already be
over 30 kt over our area and the developing LL jet will be around 50-
65 kt over southwest TX. These winds will continue to ramp up and by
0z The core of an possible 85 kt LL jet may be sitting over most of
central and southwestern LA. H85 winds across SELA, southwestern Ms,
and coastal MS will range from 40 kt over coastal MS to 75 in
southwestern MS and these will translate from WSW to ENE over the
next 3-6 hours providing rather extreme LL winds. Given the sfc low
tracking towards southwest MS sfc winds will be out of the southeast
and sse most of the afternoon and evening while h85 winds will be
out of the south. This is leading to 0-1 and 0-3km SRH of well and
honestly it really doesn't matter because the values the models are
showing is beyond sufficient but both are atleast abv 400 m2/s2 and
the 0-3 is possibly abv 700. The problem is will we be able to
actually tap into that and utilize it and that is where the question
lies. Can we recover and get into the warm sector and the more and
more we look at things this will be rather difficult but not
impossible. The front will struggle to move north most of the day
amd the issue to watch is convection through the day and early
evening. Models are trying to indicate a lot of isentropic lift and
if we get any showers ahead of this system through the afternoon and
evening then it will just make it even more difficult to become
unstable. That said it does looks like we could have a narrow window
from 00-9z (maybe more like 2-8z) where the warm sector can get to I-
10 just ahead of the prefrontal trough where the convection is
expected to be associated with.

Everything moves out Wednesday morning and should be out before
sunrise but we shall see if things slow down anymore.

Impacts tomorrow:

Winds: The sfc low is expected to track across southeast TX and into
central LA and possibly into western MS. That alone would tighten
the pressure gradient and lead to winds increasing the problem is
due how dynamic this system will be with a strong negative tile s/w
and very strong diffluence aloft the sfc low will deepen rapidly and
some models are indicating possibly as much as a 16 mb pressure job
across portions of central LA the pressure gradient will respond in
force; there could be greater than a 10mb gradient across our CWA
during the late afternoon and early evening. In addition the LL
winds will also rapidly strengthen through the day and that LL jet
(h85 winds around ranging from 50 to as high as 80kt) will track
right across swLA and into western and central MS. Even h925 winds
will be around 50-65kt. This should lead to a rather extreme
gradient wind event and that alone could cause numerous power
outages and lead to very hazardous driving conditions, especially for
high profile vehicles and along all elevated roadways. Given the
strength of the LL winds,

That said there are a few things that could keep this from getting
as rough. First the winds aren't unidirectional from the sfc to h85
so that would hurt momentum transport but this is why there is a big
concern for severe with respect to tornadoes as the fact these
winds are not unidirectional we have very strong shear. The other
concern is timing which looks to be late afternoon and through the
evening. This also hinders mixing and will hurt the overall
potential. And one other problem usually is this is a pulling wind
and typically we overestimate the win potential in that type of
regime but not all the time and given the absolute impressive
deepening that issue should not be a factor. With that we issued a
High Wind Watch for the entire area. Sustained winds of 25 to 40 mph
will be possible across the entire area with widespread wind gusts of
45 to 50 and scattered wind gusts around 55 mph or greater. Again
this is just gradient and not convective winds.

Heavy Rain: As with most convective events heavy rain is always a
concern and this system is no exception. Right now we are rather dry
but we will begin to see the total airmass start to slowly recover
early in the morning and continue to do so until our system passes
by. PWs in the morning may only be around 0.5" but by 3z could be up
over 1.75". That again combined with that very impressive diffluence
aloft and strong forcing moving across the region should make for
rather efficient storms. The positive this is this should be rather
progressive so most locations should only see very heavy rain for a
short time frame but any storms that try to develop out ahead and
then depending on the eventual track of the sfc low or meso low that
develops with the main convection could lead to a band of very heavy
rain with 1-3 inches of rain possible in a very short time frame.

Severe Weather: This as has been advertised for days now has a
rather high ceiling but it is looking more and more likely we will
struggle to recover in time and we may not get into the warm sector
until only 3-6 hours ahead of the main line which should be a
prefrontal sfc trough. And that is if and who can even get into the
warm sector. Some model solutions now barely get the warm sector
north of the coast. If we can get in the warm sector tomorrow then
the severe potential will increase especially if the warm sector can
arrive ahead of the trough and not just with it. As we have
mentioned this is a High Shear Low Cape event and the shear is
rather extreme with the helicity values mentioned earlier. If we can
actually get sfc based CAPE with no cap then the tornado threat will
be a big concern. Even just 100-200 J/kg sfc based CAPE with no cap
will be a problem. The most likely areas that could see that are in
the Enhanced Risk area thus why it is out and with a 10% hatched
tornado threat. That said the thinking is the environment is still
going to struggle to recover, even with the overall impressive
dynamics in play the cold front at 18z was draped from near Miami to
just north of the Yucatan and into the Bay of Campeche, this is WELL
to the south and with the ridge holding strong over the eastern Gulf
and across FL, any recovery initially is going to be well west of us
through at least the first half of the day. This may be why the CAMs
are very impressive with the convection over southeastern TX and into
swLA but all try to break the line up as it moves into south-central
LA around 22-2z with either a thinner line or even broken line of
convection. Why is that, I believe the lack of recovery across our
area initially combining with this line moving into a cooler
Atchafalaya Basin could be the culprit. Because of this and the
likelihood of our area struggling to recover this really appear to
be a QLCS type event.

So with that expect a line of convection to approach from the west
between 0-3z and should continue to push east and out of the CWA
possibly as early as 9z. With a QLCS, the absolute insane amount of
shear, and if we can get some sfc base CAPE with no cap then QLCS
tornadoes will be very favorable. These can develop and strike very
quickly with little warning. The other aspect is obviously winds. If
the storms are sfc based it will take very little effort to get
damaging winds to the sfc and that may end up being the primary
risk. Large hail is more of a question mark. If storms are not deep
then hail won't be a problem but if we can get a few healthy
updrafts the mid lvl lapse rates which should be steepening through
the evening may lead to large hail. /CAB/

&&

.LONG TERM...
(Wednesday through Sunday night)
Issued at 506 PM CST Mon Jan 23 2023

In the extended nothing has really changed. We cool down and dry
out behind tomorrow nights system and we could see one or two
nights of a light freeze in a few places Thursday and Friday
morning. We finally start to moderate Friday night as high
pressure slides east and zonal flow moves back overhead. The next
best chance for rain still looks like Sunday. /CAB/

&&

.MARINE...
Issued at 506 PM CST Mon Jan 23 2023

Big changes today as the Gale Watch was upgraded to a
Storm Watch. The impressive sfc low that is expected to take shape
tomorrow and move across the region could lead to a pressure
gradient of around 9-11mb. Low level winds greater than 50-60kt are
expected across the coastal waters and these4 should have little
problem mixing down to the sfc cause frequent wind gusts greater
than 50kt. all models show 0-2km winds of 50-55 kts for a short time
and this is usually a good indicator of gusts. on either side of the
Storm Watch is a SCY and the adv after the watch will last into
Thursday with winds finally starting to relax as high pressure
builds in the region. In addition to the extreme winds tomorrow and
tomorrow night the southeasterly and southerly winds will also cause
tides to ramp up and with that we have issued a coastal flood
advisory for all of our coastline for tomorrow night. /CAB/

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  37  63  42  52 /   0  60 100   0
BTR  42  69  45  54 /   0  70 100   0
ASD  40  69  49  60 /   0  50 100  10
MSY  47  70  49  57 /   0  50 100  10
GPT  41  67  51  60 /   0  30 100  10
PQL  38  67  51  63 /   0  20 100  10

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...High Wind Watch from Tuesday afternoon through late Tuesday
     night for LAZ034>037-039-046>048-056>060-064>071-076>090.

     Coastal Flood Advisory from 4 PM Tuesday to 3 AM CST Wednesday
     for LAZ058-066>070-076-078-080-082-084-086.

GM...Small Craft Advisory from 9 PM this evening to 3 PM CST Tuesday
     for GMZ530-532-534-536-538-555-557.

     Small Craft Advisory from 3 AM Wednesday to 6 AM CST Thursday
     for GMZ530-532-534-536-538-550-552-555-557-570-572-575-577.

     Storm Watch from Tuesday afternoon through late Tuesday night
     for GMZ530-532-534-536-538-550-552-555-557-570-572-575-577.

     Small Craft Advisory until 3 PM CST Tuesday for GMZ550-552-570-
     572-575-577.

MS...High Wind Watch from Tuesday afternoon through late Tuesday
     night for MSZ068>071-077-080>082.

     Coastal Flood Advisory from 4 PM Tuesday to 3 AM CST Wednesday
     for MSZ080>082.

GM...Small Craft Advisory from 9 PM this evening to 3 PM CST Tuesday
     for GMZ532-534-536-538-555-557.

     Small Craft Advisory from 3 AM Wednesday to 6 AM CST Thursday
     for GMZ532-534-536-538-550-552-555-557-570-572-575-577.

     Storm Watch from Tuesday afternoon through late Tuesday night
     for GMZ532-534-536-538-550-552-555-557-570-572-575-577.

     Small Craft Advisory until 3 PM CST Tuesday for GMZ552-570-572-
     575-577.

&&