Flash Flooding in Mississippi, January 24, 2024

Area Forecast Discussion...CORRECTED
National Weather Service New Orleans LA
500 AM CST Wed Jan 24 2024

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

.SHORT TERM...
(Today through Thursday Night)
Issued at 435 AM CST Wed Jan 24 2024

Very active 24 to 48 hours in store with multiple impacts some of
which could be significant and even possibly life threatening if
features line up in a way we would prefer not to see.

BLUF...Widespread moderate to heavy rain is the greatest concern and
the Flood Watch has been extended to include all of the area with
the exception of the immediate coast of a few LA parishes. Next is
the the possibility of severe weather with the main concerns being
straightline winds and a few tornadoes, next sea fog, finally river
flooding could become a bigger concern over the coming days.

1.  So, as mentioned the greatest concern is the potential for Flash
    Flooding. It is beginning to look more and more likely that a
    band and of heavy rain will develop and stall over the region
    with the possibility of some sites seeing upwards of 9" of rain
    in less than 24 hours. That would be before 12z Thursday with
    additional rain likely on Thursday. Where exactly that occurs is
    unknown and will determine how significant the impacts are. If
    that band stalls over generally rural parts of SELA and southern
    MS the biggest issue could be some flash flooding and then river
    flooding taking over. If that band lines up over any of our larger
    population centers it would not be out of the question that Flash
    Flood Emergencies could be issued as the rain will be heavy,
    efficient, and likely persisting for more than a few hours. Whatever
    falls today and tonight will have a barring on the impacts for
    Thursday and into Thursday night.

2.  Second concern is the risk for severe weather today although the
    risk for severe weather pales in comparison to the threat of flash
    flooding it is still there with the main concern being
    straight-line winds and a few tornadoes possible. The risk is
    contingent on a few things most notably being able to warm up into
    the lower 70s and convection firing out ahead of the main line. The
    risk should gradually decrease late this afternoon and this evening
    as convection consolidates into a line and the flash flood risk takes
    over.

3.  Sea fog remains a possible problem. This will mainly impact a more
    specific group (mainly mariners) but if sea fog can develop it could
    move inland especially over coastal MS heavily impacting locations
    like Gulfport and Biloxi.

4. 	Last river flooding, but depending on how much rain falls and
	exactly which basin it falls in. This will become more clear later
	today and tonight as we see where and how much rain has fallen.

We are already overachieving on the rain overnight as convection
was more numerous across the area shortly after midnight when we
weren't really anticipating much more than a few storms over the
far northwest by 8/9z. This convection has even had lightning as
far east as Slidell and before 9z ASD already recorded around
3/4" of rain. This is already more and earlier than expected. Lift
is increasing across the region and almost the entire suite of CAMs
and multiple HRRR runs in a row all suggest strong LL convergence
across the northwestern half of the CWA through the day and
possibly into the early evening hours. While the LL convergence
increases through the day the upper jet will sharpen up,
strengthen and slide east just enough to place the CWA close to
the RRQ while a weaker jet rounds the ridge over the central
Caribbean and Bahamas. This will enhance the diffluence aloft. The
combination of LL convergence and upper lvl diffluence almost
directly on top of each other is extremely favorable for
convective development. Obviously we know we already have
instability in place given the thunder that has already occurred
overnight when the instability was expected to be meager at that
time. We have the lift and instability so now what about forcing.
That will not be absent as we will see a steady increase in the h5
winds as multiple subtle impulses are already working through the
southwest flow aloft while the main s/w doesn't push through the
Lower MS Valley till late Thursday (and that is what could be the
main problem maker...we will hit that in just a second). Looking
at moisture availability and we will have no shortage of moisture.
This continues to increase and as of last nights sndg we were
already sitting at a very impressive 1.65" which is right at the
record for the day. The PW is expected to increase even more and
could approach 1.8" (mesoanalysis shows that 1.8" and greater PWs
is already working into the western portions of the CWA) and this
would be in the top 20 measurements for the month of Jan all time.
The is pretty extreme for this time of the year. All of this
combined suggests robust convection with locally heavy rain almost
a foregone conclusion. Add in the increasing upper lvl diffluence
and K index readings possibly approaching 40C today and again
tomorrow and the efficiency of thunderstorms will be very
high...likely 1-2" in less than 30 mins with many storms. This is
all impressive from a heavy rain standpoint but this is not the
greatest issue though, the problem is that the boundary will stall
over the CWA and will be parallel to the mid lvl flow and this is
a bad recipe. At this time it would be foolish to say exactly
where that stall could occur but it is looking more and more
likely the line will lay up somewhere across coastal central LA
and stretch to northeast through SELA and into southern MS. That
said it could easily stall 20-30 miles to the west or 20-30 miles
to the east and that now puts all of SELA in the potential...this
includes the metros of NOLA and BTR. Wherever this line does start
to hang up you can expect to see storms train over the same
locations for probably 3-6 hours if not longer. This would be a
disaster as rainfall amounts WILL quickly accumulate and it is
highly likely there will be a band of rain ranging from 5 to 8"
inches with locally higher amounts possibly greater than 11" (some
of the CAMs and multiple runs of the HRRR are indicating 10
inches or more in some areas) and this is all before 12z tomorrow.
With that we extended the Flood Watch to include all of the area
with the exception of coastal sections of Lafourche, Jefferson,
Plaquemines, and St Bernard.

As mentioned earlier this isn't it for the rain as the main s/w is
still 6 to 12 hours back and with the boundary still stalled over
the area additional storms are expected and this will just exacerbate
any issues we have today and tonight. When the next 48 hours is done
even though the setup is not exactly the same we could see similar
amounts to the March of 2016 Flood event. That was also a very
disastrous river flooding situation but for those impacts to occur
again it will be absolutely dependent on where the heaviest rain
falls.

As for the severe aspect. There are a couple of keys things that
need to happen for this to be realized and first is to get the mid
60s and possibly even upper 60s dewpoints to continue to push
farther inland and as of 8z that seemed to not be in doubt as REG
and LFT both already had dewpoints at 67 but at 10z if these were
correct, LFT and PTN were now up to 71 while ARA was sitting at
69. With dewpoints that high already (and higher than anticipated)
only very minor heating today will get the instability to climb
quickly. This will also likely lead to steep LL lapse rates which
we will need for severe weather to occur otherwise the storms may
not be able to tap into the increasing LL jet today. Speaking of
the LL jet as mentioned in the section about the rainfall a
potential LL jet is expected to develop and increase through the
day. Those winds will generally be out of the south and maybe SSW
while the sfc winds will remain out of the SSE and SE. This will
provide ample shear in the LL with 0-1 and 0-3km SRH in the
150-200 and 200 to possibly 300 m2/s2 range and that is greater
than what was expected. If we can heat up to 72/73 which seems
like a lock now, MLCAPE values could approach 1500-2k J/kg and
more of a concern is the 0-3km CAPE which could exceed 150 J/kg.
That said the deeper shear is lacking with 0-6km bulk shear ahead
of the boundary should be around 30-35kts. This is not high but
likely sufficient enough to get a few organized storms to develop.
If storms want to fully utilize that environment they will have
to develop ahead of the main band and do so before convection
tries to just start developing everywhere as the boundary
approaches. The lower bulk shear would tend to lead to somewhere
between multicellular and separate organized cells. However, with
the increasing lift and increasing instability there is a chance
that convection could quickly overwhelm the environment and we
then quickly transition to flash flooding. SPC does have a Slight
risk across much of the area given some of the key features we
discussed earlier and it looks like if severe storms were to occur
the main time frame would be mid late morning through early mid
afternoon, those initial stronger storms. By mid after we should
begin to see things congeal as the line of storms starts to really
focus in on our area as the boundary stalls.

Fog has not been as big an issue overnight but we are starting to
see sea fog develop over the coastal waters. That said the fog that
was developing earlier over the land likely mixed out due to the
increase mixing from the rain. With that fog will likely become more
attached to the coast and marine areas as the sea fog continues to
develop.

River flooding is going to be absolutely dependent on location of
the rain and how much falls. Until then at this time we will likely
see at least minor river flooding in the usual problem areas but it
would be not be a surprise to see river flooding maybe approaching
moderate category for areas along the Tangipahoa river and east to
the Pearl River (the Pearl River being a longer stem river will be
more dependent on rain fall farther north and just south of the Ross
Barnett so what ever rain falls over our area will have only minor
impacts on the Pearl). /CAB/

&&

.LONG TERM...
(Friday through Tuesday)
Issued at 435 AM CST Wed Jan 24 2024

Not much focus was placed on the extended forecast mainly as
outside of the Friday night and Saturday system the forecast looks
quiet, cool, and dry. As for the Friday night/Saturday system. It
looks like we will not get completely cleaned out Thursday night
and it will not take long to recover with PWs expected back in the
1.3 to 1.5" range. This system will bring additional rain to the
area and we could see another widespread 1-2 inches with locally
higher amounts on top of what occurs in the next 36 hours. If we
have river flooding occurring this could either exacerbate those
issues or at the least prolong it. /CAB/

&&

.AVIATION...
(12Z TAFS)
Issued at 435 AM CST Wed Jan 24 2024

All terminals will be dealing with significant impacts again
today with the inclusion of widespread shra and tsra. IFR and LIFR
cigs are expected and most terminals are already dealing with
cigs around 300-400 ft. Vsbys will continue to be an issue
especially the ones closer to the coast. Sea Fog could severely
impact GPT and NEW while lighter fog and mist will impact all of
the other terminals along with severe reductions in vsbys from
convection. /CAB/

&&

.MARINE...
Issued at 435 AM CST Wed Jan 24 2024

Sea fog is finally developing and should continue to develop and
spread ahead of an approaching cold front with the advection of
warm, moist air over cooler waters. Dew points are expected to be
about 10 degrees higher than the SSTs, so the warmer air will
condense into fog as it cools to the SSTs. Once the front moves
through the area this weekend, the fog threat will decrease as
drier air filters into the area. The other concern is gusty
gradient winds ahead of the front, but that will only get to
advisory criteria before the front clears the waters and winds
relax. /CAB/

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  70  62  71  59 / 100 100  90  60
BTR  74  66  75  62 / 100  90  90  60
ASD  71  61  72  58 / 100 100 100  70
MSY  73  63  72  60 / 100  90  90  70
GPT  67  59  68  57 /  90  90  90  70
PQL  71  61  73  57 /  80  80  90  70

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Flood Watch through Thursday afternoon for LAZ034>037-039-
     046>048-056>060-064>066-071-076>088-090.

     Dense Fog Advisory until 10 AM CST this morning for LAZ056>060-
     064>070-076>078-080-082-084-086>090.

GM...Dense Fog Advisory until 9 AM CST Thursday for GMZ530-532-534-
     536-538-550-552-555-557.

     Small Craft Advisory until 6 AM CST early this morning for
     GMZ530-532-534-536-538-550-552-555-557-570-572-575-577.

MS...Flood Watch through Thursday afternoon for MSZ068>071-077-
     083>088.

     Dense Fog Advisory until 10 AM CST this morning for MSZ086>088.

GM...Dense Fog Advisory until 9 AM CST Thursday for GMZ532-534-536-
     538-550-552-555-557.

     Small Craft Advisory until 6 AM CST early this morning for
     GMZ532-534-536-538-550-552-555-557-570-572-575-577.

&&

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