LIX issues Area Forecast Discussion (AFD) at Sep 4, 8:55 PM CDT ...New UPDATE...456
FXUS64 KLIX 050155
AFDLIX
Area Forecast Discussion
National Weather Service New Orleans LA
855 PM CDT Wed Sep 4 2024
...New UPDATE...
.UPDATE...
Issued at 839 PM CDT Wed Sep 4 2024
Not anticipating any short term changes in the forecast this
evening. There appears to be a very weak boundary noted in the
surface wind fields and radar data just to the south of New
Orleans at mid-evening that is shifting slowly northward. Overall,
rain amounts have been fairly light since sunset with no large
scale forcing evident over southeast Louisiana. However, radar
does indicate another surge of showers/storms approaching lower
Terrebonne and Lafourche Parishes. With precipitable water values
at 2.5 inches or more (and forecast to stay there until Friday),
any strong cells are going to produce very heavy rainfall, and see
no need for adjustments to the Flood Watch at this time. Largest
concerns would be the New Orleans metro and Houma/Thibodaux, where
heavy rain occurred this afternoon. Some of the convection
allowing models indicate heavy rain threat may spread to the
Mississippi coast and Northshore by sunrise.
&&
.SHORT TERM...
(This evening through Friday night)
Issued at 418 PM CDT Wed Sep 4 2024
Forecast has been and remains very difficult and confidence is
quiet low in timing, locations but conditions are in place that
there could be a few areas that see significant impacts between
now and tomorrow evening. Showers and thunderstorms are expected
to be numerous and will be efficient but the question is will we
see more showery activity or can we get more thunderstorms
embedded.
The next 6 to 9 hours it feels like we can focus on an area that
has the best chance of locally heavy rain. That would be the I
10-12 corridor and mostly in between the two. This includes the
triangle of BTR to MSY to ASD. This area has the strong LL
confluence and deep moisture convergence, VERY RICH TROPICAL air
with 2.6" PWs according to SPC meso page and almost 2.7"
according to the Blended TPW product, along with upper level
divergence (outflow channel to the north and northeast). This all
agrees with rain all but assured which we are carrying 80-100 PoPs
in that area through 3z but how much rainfall will actually occur.
The biggest negative and it is that much of a negative that it is
likely the difference between some of these areas seeing 0.5-2" of
rain in the next 6 hrs to 4-6 inches possibly higher in the next
6 hrs...lift/forcing. The sfc inverted trough associated with the
elongated low over northwestern Gulf is already over the western
fringes of the CWA and is expected to drift farther to the
northwest. That is probably the best source of lift right now as
there doesn't appear to be anything else even subtle lift/forcing
to focus on. And that is all we are really missing. We are getting
showers to develop rather quick and very easy (almost want to just
call it airmass showers as any little bit of slight lift, like
clapping your hand, will be able to get showers to develop in this
much moisture we have in place). It is easy to see this because
there is so little lightning out there, everything is rather
shallow, and cloud tops are just not really cooling. It is so easy
to get rain to develop that we are not able to get much in the way
of thunderstorms to develop which is what is needed for the
extreme efficient rain. Or we need some strong focus to get
stronger and longer persistent moderate to heavy showers. That
said it will not take much at all to cause things to really get
out of hand somewhere and it can be very subtle and then we could
see major problems if this occurs over the wrong area.
Part of the other problem is trying to focus on an area where the
heaviest rain will develop and timing. Over the next 48 hrs, the
ECMWF is so broad brushed it has widespread 3-5 inches, the GFS
really focuses hard on the inverted trough (which seems to make
the most sense) and produces a band of 6 to 10" northwest of the
area, the NAM tries to develop a very intense LL jet across the
western 3rd of the CWA with 10-12 inches of rain near the BTR
metro and to the south (some of this looks very possible but the
NAM is showing more of an extra-tropical system and see no way
this 35-40 kt LL jet develops), and then almost all of the CAMS
focus the heaviest rain through 6z Friday along the coast and this
seems very unlikely as the bullseye in the QPF it shows is right
at or just south of the marine/land interface and resemble
convection anchoring along the coast which this environment is not
the setup for that. So honestly we are sticking closer to the NBM
which is heavily weighted to the CAMS for the next 36 to 48 hrs
but did try to even some out to the northwest. Not completely
discounting the NAM but given how it tries to evolve over the
next 24 hrs just couldn't put a lot of faith into it. If I had to
choose I would like a mix of the ECMWF and GFS with the heaviest
rain probably focused just over and just off to our northwest but
because confidence is so low and again it won't take much to cause
problems anywhere will stick very close to the NBM. If the CAMs
are right then coastal MS and the southshore could have some
significant flooding problems between 9 and 18z tomorrow. /CAB/
&&
.LONG TERM...
(Friday through Tuesday night)
Issued at 418 PM CDT Wed Sep 4 2024
Well advertised cold front will push southward but will stall near
or over the coastal waters. Depending on where exactly the boundary
stalls, showers and storms should mostly be confined to the coastal
waters. Thanks to the drier continental air being pushed by the
front, dew points will get into the 60s. So in particular, low
temperatures will be below average this weekend with lows being in
the low to mid 60s possible especially as you move further north
inland. Temperatures will still be warm during the daytime hours
with highs in the 80s but will have a slight warming trend as we get
into early next week. -BL
&&
.AVIATION...
(00Z TAFS)
Issued at 639 PM CDT Wed Sep 4 2024
Conditions trending downward this evening, with most forecast
terminals reporting MVFR ceilings at forecast issuance time, and
those that aren't, should be shortly. To this point of the
evening, lightning has been rather limited, and for the first
couple hours of the forecast, the only mention of VCTS is at KNEW,
where there is lightning off to their southeast. Not particularly
confident in whether there will be direct impacts of TSRA
overnight, and use of TEMPO at several terminals may be
overstating the threat somewhat. Anticipate short term updates as
convective development dictates. Ceilings likely to deteriorate
further overnight as lower layers become more saturated with the
upper end of the IFR range likely beyond 06z. Areal coverage of
precipitation likely to be more widespread during the daytime
hours on Thursday. Do not anticipate VFR conditmions at any of the
forecast terminals during the daytime hours on Thursday. Winds may
be a bit of an issue at KNEW and KMSY with gusts to 25 knots
likely this evening.
&&
.MARINE...
Issued at 418 PM CDT Wed Sep 4 2024
An elongated area of low pressure over the northwestern Gulf
continues to slowly take shape. This did tighten the pressure
gradient over the region and winds picked up across the region.
out of the east through the day today and will continue to remain
strong to moderate overnight. Small craft headlines remain in
effect through tonight. The highest winds will be focused over
the eastern waters (tidal lakes, Sounds, and open waters east of
Plaquemines Parish) where sustained winds will be 15-20 knots with
gusts up to 25 knots and seas of 3 to 5 feet. This has also led to
some coastal flooding along east facing shores which have been
almost completely driven by the winds given we are in neap tide
right now. Waters will remain unsettled with numerous showers and
storms through Friday and will result in locally higher winds and
seas. A frontal passage will bring more offshore winds by the
weekend and potential headline criteria again. /CAB/
&&
.PRELIMINARY POINT TEMPS/POPS...
MCB 72 80 70 77 / 60 80 70 70
BTR 76 83 74 81 / 70 90 80 80
ASD 74 83 72 80 / 80 90 90 90
MSY 76 83 75 83 / 100 90 80 90
GPT 74 81 72 79 / 80 90 80 90
PQL 75 85 73 81 / 80 80 80 90
&&
.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Flood Watch through Friday evening for LAZ046>048-056>060-
064>070-076>090.
Coastal Flood Advisory until 7 AM CDT Friday for LAZ069-070-076-
078.
GM...Small Craft Advisory until 7 AM CDT Thursday for GMZ530-532-534-
536-538-557-577.
MS...Flood Watch through Friday evening for MSZ083>088.
Coastal Flood Advisory until 7 AM CDT Friday for MSZ086.
GM...Small Craft Advisory until 7 AM CDT Thursday for GMZ532-534-536-
538-557-577.
&&
$$
SHORT TERM...CAB
LONG TERM....CAB
AVIATION...RW
MARINE...CAB
Source:
LIX issues Area Forecast Discussion (AFD) at Sep 4, 8:55 PM CDT ...New UPDATE...---------------
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