366
FXUS64 KLIX 172337
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
637 PM CDT Fri May 17 2024

...New AVIATION...

.SHORT TERM...
(This evening through Saturday night)
Issued at 428 PM CDT Fri May 17 2024

Convection has struggled to develop so far today (and we aren`t
complaining). A special 18Z RAOB sounding from KLIX shows a
subsidence inversion near 850 mb in wake of last night`s MCS that
has helped to cap convection thus far. We are closely monitoring for
convection to initiate along the quasi-stationary frontal boundary
that is located just south of a line from Lafayette to Baton Rouge
to Hattiesburg. This boundary has become increasingly better-defined
as differential heating has magnified the temperature gradient
(upper 60s/lower 70s on the cool side of the boundary and mid 80s in
the warm sector). Low-level convergence has also strengthened near
the front and this will provide a source of lift, but appears to be
too shallow for convective updrafts to reach the LFC. Recent
satellite trends show the CU field is building just south of the
warm front along the Southshore, Coastal MS, and parts of coastal
southeastern LA. This is the area where storms are most likely to
initiate through mid afternoon. Expect coverage of storms to be
widely scattered given the absence of deeper forcing. However,
any storms that to break the cap has the potential to quickly
become severe with an EML overhead and strong instability rooted
in the boundary layer. These storms will be capable of producing
locally damaging winds (favorable wet microburst environment with
high instability and mid-level dry air) and large hail (steep mid-
level lapse rates).

The severe weather and flash flood risk quickly ramps up overnight.
This is when strong forcing for ascent overspreads the area as a
upper shortwave trough approaches from the Southern Plains and the
region becomes located in the right entrance region of an upper-
level jet streak. The afternoon cap erodes, allowing robust
convection to initiate and grow upscale along the aforementioned
frontal boundary. The severe and heavy rainfall threat has shifted
slightly farther south given the position of the stationary front
and anticipated southeastward movement of it tonight. This takes
the northwestern portion of the CWA out of the primary severe
wx/heavy rainfall threat area. CAMs are in good agreement showing
storms quickly growing upscale and intensifying around 1 or 2 AM
as the boundary progresses south of I-12. This would place the
Southshore and coastal MS/SE LA under the greatest risk for heavy
rainfall and severe weather during the 2-8 AM period.

Similar to last night, the greatest severe weather threat will be
damaging winds. It should be noted that the HRRR runs today has
consistently been showing intense winds near 80 kt with the
convection near the SE LA coast.

There is some uncertainty regarding where the heaviest rain falls,
but localized amounts of 3-4" will be possible. This will be
enough to produce another round of flash flooding, especially in
areas that are already saturated from the previous rounds of heavy
rainfall last night and back on Monday. There was enough
confidence to drop the Flood Watch for areas north and west of
Baton Rouge. Meanwhile the Flood Watch was expanded a tier of
zones southward to include Houma and Thibodaux but not quite all
the way down to the LA coast. The watch was also extended through
midday Saturday as it the storms will be slow to move offshore.

&&

.LONG TERM...
(Sunday through Thursday night)
Issued at 428 PM CDT Fri May 17 2024

Dry northwesterly flow aloft will be the main driver of the weather
to start the long term period. An H5 shortwave ridge axis to our
west will help transition the region into a much drier pattern
overall. This pattern sets in through at least midweek as upper
level heights and thicknesses increase over the region. This will
help most if not all of the CWFA eventually warm into the 90s
sometimes early to midweek during peak heating. At the surface, high
pressure will begin to move east across the southern Appalachians
allowing for a moisture rich onshore flow to begin across the
region...so overnight lows too will start a modest warming trend as
low level moisture begins to quickly spill northward from the Gulf
by Tuesday.

Warm and dry conditions should continue for Wednesday and Thursday
as a 594dam ridge continues to reside over northeast Mexico or Deep
South Texas. The ridge will extend northeast along the US Gulf Coast
and again this favors a hot pattern across the region. Up stream a
surface front will try to move southward on Thursday. Globals don`t
seem to agree with the progression of the front. GFS is a bit more
bullish with pushing the frontal boundary through the region despite
questionable upper levels (parent trough continues downstream pretty
quickly). For this solution to be applicable, would expect strong
convective cold pool forcing to give the front more motivation,
which given the QPF is more aggressive on the GFS is plausible. ECM,
however, keeps most of the QPF signal well north of our region
across the Mid South. This solution doesn`t have the front move
through the region...which is also a drier and warmer solution.
Regardless, this is still a week out so we will hurry up and wait
and see what eventually evolves through early to midweek next week.
For now, lower tier POPs for the northern tier seems reasonable.

&&

.AVIATION...
(00Z TAFS)
Issued at 623 PM CDT Fri May 17 2024

Frontal boundary across the middle of the CWA this afternoon has
produced rather variable conditions across the terminals, ranging
from IFR at KMCB to VFR at several terminals south of the
boundary. The primary condition this evening should be MVFR
ceilings, with a comparative lack of precipitation. As a shortwave
approaches from the southwest late this evening scattered to
numerous SHRA/TSRA will move across most of the terminals from
southwest to northeast. Direct impacts are likely to produce IFR
or lower conditions, primarily visibilities. Wind gusts to 30-40
knots will be possible with the stronger storms, and can`t rule
out higher gusts. Precipitation should begin to wind down by mid-
morning Saturday, with VFR conditions expected by afternoon.

&&

.MARINE...
Issued at 108 PM CDT Fri May 17 2024

The strongest winds thus far today has been present across the far
southeastern coastal waters, but even here observations marginally
support a SCA with occasional gusts to 20 kt. Winds have actually
become light over the lakes where the pressure gradient is light
near a stationary boundary.

Isolated thunderstorms are expected to develop near the coast late
this afternoon and early evening. Although coverage will be
limited, any of these storms have the potential to produce
locally strong winds in excess of 34 kt, large hail, heavy
downpours, lightning, and higher seas. The risk of hazardous
conditions on the waters will increase overnight and continue
through Saturday morning as storms organize along the front and
move southward. Some of our model guidance is showing the
potential for intense convective wind gusts in excess of 50 kt,
perhaps locally to hurricane strength.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  66  84  65  88 /  60  30   0  10
BTR  70  87  70  93 /  70  30   0   0
ASD  69  84  69  92 /  70  70   0  10
MSY  71  84  73  90 /  80  70   0  10
GPT  69  81  69  89 /  80  80  10  10
PQL  69  82  67  91 /  70  80  10  10

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Small Craft Advisory until noon CDT Saturday for LAZ068.

     Flood Watch through Saturday morning for LAZ036-037-039-046>048-
     056>060-064-065-071-076>090.

GM...Small Craft Advisory until noon CDT Saturday for GMZ530-532-534-
     536-538-550-552-555-557-570-572-575-577.

MS...Flood Watch through Saturday morning for MSZ077-083>088.

GM...Small Craft Advisory until noon CDT Saturday for GMZ532-534-536-
     538-550-552-555-557-570-572-575-577.

&&

$$

SHORT TERM...JK
LONG TERM....RDF
AVIATION...RW
MARINE...JK