897
FXUS64 KLIX 152111
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
411 PM CDT Sat Jun 15 2024

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

KEY MESSAGES:
1. A surge of tropical moisture will enhance rain chances starting
Sunday and will continue into the work week. Locally heavy
rainfall could lead to flash flooding especially in urban and poor
drainage areas on Monday and Tuesday. A Flood Watch may be
warranted in subsequent forecast packages.

2. Increasing onshore winds into the work week will bring a large
enough fetch to cause coastal flooding concerns. Latest forecasts
indicate coastal flooding of up to 2 to 3 feet above normally dry
ground will be possible for east and south-facing shores by
Tuesday and into the latter half of the week.

3. Hazardous marine conditions, especially for small crafts, are
expected as winds and seas increase starting Sunday. Sustained winds
are expected to be approaching 25 to 30 knots with gusts as high as
gale-force (~34 knots) by midweek. Expect headlines to be hoisted
for all of the coastal waters by Monday.


.SHORT TERM...
(This evening through Sunday night)
Issued at 404 PM CDT Sat Jun 15 2024

Dry air above 750mb is keeping our area hot and dry today, with
highs across the area expected to reach the upper 90s. This is
largely due to the ridge over the southeast CONUS. The ridge and
suppression will cause the highs today to flirt with daily records at
some sites.

Once we get into tonight and tomorrow, a subtle inverted shortwave
from the Gulf will bring in deep tropical moisture across the area,
especially south of I-10/12. The moistening up of the air column
will feature PW from 1.4 in to over 2 inches by Sunday afternoon,
which will be flirting with the daily max for that day. This deep
tropical moisture from the gulf will lead to very efficient showers
with the threat of heavy rainfall and flash flooding. South of I-
10/12 will have the highest deep tropical moisture, so the threat
seems to be maximized there Sunday afternoon through the overnight.

The rainfall seems to be more scattered in nature Sunday afternoon
and evening, so rainfall totals are about an inch during that
timeframe. Higher PW of 2.2 inches look to advect into the area
during the late evening Sunday and through Monday morning. This is
when the high-res guidance has the bulk of the rainfall in the short-
term. During the 7pm Sunday to 7am Monday timeframe, places south of
I-10/12 could see rainfall up to 2 or 3 inches with a reasonable
maximum of 5-6 inches in localized areas. Of course, the urban areas
such as the New Orleans and Houma-Thibodaux metro area would be most
susceptible to flash flooding if the high totals occur during that
time period. With the main threat being overnight and continuing
into the long term, its best to be weather aware.

&&

.SHORT TERM...
(Today through Sunday night)
Issued at 250 AM CDT Sat Jun 15 2024

A highly subsident airmass associated with deep layer ridging
centered directly over the forecast area will remain in place
today. The end result will be very warm and dry conditions
continuing with little in the way of cloud development anticipated
even during peak heating hours. Temperatures will once again
climb into the mid to upper 90s and heat index values will rise to
between 100 and 105 degrees this afternoon.

Tonight and Sunday will be a period of transition from the dry
pattern in place the past few days to a much wetter period of
weather expected for much of the upcoming week. This will be due
to a flux of deep layer tropical moisture currently parked over
the southern and eastern Gulf sliding to the northwest on the
western periphery of the deep layer ridge axis that will remain
centered over the Southeast CONUS. Tonight will remain dry
through the evening hours, but gradually increasing moisture in
the low levels will result in scattered showers and few weak
thunderstorms pushing onshore by daybreak tomorrow. Further
inland, the airmass will remain sufficiently dry and stable to
keep rain shower activity at a minimum. This is well reflected by
an expected PWAT gradient ranging from around 1.4 inches in
southwest Mississippi to near 2 inches along the immediate coast
of Louisiana by daybreak on Sunday.

As the deep layer moisture continues to feed into the region on
Sunday, PWATS will further increase to around 2 inches over the
northern third of the CWA to around 2.3 inches along the Louisiana
coast. These values are between the 90th and max PWAT readings
typically seen in mid-June. Rainfall rates of 2 to 3 inches per
hour can be expected by the afternoon hours, but storm motions of
around 20 mph will help to limit rainfall totals a bit. Still,
there will be a localized street flooding concern for areas with
poor drainage capacity. Despite the very moist airmass in place,
some lingering dry air in the mid- levels could become entrained
into the deepest convection Sunday afternoon. Thus, there could be
some stronger gusts of wind that occur with the most potent
convective cells that form. This is supported by both fairly steep
low level lapse rates in excess of 6.5 C/km and downdraft CAPE
values of around 1000 J/KG.

Conditions will remain unsettled Sunday night as a weak upper
level vorticity maxima sliding up from the central Gulf taps into
the very moist airmass in place. Despite some loss of instability
as temperatures cool into the low to mid 70s, the slight increase
in upper level lift will allow for continued development of
thunderstorms offshore where the diurnal maximum will be reached.
Mean storm motion will be onshore from the south-southeast at
around 20 mph, and this will push these offshore storms inland as
far north as the I-10 corridor through the night. Rainfall
processes will be highly efficient at this point as any mid-level
dry air is flushed out and temperatures start to warm aloft and
would not be shocked to see rainfall rates of 3 to 4 inches per
hour develop during the overnight hours. This will keep the risk
of street flooding in poorly drained areas in place through the
overnight hours. A flood watch for flash flooding will likely be
needed for portions of the forecast area starting Sunday night.

&&

.LONG TERM...
(Monday through Friday night)
Issued at 404 PM CDT Sat Jun 15 2024

Deep tropical moisture (>2.2" PWAT, near daily records) will
continue to stream into the area from the southeast through the
early part of the work week bringing with it heavy rainfall and
subsequent flash flooding concerns especially for urban and poor
drainage areas. The primary focus for our heaviest rainfall
potential will be Monday and Tuesday where high PWATs coupled with
favorable diffluent flow aloft from upper-level troughing will yield
periods of heavy rain with high rain rates of 2 to 3 inches per hour
possible. NBM guidance favors rainfall totals of 6 to 8 inches along
coastal areas and 4 to 6 inches along the I-10/12 corridor, lower
further inland, with reasonble worst case being about 2 inches
higher than the forecasted totals currently. However, it`s worth
noting that the rainfall gradient will probably much tighter from
south to north depending on the track of the broad surface low and
the deep tropical moisture position across the northern flank of the
surface trough axis. With that said, the flash flood risk is evident
so a Flood Watch may be warranted for Monday into Tuesday with
the subsequent forecast package. As this broad area of low
pressure moves toward the western Gulf coast on Wednesday, the
surge of deep tropical moisture will follow it and allow for drier
continental air from the northeast to sneak back into the area
and reduce PWAT values. Onshore southeasterly flow will keep rain
chances elevated, but more typical of afternoon convective regime
we see in the summer. This will also allow temperatures to rebound
some with more sunshine during the first half of the day on
Thursday and Friday.

Additional wind-driven hazards will also materialize as the broad
area of low pressure attempts to consolidate over the southwestern
Gulf of Mexico in association with the Central American Gyre (CAG)
and the Bermuda High noses in across the southeast CONUS. This will
quickly tighten the pressure gradient across the Gulf of Mexico and
generate a corridor of stout 850mb winds (35-45 knots) primarily to
the southwest of our area Monday through Wednesday. We can expect
upwards of 20-25 mph winds and gusts up to 30 mph across coastal
areas during this period. Though the highest winds will not reach
our area, the stout southeasterly flow will generate gusty winds
across the coastal areas and marine zones that will drive a large
fetch of water inland. Coastal flood concerns have increased since
yesterday with latest PETSS forecasts indicating inundation of water
above normally dry ground beginning on Monday and increasing through
midweek. Water heights 2 to 3 feet above normally dry ground are now
in the forecast by Tuesday into Wednesday. While southeasterly winds
do relax slightly into the latter half of the week, onshore flow
persists through the period and this could prolong coastal flood
concerns through the remainder of the week.

Aside from the broad low in the southwestern Gulf of Mexico, no
other entities are outlooked at this time and we do not expect
significant impacts from any tropical systems for the next 7 days.
That said, we will continue to monitor trends in the latest global
guidance regarding a weak surface trough that attempts to
consolidate in latest 12Z guidance and moves west under the
influence of the large Bermuda High toward the Florida Peninsula. It
is far too uncertain and inconsistent at this time to be discussing
anything with regards to potential impacts to the local area as it
pertains to any of these systems suggested by long range global
guidance.

&&

.AVIATION...
(18Z TAFS)
Issued at 1245 PM CDT Sat Jun 15 2024

Dry mid-level air will keep VFR conditions and no rain across the
area for the rest of today and into tomorrow. Much more moist air
is expected to advect into the area from the Gulf tomorrow
afternoon. When this happens, expect convective coverage to
increase, mainly at the southern terminals (MSY, NEW, and HUM)
where the moisture is more prevalent. Expect the scattered
convection to continue into the evening as well beyond this TAF
cycle.

&&

.MARINE...
Issued at 404 PM CDT Sat Jun 15 2024

A tightening pressure gradient between a high over the Southeast
CONUS and a strengthening area of low pressure in the Bay of
Campeche will result in an extended period of small craft advisory
conditions over the coastal waters during the upcoming week.
These small craft conditions with winds of 20 to 25 knots and
frequent gusts in excess of 30 knots are expected to develop as
early as Sunday night, but more likely on Monday and then continue
through Thursday. These winds extend across a long fetch from the
eastern Gulf, and this will allow for a decent swell of 2 to 3
feet to move into the area. This swell will combine with wind
waves of 5 to 7 feet to push offshore seas to around 10 feet
during the week. Wave run-up along east and south facing shores
will also be an issue through the week, and coastal flooding is
anticipated to occur during high tide cycles starting on Tuesday
and continuing through Thursday.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  70  90  72  84 /   0  70  40  90
BTR  75  93  76  87 /   0  80  40  90
ASD  75  90  76  87 /  10  80  70  90
MSY  79  90  79  87 /  10  90  80  90
GPT  78  89  77  84 /  10  80  70  90
PQL  75  91  76  89 /  20  80  70  90

&&

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

$$

SHORT TERM...TJS
LONG TERM....TJS
AVIATION...JZ
MARINE...TJS