Area Forecast Discussion
Issued by NWS New Orleans/Baton Rouge, LA

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FXUS64 KLIX 221340
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
840 AM CDT Sun Apr 22 2018

.SOUNDING DISCUSSION...
The upper air release this morning was routine with no problems
encountered. The 12z sounding indicated that the atmosphere had
destabilized and moistened considerably over the last 24 hours.
The MU CAPE was 729 J/Kg and the precipitable water value was up
to 1.67 inches. Directional and speed shear were both evident in
the sounding. The SFC-3km SRH value was 351 m2/s2. 11

&&

.PREV DISCUSSION... /issued 448 AM CDT Sun Apr 22 2018/

SYNOPSIS...

Water vapor imagery depicted the mid/upper level low over the
southeast Kansas/northeast Oklahoma border area moving eastward
with well defined vorticity maxima rotating around the low to the
south-southeast and north-northwest along the shortwave trough
axis. Very diffluent upper level flow was noted over Arkansas,
Louisiana and Mississippi in advance of the upper level shortwave.
At the surface, an elongated trough coincident with the cold
front and occluded front extended north-south along the borders of
Oklahoma/Arkansas south to Texas/Louisiana (Sabine River) with a
surface low near the Gulf coast. A warm front extended east-
northeastward from the low across southern Louisiana. Deep
moisture was wrapping into this storm system as evident with the
widespread rain, showers and thunderstorms. The surface based
thunderstorms appeared to be mostly offshore while more elevated
storms were observed mixed in with the widespread rain shield
inland.

SHORT TERM (Today through Tuesday night)...

The low pressure and frontal system will progress east today and
tonight with the cold front expected to move into northwest
portions of the forecast area late this morning and coastal areas
late this afternoon and early this evening before pushing through
the coastal waters by midnight tonight. Current synoptic,
thermodynamic and radar trends, combined with a blend of the
better resolved high-res, convective allowing models, suggest the
more surface based storms will at least initially remain offshore,
but could develop into some southeast Louisiana coastal areas
later this morning with more elevated storms over the more inland
locations. With the early arrival of the convection and the impact
of the marine layer, it appears instability will remain low,
however will still have to watch for any bowing, high reflectivity
structures for strong to possibly damaging winds, and any
rotating cells that could produce an isolated, brief weak tornado
or waterspout with the favorable low level shear profiles and
helicity that is expected this morning. More unidirectional low
level wind profiles will develop this afternoon as surface winds
veer to more westerly. Severe weather occurrences should be
isolated enough to keep the severe risk marginal.

Rain chances will be decreasing/diminishing in far western areas
late this morning to around midday, and during the afternoon from
west to east across the remainder of the forecast area as drier
air aloft is expected to outpace the cold front. Any lingering
showers over far eastern areas should end early this evening with
drier and cooler air expected to filter into the area tonight as
the cold front pushes into the eastern Gulf coast region. The
center of what will become a stacked low will be slow to advance
east across the Tennessee Valley on Monday night, so the main
forecast problem will be the wrap-around clouds that should move
into/across northern and eastern areas Monday through midday
Tuesday before clearing out late Tuesday. Temperatures are
expected to remain a rather comfortable range with lows in the
50s to near 60 and highs in the 70s.

LONG TERM (Wednesday through Saturday night)...

As our short term storm system moves northeast across the eastern
states Tuesday night and Wednesday, a reinforcing series of
shortwave troughs in the larger longwave trough over the eastern
U.S. are expected to move southeast from the northern Plains and
upper Mississippi valley across the mid to lower Mississippi
valley and southeast states through the middle to late part of the
week. The models have not shown the best agreement and run to run
consistency with timing, strength and orientation of these
shortwaves, weak cold fronts and moisture/precipitation content
as they move across the lower Mississippi Valley and central Gulf
coast regions. Because of this, there remains relatively low
confidence in exactly which periods will experience mentionable
chances of rainfall, but the latest blended guidance supports a
slight/low chance of showers Wednesday into Thursday and Friday
into Friday night. Temperatures are expected to continue to
average slightly below the seasonal normals which means nice
spring weather considering dewpoint temperatures should be mostly
in the 50s through the period. 22/TD

AVIATION...

A frontal passage with a band of showers and thunderstorms will be
the main weather feature impacting the terminals. It looks like the
band of showers and thunderstorms will impact the terminals
primarily from 09z to around 13z at KBTR and KMCB...from 11z to 15z
at KHUM, KMSY, KNEW, KHDC, and KASD, and from 12z to 16z at KGPT.
Prevailing MVFR and IFR ceilings and visibilities can be expected as
this band of precipitation moves through. Some gusty winds could
also occur with any thunderstorms. After the band of convection
moves through, gradually improving conditions are expected from 15
to 16z through the afternoon hours and evening hours. All of the
terminals will see VFR conditions in place by 06z tomorrow. 32

MARINE...
Increased gradient flow in advance of a passing low pressure today
will result in onshore winds of 15 to 20 knots and seas of 4 to 6
feet in the open Gulf waters and sounds.  Exercise caution flags
have been raised due to these conditions.   Winds are expected to
shift to the west for tonight and tomorrow, and exercise caution
flags will likely need to be extended over the open Gulf waters.
Prevailing offshore winds of 10 to 15 knots are expected from
Tuesday through Friday.  However, a brief surge of winds over the
open Gulf waters Wednesday night in the wake of a passing cold front
could push winds into the 15 to 20 knot range for few hours.  Seas
of 2 to 4 feet are generally expected for most of the week. 32

DECISION SUPPORT...
DSS code: Blue.
Deployed: None.
Activation: None.
Activities: Assessing thunderstorm risk for today
            Sandhill Crane NWR Outreach Support
            New Orleans Navy Week Support

Decision Support Services (DSS) Code Legend
Green  = No weather impacts that require action.
Blue   = Long-fused watch, warning, or advisory in effect or high
         visibility event; Marginal risk severe or excessive rain.
Yellow = Heightened impacts with short-fused watch, warning or
         advisory issuances; radar support for Slight risk severe or
         excessive rain.
Orange = High Impacts; Enhanced risk severe; nearby tropical events;
         HazMat or other large episodes.
Red    = Full engagement for Moderate to high risk of severe and/or
         excessive rainfall; direct tropical threats; Events of
         National Significance.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  54  71  54  74 /   0  10  10  10
BTR  54  74  55  76 /   0   0   0  10
ASD  58  74  56  75 /   0   0   0  10
MSY  58  74  58  76 /   0   0   0   0
GPT  60  73  58  74 /  10   0   0  10
PQL  59  74  57  75 /  30   0   0  10

&&

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

$$


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