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

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312
FXUS64 KLIX 172135
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
335 PM CST Fri Nov 17 2017

.SHORT TERM...No big changes in thinking through Monday concerning
the forecast. The area will continue to be under the influence of
a surface ridge tonight...resulting in another round of patchy fog
forming late tonight as temperatures cool into the lower 60s.
However, the more widespread dense fog observed the last couple of
nights is not expected as boundary layer winds begin to increase
during the overnight hours.

This increase in boundary layer flow is directly attributable to
an approaching fast moving shortwave trough currently moving
through the Rockies. This fast moving feature will force a strong
cold front into the Lower Mississippi Valley by tomorrow evening,
and then across the entire forecast tomorrow night. Although mid-
level moisture will remain fairly limited as the front moves
through, there should be enough forcing along the boundary to
spark off some scattered showers and very isolated low topped
thunderstorms. Have placed chance POP of 30 to 50 percent in the
forecast for tomorrow afternoon and evening to reflect this risk.
Highs will be well above average in advance of the front with
temperatures in the upper 70s and lower 80s expected.

Strong cold air advection and subsidence will rapidly take hold in
the wake of the front...along with very breezy conditions from
late tomorrow night into Sunday morning. As a deep cold pool moves
in from the northwest, temperatures will fall into the 40s and
lower 50s by daybreak on Sunday. Continued cold air advection
through the day on Sunday should keep temperatures well below
average around 60 degrees. Skies will also rapidly clear late
tomorrow night into Sunday morning as strong negative vorticity
advection and subsidence take hold. The heart of the cold pool
will be directly over the region Sunday night, and the combination
of light winds and clear skies should allow lows to cool into the
lower to middle 30s across most of the region. A light freeze may
still occur in the Pearl and Pascagoula river valleys as well as
portions of Southwest Mississippi. The cold pool will begin to
shift to the east by Monday, but cool and dry weather will
persist. Highs should only climb into the lower 60s as the fringe
effects of the 925mb cold pool linger.

.LONG TERM...

All of the model guidance has come into much better agreement
today on the evolution of the weather pattern across the Gulf
South for the middle to latter part of next week. Initially, a
fast moving and weak shortwave trough will slide through the
forecast area Monday night. This system will be moisture starved,
but it will force the surface ridge east of the area. As this
occurs, the door will be opened up for return flow off the Gulf of
Mexico to begin. Temperatures will begin to modify Monday night,
as winds shift to the south with lows only cooling into the 40s.

Tuesday will see a broad long wave trough axis begin to deepen
across the Plains and Mississippi Valley including across the Gulf
South. A strong vorticity maxima diving down from the northern
Rockies on the western side of the long wave trough will begin to
deepen as it moves into the Southern Plains. As this occurs, low
level cyclogenesis will begin to develop over the central Gulf of
Mexico. Across our region in advance of the digging longwave
trough axis, a broad area of ascent and forcing will develop. The
combination of increased forcing along with increased moisture
should spark off some scattered shower activity Tuesday into
Tuesday night. It looks like there will be little to no
thermodynamic instability, so the mention of thunderstorms has
been completely removed from the forecast. Temperatures will
modify slightly on Tuesday, but will remain below average with
highs in the middle to upper 60s and lows in the 40s and low 50s.

The shortwave feature over the Southern Plains will move into the
western and central Gulf of Mexico Wednesday into Wednesday night.
Continued low level cyclogenesis will result in a fairly strong
surface low forming over the central Gulf by Wednesday night. At
this time, it looks like the majority of forcing and available
moisture will begin to shift more toward the coastal water and
immediate coastal zones of Louisiana and Mississippi. This is
reflected in the forecast with morning isolated showers giving way
to partly cloudy skies over the northern half of the CWA by
Wednesday evening. Along the coast and offshore, scattered showers
will persist through Wednesday night. A northerly wind should also
develop on Wednesday and continue into Wednesday night as the
surface low strengthens over the central Gulf. This will result in
cooler temperatures in the upper 30s and 40s Wednesday night as
some cooler air advects in from the north.

The trough axis and surface low will continue to track to the east
on Thursday and Friday, and expect to see the region on the drier
and cooler side of the system for the upcoming Thanksgiving
holiday. However, there will be some better forcing and deeper
moisture closer to the trough axis over the eastern Gulf waters
and possibly parts of the Mississippi coast Thanksgiving Day. At
most, only some isolated showers could affect the Mississippi
coast. By Friday, the trough will be far enough to the east to
keep any showers well to the east of the area. Overall, expect to
only see partly cloudy skies and continued cool temperatures both
Thursday and Friday. Highs should only climb into the lower to
middle 60s, and lows should cool into the upper 30s and 40s
Thursday night.

&&

.AVIATION...Overall most terminals will remain in VFR through the
evening but after 6z fog could develop once again. It appears as
though fog should not be as big an issue as the past 2 nights and
this is due to the increase in winds expected overnight especially
just above the surface. That said MCB, ASD, and HDC will likely have
some patchy light fog. The bigger issue could be low CIGS but these
may not develop until 9-13z over much of the area and could remain
at or just abv 3k ft at most sites. MCB has the best chance of seeing
CIGs below 3k feet. /CAB/

&&

.MARINE...Slowly increasing onshore winds overnight due to a
tightening pressure gradient will yield exercise caution headlines
for the overnight hours over the open waters. Winds will further
increase tomorrow during the day as the strong cold front
approaches. Small Craft Advisories will be in place for the open
waters tomorrow as winds increase to 15-20 knots with frequent
higher gusts out of the southwest.

more significant issues after the cold front moves. This will occur
during the evening hours Saturday and winds will quickly veer around
to the north-northwest and increase significantly over all of the
waters. All of the waters that are not in the SCY during the
afternoon and evening hours tomorrow will be so after 3z but by 6z
all of the open waters winds will continue to increase. At this time
it looks like there could be frequent Gale force gusts and with that
have decided to issue a Gale Watch for those open waters from 6-15z
Sunday. Small craft advisories will likely continue for the open
waters through the remainder of the day Sunday.

As for seas/waves...thanks to the offshore nature of the winds sea
heights will not be too far out of hand but would not be surprised
to see 7-8 feet seas with occasionally higher possibly up to 10 feet
in the outer waters.

Winds will begin to relax Sunday night however some zones could
still see headlines into next week with winds and seas likely
becoming hazardous again Wednesday and Thursday next week. /CAB/

&&

.DECISION SUPPORT...DSS code: Blue.
Deployed: None.
Activation: None.
Activities: Gale Watch
            Small Craft Adv

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.
Yellow = Heightened impacts with short-fused watch, warning or
         advisory issuances; radar support.
Orange = High Impacts; Slight to Moderate risk severe; nearby tropical
         events; HazMat or other large episodes.
Red    = Full engagement for Moderate risk of severe and/or
         direct tropical threats; Events of National Significance.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  61  80  43  60 /   0  50  40   0
BTR  63  81  45  62 /   0  50  40   0
ASD  62  81  48  62 /   0  40  40   0
MSY  64  81  54  62 /   0  30  40   0
GPT  63  77  49  62 /   0  30  40   0
PQL  60  79  49  62 /   0  30  40   0

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...Gale Watch from late Saturday night through Sunday morning for
     GMZ550-552-555-557-570-572-575-577.

     Small Craft Advisory from noon Saturday to midnight CST Saturday
     night for GMZ550-552-555-557-570-572-575-577.

     Small Craft Advisory from 9 PM Saturday to 9 AM CST Sunday for
     GMZ530-532-534-536-538.

MS...None.
GM...Gale Watch from late Saturday night through Sunday morning for
     GMZ552-555-557-570-572-575-577.

     Small Craft Advisory from noon Saturday to midnight CST Saturday
     night for GMZ552-555-557-570-572-575-577.

     Small Craft Advisory from 9 PM Saturday to 9 AM CST Sunday for
     GMZ532-534-536-538.

&&

$$

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