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

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FXUS64 KLIX 180946
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
346 AM CST Wed Jan 18 2017

.SYNOPSIS...

Low pressure near New York City has a cold front southwest to
Atlanta, Jackson and Houston. To the north of the front, weak
high pressure is centered over Missouri and Arkansas. At upper
levels, ridging continues over the eastern Gulf of Mexico. A
strong shortwave is moving through the southern Great Lakes
States with an upper low along the Colorado-New Mexico border.
Another strong upper trough is approaching the California-Oregon
coastline.

Light surface winds early this morning have promoted the
development/transport of fog. While not being reported at all
locations, there is enough to necessitate Dense Fog Advisories
across the entire area. Temperatures are mainly in the mid and
upper 60s with a near saturated airmass.

&&

.SHORT TERM...

Through the daytime hours Friday, the main forecast concerns will
be the threat of severe weather on Thursday, as well as dense fog
issues this morning and potentially again tonight.

Don`t expect current frontal system to make much, if any, more
push southward today. Any precipitation likely to remain on the
isolated side with the best chances far northwest. A few rumbles
of thunder not out of the question near the Atchafalaya River
Basin by late afternoon.

As upper low redevelops into Nebraska over the next 24 hours,
impulse pivots around the base of the low, allowing the system to
take on a bit of a negative tilt by Thursday afternoon. While
soundings become nearly saturated through the column Thursday
morning, wind shear increases with bulk shear values in excess of
60 knots by mid-afternoon noted on forecast soundings. Instability
is a bit lacking with CAPE values less than 500 j/kg on the GFS,
and only slightly better than that on the NAM solution. Convection
that does develop will have potential to become severe, mainly
during the afternoon hours Thursday. SPC has entire area in
Marginal Risk for Thursday, and will continue to mention the
potential for severe in grids/HWO. Rain amounts of 1-2 inches
likely with Thursday convection. Precipitation should clear the
area Thursday evening.

A brief break between systems late Thursday night through the
daytime hours Friday. May be a few showers/thunderstorms,
primarily offshore and will only carry isolated rain chances.

Temperatures well above normal through Friday. Thursday will be
the coolest of the 3 days due to better precipitation chances.
Likely to be pushing record territory today and again on Friday.
Numerical guidance has tended to be a little low, and will trend
toward the warm end of the guidance envelope through Friday. 35

&&

.LONG TERM...

A couple shortwaves are quickly on the heels of Thursday`s system.
The first of the two shortwaves will bring the potential for
elevated convection late Friday night. Wouldn`t be shocked to see
a few hailers very early Saturday morning, and SPC`s Day 3 outlook
carries the area in a Marginal Risk of severe weather for that
potential. The strongest shortwave barrels through the southern
Rockies and Southern Plains states Saturday afternoon and Saturday
night. Models begin to have their timing differences with this
system, with the ECMWF being a little bit quicker. While shear
won`t be quite as strong with this system, instability is much,
much better than with tomorrow`s event, with CAPE values around
2000 j/kg and lifted indices near -10 on both GFS and NAM forecast
soundings. Primary time frame for severe weather will be Saturday
night, but could be late Saturday afternoon far west and could
linger into early Sunday morning on Mississippi coast. SPC has 15
percent threat for entire area for 24 hour period ending at 12Z
Sunday, and for Jackson and Harrison Counties in Mississippi for
Sunday morning. Of the 3 events over the next 5 days, the one late
Saturday has the highest potential to produce severe weather. With
the strength of the surface low passing to our north on Sunday,
gradient winds could also be an issue on Sunday into Monday,
particularly south of Lake Pontchartrain.

Behind the Saturday/Sunday systems, a couple of relatively quiet
days before the next front approaches Tuesday night or Wednesday.

Saturday temperature forecast will be dependent on how much
morning convection the area receives, but will trend toward the
warmer guidance for now. Beyond the Saturday system, temperatures
will be less warm than they have been, but still above normal. 35

&&

.AVIATION...

No changes with respect to low ceilings and fog this morning.
Should begin to see more SHRA activity through noon today mainly for
terminals to the west. The chance for rain will spread east through
the evening. TSRA will also begin to show up as well, and this will
be introduced in the west first during the afternoon and brought
eastward through tonight with the greatest chances showing up on
Thursday.

Looking to see how the environment is changing with respect to low
ceilings and fog while sh/ts are moving through. This is showing up
in east Texas and western Louisiana as varying ceilings and vis
restrictions as sh/ts move through. This may be the same conditions
that will present late tonight and Thursday.

&&

.MARINE...

A low pressure system will pass through the region Thursday and
Thursday night. The increased pressure gradient near this low will
push southerly flow up to around 15 to 20 knots. These conditions
will also push seas up to around 4 to 6 feet by Thursday night. High
pressure should then quickly fill back in behind this low and a
sharp decrease in the pressure gradient over the Gulf is expected
for Friday.  Winds should fall back to 10 knots or less and seas
should also drop off quickly.

This period of calmer weather will be short-lived as another strong
low pressure system moves in from the west over the weekend. Onshore
winds should increase to around 15 knots by Saturday afternoon, and
then further increase to around 20 knots and advisory levels
Saturday night and early Sunday.  Seas should also turn rough due to
these winds with seas of 5 to 8 feet expected by Sunday morning.  In
the wake of this low, model guidance is indicating that gale force
winds could impact the coastal waters beginning Sunday afternoon and
persisting through Monday.  These conditions will be highly
dependent on the strength of the longwave trough axis projected to
slide through the region Sunday.

&&

.DECISION SUPPORT...

DSS code: Blue.
Deployed: None.
Activation: None.
Activities: Dense Fog Advisories. Monitoring convective trends for
            Thursday through Sunday.

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  78  65  71  59 /  30  50  90  40
BTR  80  66  73  60 /  30  50  90  20
ASD  78  65  74  61 /  30  30  90  80
MSY  79  66  75  61 /  30  30  90  50
GPT  75  64  72  61 /  20  20  90  80
PQL  77  64  76  61 /  20  20  90  80

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Dense Fog Advisory until 10 AM CST this morning for LAZ034>037-
     039-040-046>050-056>072.

GM...Dense Fog Advisory until 11 AM CST this morning for GMZ530-532-
     534-536-538-550-552-555-557.

MS...Dense Fog Advisory until 10 AM CST this morning for MSZ068>071-
     077-080>082.

GM...Dense Fog Advisory until 11 AM CST this morning for GMZ532-534-
     536-538-550-552-555-557.

&&

$$


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