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FXUS64 KLIX 132137

Area Forecast Discussion
National Weather Service New Orleans LA
337 PM CST Fri Dec 13 2019

.SHORT TERM...Fog has never completely cleared from the middle of
Lake Pontchartrain or along the coastal waters around the mouth 
of the Mississippi River. Additionally, a low stratus deck ranging
from 200 to 400 feet above ground level remains in place over a 
large portion of the area. These conditions are the direct result 
of northeast flow keeping a shallow wedge of cooler air near the 
surface. This wedge of cooler air at the surface is being overrun 
by a warm and moist airmass coming out of the southwest just above
the surface. As a result, low stratus has been in place all day 
and will remain in place through tomorrow morning. There has also 
been some convective development embedded within the southwest 
flow regime in the mid and upper level that is directly related to
a passing upper level trough axis. The rain chances are expected 
to diminish during the early evening hours as the trough axis 
slides to the east and negative vorticity advection increases. 

The lingering low stratus deck will once again build down to the
surface and create widespread dense fog conditions this evening.
These conditions will persist through the overnight hours before
gradually clearing tomorrow morning. A dense fog advisory is in
effect for tonight to reflect the dense fog risk. Fortunately, the
fog should clear more quickly tomorrow morning, and expect to see
partly cloudy skies by late morning and early afternoon as a much
drier airmass in the mid and upper levels mixes into the boundary
layer. Temperatures will be little changed tonight from current
readings, but increased solar insolation tomorrow should allow for
highs in the middle to upper 60s by the afternoon hours.  

Strong surface ridging will remain over the area through Sunday
and expect to see clear skies and near normal temperatures through
Sunday night. Some radiation fog may develop over inland areas
Saturday night given the light boundary layer flow and relatively
high humidity values expected. By Sunday night, boundary layer
winds and increasing positive vorticity advection should be 
strong enough to prevent fog formation.

.LONG TERM...The increasing PVA will be associated with a deepening trough
moving into the eastern third of the CONUS. Strong cyclogenesis
will occur across the Midwest on Monday, and this will drive a
strong cold front out of the Plains and toward the forecast area.
Strong onshore flow Sunday night into Monday combined with
favorable jet dynamics will support the development of a line of
showers and thunderstorms in advance of the front. CAPE values of
around 1000 to 1500 J/KG on Monday are supportive of thunderstorm
development, but shear values will be on the lower end of the
spectrum. The best dynamics will be located across Southwest
Mississippi and the Florida Parishes of Louisiana where speed
shear values in the effective layer will be between 40 and 50
knots and directional shear will be around 150 to 200m2/s2. These
values would support a few strong to severe thunderstorms Monday
afternoon. Further to the south along the I-10 corridor, dynamic
support is expected to be lacking for severe thunderstorm
development at this time. Temperatures will be warmer than average
ahead of the front with temperatures approaching 80 degrees.  

A very cold airmass will advect in behind the front on Tuesday and
remain in place through Thursday. Temperatures will range from 10
to 15 degrees below average with highs in the 50s and lows in the
upper 20s and 30s. A light freeze can be expected across most of
the area both Tuesday and Wednesday nights. Only areas south of
Lake Pontchartrain and along the immediate Louisiana coast are
expected to remain above freezing. Temperatures on Thursday night
will be slightly warmer as winds turn more southerly with readings
in the 30s and 40s expected.  

Model differences are significant on Friday and confidence in the
forecast is extremely low. The GFS has a much stronger and much
wetter pattern forecasted as compared to the ECMWF. Given these
differences and extended nature of the forecast, have opted to use
a blend of the model solutions for Friday. This blend produces a
cloudy day with isolated to scattered rain showers and
temperatures slightly below normal in the lower 60s.  


.AVIATION...Main forecast problem the next few hours is how much additional, if 
any improvement above LIFR will occur at the airports still 
experiencing these low conditions. VFR conditions have moved east 
through KBTR, KMCB, KHDC and recently as far east as a KBXA-KREG-
KPTN line, so it is possible this improvement could reach near KASD-
KMSY-KHUM in the 22-24z period with airports further east more 
likely remaining LIFR. The area of RW/patchy R- is mostly moving 
east of KGPT as of 2130z, but cannot rule out some patchy DZ into 
the early evening. A blend of guidance and current observations 
suggests reductions in flight category will onset early this evening 
where lowest CIGS persist now, and a bit later in 03-06z period for 
the airports that improve or have improved to VFR. Very low 
conditions with most airports near or below airport minimums are 
then expected starting at various times overnight and persisting 
through 14-15z before improving late morning. 22/TD


.MARINE...Dense fog will remain a concern over the coastal waters
tonight into tomorrow morning. A dense fog advisory is in effect
for all of the waters. The fog should clear by tomorrow afternoon
and expect to see rather benign conditions through tomorrow night.
By Sunday, increasing gradient flow on the western side of a high
pressure system could produce onshore winds of 15 to 20 knots and
seas of 3 to 5 feet. These conditions should persist into Monday
in advance of an approaching cold front. The cold front is
expected to move through the waters Monday night, and this will
bring a sharp shift in wind direction to the northwest and then
north. Strong cold air advection over the waters will allow
boundary layer winds of 20 to 30 knots to easily mix down to the
surface Tuesday into Wednesday. Seas of 3 to 6 feet can be
expected over the sounds and lakes and 6 to 12 feet in the open
Gulf waters can be expected with these strong winds.   



DSS code: Blue.
Deployed: None.
Activation: None.
Activities: Dense Fog Advisory
            Forecast Support for the City of New Orleans.
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, or direct tropical threats; Events of 
         National Significance.


MCB  47  64  46  72 /  10   0   0   0 
BTR  48  67  47  77 /  10   0   0   0 
ASD  47  66  46  75 /  10   0   0   0 
MSY  52  66  53  74 /  10   0   0   0 
GPT  49  63  49  70 /  40   0   0   0 
PQL  47  66  47  73 /  40   0   0   0 


LA...Dense Fog Advisory from 9 PM this evening to 10 AM CST Saturday 
     for LAZ034>037-039-040-046>050-056>068-070>072.

     Dense Fog Advisory until 10 AM CST Saturday for LAZ069.

GM...Dense Fog Advisory until 10 AM CST Saturday for GMZ550-552-555.

     Dense Fog Advisory until 10 AM CST Saturday for GMZ530-532-534-

MS...Dense Fog Advisory from 9 PM this evening to 10 AM CST Saturday 
     for MSZ068>071-077-080>082.

GM...Dense Fog Advisory until 10 AM CST Saturday for GMZ552-555.

     Dense Fog Advisory until 10 AM CST Saturday for GMZ532-534-536-


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