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

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FXUS64 KLIX 172322 AAA

Area Forecast Discussion...UPDATED
National Weather Service New Orleans LA
522 PM CST Sun Dec 17 2017

The forecast grids and products have been updated to include a
Dense Fog Advisory for the entire P/CWA, including the near shore
waters, sounds, and tidal lakes. Recent surface observations
indicate that dense fog is already occurring at a few sites...
primarily near bodies of water. The fog is expected to expand in
coverage overnight and persist into Monday morning. An increase in
shower activity later tonight and Monday morning may disperse the
fog to some degree in some areas. 11


.PREV DISCUSSION... /issued 358 PM CST Sun Dec 17 2017/


Well defined frontal boundary near Interstate 20 this afternoon.
Leading edge of weakening convection has cleared all but the
extreme southern tip of Plaquemines Parish. Still a large area of
showers and a few embedded rumbles of thunder across the area east
of a line from McComb to Donaldsonville. Temperatures across the
area were generally in the 60s in a saturated atmosphere. Some
areas of lowered visibilities noted.


Short range guidance continues to indicate that precipitation
area should shift north and/or east of the area by 00z or shortly
afterward, with more precipitation developing after midnight from
the southwest. Fog could be a significant factor overnight as
well. Temperatures/dew points currently running about 10 degrees
above water temperatures over nearshore waters and tidal lakes.
Not seeing enough low visibilities to go with a pre-emptive Fog
Advisory, but evening shift will need to continue to monitor.

Expect boundary to move little over the next 24 to 36 hours with
another wave of precipitation likely during the day tomorrow. This
wave will then push the boundary further north for Monday night
into Tuesday night, waiting for the main impulse to move through
early on Wednesday. Likely POPs for much of the area Tuesday night
with this system. This is a southern stream system, so no real
cold air involved.

Temperatures will be much above normal through the short term. In
fact, current forecast temperatures may need to be raised in later
packages. Have tried to stay close to the neighboring offices
forecast temps. 35


The good news in the extended forecast is that Thursday looks dry.
Models agree on this. After that, model forecast consistency has
been non-existent. Next upper low drops into the Four Corners
region Thursday and Friday, keeping southwest flow over the area.
Trough will pass to the north of the area Friday night or Saturday
with another good chance of precipitation at some point during
that period. The wild card will be the location of the cold air.
Previous model runs had brought the colder air into the CWA, but
both 12z runs of the GFS and ECMWF now keep the area solidly in
the warmer air. Current MEX/ECX numbers are as much as 15 degrees
above current forecasts for Saturday/Sunday. Blends that are
generally used in the extended are probably going to take a couple
runs to catch up, assuming current trends continue. Have
compromised with the neighboring offices using a blended solution,
but look for these numbers to go up if trends continue. Needless
to say, forecast confidence is very low to non-existent beyond
Thursday. 35


Varying conditions from LIFR to VFR have been occurring at many
airports due to a mix of areas of rain, light fog and lower CIGS.
These varying conditions should start to trend towards more IFR to
MVFR around 00z this evening, then continuing to lower to
LIFR to VLIFR and possibly near or below some of the airport
minimums in low stratus clouds and a mix of isolated to scattered
SHRA, a few TS and fog by late tonight into Monday morning. 22/TD


A stalled frontal boundary is expected to remain inland from the
coast tonight through early in the week. Southerly winds have
continued up in the 15 to 20 knots range over southeastern coastal
waters into this afternoon, but the pressure gradient is loosening
and wind speeds are expected to be down to 10 to 15 knots over
coastal waters near or shortly after sunset, then down to around 10
knots or less overnight into Monday. The combination of lighter
onshore winds and dewpoint temperatures exceeding the cool near-
shore waters will make for a very favorable pattern for areas of sea
fog to lower the visibility to less than one mile. Have not issued a
marine dense fog advisory with the afternoon coastal waters
forecast, but it is likely going to be needed later this evening.
Fog will likely be slow to dissipate to lift with a fairly deep
marine layer expected in many areas, so improvements in visibility
may only be for a few hours around midday through mid afternoon
before fairly widespread fog develops again Monday night into
Tuesday morning.

Another surface low is expected to traverse the Lower Mississippi
Valley Tuesday night which will bring another weak cold front into
and likely stalling over the region Wednesday before lifting north
on Thursday. By the end of the week we should see a stronger cold
front pushing towards the coast, but it also may not push
offshore through the Christmas holiday. 22/TD


DSS code: Green.
Deployed: None.
Activation: None.
Activities: Monitoring fog trends.

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.


MCB  60  68  61  75 /  50  70  30  40
BTR  62  71  62  77 /  50  70  20  40
ASD  63  72  61  75 /  50  70  20  30
MSY  63  74  62  76 /  50  50  20  30
GPT  63  70  61  73 /  50  60  20  30
PQL  63  73  60  74 /  50  60  20  30


LA...Dense Fog Advisory until 9 AM CST Monday for LAZ034>037-039-040-

GM...Dense Fog Advisory until 9 AM CST Monday for GMZ530-532-534-536-

MS...Dense Fog Advisory until 9 AM CST Monday for MSZ068>071-077-

GM...Dense Fog Advisory until 9 AM CST Monday for GMZ532-534-536-538-


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