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fxus64 klix 091809 
afdlix

Area forecast discussion
National Weather Service New Orleans la
1209 PM CST Mon Dec 9 2019

Update...

Updated for 18z aviation discussion.

&&

Aviation...

A mix of MVFR and VFR conditions this morning continued at the
start of the 18z taf cycle, although they had trended mostly MVFR
just prior to 18z. Conditions are expected to remain MVFR at times
due to lower ceilings through about 20z before trending to VFR the
remainder of the afternoon into the early evening. A return to
MVFR and possibly IFR conditions, mainly due to low cigs, but
also somewhat from lower visibility from patchy fog during the
overnight hours through Tuesday morning. Isolated rain shower- and very
light rain should have only minimal/brief impacts at the terminals
through this afternoon and evening, then there is a higher chance
of rain shower which could cause longer periods of reduced visibility in
places after 06z tonight through 18z and beyond with a cold
frontal passage on Tuesday. 22/dew point

&&

Previous discussion... /issued 653 am CST Mon Dec 9 2019/

Update...

Have cancelled fog advisories, both land and marine based, as
webcams are not showing any significant fog. Can't rule out a few
small patches of ground fog, but no indications of sea fog, as
higher dew point air remains further south. Products already
issued.

Sounding discussion...

Surface inversion is very shallow, as morning sounding shows 20
knot winds at 400 feet. Airmass dries out rapidly above 3,000
feet, so we should see a fair amount of sunshine. Current high
temperature forecast appears to be attainable.

Previous discussion... /issued 358 am CST Mon Dec 9 2019/

Synopsis...

High pressure currently off the Atlantic coast extending into the
eastern Gulf of Mexico. Surface low currently over Kansas.
Southerly flow has onset across the area, but is still rather
weak, and isn't bringing particularly moist air into the area yet.
In fact current dew points noted in marine observations remains
below water temp (upper 50s dew points vs low-mid 60s water
temps). This is making for a difficult Nowcast regarding advective
sea fog. Aviation observations at Gulfport and keesler AFB
support the current dense fog advisory, but webcams are showing
little, if any, dense fog. So, what is out there is more likely
radiative and not advective.

Short term...

If fog development doesn't increase closer to sunrise, will likely
go ahead and cancel advisory prior to 7 am CST.

Onshore flow becomes somewhat better established today with mid 60
dew points across most of the area by late afternoon. This could
lead to a more substantial advective fog situation tonight.
Northern stream shortwave pushes cold front closer to area as
Kansas low moves into the Great Lakes, but other than warm
advection, no real sources of forcing today. This would allow for
some development of isolated or scattered showers this afternoon,
but no widespread precipitation. Front enters the area late
tonight and takes pretty much the entire day Tuesday to clear the
coast. At this time, it looks like most precipitation will
actually be Post- frontal, which will limit thunderstorm
development. Three hourly thunder probabilities don't get much
past 10 percent, so will be playing down any thunder mention for
now. Slow moving front means precipitation likely to linger into
at least early Wednesday before pushing offshore.

Assuming we don't get into an advective fog situation today,
anticipate high temperatures should be at or above the upper end
of the guidance envelope for much of the area...75-80. This would
be a few degrees short of records at Baton Rouge and New Orleans.
The exception to this would be along the Mississippi coast, where
the water temperatures will temper highs somewhat. Guidance is
pretty close on temperatures tonight. High temperatures on Tuesday
will be driven by the frontal location around sunrise. Calendar
day highs likely to be pre-sunrise across northwest quarter or so
of the area. Will accept blends for Tuesday night/Wednesday. 35

Long term...

Beyond Wednesday, guidance begins to struggle with timing,
strength and phasing of northern and southern stream shortwaves.
At this time, European model (ecmwf) beginning to look like an outlier solution
closing off an upper low over the Bay of Campeche over the
weekend. No support for it from CMC or the Euro ensemble. For now,
will continue use of blends, but may need to start trending toward
GFS solution in later packages. If we get precipitation in the
extended, probably on Friday, but currently looks like most
precipitation likely to be offshore. 35

Aviation...

Low level ceilings will range from vv001 to ovc005. When these
ceilings break, the next level will remain at ovc020-040 through
the day. Tonight looks to be a repeat of these LIFR to IFR
conditions. Vis will follow these same lines as it will dip to
1/2sm or less this morning and again tonight for most if not all
terminals.

Marine...

A few locations showing fog this morning and a marine dense fog
advisory will remain for these areas through 10am today. The
advisory may be issued again later today unless the fog refuses to
lift which will cause the advisory to simply be extended in time
and possibly area as well. Winds will also remain southerly around
15kt through tonight. Another cold front will approach and move
through the coastal waters Tuesday night. Fog conditions will be
replaced with widespread rain showers Tuesday night into
Wednesday. Strong northerly winds will also move in behind a cold
front. These conditions will be pressed south into the central
Gulf Wednesday night, but will rapidly move back into the coastal
waters by the end of this week.

Decision support...

Dss code: blue.
Deployed: none.
Activation: none.
Activities: forecast support to city of New Orleans and miss Gulf Coast

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.

&&

Preliminary point temps/pops...
mcb 75 62 68 40 / 40 70 100 90
btr 78 63 69 41 / 30 60 90 90
asd 77 64 75 41 / 20 30 90 80
msy 77 66 76 47 / 20 30 90 80
gpt 72 64 72 43 / 10 30 80 80
pql 76 64 77 42 / 10 20 80 80

&&

Lix watches/warnings/advisories...
la...none.
GM...none.

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