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

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FXUS64 KLIX 152352 CCA
AFDLIX

Area Forecast Discussion...CORRECTED
National Weather Service New Orleans LA
427 PM CST Sun Dec 15 2019

.SHORT TERM (Tonight through Wednesday)...

Rain chances will return from the northwest tonight as a broad
and potent, positively tilted shortwave trough moves east out of
the southern Rockies into the Plains with the associated cold
front advancing southeast into the ArkLaTex region. Generally
light southerly winds near the coast will advect greater than 60
degree dewpoints across the colder nearshore waters and land
surfaces. This will likely result in areas of fog developing near
the coast and tidal lakes during the late evening hours or after
midnight and this fog will likely persist into the early morning
hours on Monday. Have opted not to issue a dense fog advisory with
this forecast issuance, however it is possible an evening
issuance may be required if fog starts to develop early.

The strong cold front will continue to advance southeast towards the
northwest portion of the forecast area late Monday and through the
forecast area Monday night as the shortwave trough advances east
through the Plains and mid Mississippi valley. As mentioned in
previous discussions and SPC, the speed shear will be rather
impressive with modest instability also expected in the
afternoon. The large scale lift will be supported by the right
entrance region of the upper jet on the front (east/southeast)
side of the upper trough and the strong wind fields from the low
to upper levels. The main limiting factor would be possible marine
layer, low level stabilizing impacts that develop towards sunset
and in the evening in coastal areas. The SPC Day 2 Convective
Outlook kept the outlook the same as early morning with an
Enhanced Risk of severe over the northwest half of the forecast
area and a Slight Risk to the southeast. Greatest risk time
locally will be Monday afternoon (after 2 pm) and evening with
the main threat being strong, possibly damaging wind gusts, a few
tornadoes and a few instances of large hail. Have high rain
chances since coverage should be fairly widespread with near
record high precipitable water values as high as 1.6 to 1.7 inches
in a wide swath ahead and near the front. QPF amounts of 1 to 2
inches are expected with locally higher swaths possible.

Rain chances will decrease fairly quickly late Monday night and
Tuesday as the front moves offshore. This will usher in much
colder air and brisk north winds for Monday night and Tuesday.
The cold airmass will settle over the central Gulf coast region
Tuesday night into Wednesday and beyond, and clear skies should
allow for a light freeze over the northern/inland areas Tuesday
night.

.LONG TERM (Thursday through Monday)...

The cold airmass will remain settled over the central Gulf coast
region Thursday before modifying and moving east on Friday.
Another light freeze over the northern/inland areas is expected
Wednesday night.

Another potent shortwave trough with a closed low is forecast by
both the operational 12z GFS and ECMWF to move east then southeast
across the southern Plains then central to eastern Gulf coast
region during the Thursday night to Sunday period. There is a
greater spread in the models track today with the GFS taking the
deep low just north of our forecast area into the Florida
panhandle late Saturday with the ECMWF taking the low more
southeastward right over Louisiana Saturday night. The GFS is
closer to persistence with lower rain probabilities while the
ECMWF is much wetter. Have gone closer to the GFS solution and
previous forecast. 22/TD

&&

.AVIATION...

VFR conditions will prevail through the evening hours. Some fog is
possible overnight and early Monday morning at some area
airports, which could cause lower visibilities, especially at
KHUM, KGPT, KASD and KHDC. Then VFR conditions will prevail Monday
during the mid to late morning and afternoon hours. During the
afternoon and evening hours, increasing coverage of showers and
thunderstorms could cause lower ceilings and visibilities, and
inclusion in later TAF issuances will be needed. MSW

&&

.MARINE...

Moderate onshore flow to be maintained into Monday ahead of
approaching strong cold front. This front is expected to move
through the tidal lakes, sounds and coastal waters between
midnight Monday night and 9 am Tuesday morning. Cold high
pressure will build into the Gulf along Tuesday into Wednesday
with strong northerly winds well into Small Craft Advisory
criteria late Tuesday night through Wednesday morning across all
the marine area and into the afternoon across the outer waters. As
high pressure builds closer to the coast, winds are expected to
drop below 15 knots late Thursday night into Friday. 22/TD

&&

.DECISION SUPPORT...

DSS code: Orange
Deployed: None.
Activation: None.
Activities: Support for the City of New Orleans
            Slight to Enhanced Risk of Severe Thunderstorms for
            Monday afternoon/evening.

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  61  76  44  53 /  20  90  90  10
BTR  64  78  44  53 /  30  90  90  10
ASD  59  78  46  56 /  10  50  90  20
MSY  64  78  49  55 /  10  40  80  20
GPT  61  75  48  58 /  20  40  90  40
PQL  61  77  49  58 /  20  40  90  50

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...None.
MS...None.
GM...None.
&&

$$


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