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Area forecast discussion 
National Weather Service New Orleans la
331 am CST Monday Dec 22 2014

Short term...

Areas of fog will persist over the area through the first half of
the morning. As boundary layer flow begins to increase there will
be enough overall mixing in place to allow the fog to clear. The
rain currently along the immediate Louisiana coast and over the
offshore waters should diminish as overall Omega aloft decreases
in the wake of a departing vorticity lobe. Do not expect to see the
showers persist over the immediate Louisiana coast after middle-
morning...except for areas in the immediate vicinity of Head of
Passes in lower Plaquemines Parish. Southerly flow will develop
and intensify through the day. Although mostly cloudy to overcast
conditions are expected to persist...there should be a surge of
warmth and moisture into the region that will push temperatures in
the upper 60s and lower 70s by the afternoon hours.

Tonight...an upper level trough centered over the plains
will begin to rapidly deepen. This rapid deepening will be driven by
some strong northern stream vorticity descending down The Spine of
The Rockies. An attendant upper level jet in in excess of 120
knots will accompany this northern stream feature as it dives into
Texas. As this jet rounds the base of the trough...it will begin
to take on a more neutral tilt and also start to lift to the
northeast. As the jet streak rounds the base of the trough...low
level cyclogenesis will begin in eastern Texas. Warm frontogenesis
will take place across the forecast area tonight...and the
combination of a developing jet couplet aloft along with
increasing isentropic forcing in the low levels will bring
increasing rain chances to the entire forecast area. Expect to
see rainfall develop in the late evening hours and become more
likely as the night proceeds. Model soundings continue to show
a stable layer in the low levels...but there will be a region
elevated cape values above this stable layer. Expect to see
scattered showers and isolated thunderstorms begin to develop
during the late evening hours and overspread the entire area
through the remainder of the night. Lapse rates aloft combined
with increasing Omega support the development of some stronger
storms after midnight. Given that these thunderstorms will be
elevated above the stable layer...hail will be primary threat late
tonight.

Tuesday still looks to be a very active day...with the risk of
some severe thunderstorms forming during the afternoon and evening
hours. The region will remain centered beneath a jet couplet and
overall upper level forcing will be very strong. Overall
instability will increase through the day...with cape values
expected to rise to around 1000 j/kg during the afternoon and
evening hours. Shear values will also be fairly impressive with
0-3 km shear of around 30 knots and helicity values ranging from
150 to 250 m2/s2 in the afternoon and evening hours. Given these
parameters...all convective modes will be possible with this
system Tuesday into Tuesday evening. Most of the convection will
likely be pre-frontal...with the main wind shift and surge of
colder air generally coming in Tuesday evening over western zones
and during the overnight hours for eastern zones. Conditions will
clear from west to east with this frontal passage...and expect to
see lingering convection and a threat for isolated severe
thunderstorm activity over the coastal Mississippi zones through
late Tuesday evening.

The upper level trough axis will pass through the area Wednesday
and Wednesday night. The Heart of the cold pool will also pass
over the area during this time frame. 1000-500mb thicknesses will
plunge to around 5430 meters and 925mb temperatures will drop to
around 0 degrees celsius by Wednesday afternoon. This correlates
to a surface temperature in the lower 50s. With clear skies in
place Wednesday night...readings will easily cool into the 30s
over most areas. A light freeze may occur over parts of southwest
Mississippi and the Northshore late Wednesday night. Conditions
will also be very breezy as the trough axis moves through Wednesday
into Wednesday night.

Long term...

The trough axis will finally begin to pull east of the area on
Thursday...with strong subsidence and very dry air taking hold of
the area. Sunny skies and cooler than normal temperatures can be
expected. Daytime highs will only climb into the middle to upper
50s and lows on Thursday night will drop back into the lower 40s
over inland locations as conditions radiate out.

Both the Euro and the GFS indicate that another short wave trough
will slide through the area for Friday and Saturday. The Euro has
a sharper trough slide through the area...and thus Sparks off more
rainfall over the area. The GFS has a shallower trough that brings
increasing clouds...but lower overall probability of precipitation to the region for
Friday and Saturday. On Sunday...the weaker GFS also keeps a
boundary along the immediate coast with higher probability of precipitation and more cloud
cover. The Euro has a much stronger front push offshore with
clearing skies and dry weather taking hold. Given these large
model differences...and overall low confidence in weekend
forecast...have went with a blend of these solutions. This
solution brings a chance of rain to the area beginning Friday
night and continuing into Saturday and Saturday night as the
trough axis and frontal boundary move through the area. Have not
introduced thunder into the forecast at this time...although the
Euro solution would support some thunderstorm development.
Temperatures will generally be near normal for Friday and
Saturday. On Sunday...have only a slight chance of rain in the
forecast given the large model discrepancies noted earlier.

Aviation...

Shallow marine layer only about 250 feet deep will maintain IFR ceilings
and MVFR visibility around 1sm br through around 15z before improving to
prevailing MVFR ceilings remainder of taf period. Some spotty shower
activity possible from 22/12z-23/12z though khum may be close enough
to detect distant thunder well south of that location later Monday
night. 24/rr

&&

Marine...

Gradual warm frontogenesis process underway in the north Gulf will
maintain generally light to moderate easterly flow that will veer to
more onshore orientation as warm front lifts north later today.
Winds should then begin to respond to synoptic scale frontal system
developing in the plains states with deeper fetch into the Gulf to
increase winds to low end Small Craft Advisory levels Tuesday. Hard
wind shift still looks to be on track for late Tuesday night into
Wednesday morning with high end Small Craft Advisory winds and seas
developing along with some gale force gustiness at peak cold air
advection time-frame Wednesday morning. Squall line convection on
the coastal waters late Tuesday evening will likely prompt
short-fused marine warning issuances during that time-frame. Severe
weather potential should move east of MS coastal waters by daybreak
Wednesday. 24/rr

&&

Decision support...

Dss code...blue
deployed...none.
Activation...none.
Activities...slurry support
monitoring fog conditions
assessing severe weather threat for Tuesday.

Decision support services (dss) code legend
green = no weather impacts that require action
blue = long fused watch/warning/advisory in effect or high
visibility event
yellow = heightened impacts with short fused
watch/warning/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 to high risk severe
and/or direct tropical threats; events of National
significance.

&&

Preliminary point temps/pops...
mcb 70 58 70 47 / 10 30 80 80
btr 71 59 71 49 / 10 30 80 80
asd 69 60 71 53 / 10 40 80 80
msy 70 61 70 53 / 20 40 80 80
gpt 68 61 71 56 / 20 40 80 80
pql 69 60 71 59 / 20 50 80 80

&&

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

$$

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