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Area forecast discussion
National Weather Service New Orleans la
830 am CST Tue Jan 16 2018

Sounding discussion...
12z sounding is already obsolete since the cold front passed
through about an hour after the balloon launch. Surface
observations indicate low level winds have already shifted to the
northwest, and expect cold air advection to begin shortly. We will
do a special launch at 18z to gauge wintry precipitation
potential this afternoon and evening. 95/dm

&&

Previous discussion... /issued 311 am CST Tue Jan 16 2018/

Short term...

Today through this evening will be the main focus of the short
term discussion. A strong Arctic cold front is currently pushing
across northern Louisiana, and will move through the forecast area
during the morning hours. Before this front moves in, a surge of
warmer air has started to advect in from the west. This has
resulting in rising temperatures early this morning, and now
expect to see temperatures around daybreak in the upper 30s and
lower 40s at most locations. As a result, the initial
precipitation that moves will be in the form of rain.

By late morning and early afternoon, a transition over to light
sleet and snow should occur across the northern half of the
County Warning Area...mainly along and north of the I-12 corridor. Fortunately,
precip amounts will be very light as moisture divergence is still
expected to occur over southeast Louisiana today. Currently
forecasting not much more than a dusting of snow across southwest
Mississippi and the Florida parishes of southeast Louisiana by
this evening. Below the I-10 corridor, a transition from light
rain to light sleet and snow should occur in the late afternoon or
early evening hours. Again, any precipitation will be very light,
and little to no accumulation is expected. However, the risk of
some black ice forming on a few bridges has prompted the extension
of the Winter Weather Advisory to include Baton Rouge, New
Orleans, the river parishes, and Houma.

Temperatures will quickly drop in the afternoon and evening
hours, and still expect to see lows middle to upper teens north of
I-12 and the lower to middle 20s elsewhere. Hard freeze and
freeze warnings remain in effect for the region. Additionally,
strong north winds of 15 to 20 mph combined with the cold
temperatures will push wind chills into the single digits and
teens late tonight and tomorrow morning. A Wind Chill Advisory
also remains in effect for the area due to these Bone chilling
temperatures.

The Heart of the 925mb cold pool will be over the forecast area
tomorrow, and temperatures should only climb into the middle to
upper 30s at most locations. Wind chills will make it feel like
the 20s throughout the day. Winds should finally begin to decrease
tomorrow night as a surface high settles over the region, and
clear skies and low humidity values will allow temperatures to
fall back into the upper teens and lower 20s north of I-10
corridor, and the middle to upper 20s closer to the Louisiana
coast.

A more zonal flow regime will develop on Thursday as the upper
level trough and main heart of the Arctic cold pool lift to the
northeast of the region. Temperatures will modify a bit, and
expect to see highs warm into the middle to upper 40s. A dry
airmass will linger over the area, so skies should remain mostly
clear. Temperatures Thursday night will fall back below freezing
for the northern half of the cwa, but only a light to moderate
freeze is expected.

Long term...

A weakening shortwave trough over Texas will push through the
forecast area Friday night into Saturday. Increasing high level
cloud cover is expected on Friday, but the low to mid levels
should remain relatively dry. As a result, a dry forecast is in
place for Friday. Temperatures will also continue to modify, and
expect to see highs in the middle to upper 50s. Weak moisture
advection in advance of the shearing out shortwave trough will
allow for isolated to widely scattered shower development Saturday
into Saturday evening. Fortunately, lapse rates on Saturday remain
very weak, so no thunderstorm activity is expected. Temperatures
will continue to warm, and expect to see highs in the upper 60s
and lower 70s Saturday afternoon.

Saturday night appears to be the most favorable time for a
possible sea fog event to develop. Onshore flow will continue to
advect in a warm and humid airmass through the night. The
temperature and dewpoint spread compared to the nearshore water
temperatures looks favorable for sea fog to form. Additionally,
weak shortwave ridging aloft will also assist in suppressing
boundary layer turbulence. With less boundary layer turbulence and
mixing in place, fog formation is more likely. Temperatures should
only dip into the middle to upper 50s Saturday night.

Sunday should start off foggy, but increasing positive vorticity
advection in advance of an approaching shortwave trough axis and
front should result in the fog lifting by mid-day. Onshore flow
should also increase in advance of this upper level trough and a
deepening surface low in the Midwest. As temperatures warm into
the lower to middle 70s, instability should also increase. Model
soundings suggest some marginal instability with cape of 500-1000
j/kg, and this should support the development of scattered
thunderstorms Sunday afternoon into Sunday evening. A veering wind
profile from the surface into the mid-levels should support some
directional shear, and this is noted by storm relative helicity
values of around 300 m2/s2. Thus, a high shear/Low Cape
environment appears to be in place for Sunday. Severe
thunderstorms could develop, and this will need to be monitored
over the coming days.

Increasing subsidence and negative vorticity advection will take
hold in the wake of the passing front on Monday and Tuesday.
Temperatures will cool a bit behind the front, but a Pacific based
airmass will be advecting in. As a result, highs should only dip
down to the lower 60s, and overnight lows should only cool to near
40 degrees at night. These temperatures are near seasonal averages
for mid-January.

Aviation...

Low clouds will begin to build over the area today ahead of a cold
front expected to quickly surge through during the late morning
hours and should be through all of the terminals by 18z. MVFR cigs
will begin to develop from northwest to southeast beginning around
15/16z by mcb and btr. Precip should begin to slowly develop and
fall over the area also from northwest to southeast and should begin
as either light rain, maybe even light freezing rain but more likely
as sleet at btr and mcb and then likely quickly switching to snow at
mcb between 18-20z and likely around 19-21z at btr. All terminals
will likely show some frozen precip by this afternoon and into the
evening hours. All precip will be out of the area by midnight but if
icing conditions develop, it could remain until after we warm above
freezing Wednesday. This would occur on mainly protruding and
elevated structures.

Marine...

Strong offshore winds will develop in the wake of a strong Arctic
cold front with Small Craft Advisory conditions forecast by
afternoon across all coastal waters. Will keep headlines as is for
now as sustained winds should be in the mid to upper portion of
advisory criteria but there will definitely be gale force gusts
occurring with this system. Winds will remain elevated into
Wednesday night before easing on Thursday as strong high pressure
builds into the region. The high will shift to the east by the end
of the week as the next system begins to approach the area from
the western Gulf.

Decision support...

Dss code: yellow.
Deployed: none.
Activation: none.
Activities: winter weather threats today/tonight
hard freeze threat tonight and Wednesday night
dangerous wind chills tonight/Wednesday morning

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.

&&

Preliminary point temps/pops...
mcb 37 16 37 18 / 50 20 0 0
btr 40 18 39 20 / 50 30 0 0
asd 42 20 40 21 / 30 40 0 0
msy 43 23 38 26 / 30 40 0 0
gpt 45 22 39 23 / 20 30 0 0
pql 50 22 40 21 / 20 30 0 0

&&

Lix watches/warnings/advisories...
la...freeze warning from 6 PM this evening to noon CST Wednesday for
laz066>070.

Wind Chill Advisory from 9 PM this evening to 11 am CST
Wednesday for laz034>037-039-040-046>050-056>072.

Hard freeze warning from 6 PM this evening to noon CST Wednesday
for laz034>037-039-040-046>050-056>065-071-072.

Winter Weather Advisory from noon today to midnight CST tonight
for laz034>037-039-040-046>050-056>065-071-072.

GM...Small Craft Advisory from noon today to 6 PM CST Wednesday for
gmz536-538-550-552-555-557-570-572.

Small Craft Advisory from noon today to midnight CST Wednesday
night for gmz575-577.

Small Craft Advisory from noon today to noon CST Wednesday for
gmz530-532-534.

MS...Wind Chill Advisory from 9 PM this evening to 11 am CST
Wednesday for msz068>071-077-080>082.

Hard freeze warning from 6 PM this evening to noon CST Wednesday
for msz068>071-077-080>082.

Winter Weather Advisory from noon today to midnight CST tonight
for msz068>071-077-080>082.

GM...Small Craft Advisory from noon today to 6 PM CST Wednesday for
gmz538-550-552-555-557-570-572.

Small Craft Advisory from noon today to midnight CST Wednesday
night for gmz577.

Small Craft Advisory from noon today to noon CST Wednesday for

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