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Area forecast discussion 
National Weather Service New Orleans la
654 PM CST Wednesday Dec 17 2014

Sounding discussion...

Flight a little shorter than normal tonight. Lost wind data at
about 80k feet...and flight terminated over Jackson County
Mississippi in the Sandhill Crane National wildlife Refuge at a
height of 88.1k feet or 16.7 miles in altitude.

Airmass a little more moist than last night and this morning with
a precipitable water value of 0.59 inches...which is 68 percent of
the climatological normal. Sounding showed a 5 degree c inversion
between 950 and 920 mb. A little bit of moisture between 825 and
725 mb...but this is only producing a few clouds...and a little
more moisture just above 400 mb which is producing quite a bit of
cirrus. Light north to northwest winds from about 2500 feet to
10000 feet with generally west winds above that. Maximum wind westerly
at 107 knots at 44.5k feet. Freezing level near 13k feet...and
-20c level at 23.3k feet. 35


Previous discussion... /issued 416 PM CST Wednesday Dec 17 2014/

Short term...

A short wave trough currently moving through the Southern Plains
will continue to ride to the east-northeast into the Midwest and
Ohio Valley tonight and tomorrow. As this feature moves to the
north of the Gulf elongated and highly sheared region of
enhanced vorticity trailing off from the parent short wave will
slide through the area. Moisture will remain somewhat limited over
the rain chances will diminished across much of the
forecast area. However...there will be slightly higher upper level
Omega and some deeper middle and upper level moisture pooling over
the northwest portions of the forecast area. This could allow for
some isolated shower activity over the northwest County Warning Area beginning
tomorrow afternoon and persisting into the evening hours.

A second...much stronger trough will begin to eject out of The
Four Corners and into Texas tomorrow night. As this low approaches
from the west...a strong jet streak will round base of the trough
and create a region of enhanced lift over southeast Texas. At the
same area of higher baroclinicity will also be over this
region...and surface based cyclogenesis will begin to take hold.
This low will deepen through the night...and warm frontal
processes will begin to take hold over the northern Gulf of
Mexico. This warm front will move into southeast Louisiana Friday
morning and should be located along the I-10 corridor in Louisiana
and Mississippi by Friday afternoon. With increasing Omega aloft
and isentropic forcing developing in the low levels...expect to
see skies turn overcast and scattered showers and a few elevated
thunderstorms develop late Thursday and early Friday to the north
of the warm front.

By Friday afternoon...the parent surface low will move into
southwest Louisiana. A strong jet will continue to accompany the
low...and expect to see a low level jet streak of around 50 knots
develop across the forecast area. As a result...0-3 km speed shear
values will generally range from 20 to 30 knots and 0-3km speed
shear will be on the order of 35 to 40 knots. Additionally...model
soundings show a high amount of directional shear...with helicity
values ranging from 250 to 450 m2/s2 Friday afternoon. Given these
shear values...the threat for some severe convection will exist.
However...overall instability looks to be limited with this
system. Generally...any cape values will be less than 200 j/kg or
less...and this will tend to be confined to the immediate
Louisiana coast. Closer to the I-10 corridor...little if any
surface based cape will be found...with any instability confined
to the middle and upper levels. As a result...expect to see the
highest chances of surface based severe convection along and south
of a Houma to New Orleans to Gulfport line. To the north of this
line...the lowest levels of the atmosphere will be far too stable
to allow for surface based convection. Lapse rates aloft are
fairly hail will not be a threat with this system.
Isolated tornadoes and strong straight line winds will be the
primary threats from any severe thunderstorm development Friday
afternoon into early Friday evening.

The surface low will quickly race to the east Friday night with
scattered showers persisting through the overnight hours. Drier
air and increasing negative vorticity advection will finally begin
to overspread the forecast area late Friday night into early
Saturday morning. Rainfall should end by daybreak across the
entire County Warning Area...but a lingering low stratus deck trapped beneath an
elevated inversion produced by the cold air advection will persist
through the morning hours. This stratus deck should begin to mix
out during the afternoon hours as daytime heating weakens the
inversion. Temperatures will be a good 10 degrees colder on
Saturday as compared to Friday. Overall...readings will be about 5
degrees below average.

Long term...

The models have come into very good agreement on a major pattern
change across the Continental U.S. For early next week. The zonal flow regime
that has dominated the Gulf south for the last couple of weeks
will give way to a very strong trough across the eastern third of
the nation by the middle of next week. This transition will bring
another round of unsettled weather to the forecast area for

Initially...another short wave trough riding along the zonal flow
will kick out of the southwest and move through the Southern
Plains. This system will keep scattered clouds over the area for
Sunday...but precipitation is not expected for most of the region
due to a continued dry layer residing in the low levels. There
will be a low developing over the western Gulf that will slide
through the central and eastern Gulf of Mexico Sunday into Monday.
This feature should bring more clouds and a few showers to the
coastal waters both Sunday and Monday...but do not expect to see
this rainfall make a push inland during this period.

Unfortunately...a much stronger piece of northern stream upper level
energy will slide down the Lee side of The Rockies and into the
Southern Plains Monday night. As this feature dives to the
south...a very strong cold front and surface low will form over
the Midwest and Central Plains. This front will continue to race
to the south and east will begin to approach the forecast area on
Tuesday. At the same time as this front approaches...a surface low
will also begin to form over the lower Mississippi Valley. As this
low rapidly deepens in response to a strong 150 knot jet streak
moving over the region...another 50 knot low level jet will form
over the forecast area. In appears that there will
be enough residence time for some warmer and more unstable air to
advect in from the Gulf of Mexico during the day on Tuesday. Given
these conditions...have increased probability of precipitation to chance and included a
mention of convection through the day. Given the parameters some
more significant convection could develop...but confidence on this
is low for now.

The trough and associated front will quickly pull to the east of
the area Tuesday night...and a much colder airmass will begin to
move in. Fortunately...the cold air will come in after the
moisture pulls to the east. Expect to see clearing skies and much
colder temperatures late Tuesday night and Wednesday. There will
be some very strong 50 knot winds are expected at 850
mb. Even with increased friction in the low levels...gale force winds
will be possible offshore and winds over land could exceed 30 miles per hour
at times. Temperatures will average 10 degrees below normal on


High pressure will keep dry air in the lower levels of the
atmosphere through most of Thursday morning with VFR conditions
prevailing. Lower level moisture and clouds will increase from
around midday Thursday through Friday as the next weather
disturbance approaches the area. This will lead to MVFR conditions
as far east as khdc and kmcb by late Friday afternoon and evening.
22/dew point


Light easterly flow will continue through tonight over the coastal
water and seas will generally be 2 feet or less. The next
significant marine impact will begin Thursday night and continue
into Friday night as a low pressure system passes through the Gulf
south. Expect to see southeast and southerly winds of 15 to 20
knots and seas of 3 to 6 feet develop in the Gulf waters. Given
these conditions...exercise caution flags will likely be needed.
The winds will then decrease as the overall pressure gradient
wanes for Saturday and Sunday...with northwest and then northeast
winds expected. However...another very strong low pressure system
will pass over the waters Monday night and Tuesday. This system
will produce small craft winds and could potentially produce gale
force winds and very rough seas of 6 to 10 feet for Tuesday and
Wednesday. 32

Decision support...

Activities...slurry support.

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


Preliminary point temps/pops...
mcb 40 63 50 60 / 0 20 40 90
btr 43 67 53 66 / 0 20 40 90
asd 40 65 50 65 / 0 10 20 80
msy 46 67 54 68 / 0 10 20 70
gpt 42 66 49 66 / 0 10 20 70
pql 37 65 47 66 / 0 0 30 70


Lix watches/warnings/advisories...


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