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Area forecast discussion 
National Weather Service New Orleans la
721 PM CST Friday Dec 19 2014

Sounding discussion...

00z flight terminated about 4 miles south of Mobile regional
Airport at an altitude of 103.3k feet...or 19.6 miles up.

Sounding is rather similar to the 12z flight...with the
precipitable water value 1.47 inches which is about 170 percent of
the climatological normal for today. With very minor
exceptions...the sounding is saturated from the surface up to
almost 350 mb. Inversions near 1500 and 2800 feet. Northeast winds
were observed from the surface to about 970 mb...then quickly
become southwest to west by 900 mb...and remained there through
100 mb. Peak wind of 103 knots at about 210 mb...or about 38k
feet. Freezing level at 11.1k feet...and -20c level at 22.5k feet. 35


Previous discussion... /issued 404 PM CST Friday Dec 19 2014/

active weather pattern will continue through much of the period
with three different disturbances impacting the area. Best
chances of rain will be in the first 12 hours or so as the first
disturbance moves through the area. Next disturbance passes by on
Sunday with a final clearing front passing through the area some
time Tuesday.

Short term...
weak surface low will continue to move slowly toward and through
the area this evening and tonight resulting in continued
widespread showers across the area. Rainfall totals across the
region so far range from just under half an inch across much of
the South Shore to over 2 inches at New Roads. Most of the
observation sites north of the I-10/12 corridor have recorded at
least an inch of rain so far. Have kept probability of precipitation pretty high for the
first part of the 70 to 90 percent range. With little
to no lightning observed today...will keep thunder out of the
forecast for tonight...though can/T rule out one or two
thunderstorms near the coast as the low passes by. Storm total
rainfall is still expected to be in the 1 to 3 inch range for most
places. Though we could still see a few brief periods of heavier
rainfall...rain is generally expected to be light to moderate in
nature and shouldn/T cause any problems. In fact...the steadier
light to moderate rain should be quite beneficial and could help
ease some of the drought conditions in the short term.

The low is expected to elongate and weaken as it moves past the
mouth of the river with a weak frontal boundary stalling over the
northern Gulf as the driving upper short wave flattens out and flow
becomes parallel to the boundary. What this means in sensible
terms is that we can expect continued cloudiness and at least some
chance of lingering light showers through the weekend.

On Sunday...the stalled boundary will begin to lift northward as
a warm front in response to an approaching fast moving upper short
wave. This will bring isolated to scattered showers to areas near
the coast. Am generally carrying a gradient from 10 to 20 percent
across far northwestern zones to 20 to 30 percent near the coast
and around 40 percent over the Gulf waters. With unstable air
still shunted south over the Gulf...not expecting any thunder with
this activity. Rain chances will increase slightly Sunday night as
the shortwave actually passes through the area...but still no
thunder expected...and rain totals should generally be light.

Long term...
going into next week...our next decent chance of rain will be on
Tuesday as a cold front passes through the area...though rain
chances never really go away following Sunday night/S short wave.
The different models all agree that an upper trough will begin to
dig southward over The Rockies Sunday. The trough will continue to
deepen as it moves over the plains states. This trough will force
a cold front through the area most likely some time on Tuesday.
This is where the forecast gets fun as the timing and strength of
this front will have significant impacts on the temperature
forecast for Tuesday afternoon. To give you an idea of how out of
whack the different possibilities are concerning the finer details
of this system...there is a full 20 degree spread between the high
and low members of the GFS ensemble for the high temperature at
McComb on Tuesday...the low member carrying a temperature of 50
degrees and the high member 70 degrees. The Euro ensemble has a
comparatively respectable spread of only 16 degrees for the same
time frame and location. Needless to say...there is potential for
the temperature forecast on Tuesday to be very wrong. In an
attempt to avoid the windshield wiper generally
carrying temperatures that are a few degrees higher than a blend
of the ensemble means as a nod to the higher operational numbers.
Hopefully this will allow for smaller forecast adjustments moving
forward as the timing and strength becomes more clear.

Have also gone ahead and added thunder into the forecast for
Tuesday as the front comes through. There should be some decent
onshore flow ahead of the front...bringing warmer and more humid
air into the area from the Gulf of Mexico. Looks like dewpoints
could be approaching 60 degrees...which is well above normal for
this time of the time the front gets here. Model
forecast soundings indicate anywhere from 400 to 1000 j/kg of
cape across land areas with decent lift along the front. This
should be enough to support scattered thunderstorms along/ahead of
the front.

This front will clear the area with cool and dry high pressure
building in behind it. Still looks like we should see mostly
clear skies with temperatures near or below normal for Christmas
evening and Christmas day.

main forecast problem and lack of confidence is regarding the
timing of the lower LIFR category ceilings and IFR visibilities that will
mainly be associated with the more persistent and heavier rain showers and
isolated thunderstorms and rain. Have tried to time this in the short term with
observations...radar trends...and WRF/hrrr models. Overall...
conditions will trend towards IFR and LIFR through early Saturday
morning with slow improvement expected later Saturday morning as
the rain should mostly end from west to east.

low pressure off the Texas coast will move east and weaken to an
open wave as it moves south of the central Gulf Coast late tonight
and Saturday morning. Stronger winds near to above 20 knots over the
east/southeast coastal waters are expected to diminish to near 15
knots during the evening. Will carry a /small craft exercise
caution/ headline in the offshore eastern coastal waters through 9
PM. Winds and seas will ease in most location late tonight into
Saturday morning...then high pressure and cooler air will build in
from the north on Saturday resulting in northeast winds rising back
to around 15 knots in the afternoon and evening. High pressure will
shift east and the pressure gradient will weaken on Sunday into this will be a period of lighter winds and smaller
waves. A strong upper level trough and associated cold front are
expected to move through the coastal waters Tuesday. This will bring
a period of strong Small Craft Advisory conditions by Tuesday
afternoon into Wednesday...and gusts to gale force may occur.

Decision support...
Activities...slurry support
city of New Orleans dss 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 43 57 42 60 / 90 20 10 10
btr 44 58 45 61 / 80 20 10 20
asd 45 58 45 61 / 80 40 10 20
msy 50 57 51 61 / 70 30 10 30
gpt 46 57 47 61 / 70 40 10 30
pql 42 59 48 62 / 70 50 10 30


Lix watches/warnings/advisories...


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