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

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS64 KLIX 110207
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
807 PM CST Mon Dec 10 2018

.SOUNDING DISCUSSION...

This evening`s balloon launch required a second launch due to a
potential radiosonde failure at release. However, the second
launch was successful with no problems encountered. Taking a look
at the state of the atmosphere and any noticeable changes in the
past 24 hours, the main difference appears to the much drier
air working in from the north in the surface to low-levels. Clear
skies this afternoon helped develop a well-mixed PBL, allowing for
temperatures to reach the mid to upper 50`s across the station
location. The vertical wind profile continues to illustrate a
dominant cold air advection pattern, with sinking air falling to
just above the PBL creating a strengthening subsidence inversion
right around 850mb. Otherwise, the rest of the vertical profile is
primarily dry with a few other smaller subsidence inversions
aloft. With clear skies and calm winds thanks to a nearby area of
high pressure, there will be no problem with radiative cooling
occurring at the surface tomorrow morning. This will help drop
temperatures a bit colder overnight tonight compared to last
night. KLG

&&

.PREV DISCUSSION... /issued 401 PM CST Mon Dec 10 2018/

Really only two weather concerns in the 7 day forecast
period...low temperatures tonight and thunderstorm/heavy rain
threat with the frontal system later this week.

Regarding tonight, expect temperatures to plummet after sunset as
clear skies, light winds and low dewpoints allow for efficient
radiative cooling. Expect to see widespread freezing temperatures
across the area. Temps forecast to bottom out in the upper 20s and
lower 30s along/north of the I-10/12 corridor and generally in the
mid to upper 30s south of the tidal lakes. Where it gets a little
tricky is through the river parishes and along the west bank of
the Mississippi River through the south shore. One thing is fairly
certain...and that is that the more urban areas should remain
above freezing. All of the available guidance even suggests Belle
Chasse, which is one of the typically colder spots, will remain
just above freezing. However, the swath from Houma up to the river
and northwestward will likely hit freezing for a few hours near
daybreak. Per local policy, freeze warnings are issued for every
freeze across areas for areas from Donaldsonville to Lake
Pontchartrain and southward. This means that a freeze warning will
be issued for Assumption, St. James, St. John, St. Charles, Upper
Terrebonne and Upper Lafourche from 3am until 8am tomorrow
morning.

Temps should rise fairly quickly in the morning once the sun comes
up. Expect highs in the mid to upper 50s. Lows should be a few
degrees warmer tomorrow night, bottoming out in the mid 30s north
and the upper 30s and lower 40s south.

Return flow starts to kick in on Wednesday as high pressure shifts
eastward and winds turn southerly ahead of a vigorous frontal
system taking shape over the plains states. Models remain in
fairly good agreement with respect to timing on this system, but
still have differences in both the intensity and placement of the
surface and upper lows. GFS continues to have the deeper and
farther north solution. Again, while these differences will affect
some nuances of the forecast, the overall pattern is fairly
similar and a general blend of the guidance has been used for our
local weather.

Both the GFS and the Euro indicate a vigorous trough will dive
southward Wednesday night into Thursday morning causing a surface
low to deepen over Texas as it moves eastward toward the middle
Mississippi Valley. The tightening pressure gradient should result
in stronger winds, locally which will help pump Gulf moisture
back into the area. Dewpoints are forecast to recover into the low
to 60s by Thursday afternoon. There still appears to be a fair
amount of instability and lift along the front as it passes
through the area late Thursday afternoon and Thursday evening.
Depending on how the exact details of this system play out, there
could be a squall line along/ahead of the front. There will likely
be a non-negligible severe weather threat with this system, but
for the time being, the probability of severe weather is below the
threshold to be included in SPC`s severe weather outlook. The
system definitely bears continued monitoring over the next few
days as details become clearer.

Behind the front, both models suggest wrap around moisture will
keep things cloudy with some lingering showers into Friday, so
will maintain the 20 to 30 percent POPs for Friday and Friday
afternoon. Of note, this kind of stacked and well-developed system
will typically include a dry slot feature, so as the details of
the dry slot feature become clearer, POP forecasts may need to be
adjusted. Behind the front, expect temperatures to fall to near or
just below normal for this time of year through the weekend.

.AVIATION...

VFR conditions are expected for all taf sites through Wednesday
00z.

.MARINE...

Cold air advection will decrease this evening and overnight.
However, north winds will increase slightly this evening through
midnight and ease after 2am Tuesday morning. Surface high pressure
system becomes more centered over the Deep South. As a result,
winds will decrease early Tuesday and continue subsiding through
Wednesday. In addition, wind direction will veer from the north to
the southeast during this period and seas will subside as well.

Another strong storm system will begin to affect the waters Thursday
into Thursday with south and southwest winds increasing back into
the 20 to 25 knot range and seas climbing back above 7 feet over the
Gulf waters and sounds. Due to the close proximity of the surface
low and deep upper level low over the Deep South, strong westerly
winds are expected Thursday night through Saturday morning and a
period of gale force winds may develop as the strong storm system
pulls away from the region. West to northwest winds will finally
ease to 10 to 15 knots by late Saturday night.

.DECISION SUPPORT...

DSS code: Blue.
Deployed: None.
Activation: None.
Activities: Small Craft Advisory.
            Widespread freezing conditions tonight.

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 rain; direct tropical threats; Events of National
         Significance.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  28  57  34  63 /   0   0   0   0
BTR  29  56  36  64 /   0   0   0   0
ASD  30  56  34  64 /   0   0   0   0
MSY  35  54  40  65 /   0   0   0  10
GPT  32  55  37  62 /   0   0   0   0
PQL  32  57  34  65 /   0   0   0   0

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Freeze Warning from 3 AM to 8 AM CST Tuesday for LAZ056>060-065.

GM...None.
MS...None.
GM...None.
&&

$$



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.