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

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FXUS64 KLIX 040612

Area Forecast Discussion
National Weather Service New Orleans LA
1212 AM CST Sun Dec 4 2016

Conditions will continue to deteriorate through MVFR to IFR
conditions due to a combination of low CIGS and VSBY restrictions
in RA/SHRA/TSRA and some BR through this morning. A line of
SHRA/TSRA will impact some of the airports along and south of the
I-10/12 corridor in Louisiana with temporary lower conditions as
it moves east at 18 knots. Many airports should see improvement to
MVFR or possibly VFR conditions in the afternoon before lowering
to IFR again Sunday night. 22/TD


.PREV DISCUSSION... /issued 959 PM CST Sat Dec 3 2016/

Surface analysis indicated a warm front extended near the entire
southern Louisiana coast. A squall line of thunderstorms extended
from north of the boundary over southwest Louisiana, southwestward
into the northwest Gulf of Mexico just ahead of an inverted
trough/elongated surface low pressure system. As the parent upper
level storm system moves slowly east over northwest Mexico, the
squall line is expected to continue moving east towards the
southwest portion of the forecast area after midnight, probably
quite a bit slower than indicated by the convective allowing,
high-resolution HRRR based on current eastward movement of only
11-12 mph.

The warm front should move slowly inland and approach I-10
sometime late tonight or tomorrow morning. Any thunderstorms that
maintain integrity and exhibit bowing segments could produce a few
strong to severe wind gusts in the 40 to 60 mph range over areas
generally south of I-10 and west/southwest of metro New Orleans
between 2 and 7 am. Regarding heavy/excessive rainfall potential,
most areas of south central and southeast Louisiana and south
Mississippi are still coming out of a drought, and flash flood
guidance is on the high side ranging from 2.5 to 3 inches in 1
hour, 3 to 3.5 inches in 3 hours and 3.5 to 4 inches in 6 hours.
While locally high amounts in these ranges have a chance of
occurring, the risk is not great enough to warrant a flash flood
watch at this time. Since additional heavy rainfall potential is
forecast through Monday, it is possible a flash flood watch could
eventually be issued for portions of the forecast area. Updates
have been issued that now include mention of thunderstorms and
heavy rain potential tonight. 22/TD


Strong east winds in the 20 to 30 knots range have been occurring
on Lake Pontchartrain and Lake Borgne most of this evening, so
upgraded the "Small Craft Exercise Caution" to a "Small Craft
Advisory" with an update earlier, and that advisory is in effect
until midnight tonight. Otherwise, remainder of Coastal Waters
Forecast looked on track at this time. 22/TD

PREV DISCUSSION... /issued 358 PM CST Sat Dec 3 2016/

Modest isentropic lift underway over a very dry low level column.
some light rain reaching surface with some slow saturation from
top-down taking place. meanwhile, mechanically enhanced baroclinic
zone noted along the texas coast, per radar presentation of very
thin surface base convective band along frontogenetic zone.
Satellite imagery shows large baroclinic leaf structure with jet
streak downstream of large cut-off low. 12Z chart analysis
indicated much stronger jet dynamics occurring on the upstream
side of the low, indicative of much deeper digging of the low to
still occur over the next 24 hours or so. As upper system begins
to eject, the warm sector is expected to surge northward to place
the forecast area squarely in the better surface based instability
region monday. There may some inhibition afforded due to cooler
marine layer, but high helicity storms may be deep enough to
penetrate higher wind gusts and a few funnels that may be capable
of reaching close to the surface immediately ahead of the front.
The ECMWF is faster with the evolution and passage of the surface
low in the late afternoon, whereas the GFS is about 6 hours later.
Best timing does appear to be in the Monday late afternoon and
early evening hours. Next issue will be heavy rain potential, with
storm total QPF amounts coming in generally between 2 to 4 inches
over the northwestern portions of the forecast area, 1 to 2 inches
elsewhere. Area rivers can handle some runoff, but will have to be
monitored over the next few days. Went close to model blends on
temperatures and PoPs, but held off mention of thunder until
Sunday as most weather tonight will be elevated isentropic until
convective instability can deepen much later tonight into sunday

Strong cold front swings through the area Monday night as surface
low lifts into the western Tennessee Valley. Tuesday should be
drier and considerably cooler though the much colder air rushing
into the region on arctic surge Wednesday night. Models showing a
band of precipitation accompanying the front, but the GFS is less
pronounced and faster at flushing out the moisture Thursday, while
the ECMWF actually shows a weak surface low over the northeast
gulf that may maintain wrap-around moisture over the area for much
of Thursday. At this time, will indicate chance PoPs for mainly
Thursday morning. Temperatures are expected to warm very little on
Thursday, perhaps remaining in the 40s most of the day, then
freeze conditions in the interior sections of the area for Friday
morning and again Saturday morning. Another surge of arctic cold
moves in Sunday with a very cold over-running rain anticipated.
Though not calling for it this far out, some patchy sleet may not
be out of the question next Sunday if low level temperatures fail
to modify between systems. More on this later. 24/RR

Most rain remaining just north and west of KMCB and KBTR, with other
terminals dry. Have started to see ceilings lower into the MVFR
category at most terminals in the last hour or so. Expect these
ceilings to continue to lower through the evening with IFR ceilings
possible at most terminals after about 08z. Mesoscale modeling
indicates rain spreading back into the remainder of the area after
about 04z, so could see visibilities reduced below what forecast
currently shows. Little, if any, thunder expected through 18z
Sunday. 35

Surface ridge located over the upper Mississippi River and low
pressure in northern Mexico will keep moderate persistent easterly
flow with a tight pressure gradient. Thus will keep small craft
advisory going in offshore waters and exercise caution headline over
tidal lakes and Lake Borgne. A gradual reduction in wind speeds will
take place late tonight and Sunday as the sfc low is finally
expected to lift toward the region. Winds will shift to the west and
increase to near small craft Monday night as a cold front associated
with that low pressure moves through. Expect this wind shift to be
brief as the pressure field breaks down before a very strong cold
front marches through Thursday morning. 35

DSS code: Blue.
Deployed: None.
Activation: None.
Activities: Small Craft Advisory. Monitoring convective and heavy
            rainfall trends through Monday night.

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.


MCB  52  62  54  66 /  90  90  70  80
BTR  58  67  57  68 /  80  90  70  80
ASD  61  71  60  70 /  70  70  60  80
MSY  64  73  63  72 /  70  70  60  80
GPT  61  72  61  70 /  70  70  60  80
PQL  59  72  62  71 /  60  80  60  80


GM...Small Craft Advisory until 3 AM CST Sunday for GMZ530-532-534-

     Small Craft Advisory until 3 PM CST Sunday for GMZ555-557-570-

GM...Small Craft Advisory until 3 AM CST Sunday for GMZ532-534-536-

     Small Craft Advisory until 3 PM CST Sunday for GMZ557-570-572-


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