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

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878
FXUS64 KLIX 171422
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
822 AM CST Sun Dec 17 2017

.SOUNDING DISCUSSION...
Significant moistening of the column evident in this morning`s
sounding. Precipitable water from the 12z sounding today is 1.77
inches compared to 0.94 inches at the same time yesterday and 1.14
inches just 12 hours ago. The 1.77 inch PW observation is actually
a record high for this date according to SPC`s sounding
climatology page. That being said, showers and storms today will
be capable of producing some efficient rainfall. This is also
evident in earlier ASOS observations from KBTR as the line moved
through that area. The ASOS recorded 0.41 inches of rain in 14
minutes for a rainfall rate of just under 2 inches per hour, which
is fairly impressive for December. All of this continues to
suggest the main threat from today`s showers and thunderstorms
will be locally heavy rainfall that can lead to ponding of water
in low lying and poor drainage areas.

&&

.PREV DISCUSSION... /issued 408 AM CST Sun Dec 17 2017/

Discussion...
Rain is starting to spread across the western
portions of the forecast area this morning. Current radar trends
show that this area of rain has little to no lightning associated
with it. In fact, the area has been weakening as it pushes our
way. This may be the only period of rain for today. The rain could
be done for most of the area by this afternoon if this trend
holds up. This is the beginning of an active period for us over
the next week. An area of low pressure is moving across the Lower
Mississippi Valley today and will move a front across the region.
The front is expected to stall and be the focal point for periods
of rainfall over the next couple of days. Guidance still suggests
that the atmosphere will be rich in moisture for this time of the
year as precipitable water values are expected to top out around
the 1.8 to 1.9 inch range. This would be a record amount if we can
observe it in a sounding at 12z or 0z tonight. The Weather
Prediction Center has the entire area outlooked in a Marginal Risk
for Excessive Rainfall on today. We are expecting rainfall totals
in the 2 to 4 inch range for much of the area. Higher totals will
certainly be possible if any specific areas see multiple rounds
of heavier storms. Some localized ponding of water in low lying
and poor drainage areas is possible. Will have to monitor the
flooding situation carefully as this evolves into a multi-day type
event with moisture rich environment and a stalled front over the
forecast area through at least Tuesday. The severe weather aspect
of this system today looks very weak. Right now, lightning is not
being observed with the area of rainfall. The Storm Prediction
Center has placed the area along the Louisiana coast in a Marginal
Risk for Severe Weather for today. They have also removed the
marginal risk for Monday as well as the parameters for severe
weather are not conducive for strong convection.

Guidance still points to another upper low ejecting out over the
southern Plains with another surface low moving across the area by
midweek. This will bring another round of showers and
thunderstorms to the region Tuesday night into Wednesday morning.
This could be a little bit more robust in terms of thunderstorms
than the system for today. The sounding profiles are starting to
hone in on this particular system for a better chance of severe
weather. This one may be the one to watch for severe convection.
We should be between systems Thursday...and guidance then also
points to another major system into the region next weekend. Still a lot
of uncertainty going into the Christmas holiday regarding the timing
and strength of the next cold front. With that being said have
maintained a blend in the latter part of the forecast period.
13/MH

AVIATION...
MVFR conditions have taken over at most sites and
the few that aren`t there will be shortly. Conditions will continue
to deteriorate during the morning and midday hours today as low
ceilings and widespread showers and a few thunderstorms impact the
terminals. MVFR to IFR category conditions will be widespread from
Sunday morning into Sunday evening. Convection will diminish
somewhat from west to east this afternoon and evening. CAB/11

MARINE...
Weak sfc low moving through western and central LA
will continue to lift to the northeast driving a cold front into the
region however that front will stall before reaching the coast and
thus onshore flow will continue. Winds though will relax as the
pressure gradient relaxes through the day today. The boundary will
remain draped across southern LA and into southern Ms and central
LA through Monday before finally surging back to the north as a warm
front. By midday Tuesday another weak sfc low will develop over
southern TX keeping winds around the 10-12 kt range through Tuesday.
As this low lifts to the northeast it will deepen some over the
Lower MS Valley and this could bring a temporary time frame of SCS
headlines again overnight Tuesday and into Wednesday. This time the
cold front should finally push into and through the coastal waters
but it will not be strong and could very well not be a clean push
through as winds will quickly veer around to the southeast Thursday
afternoon with as a much strong surface low developing over the
central/southern Plains. A much stronger cold front could
finally push through during the weekend before Christmas. /CAB/

DECISION SUPPORT...
DSS code: Blue.
Deployed: None.
Activation: None.
Activities: Small Craft Adv.

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  68  59  68  60 / 100  50  70  30
BTR  70  61  70  62 / 100  50  70  20
ASD  71  62  71  60 / 100  50  70  20
MSY  71  62  72  62 / 100  50  50  20
GPT  69  62  69  61 / 100  50  60  20
PQL  72  62  71  60 / 100  50  60  20

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...Small Craft Advisory until 9 AM CST this morning for GMZ552-572.

     Small Craft Advisory until noon CST today for GMZ555-575-577.

MS...None.
GM...Small Craft Advisory until 9 AM CST this morning for GMZ572.

     Small Craft Advisory until noon CST today for GMZ575-577.

&&

$$



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