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

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FXUS64 KLIX 192104

Area Forecast Discussion
National Weather Service New Orleans LA
404 PM CDT Thu Oct 19 2017


Low and mid-level moisture will continue to stream into the area
through tomorrow night on the back of increasing southeast flow.
A few showers have managed to develop along the Louisiana coast
today in response to this moisture increase, and expect to see
this trend persist through tomorrow afternoon. Have POP of 20 to
30 percent in the forecast for tomorrow afternoon mainly to the
south of the I-10 corridor where deeper moisture will be available
to tap into. Temperatures will remain warmer than average with
overnight lows only upper 50s and lower 60s north of I-10 and the
middle to upper 60s south of I-10. Tomorrow will continue the
trend of above normal temperatures as highs warm back into the
middle 80s. Any influence of the upper level ridge will be gone by
tomorrow night as a strong longwave trough axis approaches from
the west. With increasing positive vorticity advection taking hold
tomorrow night, expect to see POP values south of I-10 increase
into the 30 to 40 percent range. North of I-10, lower overall
moisture content and some weak low level stability should keep
shower activity at bay through the night. Lows will continue to
modify tomorrow night, with temps only cooling into the middle to
upper 60s north of I-10, and the lower 70s south of I-10.

Increased omega values in the mid and upper levels associated with
the increasing positive vorticity advection in advance of the
approaching trough will support scattered to numerous showers and
thunderstorms on Saturday. Convective activity should start out
more scattered in the mid to late morning hours, but as
temperatures climb back into the middle 80s in the afternoon, more
numerous showers and thunderstorms are expected to develop. It
looks like an area of enhanced low level moisture convergence
could set up near the Atchafalaya Basin, and have the highest POP
in place for areas west of the I-55 corridor. MLCAPE values of
around 1000 J/KG and weak shear support more typical Summer like
convective development on Saturday. At worst, a few strong storms
may try to fire up in the afternoon hours and produce locally
heavy downpours and gusty winds below severe limits. POP values
range from 30 to 50 percent east of I-55 to 50 to 70 percent west
of I-55 Saturday afternoon. Although ample moisture will remain in
place Saturday night along with decent forcing throughout the
atmospheric column, the loss of daytime heating will decrease
convective potential and POP values for Saturday night. Expect to
see only a 20 to 30 percent chance of showers and thunderstorms
late Saturday night as temperatures cool into the upper 60s and
lower 70s.


Sunday into Sunday night will be the most active portion of the
extended forecast period as a strong upper level trough axis and
associated surface low and cold front sweep through the forecast
area. There will be little in the way of convective inhibition as
MLCAPE values rise back to around 1000 J/KG. Expect to see shower
and thunderstorm activity begin to become more numerous through
the mid and late morning hours in advance of the approaching low
and cold front, and expect to see a few of these thunderstorms
turn strong to possibly severe. This severe concern is largely
driven by some weak directional shear resulting in storm relative
helicity values around 150m2/s2 over the northern parts of the
CWA. Longer lived and deeper rotating updrafts combined with low
wet bulb zero heights of around 12,000 feet should support some
large hail development in the strongest storms Sunday morning.
Lingering dry air in the mid-levels could also entrain into any
downdrafts creating a favorable environment for locally strong
winds as well.

Heading into the afternoon hours, the a pre-frontal trough and
associated line of convection should race across the forecast
area. This band of showers and thunderstorms could also contain
some severe storms, but the severe threat should shift from a
large hail event to a damaging straight line wind event as
directional shear decreases and speed shear increases. Have a high
POP of 80 percent or greater for much of the forecast area Sunday
afternoon, as the line is expected to move rather quickly from
west to east. Temperatures in advance of the line of convection
should easily rise back into the middle 80s. The actual cold front
and area of low pressure should then sweep through the area Sunday
night. The severe threat should diminish Sunday night in the wake
of the pre-frontal trough as a shield of light to moderate rain
with some embedded elevated thunderstorm activity trails the
leading squall line. This should greatly stabilize the
environment as the actual front moves through. This is easily
noted by reduced MLCAPE values of less than 500 J/KG and decreased
low level lapse rates of less than 6.0C/km in the latest model
soundings. Only expect to see showers and a wind shift to the west
accompany the actual frontal passage.

Expect to see the front clear the Mississippi coastal counties
early Monday morning with decreasing rain chances and clearing
skies through the morning hours on Monday. Deep layer northwest
flow, strong subsidence, and dry air advection will dominate the
forecast area from Monday afternoon through the remainder of the
week. A reinforcing frontal boundary should sweep through Tuesday
into Tuesday night as a strong shortwave trough passes through in
the upper levels. The northwest flow regime will also transport in
some much cooler air, and expect to see lows will dip into the
middle to upper 40s over the northern half of the CWA both Tuesday
and Wednesday nights. Highs will be cooler than average in the
middle 70s as well.



VFR category conditions are expected at each of the terminals
throughout the TAF forecast period. A brief period of patchy fog
will lower vis to 5 miles at times at KMCB and KBTR around 12z.



A persistent east wind of 15 to 20 knots and seas of 3 to 6 feet
are expected over the open Gulf waters through Saturday as a tight
pressure gradient lingers over the northern Gulf of Mexico. The
gradient should relax slightly Saturday night and Sunday as a low
pressure system approaches from the west. Still expect to see
southeast flow of 10 to 15 knots and seas of 2 to 4 feet in
advance of this system. The low and an associated cold front
should then slide across the coastal waters Sunday night, and
a much colder and drier airmass will build in for Monday and
Tuesday. Winds should shift to a northwest direction at 15 to 20
knots over the open waters early next week as high pressure builds
in. 32



DSS code: Green.
Deployed: None.
Activation: None.
Activities: None.

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  56  84  65  84 /   0   0  10  40
BTR  60  85  68  85 /   0  10  10  60
ASD  60  84  66  84 /  10  10  20  50
MSY  68  84  72  84 /  10  20  30  50
GPT  63  83  69  83 /  10  10  10  30
PQL  60  84  66  83 /   0   0  10  30




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