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

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000
FXUS64 KLIX 230053
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
753 PM CDT Tue Aug 22 2017

.SOUNDING DISCUSSION...

Routine flight terminated over Mandeville at a height of 89,000
feet or almost 17 miles above the surface.

Airmass remains very moist and unstable with a precipitable water
value of 2.12 inches, a MUCAPE around 3400 J/kg and a Lifted Index
of -7. Mid level lapse rates were at 5.6C/km. Freezing level this
evening was near 16,000 feet, the -20C level at 27,000 feet and
the wet bulb zero at just under 14,000 feet.

Low level winds were light and generally out of the south, with
wind speeds not reaching 10 knots until the balloon reached 500
mb. Winds remained southerly through 200 mb, then gradually backed
to the east by 150 mb. Max wind of 22 knots was measured at 45,000
feet. 35

&&

.PREV DISCUSSION... /issued 347 PM CDT Tue Aug 22 2017/

SHORT TERM...

Current convective activity should mostly come to an end by this
evening as temperatures fall into the 70s and lower 80s and
instability value decrease. Expect to see a mainly dry night for
land based zones, but the marine areas should see thunderstorm
activity redevelop later tonight into early tomorrow morning.

Heading into tomorrow, ample deep layer moisture will remain in
place as noted by precipitable water values in excess of 2.2
inches. Additionally, an upper level trough axis will continue to
linger over the area that is associated with an upper level low
pushing into the western Gulf of Mexico. Additional forcing will
be supported by another northern stream trough axis and associated
weak frontal boundary also slipping into the region from the
north. All of these features combined together will support
scattered to numerous shower and thunderstorm activity from the
mid-morning hours through the early evening, and have placed a POP
of around 60 percent in the forecast for much of the CWA to
reflect this risk. Temperatures should be slightly cooler than
average in the upper 80s and lower 90s due to the more extensive
cloud cover expected from the convection. Although convective
coverage should decrease tomorrow night as instability decreases,
the presence of the weak frontal boundary in the area should keep
some isolated shower and thunderstorm potential in play through
the overnight hours. The highest POP will once again be in the
offshore waters tomorrow night.

Thursday and Thursday night should be little changed from
Wednesday and Wednesday night with the upper level trough axis
and associated plume of deeper tropical moisture continuing to
interact with a weak surface front stalled along the Gulf Coast.
Expect to see another day of decent convective coverage with
likely POP of 60 percent in the forecast. The potential for
locally heavy rainfall will persist from Wednesday into Thursday
as precipitable water values remain above 2.2 inches. Temperatures
should be slightly cooler than average on Thursday due to the
increased convective potential through the afternoon hours.

LONG TERM...

The forecast becomes more uncertain beginning on Friday and
persisting through next Monday. Although all of the global model
guidance is in good agreement that a tropical low should develop
over the Bay of Campeche and then move into Texas by Friday, there
is lower confidence on where this tropical low will head over the
weekend and into early next week. Currently, the forecast calls
for the low to linger over Texas through the weekend before
pulling to the northeast early next week. This will push the low
through the Lower Mississippi Valley toward the end of the
forecast period. The biggest concern is timing the passage of this
low. If the low is picked up by a passing northern stream trough
earlier, than the rain chances will diminish more quickly for the
CWA. However, if the low is not picked up and transits more slowly
across the region, then rain chances will be significantly higher
and the threat of heavy rain will persist longer.

At this point in time, have opted to stick with a blend of the
model guidance given the uncertainty. This keeps the region under
an area of deeper tropical moisture and enhanced forcing aloft
resulting in a prolonged period of higher heavy rainfall potential
through the weekend and into early next week. Overall POP values
remain in the likely category of 60 to 70 percent from Friday
through Monday. Due to the extensive cloud cover and rainfall
expected, temperatures should be significantly cooler than normal
with highs in the lower to middle 80s and lows in the lower to
middle 70s. Current thinking in terms of storm total rainfall from
Thursday through Monday ranges from 3 to 5 inches in the CWA, but
some locally higher amounts cannot be ruled out.

AVIATION...

Away from the convection, VFR conditions are expected to prevail.
Another brief period of light fog is possible around 12z
Wednesday, then another round of scattered SHRA/TSRA is expected
with daytime heating on Wednesday. 22/TD

MARINE...

The biggest issue will the potential for a tropical low to form in
the southwest Gulf over the next couple of days and then move into
Texas. If this occurs, a general increase in southeast winds from
5 to 10 knots tomorrow to closer to 15 to 20 knots over the
western waters can be expected from Friday through the weekend.
The eastern waters are currently expected to be in the 10 to 15
knot range. The bigger concern will be swells emanating from the
tropical low resulting in higher seas over the open Gulf waters
beginning on Thursday and persisting through the weekend. Seas
could potentially increase to 6 to 8 feet over the western outer
waters due to these swells. There is fair degree of uncertainty in
the forecast at this time.

DECISION SUPPORT...

DSS code: Blue.
Deployed: NOHSEP.
Activation: None.
Activities: DSS support for NOHSEP; Monitoring Convective trends.

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  73  89  73  88 /  30  60  50  60
BTR  74  90  75  90 /  30  50  50  60
ASD  76  90  76  92 /  30  40  40  40
MSY  78  90  77  92 /  30  30  20  40
GPT  77  89  77  89 /  20  40  30  40
PQL  75  89  75  90 /  20  30  30  50

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...None.
MS...None.
GM...None.
&&

$$



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