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

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FXUS64 KLIX 191755
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
1255 PM CDT Tue Sep 19 2017

.UPDATE...
Update for 18z aviation.

&&

.AVIATION...
Some scattered showers and storms this afternoon, but coverage is
low and should only impact terminals temporarily. Patchy fog
possible early Wednesday morning, mainly at MCB and ASD.

&&


.PREV DISCUSSION... /issued 810 AM CDT Tue Sep 19 2017/

..SOUNDING DISCUSSION...
PW is up some to 2 inches due to more moisture in the sfc to 700
mb layer. A broad subsidence inversion is developing just above at
about 650 mb. Winds are very light at low levels and then become
westerly above 500 mb. Expect scattered showers and storms again
later today, though without much forcing coverage should be low.

Krautmann

PREV DISCUSSION... /issued 340 AM CDT Tue Sep 19 2017/

SHORT TERM...

Little change in the overall forecast as a it still looks like a
low to mid-level ridge axis will remain parked directly over the
Gulf South through Thursday, and a broad and weak upper level
trough axis also settles over the region. This pattern will
contribute to somewhat typical conditions for mid to late
September. However, the ridging in the low and mid-levels should
allow for slightly warmer than normal temperatures through
Thursday. Daytime highs will easily climb into the lower 90s at
most locations, and overnight lows should only cool into the lower
to middle 70s due to southerly winds in the low levels pumping in
ample moisture from the Gulf.

The wind field will be very light throughout the atmospheric
column as ridging remains centered over the area. With light
winds in place, the prospect for some patchy fog to form each
night will exist as a weak low level temperature inversion forms.
Any fog should quickly burn off after sunrise, and expect to see a
well mixed boundary layer in place by mid- morning. The other
weather impact each day will be the risk of some scattered
afternoon and early evening showers and thunderstorms forming each
day. MLCAPE values will be fairly consistent each afternoon at
around 1500-1600 J/KG, and the convective temperature should be
right around 90 degrees. Once this convective temperature is
realized in the early afternoon hours, scattered thunderstorms
should start popping up. The storms should be fairly stationary,
and should not turn very strong due to the lack of wind shear
aloft. Basically, any storms should fire up, and then quickly fall
apart as the updraft is cut off by rain cooled air. Thus, the
threat of heavy rainfall and severe weather is low. At most, some
gusty winds could develop with a few of the stronger cells that
manage to form. Overall rain chances will be around 20 to 30
percent each day through Thursday.

LONG TERM...

A broad closed mid to upper level low will gradually form over the
Gulf of Mexico and the forecast area over the weekend. This low
will slowly push to the west on the southern periphery of a
strengthening upper level ridge across the Midwest and Great Lakes
region of the country. Some increased moisture advection and
convergence should develop on the eastern periphery of the upper
level low, and this area of deeper moisture will spread into the
forecast area by Saturday and Sunday. With slightly stronger
forcing in place aloft, expect to see somewhat higher POP values
for Friday of 30 to 40 percent. Temperatures will be very near
average for late September, and expect to see a continued largely
diurnal pattern of development for showers and thunderstorms both
Friday and Saturday with peak convective coverage taking place in
the afternoon hours. By Saturday and Sunday, the deeper pool of
moisture should move in as noted by precipitable water values
increasing from around 1.75 to 2.0 inches. Convection should
develop more readily over the coastal waters in the overnight
hours, and this convection should then spread inland through the
morning and early afternoon hours. The forecast calls for POP
values of of 40 to 50 percent in the forecast both Saturday and
Sunday.

Early next week, the upper level low should have pushed further
to the west into Texas, and another deep layer ridge should begin
to take hold of the region. Fairly strong subsidence and a surge
of drier air is expected to advect into the area by Monday, and
have very low POP values of only 10 to 20 percent in the forecast
Monday and Tuesday afternoons. The highest POP will tend to be
near the coast where the seabreeze front could provide just enough
low level forcing to overcome the mid-level cap. However, most
areas should be warm and dry early next week with highs easily
climbing back into the lower 90s. The drier airmass at night
should also allow for somewhat cooler temperatures in the upper
60s and lower 70s. Given the light wind field expected, patchy fog
may once again try to form mainly over inland areas and river
valleys.

AVIATION...

Ceilings and vis restrictions will cause a few terminals
to fall into IFR conditions this morning and again tonight. Decks
and vis will rapidly improve after sunrise. Chance sh/ts activity
today. Some VCTS will be used for those terminals that are most
likely to be affected.

MARINE...

High pressure will remain across the eastern half of the
gulf keeping winds weak and southerly. These conditions are expected
to be maintained through the week.

DECISION SUPPORT...

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.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  90  71  91  69 /  30  30  30  20
BTR  92  72  91  71 /  30  30  30  20
ASD  90  72  91  71 /  20  20  30  10
MSY  90  75  90  74 /  20  20  30  10
GPT  89  73  89  72 /  30  30  20  10
PQL  90  71  89  70 /  30  20  20  10

&&

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

$$



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