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

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FXUS64 KLIX 210842
AFDLIX

Area Forecast Discussion
National Weather Service New Orleans LA
342 AM CDT Thu Sep 21 2017

.SHORT TERM...

The upper level pattern across the Gulf South will be little
changed through Saturday as a broad and weak upper level trough
extends across the Southeastern CONUS, and a broad area of high
pressure remains parked over the Plains and Midwestern states.
The region will remain in an area of higher upper level vorticity
and resultant forcing. This would lend toward greater convective
coverage, but a significant limiting factor will be in play
through Saturday. This factor is the advection of drier mid-level
air into the area that is expected to push precipitable water
values from around 1.7 to 1.8 inches today to 1.5 inches by Friday
and Saturday. This drop in available moisture and a slight
increase in mid-level temperatures will reduce lapse rates and
increase convective inhibition as the short term period progresses
forward. Expect to see scattered showers and thunderstorms
redevelop from late morning into the afternoon hours today with
the high POP values near the seabreeze boundary along the
Mississippi coast. Tomorrow will see slightly less coverage, and
by Saturday only expect to see isolated showers and thunderstorms
develop during peak heating hours in the afternoon. The drier
airmass and less cloud cover should allow for continued above
normal daytime highs in the lower 90s. Overnight lows will be
slightly above normal in the lower to middle 70s.

.LONG TERM...

The upper level pattern will shift slightly on Sunday and Monday
with a broad and weak upper level low becoming more centered over
the forecast area. As this upper level low moves in, it will also
advect in greater mid-level moisture pushing precipitable water
values back to near 2 inches. With continued vorticity and higher
omega values in place in the upper levels and less convective
inhibition, expect to see greater convective coverage and higher
POP values develop for both Sunday and Monday. Scattered showers
and thunderstorms should fire up from mid-morning into the early
evening hours with more isolated activity along the coast and
offshore in the overnight hours. POP values will range around 30
to 50 percent during this period, and also expect to see daytime
highs fall back to more normal levels in the upper 80s during this
period.

The upper level low will rapidly move northward into the Tennessee
Valley and weaken in response to a much stronger longwave trough
diving into the Rockies. As the low pulls out, a shortwave ridge
axis will build over the area resulting in increased negative
vorticity and subsidence throughout the atmospheric column. This
sinking airmass will warm temperatures in the mid and upper
levels, and also dry out the atmosphere. A fairly strong mid-level
cap should develop by Tuesday and remain in place through
Wednesday. Precipitable water values should also fall below 1.5
inches by Wednesday. With little in the way of available moisture
or forcing in place, only expect to see very isolated showers and
thunderstorms on Tuesday, and even lower rain chances by Wednesday
when the ridging aloft is expected to be strongest. The lack of
cloud development and rain activity will also allow daytime highs
to climb back above normal in the lower to possibly middle 90s.
The drier airmass should allow for temperatures to cool into the
upper 60s and lower 70s at night, and light boundary layer flow
should promote some patchy fog development from Monday night
through Wednesday night.

&&

.AVIATION...

A couple terminals like KMCB and KBTR will likely begin to see some
light BR by 10-12z and persisting through 13-14z this morning. After
14z all terminals should move back into VFR status through the
evening hours. Isolated to scattered SHRA/TSRA are expected in the
afternoon so will likely carry VCTS or a PROB30 at most TAF
airports. There is a chance that a greater number of airports could
experience MVFR to possibly IFR conditions in BR late tonight and
early Friday morning, but not likely at KMSY. 22/TD

&&

.MARINE...

A large ridge of high pressure over most of the eastern U.S. is
expected to maintain a generally weak pressure gradient resulting in
mostly light winds of 5 to 10 knots and low wave heights/seas across
the lakes, sounds and coastal waters today into Friday morning. A
subtle tightening of the pressure gradient going into the weekend
and early next week should produce easterly winds up in the 10 to 15
knots range over offshore waters Friday afternoon and Friday night,
and across most of the coastal waters Saturday into Sunday. Seas
should also respond with significant wave heights rising to around 3
feet offshore. These winds should ease a bit and turn more southeast
on Monday. 22/TD

&&

.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  91  70  91  70 /  30  20  20  20
BTR  93  71  92  71 /  20  20  20  10
ASD  90  71  90  70 /  30  20  30  10
MSY  91  75  91  75 /  20  10  20  10
GPT  89  73  89  71 /  40  20  30  10
PQL  90  70  89  69 /  40  20  30  10

&&

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

$$

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