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

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FXUS64 KLIX 201313

Area Forecast Discussion
National Weather Service New Orleans LA
813 AM CDT Wed Sep 20 2017

The sounding this morning is unstable with mixed layer CAPE at
1100 J/KG. Showers and storms will develop later today, though
with high pressure in place coverage should be isolated and will
be primarily over the cities or along sea/lake breeze boundaries.
Winds are southerly at about 10 kts from the sfc to 800 mb then
become westerly aloft.



.PREV DISCUSSION... /issued 342 AM CDT Wed Sep 20 2017/


A broad and weak upper level trough axis extending along the
Southeastern coast and into the Gulf South will be the main
feature impacting the forecast through Friday. The weak trough
axis aloft will provide just enough upper level forcing to
overcome some weak convective inhibition in the mid-levels.  In
the low levels a typical seabreeze cycle will also serve as a
catalyst for the weather expected over the next few days. Expect
to see convection fire up along the seabreeze front from late
morning through the afternoon hours each day. As temperatures wane
and seabreeze weakens in the evening hours, overall convective
activity will also decrease. In fact, the overnight hours should
remain fairly dry both tonight and tomorrow night.

Another driver of our weather will be an increase in overall
atmospheric moisture content both today and tomorrow. Precipitable
water values will range around 1.9 to 2.1 inches through Friday,
and this will allow for slightly more convective coverage through
the short term period. Have chance POPs of 30 to 40 percent in the
forecast each afternoon. Temperatures will remain slightly warmer
than average during the daytime hours with highs in the lower 90s
expected across most areas due to continued weak mid-level
subsidence. The increase in atmospheric moisture and dewpoints
will also keep overnight lows somewhat elevated in the lower to
middle 70s. With light boundary layer winds and ample moisture in
place, some patchy fog can be expected to develop each night.


Over the weekend, a broad and weak closed upper level low should
form on the tail end of the broad trough extending across the
Southeast CONUS. This weak upper level low will keep some
increased omega and forcing in place in the upper levels. Overall,
little change in the sensible weather pattern is expected from
Saturday through Monday from that seen today through Friday. A
seabreeze cycle will continue to dominate the low level weather
pattern, and expect to see scattered showers and thunderstorms
fire up on the seabreeze front beginning in the late morning and
persisting through the afternoon hours. As the seabreeze weakens
due to the loss of differential heating in the evening, convective
activity will also wane. The overnight hours should remain dry,
and some patchy fog could develop each night as boundary layer
flow remains weak. The one big change will be a return to more
normal temperatures as the mid-level ridge and resultant
subsidence comes to an end in response to the upper level low
translating down into the mid-levels. Highs in the upper 80s and
lows in the upper 60s and lower 70s can be expected.

By Tuesday and Wednesday of next week, the broad upper level low
over the area should shift to the north and east as a ridge of
high pressure begins to move in from the west. A deep layer ridge
axis should be firmly in place by Tuesday afternoon, and expect to
see a reduction in precipitable water values to around 1.5 inches
as subsidence increases throughout the atmospheric column. A
strong mid-level cap will also inhibit most convective initiation
throughout the day both Tuesday and Wednesday. As a result, have a
very low POP of less than 20 percent in the forecast. At most, an
isolated shower or thunderstorm may try to form, but the chances
are very low of that occurring. Temperatures will also warm back
into the lower 90s in the afternoon in response to the increased
subsidence and lack of cloud development. However, the drier
airmass will also allow temperatures to cool to more normal levels
at night in the upper 60s and lower 70s. As a result, some patchy
fog could continue to form each night.


As temps climb and lows reach mid 70s FG will be tougher
to come by. But BR will still remain a nightly restriction and will
vary from around 1sm up to 5sm. There is a 30% chance of sh/ts
activity today. But any particular site being affected will be
relatively small and mention will only be made for those that will
have the best opportunity at getting that 30%.


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 weekend.


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  91  70  91  69 /  30  10  30  20
BTR  92  71  92  71 /  30  10  40  20
ASD  91  72  90  70 /  30  10  30  20
MSY  91  74  91  74 /  30  10  30  20
GPT  90  73  89  72 /  30  10  30  20
PQL  90  71  90  70 /  20  10  30  20



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