Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 032328

Area Forecast Discussion
National Weather Service Mobile AL
528 PM CST Sat Dec 3 2016

.DISCUSSION...Updated for latest aviation discussion below.


00Z issuance...Abundant cloud cover continues to stream over the
southern US this evening. Cigs are forecast to range around 5 kft
this evening, but satellite shows some lower cigs (into MVFR
categories) moving in from the south over a warm front across the
Louisiana coast. This forecast considers this, lowering cigs to MVFR
categories by close to next issuance. A few sprinkles this evening
possible. Chance of shra, possibly a few tsra possible between 12 and
16Z Sun AM. Northeast winds 8 to 13 knots this evening becomes more
easterly overnight. /10


.PREV DISCUSSION... /issued 317 PM CST Sat Dec 3 2016/

NEAR TERM /Now Through Sunday/...Next main upper trof digging
south over the sw conus and California Baja will begin to shift
eastward late tonight and during the day on sun resulting in a
persistent mean flow aloft at around 50 kts from the southwest
tonight and during the day on sun. At the surface a strong ridge of
high pressure centered near mid Atlantic and TN/OH river valleys
stretching southward towards the northern gulf states...which
continues to lead to a dry cool wedge of stable air over much of the
deep south along the northern gulf...will begin to slowly retreat
northward early sun and continue through sun afternoon in response to
the deep upper trough to the west shifting eastward. With this
pattern expect a continued stratus deck at around 10 kft to continue
through early this evening then lowering overnight and early sunday
as the depth of the cooler/stable air begins to decrease. With this
cloud bases lower to around 1.5 kft by early sun morning resulting in
a low status deck slowing eroding generally from south to north
during the day on sun. With the better layer lifted noted light rain
and fog will also be possible by early sun becoming better developed
through sun afternoon. Although we are not seeing much in the way of
widespread glaciated convection with this pattern through sun will
continue to mention a slight chance to chance thunderstorms also
which will be embedded mainly in the heavier rain.

As for temps will continue to lean towards the cooler mav guidance
through sun afternoon due to the cool wedge slowly eroding through
the next 24 hours. Lows tonight will range from the upper 40s to
lower 50s over northern sections of the forecast area and the middle
to upper 50s over middle and coastal sections. Highs sun will
eventually climb to the lower to middle 70s generally from I-10 and
southward and he middle 60s lower to lower 70s further north. 32/ee

SHORT TERM /Sunday night Through Tuesday night/...A cut off upper
low over northern Mexico weakens to an open wave and ejects across
the southeast states Monday night into Tuesday afternoon. An
associated surface low meanwhile moves from near the Texas coast to
the lower Mississippi river valley Monday evening, then to near the
Ohio river valley Tuesday morning. A surface frontal boundary will be
oriented along the northern Gulf coast early Sunday evening,
including being slightly inland over the coastal counties of the
forecast area due to a surface wave on the boundary which exits to
the east. A surface ridge over the southeast states will be an
impediment to the frontal boundary making much northern movement into
the forecast area with the approach of the surface low at least
through Monday, after which the frontal boundary pushes northward
through the forecast area Monday night. With the frontal boundary
positioned near the coast Sunday night into Monday, this sets up an
extended period of isentropic lift (overrunning flow) over much of
the forecast area until the front pushes northward Monday night
opening the remainder of the area to a warm sector. This period of
extended precipitation north of the frontal boundary will have
important impacts on the forecast, especially shear and instability.
Numerical visibility guidance  is indicating fog at least over the
southern portion of the area Sunday night which lingers near the
coast Monday morning, and have added this to the forecast. Tabular
guidance also indicates fog could affect essentially the entire
area and the mention of fog may need to be expanded, although
visibilities overall look to remain above dense fog advisory

Assessing instability, MLCapes will range from 200 J/kg and less
over most of the forecast area through Monday, transitioning to
values near 1000 J/kg well offshore. Along the coast, values near 500
J/kg will be possible. Values near 500 J/kg will spread further
inland Monday night, but likely make it to at best midway through the
area, while values near 200 J/kg persist well inland. A negative
item is that the conditional probabilities for thunder only get to
35% near the coast Monday afternoon, then will be around 40% Monday
night before falling off into the single digits.

Assessing the shear profile, despite the frontal boundary pushing
northward through the area, model soundings show that the lower
levels of the atmosphere, such as below 900-950 mb, are at best
isothermal. This in turn indicates that convection will be elevated
and not have access to the best veering wind profile. That means that
should these soundings be correct (this is a very important caveat), the
shear available to convection will be much less. 0-3 km helicity
values are indicated to be around 200 m2/s2, but values where the
convection will be based (above the low level veering), if the
current soundings are correct depicting the isothermal layer, then
these values will be lower - possibly as low as near 50 m2/s2.

The only exception to this is at the immediate coast where the
isothermal layer is very shallow, as this transitions to a surface
based environment over the northern Gulf. This indicates that the
best potential for surface based convection which would have access
to the higher shear values will be at the immediate coast. Timing
wise, this looks to be generally during Monday afternoon, but could
begin late Monday morning and also stretch into Monday evening. The
anticipated evolution of surface based convection moving northward
from the northern Gulf into the immediate coastal areas looks good,
so at this point the only potential for surface based convection is
at the immediate coast, after which convection becomes increasingly

Despite this assessment and taking into consideration that later data
may indicate a more favorable environment - especially if the low
level sounding data proves incorrect - will stay with the current
hazardous weather outlook which has mention of some severe storms
possible across the entire area Monday into Monday night. That said,
currently this potential appears to be confined to near the coast.

Have gone with likely pops across the area Sunday night followed by
categorical pops on Monday as the isentropic lift strengthens north
of the frontal boundary. High pops continue for Monday evening, then
taper off late Monday night to dry/slight chance pops on Tuesday as
the upper trof passes the area with subsidence following in the wake
of the upper system. Lows Sunday night range from the lower/mid 50s
well inland to the lower 60s at the coast with a bit warmer values
for Monday night. Tuesday night will be cooler as dry high pressure
builds into the region, and range from the mid 40s inland to the
lower 50s at the coast. Highs on Monday range from the mid 60s well
inland to the lower 70s at the coast, then for Tuesday highs will
range from the mid 60s inland to the upper 60s closer to the coast.

LONG TERM /Wednesday Through Saturday/...An evolving longwave trof
at the beginning of the period over the western and central states
continues eastward across the eastern states through Friday, and into
the western Atlantic on Saturday. An associated surface low way to
the north brings a reinforcing cold front through the forecast area
Wednesday night (with no return flow ahead of the front), and ushers
what could be the coldest air of the season into the region. Will
have slight chance pops for Wednesday night and Thursday as layer
lifting produces isolated to scattered patches of rain, but otherwise
dry conditions are expected.

Highs on Wednesday will range from the lower 60s well inland to the
mid/upper 60s near the coast, then trend much cooler through Friday
to range from the lower 40s inland with upper 40s near the coast.
Saturday will see some recovery with highs near 50. Lows Wednesday
night range from the lower 40s well inland to the lower 50s at the
coast, then very cold nights are expected for Thursday night and
Friday night with lows ranging from mid 20s inland to the lower 30s
at the coast. /29

MARINE...A strong easterly wind flow will continue over the northern
gulf through tonight ahead of deep trof approaching from the west
combined with strong high pressure to the north and east. A moderate
to strong southerly flow will eventually develop during the day on
sun and continue through mon night as warmer air continues to shift
northward ahead of the main upper system to the west reaching the
northern gulf and inland coast later in the day. As a result a small
craft advisory will continue over the open gulf waters through sun
afternoon and the lower end of mobile bay and ms sound through this
afternoon. Rain chances also increased sun through mon with a few
strong thunderstorms possible by late mon and mon night. A cold front
moves east of the marine area in the wake of this system by tue night
increasing late in the week as a stronger surge of colder air moves
south over much of the northern gulf. 32/ee


AL...High Rip Current Risk through Sunday afternoon for ALZ265-266.

FL...High Rip Current Risk through Sunday afternoon for FLZ202-204-

GM...Small Craft Advisory until 6 PM CST Sunday for GMZ650-655-670-

     Small Craft Advisory until 6 PM CST this evening for GMZ631-632.



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