Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 160342

Area Forecast Discussion
National Weather Service Mobile AL
942 PM CST Sun Dec 15 2019

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


16/00Z issuance...Had to amend MOB TAF due to ceilings lowering
and becoming MVFR earlier than anticipated. Otherwise TAFs are on
track this evening. Expecting prevailing MVFR ceilings around 1500
feet agl overnight with areas of fog also some brief periods of
IFR visibility in fog of 1 to 3 miles. Some locations could see
LIFR visibility with 1 mile or less late tonight. Surface winds
through Monday generally south or southeasterly around 10 knots or
less. Showers or possible thunderstorms possible late in the day
on Monday. /12


.PREV DISCUSSION... /issued 408 PM CST Sun Dec 15 2019/

NEAR TERM /Now Through Monday night/...

Active weather can be expected through the near term with the
threat for dense fog tonight into Monday morning, followed by a
severe weather threat from late Monday afternoon through Monday

Low level moisture advection will begin in earnest later tonight
and continue through the day Monday. A surge of low level
dewpoints into the mid and upper 60s is expected near the coast
later tonight into Monday morning. This moist low level airmass
will overspread the relatively cooler waters of local bays and the
near offshore shelf waters. This should result in the development
of sea fog as the dewpoint/water temp spread should approach the
critical 10 degrees. Have issued a Marine Fog Advisory for these
coastal waters along with a Fog Advisory for counties adjacent to
the coast. The best timing for dense fog appears to be from
midnight tonight through mid morning Monday.

A potent mid and upper level shortwave will quickly eject from the
Four Corners region of the Intermountain West today across the
Lower Mississippi and Tennessee Valley`s by Monday night.
Favorable upper tropospheric jet coupling will help to enhance
large scale ascent over northwest portions of our forecast area by
late Monday afternoon and evening. This large scale ascent will
gradually weaken and lift northeastward by late Monday night as
the upper jet dynamics pull away. At the surface, low pressure
will develop in the vicinity of the Arklatex region and move
northeast across the lower Tennessee River Valley through Tuesday
morning. This surface low should help to keep low level winds
backed through the evening and support enhanced low level
effective storm relative helicity. The latest CAM/SREF guidance,
along with operational runs of the GFS/ECMWF, suggests that there
will be a favorable corridor of overlapping low level shear and
instability over areas northwest of Interstate 65 from late Monday
afternoon through Monday evening. MLCAPES in this region are
during this time are progged to be at least 1000j/kg, as 0-1km
SRH values exceed 150m2/s2. In addition, deep layer shear should
exceed 50 kts by this time as the low level jet strengthens and
lifts across the aforementioned areas. The deep layer shear vector
remains quasi-orthogonal to the initiating boundary, therefore a
mixed storm mode of short-line segments and supercells are
anticipated. Many severe weather parameters appear favorable for
all severe weather hazards, including the potential for damaging
winds, hail up to one inch, and a few tornadoes. The only
potential limiting factor may be the overall degree of mid level
lapse rates and potential residual mid level capping inversion.
But considering the degree of thermodynamic instability and modest
height falls near the initiating boundary over northwest sections
of the CWA, we believe that the threat for severe storms over
these locations remains relatively likely. This region remains
outlooked for an Enhanced Risk for Severe Thunderstorms by the SPC
and we agree with this assessment.

By later Monday night, mainly after midnight, there may be an
overall weakening of the ongoing convection and severe weather
potential with southeastward extent (southeast of Interstate 65).
The best dynamic forcing for ascent begins to lift out to the
northeast which will result in weakening instability. In addition,
the low level wind shear will also begin to gradually weaken as
surface winds veer in response to the departing surface low. So
the overall threat is relatively lower to the southeast of
Interstate 65 as compared to areas further to the northwest.
Although the severe threat will be diminishing, it cannot be
completely ruled out. Another potentially limiting factor near the
coast will be the relatively cooler offshore shelf and area bay
waters. This may result in a shallow marine inversion that could
keep storms slightly elevated in these areas. This inversion
should mix out with inland extent. We plan to launch special
research soundings (USA collaborative project) by early Monday
afternoon to assess the atmospheric profile near the coast.

In addition, the latest CAM guidance indicates the potential for
showers and thunderstorms to develop by Monday afternoon along the
western Florida Panhandle within a zone of enhanced moisture
advection. As the atmosphere destabilizes and deep layer shear
increases by Monday afternoon, we cannot rule out a strong to
severe storm in these locations ahead of the overnight convection.
There remains more uncertainty with regard to this potential as a
residual capping inversion may hinder the strength of convection
developing in these areas. /JLH

SHORT TERM /Tuesday Through Wednesday night/...

The short term will begin as an upper level trough progresses
over the Mississippi and Ohio Valleys, with mainly zonal flow for
the northern Gulf area. A surface cold front will also make its
way over the region, bringing a line of showers over the area into
Tuesday morning. The intensity of this line is expected to be on
the downward trend Tuesday morning; however, there will still be
the possibility for a few embedded thunderstorms due to MLCAPE
values between 150 and 300 J/kg. Although, SRH values will have
decreased substantially to be negligible by this time. The
majority of shower activity is expected to move out of the area by
late Tuesday afternoon, with dry conditions the main story
through midweek as a large area of surface high pressure as well
as upper level ridging begin to build in from the west.

High temperatures Tuesday are likely to occur in the morning, due to
FROPA and CAA behind the front. Expecting highs early Tuesday to be
around 50 degrees in the interior SW AL/SE MS, in the 60s along the
AL coast, and in the mid to upper 60s for areas southeast of I-65.
Temperatures will then decrease through the day. Overnight lows
Tuesday night into Wednesday morning will be chilly, with values in
the upper 20s in the interior SW AL/SE MS and in the lower to mid
30s elsewhere. Highs Wednesday will then only reach the lower 50s.
Overnight lows into Thursday will be the coldest of the forecast
period, with temperatures in the upper 20s to around 30 degrees
likely area-wide. /26

LONG TERM /Thursday Through Sunday/...

Upper level ridging and surface high pressure will maintain their
hold over the region Thursday, keeping dry conditions for the
northern Gulf coast during this time. However, these features will
progress eastward and a surface low pressure, with associated
cold front, will form and move over the Plains through late week.
At the same time, an upper level trough will also progress over
the Plains towards the eastern CONUS. However, timing
discrepancies exist between models for this late- week system.
Therefore, have opted to stick close to the blended solution of
slight to chance PoPs for Friday and into the weekend.

Temperatures for the long term period will generally run slightly
cooler than the seasonal norms for this time of year. Expecting
highs in the mid to upper 50s Thursday, but will gradually warm to
be in the lower 60s by late weekend. Furthermore, lows will begin in
the lower to upper 30s Friday morning, but then generally be in the
upper 30s to lower 40s through Sunday morning. /26


Light to moderate southerly flow will persist through Monday
night ahead of a strong cold front approaching from the west.
Dense sea fog is also expected over the bays and near shore waters
late tonight into early Monday morning. The cold front will move
through the marine area late Monday night into Tuesday morning,
bringing increased chances of showers and thunderstorms during
this time. Strong northerly flow will then take hold in the wake
of the cold front. Gusts to gale force are possible over the open
Gulf waters Tuesday evening through Wednesday morning. Thus, a
Small Craft Advisory will likely be needed beginning Tuesday
morning and continuing into Wednesday night. Light to moderate
easterly flow will then be likely Friday, lasting into the start
of the weekend. /26


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

     Dense Fog Advisory until 10 AM CST Monday for ALZ261>266.

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

     Dense Fog Advisory until 10 AM CST Monday for FLZ201>206.

GM...Dense Fog Advisory until 10 AM CST Monday for GMZ630>636-650-655.



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