Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 210523 AAD
AFDMOB

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1123 PM CST Fri Jan 20 2017

.DISCUSSION...See updated information for land areas below.

&&

.UPDATE...Tornado Watch #10 now in effect for all southeast
Mississippi zones thru 7 AM Saturday. Headlines added to zones. /10

&&

.PREV DISCUSSION... /issued 1052 PM CST Fri Jan 20 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...
06Z issuance...Still same thinking with current package. Ahead of
shra/tsra development later tonight, CIGS are expected to drop into
low end IFR/LIFR levels, with local drops in VISBYS. Wild card in
the VISBY drop is winds at times rising to around 5 knots over the
fa, creating a stratus deck from any fog. Basically, am expecting
VISBYS to oscillate up and down. As the precip arrives after 09z,
this will become the primary governor of operations. IFR to low end
MVFR conditions expected. A reprieve is expected after noon into the
early evening, but another round of shra/tsra is expected tonight.

/16

PREV DISCUSSION... /issued 1006 PM CST Fri Jan 20 2017/

.ENHANCED RISK OF SEVERE WEATHER IMPACTING MUCH OF THE REGION
STARTING LATE TONIGHT AND CONTINUING INTO SATURDAY...

DISCUSSION...See updated information for land areas below.

UPDATE...The forecast remains on track with concerns to a potential
significant severe weather event impacting the forecast area starting
late tonight and continuing through the morning hours Saturday. Mid-
evening water vapor imagery shows a potent shortwave trough lifting
northeastward across southeast TX/southwest LA within the southwest
mid level flow. Radar imagery already shows scattered to numerous
showers and thunderstorms developing in this same vicinity this
evening, a few of which have become severe.

Short range models are in good agreement with ejecting the shortwave
trough northeastward across the Lower Mississippi Valley and Central
Gulf Coast region late tonight into early Saturday morning. Low level
southerly flow/warm advection will increase across our forecast area
overnight in response to this ejecting feature, with the latest RAP
guidance indicating a 45-55 kt 850 mb jet lifting across southeast MS
and adjacent southwest AL between 09-12Z (3-6 A.M.) The past few runs
of the HRRR have been in agreement with bringing an eastward
developing complex of thunderstorms toward western portions of our
forecast area during this same time frame, with indications of an
embedded linear complex of severe storms and discrete supercell
structures also developing in advance of the line. A fairly unstable
environment will be in place across our area overnight within the
warm/moist southerly low level regime, with MLCAPE values between
500-1000 J/KG. This instability in conjunction with strengthening
low level flow/shear and steepening mid level lapse rates will create
an environment conducive to severe storms with all modes possible
including damaging winds, large hail and a few tornadoes with
discrete supercells and within the main line. There is also an
increasing concern that a strong/significant tornado (EF2+) threat
could develop south and west of a New Augusta, Leakesville, Bay
Minette, Gulf Shores line late tonight into early Saturday morning,
where strongly curved hodographs/low level shear parameters will
favor that potential in this region (RAP showing 0-1 km storm
relative helicity as high as 300-400 m2/s2). Tornado Watches will
likely be issued overnight. Please see our latest graphicast at
www.weather.gov/mob

We encourage everyone especially west of a Camden to Gulf Shores line
to have multiple ways to receive warnings overnight and stay vigilant
for the potential of severe weather. /21

PREV DISCUSSION... /issued 529 PM CST Fri Jan 20 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...
00Z issuance...This package is reflecting the first of several rounds
of inclement weather expected this weekend. As the evening
progresses, am expecting CIGS/VISBYS to drop into IFR or lower
levels, especially close to the coast and over SE Mississippi/SW
Alabama as onshore flow brings in stratus. With winds staying up as
the next system approaches, dense fog will be localized. Timing of
shra/tsra moving over the area is in the 09z reaching SE Mississippi,
the spreading east and east of the fa by 21z.

/16

PREV DISCUSSION... /issued 303 PM CST Fri Jan 20 2017/

NEAR TERM /Now Through Saturday/...Two rounds of potentially severe
weather will affect the forecast area, the first occurring late
tonight into Saturday morning, followed by a second round developing
Saturday afternoon.  An upper trof over the central and southern
Plains strengthens through Saturday as a second system surges from
the western states and into the southern Plains.  With a continuing
moist southerly low level flow and a series of vigorous shortwaves,
strong deep layer lift shifts into much of the forecast area after
midnight tonight, then shifts eastward across the eastern portion
during Saturday morning, except lingering into the early afternoon
hours near the western Florida panhandle.  A second region of deep
layer lift then advances into the forecast area Saturday afternoon,
which will represent the second round of potentially severe weather.

One forecast concern regards the timing of a warm front expected to
develop over the northern Gulf, and then advance northward through
the forecast area during the period.  The advance of the warm front
may be delayed if abundant precipitation develops north of the
boundary tonight, but at this time expect the warm front to advance
into the southern portion of the forecast area late tonight, then
continue northward through the remainder of the area by around
midday Saturday.  MLCape values increase to around 500 J/kg for much
of the area late tonight, with values around 750 J/kg generally over
southeast Mississippi.  Based on the timing of the deep layer lift
and anticipated position of the warm front, along with abundant
shear which will be in place, expect that the highest potential for
severe storm development will be over southeast Mississippi and
adjacent southwest Alabama counties late tonight.

For Saturday morning as MLCape values increase to 1000-1500 J/kg,
expect severe storm potential for essentially the entire forecast
area early Saturday morning which then will be mostly along and east
of the I-65 corridor later Saturday morning as the best deep layer
lift shifts eastward.  While the 850 mb flow decreases to around 30
knots for much of the area Saturday afternoon, higher values of 35-
40 knots are expected generally along and east of I-65.  Expect 0-1
km helicity values of 100-125 m2/s2 west of I-65, with values of 150-
175 m2/s2 east of I-65.  MLCape values Saturday afternoon increase
to range from 1500 J/kg well inland to 2000-2500 J/kg closer to the
coast.  There may be a brief lull in convective activity around
midday Saturday, but this will be followed by convection again
increasing in coverage as the second region of deep layer lift
shifts into the area. Considering that the placement of the best
instability and shear will be generally along and east of I-65
Saturday afternoon, expect that this portion of the forecast area
represents the highest potential for severe storm development.
Severe storms that develop during the entire period will be capable
of damaging wind gusts and large hail, with isolated tornadoes
possible.

A general 1.0-2.0 inches of rain will fall across the forecast area
during the period, with locally higher amounts possible.  Will
continue with a Limited Risk of Flooding for much of the forecast
area through Saturday, except for an Enhanced Risk over southeast
Mississippi and southwest Alabama where the greatest potential
exists for locally high rainfall amounts.  With very high dewpoints
in place in the area flowing over cooler near shore waters, have
gone with a dense fog advisory for much of the coastal counties
until about 09Z, at which point mixing due to convective activity
will act to dissipate the fog. /29

SHORT TERM /Saturday night Through Monday night/...A dry slot will
advance through the forecast area Saturday night, but until this
occurs an 850 mb jet is expected to increase to 40-50 knots
generally east of I-65 Saturday evening in an environment with
MLCape values of at least 1500 J/kg initially decreasing to 750-1000
J/kg by midnight. So, we expect continued severe storm potential
mainly along and east of I-65 Saturday evening which gradually
decreases in potential late Saturday evening, possibly continuing
into the early morning hours Sunday before ending as the dry slot
sweeps through and shuts down convective activity. While convective
activity shuts down Sunday into Sunday night, the impressive size
and strength of the surface cyclone will allow for persistent wrap
around low-level moisture transport from north to south into the
central Gulf states through Monday, leading to lingering cloud cover
and perhaps some gusty winds. Skies then expected to clear from west
to east Monday night as ridging builds in from the west.

Highs in the upper 60s to low 70s Sunday then cool off to upper 50s
and low 60s Monday. Lows in the upper 50s to low 60s Saturday night
take on a similar cooling trend, with upper 40s to low 50s expected
Sunday night and low to mid 40s Monday night. 29/49

LONG TERM /Tuesday Through Friday/...The surface and upper-level
ridges provide clear skies and pleasant weather Tuesday before moving
off to the east ahead of the next approaching system. The next trough
begins to dig across the eastern CONUS Wednesday, with another
frontal passage supporting a chance for showers across the central
Gulf states Wednesday night into Thursday. High pressure then builds
back into the region during the latter half of next week as zonal
flow sets up aloft along the southern periphery of deepening low
pressure over eastern Canada.

A warming trend sets up Tuesday into Wednesday as southerly flow
ahead of the next approaching front returns Gulf moisture to the
area. Expect mid 60s across the area Tuesday and low 70s Wednesday.
Cooler temps in the upper 50s to mid 60s expected Thursday and
Friday in the wake of the frontal passage. Overnight lows will
likewise see a warming trend with the return of Gulf moisture Tuesday
and Wednesday nights (mid 40s inland to low 50s near the coast)
before dipping into the mid 30s in the wake of the cold front during
the latter half of next week. /49

MARINE...Expect areas of dense fog to develop over bays and near
shore waters early this evening and continue until approximately 09Z
when mixing due to convective activity will dissipate the fog.  A
strong cold front approaching from the west moves across the marine
area on Sunday.  A southerly flow ahead of the front becomes strong
on Saturday, then switches to a northwesterly flow in the wake of
the front with Gale conditions expected for much of the area.  The
strong offshore flow subsides Monday into Monday night.  Have gone
with a Small Craft for the 20-60 nm portion beginning Saturday
afternoon, followed by nearly the entire area for Saturday night
through Sunday.  Have gone with a Gale Watch fog nearly the entire
area for Sunday night, which ends early Monday morning. /29

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...High Rip Current Risk from 6 AM CST Saturday through late Sunday
     night for ALZ265-266.

     Dense Fog Advisory until 3 AM CST Saturday for ALZ263>266.

FL...High Rip Current Risk from 6 AM CST Saturday through late Sunday
     night for FLZ202-204-206.

     Dense Fog Advisory until 3 AM CST Saturday for FLZ202-204-206.

MS...Dense Fog Advisory until 3 AM CST Saturday for MSZ078-079.

GM...Dense Fog Advisory until 4 AM CST Saturday for GMZ630>635-650-
     655.

     Gale Watch from Sunday evening through late Sunday night for
     GMZ631-632-650-655.

     Small Craft Advisory from 6 PM Saturday to 6 PM CST Sunday for
     GMZ631-632-650-655.

     Gale Watch from Sunday evening through Monday morning for GMZ670-
     675.

     Small Craft Advisory from noon Saturday to 6 PM CST Sunday for
     GMZ670-675.

&&

$$

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