Special Marine Warning 71, NWS Mobile, AL

Area Forecast Discussion
National Weather Service Mobile AL
1236 AM CDT Sat Apr 10 2021

.AVIATION...
06Z issuance... A complex aviation forecast lies ahead for tonight
into Saturday. Mostly VFR conditions prevail over the region for
the time being, with some isolated spots of MVFR to IFR along
coastal sections of AL and the FL panhandle. A line of strong to
severe thunderstorms is organizing over MS and will continue to
develop and push eastward as a powerful MCS/derecho. This will
result in periods of IFR to LIFR conditions, heavy thunderstorms,
and intense variable winds sustained greater than 30 to 40kts,
gusting to near 50kts or greater at times. This is reflected in
the most recent TAFs with approximate timings via tempo groups.
Ahead of the line, isolated strong to severe thunderstorms are
possible, and will pose the threat for damaging winds, large hail,
and isolated tornadoes. Once the line has pushed through by mid
morning, expect MVFR conditions to prevail amidst a stratiform
rain region over the area that will clear by early afternoon. MVFR
to IFR conditions prevail during the afternoon into evening hours
today. MM/25

&&

.PREV DISCUSSION... /issued 933 PM CDT Fri Apr 9 2021/

NEAR TERM /Now Through Sunday/...A significant severe weather event
is expected to occur over the area mainly from late tonight into
Saturday morning as a powerful squall line plows through the area.
This powerful squall line will likely have become what is known
as a "Derecho", which is a widespread, long-lived and fast moving
line of storms producing (predominately) straight-lined winds. The
cause of this upcoming event is associated with an upper trof
over the Plains advancing towards the Mississippi River valley
through Saturday morning along with the important interaction of
two upper jets and unusually favorable thermodynamics over the
region. The first of these upper jets is located in the eastern
portion of the upper trof and strengthens while the second upper
jet will be in place over Texas into the northwestern Gulf. As the
upper trof advances eastward, this brings the favored right
entrance region of the upper jet on the eastern portion of the
upper trof into phase with the favored left exit region of the
southern stream jet, resulting in what's known as a "coupled jet
structure" with a region of strong upper level divergence.
Currently, a broken line of strong storms extends from northern
Louisiana into northeastern Mississippi, and a second line of
strong storms is located mainly over eastern Oklahoma. As the
coupled jet structure evolves, these two lines of storms merge
into one line - the Derecho - which then pushes quickly through
the forecast area late tonight into Saturday morning, likely
exiting around 10 am. Some other important ingredients in the
development of the Derecho is that despite the late night/early
morning timing, significant instability will be present along and
ahead of the system with MLCAPE values of 1500-2500 J/kg,
potentially even higher, along with a large dome of rain cooled
air which develops immediately in the wake of the fast advancing
storms. Another important ingredient in this severe weather event
is that unusually steep lapse rates around 8C/km from 800-650 mb
are noted in model soundings ahead of the approaching Derecho. In
addition, 0-6 km bulk shear values will be 50+ knots, and 0-3 km
helicity values will be around 300 m2/s2, potentially 400 m2/s2.
Isolated to scattered supercells will also develop in advance of
the Derecho. Damaging winds, large hail and tornadoes are all
possible as the storms move through. Through tonight, the Storm
Prediction Center (SPC) has a Moderate Risk for interior southeast
Mississippi and much of southwest Alabama, with an Enhanced Risk
over nearly all of the remainder of the area. For Saturday
morning, SPC has an Enhanced Risk along and east of I-65 with a
slight risk over much of the remainder of the area as the Derecho
and leading supercells move through the remainder of the area.
Please ensure that you have a way to receive weather alerts and
warnings for this weather event. Categorical pops remain in place
overnight into Saturday morning, then dry conditions develop first
over western portions of the area early Saturday afternoon then
the remainder of the area later in the afternoon. Dry conditions
are expected to prevail Saturday night into Sunday. A High Surf
Advisory is in effect from 1 am tonight until 7 am Sunday due to
breaking waves of 5 to 7 feet in the surf zone. A High Risk of rip
currents remains in effect through Sunday afternoon as well. Lows
tonight will be in the lower 60s except for mid 60s at the coast.
Highs on Saturday will be mostly in the mid to upper 70s. Lows
Saturday night range from the lower to mid 50s along and west of
I-65 with mid 50s to lower 60s further to the east. Highs on
Sunday mainly range from the upper 70s to lower 80s. /29

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...High Rip Current Risk through Sunday afternoon for ALZ265-266.

     High Surf Advisory until 7 AM CDT Sunday for ALZ265-266.

FL...High Rip Current Risk through Sunday afternoon for FLZ202-204-
     206.

     High Surf Advisory until 7 AM CDT Sunday for FLZ202-204-206.

MS...None.
GM...Small Craft Advisory until 4 PM CDT this afternoon for GMZ631-
     632-650-655.

     Small Craft Advisory until 7 PM CDT this evening for GMZ670-675.

&&

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