Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 230452
AFDMOB

Area Forecast Discussion
National Weather Service Mobile AL
1152 PM CDT Thu Apr 22 2021

.AVIATION...
06Z issuance...VFR conditions prevail through Friday afternoon,
with the exception of MVFR conditions in patchy overnight fog. For
Friday evening, numerous to widespread showers and storms spread
quickly into southeast Mississippi and southwestern Alabama,
potentially into south central Alabama and the western Florida
panhandle. Light northeasterly winds become southeast 10 to 15
knots on Friday. /29

&&

.PREV DISCUSSION... /issued 942 PM CDT Thu Apr 22 2021/

NEAR TERM /Now Through Saturday/...A nearly zonal flow pattern will
be in place over the area through Friday morning while an upper trof
advances across the southwestern states.  An associated surface low
will be located near the Texas panhandle by Friday morning while a
large surface ridge over the eastern states begins to retreat
eastward and allows for a southeasterly flow to become established
over the region.  The upper trof advances across the south central
states Friday night then moves into the southeastern states on
Saturday, with the surface low tracking eastward with the system and
moving to near central Tennessee or eastern Kentucky by late
Saturday afternoon.  A strong shortwave located in the eastern
portion of the upper trof results in an expansive region of strong
deep layer lift shifting from over eastern Texas/western Louisiana
Friday afternoon to across Mississippi and Alabama Friday night into
Saturday.  A warm front extending from the surface low into the
north central Gulf will lift northward into the interior portion of
the forecast area Friday evening then continues north of the
forecast area into central Alabama later Friday night. In response
to the strong deep layer lift, it appears that a secondary surface
low will have developed on the portion of the warm front over
northern Louisiana Friday evening then shifts into west central
Alabama Friday night, moving along the warm front in the process.
This secondary surface low then continues into approximately
northern Georgia by midday Saturday.

The warm front lifting northward through the forecast area ushers
lower to mid 60s surface dewpoints into the area. Instability will
initially be rather limited due to unfavorable lapse rates from 850-
600 mb, but these lapse rates improve significantly later in the
night in response to the strong deep layer lifting shifting into the
region. Despite the nocturnal timing, with the improving lapse rates
aloft and surface dewpoints in the lower to mid 60s, MLCAPE values
of 500-1000 J/kg will be realized by 12z Saturday over much of the
area except for easternmost portions.  MLCAPE values then increase
to 1000-2000 J/kg Saturday morning with similar values continuing
into much of the afternoon hours.  With the passage of the warm
front, model soundings exhibit a rather favorable veered profile
within the lowest 3 km with 0-3 km helicity values of 400-600 m2/s2.
Similar helicity values continue over the area through 12Z Saturday
then decrease to around 200 m2/s2 by 15Z Saturday due to the surface
flow becoming southwesterly, then decrease further to around 100
m2/s2 by 21Z Saturday.  0-6 km bulk shear values will be rather high
Friday evening through Saturday and range from 50-70 knots.

The somewhat complex evolution of surface features along with the
strong deep layer lift looks likely to play out as a severe weather
event with two main periods of severe weather (*see last sentence
though).  The first of these will occur as the warm front lifts
northward through the forecast area and brings high shear values and
gradually improving instability.  If convection is able to develop
sufficiently, then all modes of severe weather will be possible -
damaging winds, large hail and tornadoes.  The second period will
involve a squall line of storms that develops in response to the
secondary surface low, with the squall line progressing eastward
through the forecast area late Friday night into around midday
Saturday. By the time the squall line begins to enter our forecast
area (~ 3 am Saturday morning), instability will have increased to
at least around 500 J/kg and likely range from 500-750 J/kg in a
highly sheared environment.  Shear remains high through mid Saturday
morning which along with increasing instability supports the
potential for all three modes of severe weather (damaging winds,
tornadoes, large hail) with the squall line.  Helicity values will
be decreasing later in the morning though, so while bulk shear
values remain high, this suggests that the severe threat evolves to
being mainly a damaging wind threat before exiting to the east. Note
that a severe weather potential will also exist with isolated
convection expected ahead of the squall line.  There is a small
possibility that yet a third round of severe weather could develop
along a cold front which advances into the western portion of the
area late in the afternoon, but at this point forcing looks way too
weak and drier air will also be flowing into the area.

There will also be a potential for locally heavy rainfall mainly
from Friday night through (early) Saturday afternoon.  A high risk
of rip currents is in effect beginning Friday evening. Lows tonight
range from the mid 40s well inland to the lower 50s near the coast,
then warmer overnight temperatures are expected Friday night ranging
from around 60 well inland to the mid 60s near the coast. Highs on
Friday will be 70 to 75 and Saturday will be mostly around 80. /29

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...High Rip Current Risk from Friday evening through Monday morning
     for ALZ265-266.

FL...High Rip Current Risk from Friday evening through Monday morning
     for FLZ202-204-206.

MS...None.
GM...Small Craft Advisory from 7 PM Friday to 10 PM CDT Saturday for
     GMZ631-632-650-655-670-675.

&&

$$

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