Special Marine Warning 250, NWS Mobile, AL

Area Forecast Discussion
National Weather Service Mobile AL
643 AM CDT Thu Aug 12 2021

.AVIATION...
12Z issuance...VFR conditions will prevail through much of the
period. Scattered to locally numerous showers and thunderstorms
will develop through the afternoon with brief reductions in
ceilings and visibilities expected. /13

&&

.PREV DISCUSSION... /issued 445 AM CDT Thu Aug 12 2021/

NEAR TERM UPDATE /Now Through Friday/...A broad upper trof over
the northern Plains and Great Lakes region slowly amplifies while
moving into the interior eastern states through Friday. Another
upper trof mainly over the Florida peninsula and into the Florida
Straits meanwhile slowly shifts into the eastern Gulf and
strengthens slightly, and this feature along with the upper trof
moving into the interior eastern states will gradually weaken an
upper ridge over the southeastern states. A surface ridge which
extends from the western Atlantic to along the northern Gulf coast
appears to break down slightly and form a modest surface high
centered roughly over the north central Gulf coast. During this
transition, light southerly surface winds over the forecast area
continue through today then switch to a light northerly flow on
Friday except for a weak southerly flow resuming during the
afternoon near the coast. Abundant deep layer moisture remains in
place over the region with precipitable water values near 2.0
inches, and MLCAPE values increase to near 2000 J/kg this afternoon
with similar values expected by Friday afternoon. While no synoptic
scale forcing mechanisms of note will be present through the period
within this convectively favorable environment, there will be a
modest land and sea breeze circulation which cycles through the
period. This will serve to initiate convection first over the marine
area overnight with the convection then spreading inland and
increasing in coverage during the day aided by  daytime heating and
outflow boundaries generated by convection. The environment is
weakly sheared which implies slow moving storms along with the
potential for mainly nuisance type flooding. A few strong to
possibly marginally severe storms are possible each day as well.
Highs will be in the lower to mid 90s, and afternoon heat index
values reach 101-106 and should remain just below heat advisory
criteria of 108. Lows will generally range from the lower 70s inland
to the mid 70s at the coast. A low risk of rip currents is expected
through the period. 29/13

SHORT TERM /Friday night Through Saturday night/...An upper level
trough stretches south across the western half of the southeast and
Lower Mississippi River Valley, with high pressure centered off the
Carolina coast and over the western Conus. The basic upper pattern
remains the same through the Short Term as TD Fred moves into the
Gulf of Mexico Saturday morning, then moves north along the Florida
Gulf coast. Current guidance places the center of Fred near Tampa
Bay by Sunday morning. The airmass over the forecast area and nearby
remains on the moist side of normal for the current time, with
precipitable h20 values ranging between 1.9" and 2.2" through the
period, even with mean low level flow becoming northeasterly on
Saturday.

With the high moisture values and presence of an upper level
shortwave trough moving west over the northcentral Gulf coast,
scattered to numerous showers and thunderstorms are expected
Saturday, above seasonal norms. For Friday and Saturday nights, am
expecting the typical pattern of decreasing coverage of convection
in the evening over land, with redevelopment offshore after
midnight. Depending upon the final speed and makeup of Fred,
southeastern portions of the marine portion of the forecast area may
start seeing rainbands late Saturday night. Looking at the chance of
strong to severe storms in the Short Term, MlCapes rise into the
2000-2500J/kg range across the forecast area. DCapes, on the other
hand, peak in the 700-900J/kg range over areas along and southeast
of I-65 as the upper level shortwave trough moves west over the
northcentral Gulf Coast, 500-700J/kg northwest. With marginal
instability, a few strong to severe storms are possible, mainly
along and southeast of I-65.

With a weak upper ridge stretching east over eastern portions of the
Southeast, enough upper subsidence is present for temperatures a bit
above seasonal norms. Lows Friday and Saturday night drop into the
low to mid 70s inland, upper 70s along the coast. High temperatures
Saturday rise into the 89-94 degree range, with Heat Indices topping
out in the 97-102 degree range.
/16

EXTENDED TERM /Sunday Through Wednesday/...Tropical Depression Fred
continues to be the main feature of concern during the period. This
system is currently forecast to move into the SE Gulf of Mexico
Saturday morning, then be located west of the central FL Peninsula
by Sunday morning, and making landfall somewhere across the FL
Panhandle by late Sunday night/early Monday morning. The track
forecast continues to have quite a bit of uncertainty as the day 5
western edge of the cone ranges from the Alabama/Mississippi
border to near Jacksonville, FL on its eastern edge. Regardless,
NOW IS the time to be making preparedness plans in the event you
have to enact them. The potential for wind impacts has decreased
slightly due to a slight eastward shift in the forecast track.
There is now about a 1 in 10 chance of seeing 39 mph or higher
across south central Alabama and the western Florida Panhandle.
Also, the rip current risk will become HIGH over the weekend and
will remain well into next week. The current earliest reasonable
arrival time of tropical storm force winds would be Sunday morning
along coastal Alabama and northwest Florida, but most likely
later in the day on Sunday. There are a lot of factors that will
ultimately determine the precise path of the storm and future
adjustments will be necessary. Therefore, keep watching the trends
and we'll keep breaking down the spatial and temporal impacts.
Given our current sensible weather forecast, on Sunday, showers
and thunderstorms will advance into the region from the southeast
on Fred's western periphery. Rain chances remain elevated across
the area Sunday Night, Monday, Tuesday and into Wednesday as
Fred's moisture envelope continues to diminish in areal extent,
but still remains prevalent enough to increase rain chances over
our region. Although, the latter portion of the extended will
ultimately depend on what Fred does. As for temperatures, highs
will range from the upper 80s to low 90s. Overnight lows will
generally be in the low to mid 70s inland and upper 70s to low 80s
along the coast. /23 /14

MARINE...Light southeasterly winds prevail through Saturday with
an offshore flow possible late each night into the early morning
hours. Scattered showers and storms are expected through Saturday.
Tropical Depression Fred may bring increased winds and waves, as
well as increased coverage of showers and storms, on Sunday into
Monday. Refer to the official National Hurricane Center advisories
for more information regarding Fred. /13

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...None.
FL...None.
MS...None.
GM...None.
&&

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