Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 241735 AAB

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1235 PM CDT Sun Sep 24 2017

.DISCUSSION...See updated information for land areas and aviation
discussion below.


.UPDATE...Convective debris from storms over the Gulf continues to
stream northward across the area. This will likely serve to reduce
the coverage of showers and thunderstorms this afternoon,
especially along the coast. As a result, have updated the forecast
to lower rain chances slightly along the coast for the remainder
of the afternoon. /13


18Z issuance...VFR conditions expected outside of scattered
showers and thunderstorms through the late afternoon hours. This
convection will bring ceiling and visibility drops to MVFR levels,
and briefly IFR in some locations. Locally strong gusty surface
winds possible near the thunderstorms. /13


.PREV DISCUSSION... /issued 638 AM CDT Sun Sep 24 2017/

DISCUSSION...Updated for latest aviation discussion below.

24/12Z issuance...General VFR CIGs/VISBYs expected through much of
the forecast period, but another round of SHRA/TSRA are expected
across the area today. SHRA/TSRA offshore at the beginning of the
period this morning, developing inland and becoming numerous in
some locations by this afternoon. This convection will bring
ceiling and visibility drops to MVFR levels, and briefly IFR in
some locations. Locally strong gusty surface winds possible near
the thunderstorms, some small hail also possible. 12/DS

PREV DISCUSSION... /issued 441 AM CDT Sun Sep 24 2017/

NEAR TERM /Now Through Sunday night/...Water vapor imagery indicates
mid-upper low spinning over the Gulf coast region, generally
centered over our forecast area. Models are in good agreement that
this upper low will drift slowly west through tonight and be
situated just to the west of our local area by late tonight. As this
low migrates to the west, associated mid-upper level wind flow will
become more more southerly across the forecast area, with deeper
layer moisture increasing through tonight. This trend of increasing
moisture is already noted in differences between profiles of 24/00z
and 24/06z soundings at both LIX and BMX (as both offices did 24/06z
soundings in support of NHC). GFS indicates that PWAT`s will
increase to near 1.9 inches across much of the forecast area by late
this afternoon and tonight. With this abundant moisture, combined
with daytime heating and instability (with SBCAPE approaching 2500
j/KG over southern third of forecast area this afternoon), expect
showers and storms to be more numerous today than past several days
(especially over southern half of forecast area). As the deeper
layer flow switches from northeasterly to more southerly with
movement of the mid-upper low, storm motion is expected to be more
to the west across the area today as opposed to the storms moving in
from the northeast as they have over the past few days. The
increasing deep layer moisture, will somewhat limit downdraft
potential today, although storms will continue to have the potential
for strong gusty winds up to around 35 or 40 mph. Evening upper air
analysis indicate 500 MB temps still in the -10C to -11C range over
much of the area (in association with upper low). This will continue
to result in the potential that some of the stronger storms that
develop today will be capable of producing small hail (pea to nickle
sized), as has been the case for the past several days. Potential
for severe storms is low, but one or two isolated severe storms
could be possible this afternoon. Any lingering convection will
diminish during the early evening hours with the loss of daytime
heating and instability. As has been the case for the past several
nights, there could again be some patchy light fog tonight,
especially for those areas that receive rainfall today. Surface
high pressure ridging in from the northeast will continue to
maintain a light, predominately east wind flow across the region
today, with a slight southeasterly component near shore this
afternoon in association with the sea breeze circulation. Highs
today in the upper 80s over most of the interior, and mid 80s
along and near the coast. Lows tonight expected to range from the
upper 60s over most of the interior to the low to mid 70s near and
along the coast. 12/DS

SHORT TERM /Monday Through Tuesday night/...To start the short
term, mid-upper low transitions to a negative tilt shear
zone/trof positioned from AR to the central Gulf coast on Monday.
Considering the orientation of the trof axis, deep layer ascent
operating on presence of deep layer moisture (pwats 1.7 to 1.9
inches) and daytime instability favors a modest chance of storms
during the day Monday. Some of the storms could become locally
strong, producing brief strong wind gusts, locally heavy rains,
frequent lightning and possibly some small hail. High level trof
axis pushes southeast Tuesday, becoming more aligned across the
northern Gulf as Hurricane Maria lifts northward east of the Mid-
Atlantic coast. A net lowering in deep layer moisture over the
interior northeast, combined with increased subsidence on the
southwest side of Maria`s large scale circulation suggest chances
of showers and storms will be on low side (10% or less) along and
north of a line from Waynesboro MS to Crestview FL Tuesday.
Forecasters call for a slight chance of storms south of this line
where better moisture lingers. Daytime highs 85 to 90 on Monday
and upper 80s to lower 90s for Tuesday. Overnight lows in the mid
to upper 60s interior to lower to mid 70s beaches. /10

LONG TERM /Wednesday Through Saturday/...A narrow mid level ridge
builds over the forecast area into the middle of the week which
would increase larger scale sinking motions and little to no
convective activity. A frontal boundary drops southward over the
Lower MS River Valley Thursday, making passage into the northern
Gulf Friday. Highs in the upper 80s to lower 90s Wednesday and
Thursday trend lower to end the week, settling lower by a category
or two Saturday with highs ranging from 80 to 85 to start the
weekend. Coolest overnight lows in the outlook is Saturday morning
with numbers ranging from the lower to mid 60s along and
northwest of I-65. 65 to 70 at the beaches. /10

MARINE...High pressure ridging into the marine area from
the northeast will continue through most of the forecast period,
with a primary light to occasionally moderate easterly wind flow
continuing over the marine area. Still expect a slight offshore
component during the late night and early morning hours and a slight
onshore component during the afternoon and early evening hours. Flow
near or just below Exercise Caution levels today as gradient
tightens between high pressure to the north and Hurricane Maria
making its closest approach to the east coast of Florida. Flow
relaxes once again by Tuesday as Maria continues to move up the
Atlantic east coast, weakening the ridge to the north. Winds
become lighter and more variable early Wednesday before becoming
more northerly and increasing by Thursday in the wake of a cold
frontal passage. Seas initially around 3 to 4 feet, decreasing to
1 to 2 feet in the middle part of the forecast period, then
increasing once again late in the period in the wake of the front.




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