Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 130553 AAA

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1153 PM CST Tue Dec 12 2017

.DISCUSSION...Updated for latest aviation discussion below.


06Z issuance...VFR conditions through 14.06z. Winds will be north
at 2 to 6 knots through about 13.14z shifting southwest at 6 to
10 knots by late morning continuing through 14.06z. 32/ee


.PREV DISCUSSION... /issued 711 PM CST Tue Dec 12 2017/

DISCUSSION...Updated for latest marine discussion below.

MARINE...The Small Craft Advisory that was in effect for the Gulf
marine zones was allowed to expire at 6 PM. Still looking at Small
Craft Exercise Caution conditions over Gulf waters through for the
remainder of the night, with northerly winds of around 15 to 20
knots, with an occasional higher gust. Seas still running in the 4
to 6 foot range well offshore. 12/DS

PREV DISCUSSION... /issued 601 PM CST Tue Dec 12 2017/

DISCUSSION...Updated for latest aviation discussion below.

00Z issuance...VFR conditions through 13.18z. Winds will be north
at 2 to 6 knots through about 13.14z shifting southwest at 6 to
10 knots by late morning continuing through the afternoon. 32/ee

PREV DISCUSSION... /issued 350 PM CST Tue Dec 12 2017/

NEAR TERM /Now Through Wednesday/...Surface high pressure from
the Lower Mississippi River Valley to Texas, slips southeast into
the Gulf by Wednesday. A dry weather period continues through the
near term. Under clear skies, light winds and low dewpoints,
effective radiational cooling processes overnight favors lows
dipping to near freezing or a few degrees below freezing over the
interior. /10

SHORT TERM /Wednesday night Through Friday night/...Shortwave
energy dives south into a seemingly ever present upper trough over
the eastern Conus. The southern end of this shortwave energy
becomes attached to a piece of energy noodling around over the
California/Mexico border Friday, slowing the south end of the
energy. This energy then stretches the energy across the Southern
Plains and Southeast by Saturday morning. This
anchoring/stretching of the upper shortwave as it moves south
pushes a weakening cold front across the forecast area into
Thursday night at an ever slowing pace. But later Thursday night
into Friday night, as the middle/northern end of the shortwave
swings through, the surface front gets a more solid push, pushing
it south of the coast over the central Gulf, where it stalls early
Friday night. The energy that had been noodling around along the
US/Mex border gets an eastward push, to over the Rio Grande Valley
by Saturday morning, and with that, a surface low begins to form
along the surface boundary over the western Gulf. A piece of
shortwave energy gets a push east ahead of the US/Mex energy,
forming isentropic upglide shra over mainly the northern Gulf late
Thursday night into Friday night. A few shra may make it far
enough north to dampen the coast, but am expecting the majority of
the land portion of the forecast area to remain dry through the
Short Term. The front does bring a surge of cooler air to the
forecast area, with around seasonal temps Wednesday night into
Thursday night, dropping to below seasonal Friday through friday
night. May even see a return of near freezing temps Friday night
over the northern half of the forecast area.


LONG TERM /Saturday Through Tuesday/...The extended this package
is a compromise, especially later Sunday night on. Medium range
guidance is consistent in more shortwave energy diving south over
the western Conus, pulling the upper trough west and building an
upper ridge over the eastern Conus through the weekend. Both are
consistent in taking the surface low that had formed over the
western Gulf north-northeast, bringing increasing southerly flow
over the Lower Mississippi River and Southeast. The problem lies
in the speed of the driving upper shortwave energy moving east
Sunday night on. The 12z ECMWF is advertising a much quicker
solution than the GFS, pushing the upper shortwave over the Lower
Mississippi River Valley to the southeast Sunday night into
Monday. The GFS meanders the energy eastward, with the shortwave
moving over the Mississippi River Monday night into Tuesday. Both
are advertising the shortwave moving east just north of the
northern Gulf coast, developing a strong 60knot 850m jet over the
Lower Miss river Valley, then taking it northeast. Both are
advertising decent mid/upper level jet structures bringing upper
support to the system. Both are advertising the best combination
of jets and low level southerly flow off the Gulf remaining west
of the forecast area. Timing varies, with the GFS advertising
Sunday into Sunday night whilst the ECMWF is advertising Saturday
night. The ECMWF solution is also stronger than the slower GFS.
Either way one cuts it, feel it is wise to keep an eye on this
weekend system for any severe weather. As for the forecast, went
with a blend of GFS/ECMWF timing, hanging on to the rain longer
then the ECMWF is advertising. Through the weekend, with a
building upper ridge to the east and moderate low level flow,
temps rise to above seasonal levels through the weekend. From
there, with post system northerly flow slower to move in than the
ECMWF, have went with temps easing down to closer to seasonal
levels by Tuesday.


MARINE...Will maintain small craft advisory criteria through this
evening for moderate offshore flow and seas. Moderate to strong
offshore flow returns by the end of the week following next
frontal passage. /10




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