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Area forecast discussion...updated
National Weather Service Mobile Alabama
1013 am CST Wed Dec 13 2017

Discussion...see updated information for land areas below.


Update...effective mixing of a deep layer dry airmass in place
over the Gulf Coast has resulted in a much lower dewpoints,
requiring an update to gridded relative humidity fields. A gradual
moderation in dewpoints expected by late in the day as high
pressure axis shifts to the south over the Gulf. This results in
an evolving south to southwest component of wind flow off the
Gulf. /10


Previous discussion... /issued 520 am CST Wed Dec 13 2017/

Discussion...updated for latest aviation discussion below.

12z issuance...VFR conditions expected through the period, with
light northeast winds this morning veering to southwesterly by
this afternoon. /49

Previous discussion... /issued 430 am CST Wed Dec 13 2017/

Near term /now through Wednesday night/...surface high pressure
builds over the eastern Gulf through the near term, beneath a
zonal flow pattern aloft. Local winds shift to more southwesterly
and we see a slight bump in moisture as a result. Otherwise, the
near term remains dry beneath mostly clear skies. Southwest flow
and bump in moisture means low temps stay above freezing for most
tonight, with the exception being areas north of Highway 84 where
temps could dip to around freezing. /49

Short term /Thursday through Friday night/...zonal flow aloft
will persist over the region downstream of a developing upper low
pressure area over northwest Mexico. Light southwesterly to
westerly winds on Thursday afternoon will precede a weak cold
front approaching from the north. This front will move south
across the area Thursday night, bringing a northerly flow back
into the region on Friday. A few showers may occur along the coast
Friday morning with the passage of the front. Otherwise, rain-
free conditions will persist through the short term. /22

Long term /Saturday through Tuesday/...the upper closed low
pressure area over northwest Mexico will lift northeast over
western Texas, and evolve into an open wave as it merges with a
strong upper level shortwave moving from west to east from the
Great Plains to the East Coast early next week. Meanwhile, a
1025mb surface high pressure area over the southeast states
Saturday morning will move northeast as a developing surface low
pressure area across the western Gulf of Mexico lifts northeast
across the southern Mississippi River region. Flow will follow
late Saturday and Sunday as the center of high pressure moves east
of the area.

There is better timing with this system between the European model (ecmwf) and GFS
this run, with a 40 to 60 knot low level jet setting up over the
forecast area on Sunday. A 120 to 140 upper level jet is still
expected to develop over the eastern conus, with the forecast area
residing in the favorable right entrance region on Sunday.
Adequate wind shear will certainly be in place early next week
with this system, along with upper level diffluence and mid level
lift, instability in the low to mid levels is still undefined.

The dry period will persist through midnight Saturday night,
followed rain increasing from west to east after midnight. A 90
percent chance of precipitation is forecast for Sunday. Given
the strong dynamics advertised by the models, increased to a
chance of thunderstorms embedded in the definite rain showers.
This system may bring a round of strong to marginal severe
weather with damaging winds and possible tornadoes being the main
threats if higher MLCAPE values are realized. /22

Marine...offshore flow weakens today before becoming
southwesterly and increasing back to exercise caution levels
tonight into early Thursday. A lighter west flow then sets up
Thursday before another front moves through and moderate offshore
flow develops Friday. Moderate to strong onshore flow returns by
Sunday ahead of yet another approaching front. Seas generally
around 2 to 4 feet through the end of the week, then building to 5
to 8 feet Sunday due to the onset of stronger onshore winds. /49


Mob watches/warnings/advisories...

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