Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 050454 AAC

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1054 PM CST Sun Dec 4 2016

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


06Z issuance...Frontal boundary slips southward off the coast
tonight with low stratus lowering the IFR/potentially LIFR
categories. -RA/-DZ anticipated during the pre-dawn hours Monday.
Forecast of vsby is problematic as various guidance solutions differ
on reductions. For now, MVFR vsby will be mentioned in the TAF`s.
Light northeast to east winds overnight. Gulf boundary returns
northward as a warm front on Monday, bringing the potential of an
east to west oriented area of convection northward across the Gulf
coast late AM into early afternoon. Another band of convection comes
in from the west later in the day. IFR to LIFR cigs linger into the
day Monday. /10


.PREV DISCUSSION... /issued 846 PM CST Sun Dec 4 2016/

DISCUSSION...See updated information for land areas below.

UPDATE...Unsettled weather continues overnight. Center of deep
upper level low this evening was far to the south, positioned close
to Mazatlan Mexico. East of this feature, a well pronounced high
level southwest flow continues to stream deep moisture north and east
out of TX and into the Mid-Atlantic and southeast US. Oriented
parallel to the mean flow aloft is a quasi-stationary surface frontal
boundary draped from southern Georgia to off the southeast Louisiana
coast into the northwest Gulf. Mid level impulses embedded in the
southwest flow aloft are riding northeast atop the surface boundary
which is contributing to the deep layer ascent of anomalously high
precipitable water (pwat) values over the central Gulf coast. Pwat
values ranging from 1.4 to 1.7 inches is now some 160 to 200% of
climatic normals per the 05.00Z evening analysis. The result has
been very beneficial rainfall over the local area that has been
experiencing severe to extreme drought conditions.

There have been reports of general 1 to 3 inch rain amounts today
with a few areas exceeding 4 inches. Radar shows the leading edge of
a broken line of showers/embedded thunder moving east across the
coastal waters with a large area of light to embedded moderate rains
back behind, moving inland across southeast LA and spreading
northeast from there into southern MS, southwest AL. Have made some
updates to current weather coverage given radar trends. Will maintain
the potential of patchy fog over bays and sounds overnight. Have the
warmer overnight lows (lower to mid 60s) generally along and
southeast of I-65 where these areas are southeast of the stalled
frontal zone. Low to mid 50s interior northwest zones. /10

PREV DISCUSSION... /issued 538 PM CST Sun Dec 4 2016/

DISCUSSION...Updated for latest aviation discussion below.

00Z issuance...A complex pattern forecast overnight with a warm front
near the Gulf coast producing low stratus, passing rains and
decreased vsby. Forecast calls for cigs to lower to IFR categories
with a mix of MVFR/IFR vsby. The warm front currently is analyzed
along the I-10 corridor over the northwest FL Panhandle to Mobile Bay
and southwest across the Mississippi Sound. MOB/BFM is forecast to
see north to northeast winds northwest of the boundary while PNS sees
winds generally from a southwest direction early on. Northeast flow
should set up at PNS overnight as the warm front gets a push south
as high pressure builds across the Mid-South. /10

PREV DISCUSSION... /issued 340 PM CST Sun Dec 4 2016/

NEAR TERM /Now Through Monday/...A cut off upper low over northern
Mexico advances across the Rio Grande then breaks down to an open
wave on Monday while continuing across Texas. The surface pattern
evolution which responds to this transition will play a critical role
in determining where, and if, severe convection will occur through
Monday afternoon. At the beginning of the period, a quasi-stationary
frontal boundary will be oriented east-west over the coastal
counties, extending to near the Mississippi River delta then back to
a surface low over the far west central Gulf. As a surface ridge
builds from the southern Plains into the interior southeast states,
the frontal boundary shifts southward tonight to extend from south of
the mouth of the Mississippi River to near Panama City, Florida.

The frontal boundary remains south of the forecast area on Monday,
except for advancing to near Destin, Florida late Monday afternoon.
The WPC graphics depicting the frontal boundary movement represent
this well.  Model soundings show lapse rates that are unfavorable up
to nearly 850 mb, and convection will be elevated and unable to tap
into the veered profile that exists at low levels. The only area of
concern that looks to exist is very late Monday afternoon at the
immediate coast of the western Florida panhandle where the
inversion/isothermal layer is very shallow and the frontal boundary
will be close. Am concerned though that even at this location the
lapse rates up to about 875 mb do not look particularly favorable.

This will need to be reassessed by the midshift given the apparently
unfavorable conditions for strong/severe storms through Monday
afternoon, with the only potential at this point existing at the
immediate coast of the western Florida panhandle in the late
afternoon. Will have the highest rain chances this evening generally
east of I-65 where gradually weakening convection will be located,
with good chance pops elsewhere. Rain chances increase over the
western portion and coastal portion of the area late tonight, then
steadily increase across the entire area through Monday as an
overrunning flow strengthens. Lows tonight will range from the lower
50s well inland to the lower 60s near the coast, then highs on Monday
range from the lower 60s well inland to the lower 70s close to the
coast. /29

SHORT TERM /Monday night Through Wednesday night/...A warm front
draped just offshore Monday will lift north through the forecast
area Monday night as a deepening surface low in the northwestern
Gulf tracks northeast into the lower Mississippi Valley. Ahead of
this front, ongoing elevated convection will produce widespread rain
across the forecast area, but pose little threat in terms of severe
weather. South of the front, a more surface-based instability (CAM
soundings suggesting 400 to 600 J/kg MLCAPE) and plentiful deep
layer shear of 40 to 60 knots could support the development of some
low-topped supercells, with the primary severe threats being
damaging winds and a brief tornado or two. As the low continues
northeast into the Tennessee Valley early Tuesday morning, an
attendant cold front pushes through the forecast area from the west,
bringing with it another line of convection that could likewise
support strong to low-end severe straight line winds, although this
threat remains more uncertain as the dynamics weaken and the upper
level energy lifts to the north with the low. The post-frontal
environment will bring clearer skies and drier conditions into

High temperatures will range from mid 60`s well inland to around 70
near the coast on Tuesday afternoon, and low 60`s inland to upper
60`s near the coast Wednesday afternoon. Look for overnight lows to
remain somewhat warm Monday night as the lifting warm front and
clouds keep temps in the mid 50`s inland to low 60`s near the coast.
Lows will then cool off Tuesday and Wednesday nights in the post-
frontal environment, with low to mid 40`s inland to around 50 near
the coast each of those nights. /49

LONG TERM /Thursday Through Sunday/...Models currently uncertain as
to the exact timing of the next system, but at this time the next
cold front is progged to pass through the area sometime Thursday
morning. Models are, however, in relative agreement that this next
cold front will bring a more significant shot of cooler and drier
air in its wake, with overnight lows Thursday and Friday nights
expected to dip below freezing and into the mid 20`s areawide.
Likewise, afternoon highs look to be much cooler than normal to end
the week, with temps struggling to break 50 degrees Friday afternoon
and only reaching the low 50`s Saturday afternoon. Things warm
slightly Saturday night into Sunday, with lows in the mid to upper
30`s and highs in the low to mid 60`s. /49

MARINE...Visibilities have improved over Mobile Bay and will cancel
the marine dense fog advisory with the afternoon package issuance. A
frontal boundary will move southward across the marine area tonight
with moderate southerly winds becoming northerly. Will continue with
a small craft advisory through 00Z Monday to allow sufficient time
for the seas to subside below advisory criteria. The frontal
boundary returns northward on Monday as a warm front, then moves
inland Monday night. A surface low which meanwhile approaches from
the northwestern Gulf and passes north of the area, bringing a cold
front across the marine area late Monday night into Tuesday morning.
The onshore flow becomes strong Monday night ahead of the front, then
switches to the northwest and subsides in the wake of the front. A
stronger cold front moves through the marine area on Thursday. /29




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