Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 191227
AFDMOB

Area Forecast Discussion
National Weather Service Mobile AL
627 AM CST Thu Jan 19 2017

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

&&

.UPDATE...Updated the forecast as we had to expand the Dense Fog
Advisory (valid until 10 am) inland over portions of south central
Alabama. Otherwise no changes to previously issued forecast from
earlier this morning. 12/DS

&&

.AVIATION...
19/12Z issuance...Areas of dense fog persist over portions of the
region this morning, especially for areas near the coast and inland
over interior southwest and south central Alabama. LIFR conditions
will remain likely for many locations through around 16z this
morning afterwards MVFR to brief periods of IFR except where
remaining lower due to increasing showers and thunderstorms. Some
isolated showers have already developed around the MOB area, mixing
out the fog somewhat. Additional showers and storms, a few isolated
ones on the strong to possibly severe side, develop and move west to
east across the region this afternoon and this evening. Surface winds
generally southerly at 10 to 15 mph, but higher gusts near
thunderstorms. 12/DS

&&

.PREV DISCUSSION... /issued 437 AM CST Thu Jan 19 2017/

NEAR TERM /Now Through Thursday night/...Dense fog will continue to
be widespread over southern half of forecast area early this morning,
especially closest to the coast. May see fog spread inland a little
further toward daybreak, and may have to adjust the current Dense Fog
Advisory a little further inland accordingly. Will monitor for this
possibility. Otherwise, primary forecast issue for today will be the
development of showers and thunderstorms to the west of the forecast
area this morning which will move east across the region this
afternoon and evening as a shortwave ejects northeast out of the
southern plains, sending a weakening cold front into our area. The
associated upper trof, oriented from the central Plains to Louisiana
this morning takes on an increasing negative tilt while sweeping
across the southeast states tonight. Even with the increasing
negative tilt however, most of the stronger mid/upper level dynamics
lift well north of our forecast area through this evening as the
aforementioned shortwave ejects northeast.

Enhanced deep layer lift to the west of the forecast area early this
morning has created an active line of convection, which is expected
to advanced eastward across most of the western/central forecast
area this afternoon, then continue across the western Florida
panhandle and south central Alabama this evening before exiting to
the east. The effect of this "early-day" precipitation to the west
and associated clouds spreading east across our area will likely
limit substantial diurnal heating somewhat today. This, combined
with relatively poor low to mid level lapse rates (generally around
6.0 degrees C/km), and SBCAPEs projected to only be in the 600-800
J/KG range (across interior counties), along with strongest upper
support passing well to the north, look for only a minimal chance
for severe weather with the passage of this system.

The Storm Prediction Center (SPC) has placed our entire forecast
area under a marginal risk for severe storm development today into
this evening, however they too indicate above discussed parameters
will act to limit the severe weather risk. Some slight directional
shear noted, but Storm Relative Helicity generally no more than 125
M2/S2 during peak of event with Surface to 500 mb Bulk Shear of
35-45 knots. With this, agree with SPC that primary marginal severe
risks would be isolated strong to locally damaging winds as
convection attempts to form into short line segments. A brief
embedded tornado is possible, but overall this threat is quite
limited.

Some locally heavy rainfall will be possible, but the system will be
a relatively fast mover, with 1.25 inches of rain at the
most (primarily over coastal counties). As mentioned in earlier
discussions, this will pose only a limited threat of nuisance type
flooding across the area.

SHORT TERM /Friday Through Saturday night/...Severe weather
potential rises. A deep layer trof over the Nation`s heartland will
maintain an active high level southwest flow over the central Gulf
coast as a series of mid level energy maximums move across. The lead
area of deep convection that passes east of the area tonight will
have worked the environment over for Friday with thunderstorm
probabilities in the chance categories. At the base of the long wave
trof, next mid-level impulse over Texas coast Friday afternoon ejects
northeast and will cause deep layer moisture (pwats) to surge back
to a range of 1.3 to 1.5 inches by Friday night into Saturday
morning. At same time, low level wind profiles will begin to
strengthen and coupled with steepening mid-level lapse rates, the
approach of relatively cool mid-level temperatures and an increase in
deep layer ascent, will contribute to an atmosphere supportive of
severe storms generally Friday night into Saturday morning. The main
impacts being large hail and damaging straight line winds in excess
of 60 MPH. Locally heavy rains and frequent lightning activity will
be common in the strongest storms as well. Going into the day
Saturday into Saturday night, well pronounced southern stream storm
system ejects across the Red River Valley of Texas/Oklahoma, causing
a frontal wave of low pressure to deepen/occlude over Arkansas while
attendant cold front approaches from the west. Ahead of the front,
steep mid-level lapse rates in the warm sector will continue to
support the potential of severe storms which could contain large hail
along with a damaging wind threat.

LONG TERM /Sunday Through Wednesday/...Strong frontal wave of low
pressure, taking up a large area of the deep south on Sunday,
advances across the Virginias/Carolinas on Monday. Will maintain a
modest chance of storms on Sunday, then we transition to the cooler
and more stable side of the departing low`s wrap around moisture
field to our northeast by Monday morning with predominant mode of
weather being light rain then. Sunday and Monday likely to be breezy,
especially closer to the coast. Daytime highs cooler on Monday, in
the lower 60s which will be close to seasonal then moderating back to
a range of 69 to 73 by Wednesday. Mild lows Sunday morning trend
cooler into the mid 40s interior to lower 50s coast by Tuesday
morning. /10

MARINE...Dense fog will continue over the near shore Gulf waters
and area bays this morning and after dissipating somewhat during the
day, redevelops again tonight, although possibly being less
widespread and persistent tonight than previous few days. High
pressure over the marine area will break down today as a weakening
cold front approaches from the west. A series of additional frontal
systems will then move west to east across the marine area Friday
night through Sunday. Periods of showers and thunderstorms, some
strong, should be expected through the weekend. Onshore flow will
gradually increase through the weekend, then become strong west to
northwest in the wake of the fronts by late Sunday. Seas gradually
build through the weekend then increase sharply during the early part
of next week. Small Craft Advisories will be likely toward the end
of the weekend into early next week. 12/DS

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...Dense Fog Advisory until 10 AM CST this morning for ALZ054>060-
     261>266.

FL...Dense Fog Advisory until 10 AM CST this morning for FLZ201>206.

MS...Dense Fog Advisory until 10 AM CST this morning for MSZ075-076-
     078-079.

GM...Dense Fog Advisory until noon CST today for GMZ630>635-650-655.

&&

$$

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