Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 171132

Area Forecast Discussion
National Weather Service Mobile AL
532 AM CST Sun Dec 17 2017

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


12Z issuance...MVFR conditions currently over southeast
Mississippi and southwest Alabama expected to continue spreading
east across the remainder of the area today. Showers and storms
move in from the south and west throughout the day, resulting in
locally lower cigs/vis and perhaps some gusty winds. Otherwise, south
winds around 10 to 15 knots this afternoon become light overnight
tonight as patchy fog develops near the coast and ceilings lower
to IFR levels. Fog could be dense in spots, with visibilities down
to under a mile at times. /49


.PREV DISCUSSION... /issued 446 AM CST Sun Dec 17 2017/

NEAR TERM /Now Through Sunday night/...Southwest deep-layer flow
persists over the area through the near term between a large upper
high over the northern Caribbean and a deep longwave trough over
the western CONUS. Embedded within this southwest flow, a potent
shortwave trough kicks quickly northeast from Texas to the Great
Lakes region to start the period. A weak surface low associated
with this upper impulse similarly moves northeast, albeit from the
western Gulf into the Lower Mississippi River Valley. As it
approaches our area, this low pushes a warm front north towards
the I-20 corridor. South of this front, the gradient between the
surface low to our west and building high pressure to our east
forms a well- established onshore flow over our area, which
results in a rapid influx of Gulf moisture. The surface low then
weakens and passes to our north as it is quickly outrun by its
parent shortwave, which dampens and merges with the northern jet
over the Atlantic.

What does this mean for our area? A remnant boundary, leftover by
the weakening surface low will act on a moistening and destabilizing
boundary layer, leading to an increase in rain chances from west
to east today and tonight. Some locations along the coast could
see a few showers and storms out ahead of this boundary as well.
Patchy dense fog will also become increasingly possible today and
tonight (particularly closer to the coast) as the warm, moist
boundary layer air overruns the cold nearshore and bay waters. It
is possible that some areas along the immediate Alabama shoreline
see patchy fog develop as early as this morning, with locations
farther east seeing it begin to develop later in the day and
overnight tonight as the low-level moisture plume expands in that
direction. /49

SHORT TERM /Monday Through Tuesday night/...Upper high pressure
area centered over the Bahamas will drift slowly southwestward
through the short term, while an upper cutoff low over the
southwest conus/northwest Mexico border drifts slowly eastward to
the lower Mississippi River region. A weak surface high pressure
ridge will remain across the southeast states through Tuesday
afternoon, and then weaken and shift southeast as an area of low
pressure forming over east Texas on Tuesday moves east to the
lower Mississippi River region by late Tuesday night. Deep layer
moisture will fluctuate across the region, with precipitable water
values ranging from 1 to 1.9 inches.

Scattered to numerous rain showers along with a few embedded
thunderstorms will occur on Monday as upper level impulses
traverse the region, followed by rain tapering off Monday night.
Rain chances will then gradually increase again Tuesday and
Tuesday night as the upper low approaches the region. The best of
precipitation will occur after midnight with scattered to numerous
rain showers, along with a few embedded thunderstorms, spreading
from west to east.

Widespread rainfall amounts through the short term will range
from 0.4 to 0.6 west of the Alabama River. Locally higher amounts
near 1 inch will be possible across the inland portions of
southeast Mississippi and southwest Alabama. Widespread rainfall
amounts east of the Alabama River will range from 0.2 to 0.4, with
locally higher amounts near 0.75 across south central Alabama.

Sea fog will continue to develop through the short term as surface
dewpoints in the mid to upper 60s move northward over the cool
gulf and bay waters. Light southerly winds will allow the sea fog
to advect far inland at times. While the inland areas will see the
fog dissipate during the daytime hours, some of our coastal zones
could see extended periods of dense fog lasting throughout the
daytime hours. /22

LONG TERM /Wednesday Through Saturday/...The cutoff upper low
pressure area will move slowly eastward and evolve into an open
wave as it passes over the southeast conus. Scattered to numerous
rain showers along with a few embedded thunderstorms Wednesday
morning will taper off from west to east throughout the day as the
upper wave moves east of the region. A dry period will then occur
Wednesday night through Thursday evening, followed by
precipitation chances increasing once again as a longwave upper
trough digs south over much of North America. Additional
widespread rainfall amounts late Thursday through Saturday are
expected to range from 0.25 to 1 inch. /22

MARINE...A moderate to strong southeasterly flow continues this
morning, before slowly subsiding and shifting to southerly today
through midweek. A Small Craft Advisory remains in effect for open
Gulf waters out 20 to 60 nm until 9AM CST this morning, with
exercise caution level winds thereafter into Monday. In the wake of
the recent cold spell, the near shore waters and area bays are still
quite cold. The combination of these cold waters and warm, moist
onshore flow will result in the formation of marine fog through
midweek, likely becoming widespread and perhaps dense at times. Will
continue to wait and see how late today the more southerly flow and
better moisture develops before issuing a Marine Dense Fog Advisory.
Will likely need an Advisory for the marine area at some point
during the early part of this week, as dense fog will lead to an
increased probability of hazardous boating impacts. /49


GM...Small Craft Advisory until 9 AM CST this morning for GMZ670-675.



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