Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 210501 AAB
AFDMOB

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1201 AM CDT Sat Oct 21 2017

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

&&

.AVIATION...
06Z issuance...VFR conditions will prevail for the next 24 hours.
Some light patchy fog is possible late tonight. Isolated showers
and storms are expected over the western portion of the area
mainly during Saturday afternoon. Light easterly winds will
increase to 5 to 10 knots over interior areas by mid Saturday
morning, with 10 to 15 knots closer to the coast. /29

&&

.PREV DISCUSSION... /issued 945 PM CDT Fri Oct 20 2017/

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

UPDATE...Will update to adjust temperature trends for the rest of
the night based on a quicker than expected cooling this evening.
Current forecast overnight lows still look reasonable but will
monitor.  Made other minor adjustments. /29

PREV DISCUSSION... /issued 715 PM CDT Fri Oct 20 2017/

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

UPDATE...Have updated to make some adjustments to winds and
cloud cover for much of the period, with some other minor
adjustments. /29

AVIATION...
00Z issuance...VFR conditions will prevail for the next 24 hours,
with the potential for some isolated showers over the western
portion of the area Saturday afternoon. May see some patchy light
fog late tonight. Light easterly winds increase by mid Saturday
morning to 5 to 10 knots inland ranging to 10 to 15 knots closer
to the coast. /29

PREV DISCUSSION... /issued 352 PM CDT Fri Oct 20 2017/

NEAR TERM /Now Through Saturday/...A series of shortwaves move
into, with some moving through, an upper trough over the western
Conus, pushing an upper high/upper ridge stretching north over the
eastern Conus eastward. A surface high centered over the Mid
Atlantic region gets pushed east off the Atlantic coast as a result,
with low level flow transitioning from a general easterly to a more
southeast direction by Saturday evening. This shift in low level
flow does bring increased moisture levels, but for the forecast
area, mainly to areas along and west of the AL/MS state line.
Combined with moderate to strong flow south of the coast and a
drier airmass working its way west over the northeastern half of
the forecast area due to the shifting upper ridge, am keeping any
mention of shra/tsra over the western border of the forecast area,
including the marine area. This increase in moisture levels does
filter inland a bit, with greatest inland push being over
Mississippi. With increasing cloud cover, these portions of the
area combined with subsidence from the upper high, a southwest to
northeast gradient of overnight lows sets up, ranging from well
above seasonal mid 60s along the coast and inland southeast
Mississippi to a still above seasonal, but not as much mid 50s
northeastern portions of the forecast area. Saturday, the entire
area is expected see continued but decreasing subsidence from the
shifting ridge. Combined with good daytime heating to mostly sunny
skies, daytime highs in the mid 80s are expected, about 5 to 8
degrees above seasonal. /16

SHORT TERM /Saturday night Through Monday night/...Will be
transitioning to a more unsettled weather pattern in the short term.
A vigorous upper level storm system over the Plains Saturday night
ejects eastward over the ARK-LA-TEX Sunday. East of this system, an
evolving, increasingly diffluent high level southwest flow over
the MS River Valley favors the eastward spread of deep layer
ascent over the central Gulf coast within a zone of increasing
deep layer Gulf moisture. Initially, highest precipitable water
(pwat`s) ~1.7" will be generally confined over the mid and lower
MS River Valley region Saturday night with much of the night
looking to remain rain-free, although an isolated shower or storm
will be possible near the coast and offshore. Sunday, Gulf
moisture opens up further and spreads east over the local area.
PWAT`s surge to 1.9 to 2.1 inches by Sunday night over the
forecast area. These values would stand between 1 and 2 standard
deviations above the climatological mean. Given the high amounts
of deep moisture, approach and movement of the best forcing over
the area Sunday and Sunday night, coverages of showers and storms
increase markedly with some concern for locally heavy rain in the
equation mainly by Sunday night. Latest storm total qpf for our
region looks to range between 1 to 2 inches with isolated amounts
possibly over 4" not out of the question. As for severe weather, will
maintain a low to marginal threat of occurrence on this package
mainly Sunday night as continuity amongst the global models on
shear and instability continues to differ. Out of respect for the
20.12z ECMWF though and the overall appearance of the vigorous
geo-potential height field aloft, its continued trend of a
stronger low level jet appears more plausible than other guidance
which shower weaker wind fields. The ECMWF solution would imply
some potential for a few stronger embedded storms, a few of which
could become briefly severe and exhibit some weak rotation in the
main convective line. Will continue to monitor closely over the
next few forecast cycles.

Upper level low/trough axis is progged to move east across our
area through the day Monday. Numerous to widespread showers and
embedded storms will be ongoing through Monday morning across
eastern portions of the forecast area, with rain chances lowering
over the western zones as a cold front pushes east across our
region. A few lingering light rains are possible into Monday
evening within the wrap-around moisture in the wake of a frontal
low pressure system lifting out to the northeast up across the
Appalachians.

Highs Sunday are expected to be close to 80 over the western zones
and lower half of the 80s east. Monday, a category or so lower on
highs with mid to upper 70s which will be closer to seasonal
normals. Overnight lows a mild to muggy mid 60s interior to lower
70s coast Saturday night will be cooling into the mid to upper
50s interior to mid 60s coast by Monday night. /10

LONG TERM /Tuesday Through Friday/...A reinforcement of cool/dry
air spreads into our area late Tuesday with much cooler overnight
lows expected Tuesday and Wednesday nights with overnight lows
plunging into the 40s for much of the area both nights. The
exception is along area beaches where lower to mid 50s forecast.
Coolest highs on Wednesday with numbers not looking to get out of
the 60s. Some modification in daytime highs into the 70s forecast
to close out the week. /10

MARINE...A surface high over the Mid Atlantic region shifts
southeast to off the Atlantic coast. The southward shift tightens
the pressure gradient, bringing moderate to strong easterly flow
to mainly unprotected waters. Will be issuing a small craft
advisory with this package. A deep upper trough shifting east
over the western conus pushes a moderate front west to east
across the area late Sunday into Monday afternoon. This will bring
unsettled weather to area waters into the coming week, with
strong offshore flow following. /16

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...None.
FL...None.
MS...None.
GM...Small Craft Advisory until 7 PM CDT Sunday for GMZ650-655-670-
     675.

&&

$$

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