Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 300337 AAC
AFDMOB

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1037 PM CDT Sat Apr 29 2017

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

&&

.UPDATE...Tidal gauges in northern Mobile Bay are showing tide
levels approaching near 2 feet above normal. Given this, felt it
was prudent to expand the Coastal Flood Advisory to cover central
Mobile and Baldwin Counties as minor coastal flooding will be
possible along the shores of northern Mobile Bay, particularly
tomorrow afternoon. 34/JFB

&&

.PREV DISCUSSION... /issued 954 PM CDT Sat Apr 29 2017/

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

UPDATE...No additional updates are needed for the rest of the
evening. Gusty southeast winds will persist along coastal areas
overnight with gusts of 30 to 35 mph. This southeast wind will
result in little drop in temperatures with lows in the 70s across
the area.

There is still uncertainty with the timing of storms and severe
weather threat for tomorrow. The last several runs of the HRRR
organizes the convection over east TX and develops a squall line
that approaches the MS river by 7am and southeast MS by noon. The
HRRR maintains this squall line as it approaches the MS/AL line
and indicates the development of an organized cold pool which
would allow this complex to continue east through the rest of the
area during the afternoon. This seems plausible given the amount
of instability present (2000-2500 J/kg MLCAPE and significant mid
level dry air to support the cold pool.) However, deep layer shear
quickly decreases with eastward extent which would suggest some
weakening is possible. If this more organized earlier line comes
to fruition, then the severe threat for the evening/overnight
becomes less certain due to the atmosphere being substantially
worked over.

Other guidance shows this first convective line weakening as it
nears us (likely due to the quick decrease of deep layer shear) with
the main thunderstorm activity/severe threat developing along the
cold front and impacting our region late afternoon through the
early morning hours.

We will have to watch things very closely overnight. Given the
strong elevated mix layer present over the area, we can see a
strong cold pool becoming organized and supporting the earlier and
more vigorous convective line, but confidence is not high.

Winds will become very gusty across the entire area tomorrow after
sunrise with gusts up to 40 mph outside of thunderstorm activity.
There will also be a chance of minor coastal flooding tomorrow
afternoon during high tide, especially for the typical flood prone
areas in and around Mobile Bay. 34/JFB

PREV DISCUSSION... /issued 646 PM CDT Sat Apr 29 2017/

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

UPDATE...Isolated warm air advection showers have developed over
parts of inland SE MS and across the state line into far western
AL. Updated the forecast to maintain a slight chance of showers
through the next few hours. While we will let the inland portion
of the Wind Advisory expire at 7pm, we are issuing another Wind
Advisory for Sunday/Sunday evening as winds are expected to
quickly strengthen after sunrise. Frequent gusts of 30 to 40 mph
are expected given forecast soundings showing 35 to 45 knots in
the mixed layer. Will maintain the wind advisory for the coastal
counties tonight. While there will may be a lull in gusts this
evening, expect gusts around 30 mph, especially along the coast
overnight as the low level jet begins to strengthen, increasing
further after sunrise. 34/JFB

AVIATION...
00Z issuance...MVFR ceilings inland and IFR ceilings near the coast
expected to continue through the 24 hour period. Strong southeast
winds will persist as well, with wind speeds of 15 to 20 knots
gusting to 25 knots overnight strengthening to a sustained 25 knots
gusting to 35 knots by Sunday afternoon. A cold front with showers
and thunderstorms will approach from the west Sunday night, with
additional storms possible ahead of this line during the afternoon
hours. Some storms could become strong to severe, with gusty winds,
large hail, and frequent lightning possible. Heavy downpours will
lower vis/cigs in and around storms as well. /49

PREV DISCUSSION... /issued 305 PM CDT Sat Apr 29 2017/

Coastal Hazards Persist Through the Weekend...

Slight Risk of Severe Storms and Locally Heavy Rains Approach
from the West Late in the Day Sunday...

NEAR TERM /Now Through Sunday/...A strong 1027mb surface high
pressure area will remain centered over the western Atlantic with
a ridge extending west across the eastern Gulf of Mexico and
southeast states through Sunday afternoon. A band of showers and a
few thunderstorms are currently ongoing across the lower Great
Lakes region along an associated warm front lifting northward
through that region. Additional convection is also occurring
across central and eastern Oklahoma, southeast Kansas, and much of
Missouri associated with an area of low pressure over eastern
Oklahoma, while a cold front advances southeast through the central
and southern Great Plains. Also noted is a cluster of showers and
thunderstorms lifting northward over northern Louisiana. A deep
strong southerly wind flow will persist through Sunday afternoon
across our region between these two systems, and a Wind Advisory
will remain in effect across our coastal zones through the
overnight hours and the advisory will likley be expanded northward
across the remainder of the forecast area on Sunday.

A quasi-linear convective system (QLCS) will advance east across
the lower Mississippi River ahead of the cold front as the surface
low and upper low lifts north-northeast across the central
plains. This line of showers and thunderstorms will approach the
forecast area late Sunday morning, and advance eastward to the
Alabama River through late Sunday afternoon. Additional showers
and few thunderstorms are expected to develop ahead of the main
line across much of the forecast area through Sunday afternoon. A
deeply moist airmass with precipitable water values locally
enhanced over 2" along the line will bring potential for heavy
rainfall west of the Alabama River. Locations generally along and
west of the Alabama River remain most favored for severe weather
potential Sunday afternoon where a 40-50 knot low level jet will
be aligned with up to 1500 J/KG of available MLCAPE. Damaging
winds and possibly a few tornadoes embedded within the line will
be the primary severe weather hazards, though marginally severe
hail will also be possible.

The persistent strong southerly flow, high period swell, and high
tidal ranges will bring a high risk of deadly rip currents to the
local beaches through late Sunday. Surf heights will also continue
to build through the weekend, with breakers forecast to range as
high as 5-8 feet by Sunday. Minor coastal flooding may also occur
through the weekend as the persistent southerly fetch continues,
particularly in low lying areas as tides average around 1 foot above
predicted levels (which may equate to the 2 to 3 foot MSL range near
times of high tide).

Low temperatures tonight will range from 70 to 75 degrees. High
temperatures Sunday will range from 82 to 87 degrees inland areas,
with lower 80s across the coastal section. /22

SHORT TERM /Sunday night Through Tuesday night/...Heavy rainfall
and potential of severe weather still looks on track to impact
much of our forecast Sunday night. A vigorous upper level storm
system with a large area of significant mid level height falls of
some 15 to 25 decameters exits the central High Plains and is
progged to lift up across the Upper Mid-West thru the night.
Attendant cold front positioned over the Mississippi Valley at the
start of the short term period crosses the forecast area Sunday
night. Synoptic scale lift/deep layer forcing along and ahead of
the frontal zone will result in an ongoing and organized QLCS that
advances eastward through our forecast area overnight Sunday
night/early Monday morning. Storms moving within an anomalously
high, deeply moist airmass, with precipitable water values locally
enhanced over 2" supports potential for heavy rainfall. Amounts
between 2-4" with locally higher totals possible. Locations
generally along and west of the I-65 corridor remain most favored
for severe weather potential Sunday evening where a 40-50 knot low
level jet will be aligned with up to 1500 J/KG of available
MLCAPE. Damaging winds and possibly a few tornadoes embedded
within the line will be the primary severe weather hazards, though
marginally severe hail cannot be ruled out. The severe weather
risk may decrease with time late Sunday night as instability wanes
and 850 mb flow gradually veers and decreases, but will monitor
for a few embedded strong to marginally severe storms in addition
to locally heavy rainfall as convection spreads east overnight.

Front makes passage Monday morning with potential of lingering
showers/storms over the eastern zones before chances quickly drop
off. Quiet weather conditions are expected Tuesday, after the
passage of Monday`s cold front. Highs Monday 75 to 80 much of the
area and mid 80s interior to near 80 coast for Tuesday. Tuesday
morning lows coolest over the interior, dipping into the lower to
mid 50s while the coast will be more mild, in the upper 50s/lower
60s. /10

LONG TERM /Wednesday Through Saturday/...Overall, continuity will
be maintained in the longer term. Another storm system deepens as
it heads from the Rockies into the Plains. Downstream amplification
of the upper flow should aid in bringing the weakening front
(over the Gulf) back northward into our area along with return
of deepening layer moisture profiles. In addition, we also expect
the development of another frontal system to our north and west.
All-in-all, it looks like wet weather conditions are again
expected for mid to late week. Showers/thunderstorms will likely
begin during the day Wednesday. It looks like most of the rain
should fall over the region Thursday and Thursday night as the
synoptic scale system traverses the region. At this stage, we
can`t rule out the possibility of heavy rainfall with this storm
system, although integrated water vapor transport anomalies are
not nearly as great as they are with tomorrow`s storm system.

The cold front should move through the region Thursday/Thursday
night, with drier air advecting into the region in wake of the
front. Even with the drier airmass moving into the area, enough
synoptic scale forcing should remain from the upper system to allow
for a small possibility of lingering showers Friday over the
zones before ending. Daytime highs in the lower to mid 70s trend
much below seasonal normals Thursday thru Saturday. Overnight lows
could possibly settle into the upper 40s by Saturday morning over
the interior. /10

MARINE...A strong onshore flow will continue ahead of an
approaching storm system through Sunday night, and Small Craft
Advisories remain in effect through the weekend. Adverse conditions
are expected Sunday through early Monday morning as a strong storm
system brings the possibility of strong to severe thunderstorms
ahead of a cold front. The front should move east of the region
Monday, allowing conditions to gradually improve. Expect diminishing
offshore winds and subsiding seas for Monday and Tuesday. An onshore
flow should return by Tuesday night as another storm system
approaches from the west for mid to late week. /22

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...Wind Advisory from 7 AM Sunday to midnight CDT Sunday night for
     ALZ051>060-261-262.

     Wind Advisory until midnight CDT Sunday night for ALZ263>266.

     Coastal Flood Advisory until 4 AM CDT Monday for ALZ263>266.

     High Rip Current Risk through late Sunday night for ALZ265-266.

     High Surf Advisory until 4 AM CDT Monday for ALZ265-266.

FL...Wind Advisory until midnight CDT Sunday night for FLZ201>206.

     Coastal Flood Advisory until 4 AM CDT Monday for FLZ202-204-206.

     High Rip Current Risk through late Sunday night for FLZ202-204-
     206.

     High Surf Advisory until 4 AM CDT Monday for FLZ202-204-206.

MS...Wind Advisory from 7 AM Sunday to midnight CDT Sunday night for
     MSZ067-075-076-078-079.

GM...Small Craft Advisory until 10 AM CDT Monday for GMZ650-655-670-
     675.

     Small Craft Advisory until 4 AM CDT Monday for GMZ630>632.

     Small Craft Advisory from 6 AM Sunday to 4 AM CDT Monday for
     GMZ633>635.

&&

$$

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