Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 281127 AAA
AFDMOB

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
627 AM CDT Fri Apr 28 2017

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

&&

.AVIATION...
12Z issuance...MVFR to IFR cigs and visibilities through 28.15z
this morning, followed by mostly MVFR cigs through 29.03z,
followed by possible IFR cigs and visibilities through 29.12z.
Scattered showers will also be possible through 28.18z. Winds will
continue from the southeast at around 10 knots through 28.15z
increasing to around 16 knots with gusts from 20 to 25 knots this
afternoon and early this evening diminishing to 12 to 16 knots
through 29.12z. 32/ee

&&

.PREV DISCUSSION... /issued 453 AM CDT Fri Apr 28 2017/

NEAR TERM /Now Through Friday night/...A broad mid to upper trof
over much of the western half of the country will dig over the
southwest conus through tonight with upper ridge amplifying along
the se Atlantic coast stretching well offshore over over the
adjacent Atlantic waters through Sat morning. Near the sfc a broad
ridge of high pressure will continue to stretch from the western
Atlantic well off the Carolina coast to the north central gulf
coast through Saturday morning combined with a developing cold
front drifting eastward over much of western Texas. With this
pattern the main concern through tonight will be an increasing
southerly wind flow over the entire forecast area today and
tonight with winds gust from 20 to 25 knots occurring along the
immediate coast beginning this afternoon and continuing through
most of the tonight. With the strong onshore flow low level
moisture content will continue to steadily increase from south to
north over the forecast area resulting in surface dewpts in the
lower to middle 70s for most locations in the forecast area by
late this afternoon and this evening leading to very humid/muggy
conditions going into the first part of the weekend. Low status
clouds and patchy fog will also be likley early this morning
followed by a broken deck at around 2.5kft through early this
evening then lowering again late this evening continuing through
tonight. Daytime temps will also be modified somewhat with high
ranging from the middle 80s for most inland areas and the upper
70s to lower 80s along the coast. Lows tonight will continue to be
on the warm side ranging from the lower to middle 70s for most
inland areas and the middle 70s along the coast. 32/ee

SHORT TERM /Saturday Through Sunday night/...A southerly to
southwesterly mid level flow pattern will persist across the
forecast area Saturday and Saturday night between a strong ridge
of high pressure centered over the western Atlantic Ocean and the
next vigorous upper level storm system that will be moving from
the vicinity of the Four Corners Region early Saturday morning
toward the southern Plains states through Saturday night. Breezy
southeasterly to southerly surface flow will persist across our
forecast area Saturday into Saturday night as there will be a
tight MSLP gradient between a surface ridge of high pressure over
the southeast states and the deepening low pressure system over
the southern Plains. The southerly onshore flow will keep a very
warm and moist airmass in place across our forecast area through
Saturday night, with continued high surface dewpoints in the upper
60s to lower 70s. A rather dry deep layer airmass will remain in
place Saturday morning, but a gradual moistening is expected to
occur over southeast MS and adjacent southwest AL by Saturday
afternoon, with precipitable water values forecast to improve to
around 1.25" to 1.5". Weak impulses embedded in the southwest
flow may interact with the available moisture and instability and
perhaps enough heating to erode the cap to aid in the development
of isolated to scattered showers and thunderstorms Saturday
afternoon. Highs Saturday are forecast to be quite warm, ranging
from the mid 80s to around 90 inland, with readings along the
immediate coast ranging from around 80 to the lower 80s. High
surface dewpoints and continued elevated southerly flow will keep
overnight lows warm Saturday night, with readings around 70 inland
to the lower to mid 70s near the coast.

The forecast for the rest of the short term period will focus on a
potential severe weather and heavy rain event to impact much of
the area Sunday into Sunday night. The upper level low over the
Plains states will eject northeastward across OK/KS during the day
Sunday, then across the Upper Midwest Sunday night. A cold front
will spread eastward from the vicinity of far eastern OK/TX Sunday
morning to the Mississippi Valley by late Sunday afternoon, then
eastward across the Tennessee Valley and central Gulf Coast region
Sunday night into Monday morning. Deep layer forcing, along with
enhanced deep layer moisture along the frontal zone will support
the development of an organized QLCS that will spread eastward
from the Mississippi Valley Sunday and across much of our region
late Sunday afternoon into early Monday morning. The latest
forecast will continue to favor a slower progression of the
linear complex Sunday into Sunday night, with expectation that the
line will begin to impact our far western zones (i.e. southeast
MS) by mid to late Sunday afternoon, with a gradual eastward
progression across the forecast area Sunday evening into the
overnight Sunday night/early morning hours Monday. We have
adjusted POPs and rainfall amounts accordingly in coordination
with surrounding WFOs and WPC. Plentiful deep layer moisture with
pre-storm environmental precipitable water values up to around
1.75" will be in place, with indications of precipitable water
values locally enhanced to between 2.00" and 2.40" inches along the
lines per the GFS and NAM. This deep moisture will favor locally
heavy rainfall across the region, with the heaviest amounts expected
along and west of the I-65 corridor. Rainfall amounts between 2" to
4" with localized higher amounts will be possible, along with
localized flooding also possible.

Severe weather is also a concern, particularly Sunday afternoon
into Sunday evening as a 40-50 knot low level jet strengthens
along and ahead of the line, and deep layer shear around 50 knots
overspreads the region. MLCAPE values will also average between
1000-1500 J/KG along and ahead of the line. The primary concerns
will be damaging straight line winds within the line, and perhaps
some marginally severe hail. 0-1km storm relative helicity between
200-400 m2/s2 will also be supportive of a few tornadoes within
the line (or possibly within any discrete cells that manage to
develop ahead of the line). The latest Day 3 outlook from SPC
advertises a slight risk of severe weather across our western
zones along the line Sunday afternoon and evening, but will need
to monitor for a possible eastward expansion into Sunday night as
the environment could remain supportive of a few continued severe
storms late. /21

LONG TERM /Monday Through Thursday/...The back edge of convection
may finally exit eastern portions of our forecast area by mid to
late Monday morning. Dry weather returns Monday and Tuesday in the
wake of the frontal passage. Temperatures will also average slightly
cooler Monday. Scattered to numerous showers and thunderstorms
return to the forecast Wednesday into Thursday as the next upper
level trough moves across the region. This system will bring
potential for more locally heavy rainfall, and possibly a few strong
storms. We will continue to monitor closely. /21

MARINE...Southerly winds and seas will begin to increase today and
continue through early Sunday evening in response to a strong upper
level system and surface cold front approaching from the west,
combined with a deep ridge of high pressure continuing along the
eastern seaboard. Seas will build to 8 to 10 feet from 0 to 60 nm
out by late Sat continuing into Sun. As a result a Small Craft
Advisory has been issued for the offshore waters tonight and the
entire marine area by early Sat morning continuing through Sun
evening. A light northwest flow is expected in the wake of the front
developing by early Mon morning, continuing through early Tue
morning, then shifting north to northeast by mid morning on Tue. A
light to moderate onshore returns by late Tue continuing through
midweek ahead of another week cold front approaching from the west.
Showers and thunderstorms are expected with this pattern with mostly
scattered coverage early Sat, then numerous coverage moving in from
the west by Sun afternoon and Sun evening. A few strong to severe
thunderstorms will be possible mainly on Sunday.  32/ee

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...High Rip Current Risk through late Sunday night for ALZ265-266.

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

MS...None.
GM...Small Craft Advisory from 7 AM Saturday to 10 PM CDT Sunday for
     GMZ630>635.

     Small Craft Advisory from 10 PM this evening to 1 AM CDT Monday
     for GMZ650-655-670-675.

&&

$$

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