Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 280619 AAA
AFDMOB

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
119 AM CDT Tue Mar 28 2017

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

&&

.AVIATION...
06Z issuance...MVFR to IFR cigs and visibilities through about
28.14z followed by mostly VFR conditions through about 29.03z then
MVFR to IFR cigs and visibilities through 29.06z. Lower cigs and
visibilities mostly from low stratus and patchy fog this morning
and again late tonight and early wed morning. Winds will be
mostly south at 5 knots or less early this morning then 8 to 13
knots with gusts to around 20 knots during the late morning and
afternoon hours diminishing to 3 to 5 knots by mid evening. 32/ee

&&

.PREV DISCUSSION... /issued 958 PM CDT Mon Mar 27 2017/

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

UPDATE...Upstream deep convection on the wane as forecast. Will
have to watch because there are still some marginal strength
updrafts updraft for SPS/sub-severe criteria. Those updrafts will
continue moving very slowly east in the flow until they weaken
altogether in the next few hours. MLCAPES down 200-600 J/KG from
3 hours ago. Did have one multicell with a large weak echo region
/ very reflective elevated mid-level reflectivity core that moved
across Wayne Co., MS and into Choctaw/Washington Co., AL border
region. Getting some verification from that where we suspect hail
diameters from between 1-1.5" diameter. We appreciate the reports
received thus far.

Did a quick update to the grids and text products to better
describe the fog evolution overnight. Also left a slight rain
chance to the north through midnight. Finally, removed headline
for high rip current risk (see earlier discussion).

MARINE...42012 is back on-line (now with dewpoint). Significant
wave heights across the marine area generally anywhere from 2-3
feet with wave periods 5-6 sec. No changes to marine package
overnight.

PREV DISCUSSION... /issued 628 PM CDT Mon Mar 27 2017/

DISCUSSION...See updated information below.

UPDATE...Main impact to our area next few hours will be broken
line of multicells-supercells now moving into NW corner of our
area (Choctaw Co., AL). These updrafts will be capable of
supporting severe hail (1-1.5" in diameter) and possibly damaging
straight line winds. Developing updrafts further SW, to affect our
interior SE MS counties within the hour. As thought all along,
Choctaw- Wayne, Clarke and Wilcox stand the greatest chances of
experience an isolated severe thunderstorm through mid- evening.

The line will proceed east early this evening and main impact
will be large hail and possible damaging straight line winds.
MLCAPEs in ambient environment ahead of line range from 750 J/KG
NW zones to near 1500 J/KG closer to the coast. Instability trend
is down past few hours, of course given time of day. Nearly 35-40
kt of bulk shear supporting both multicells (with observed new
cell growth on low-level SW flank) and occasional supercells
(observed deep persistent rotation where propagation forces storm
motion to right, temporarily). Latest guidance shows these storms
will be affecting our region with waning intensity through 9-10 PM
and that should just about do it.

Will allow High Risk of rip currents to finally go to `Moderate`
around dark, thus RP.S will expire.

Did make some tweaks to grids to reduce rainfall amount for
interior sections tomorrow afternoon. /23 JMM

AVIATION...
00Z issuance...Pretty much a forecast of persistence overall, as
with last night. With sunset, will see mainly stratus rolling
inland again by 03Z from S to N with mainly CIGs falling into
IFR-LIFR range through midnight. Condition persists with
occasional sfc vsbys at some TAF sites dropping below 1 SM from
6-10 UTC. After sunrise, VFR conditions by 15 UTC regionwide.
Tomorrow afternoon, cloud streets will keep some occasional broken
MVFR CIGs around int he afternoon. Light S winds with low end
gusts subsiding by 02 UTC and resuming after 16 UTC tomorrow,
especially near the coast. /23 JMM

PREV DISCUSSION... /issued 413 PM CDT Mon Mar 27 2017/

NEAR TERM /Now Through Tuesday/...Thunderstorms developing over
central MS to northern and central LA within an airmass
characterized by increased afternoon instability (MUCAPES having
increased to 2000-3000 J/KG). Latest high resolution HRRR is more
robust on carrying activity eastward, showing a series of
organized convective clusters passing over the interior zones this
evening. A few of the storms could produce marginally severe hail
and localized damaging wind gusts. The other guidance, NSSL WRF-
ARW and the 3 KM NAM are not as pronounced with their respective
solutions. Late afternoon and evening updates to storm coverage
over the interior will likely be required depending on convective
trends upstream.

Upper level storm system/trof axis over the Mid-South to begin
the near term, makes steady eastward progression across the
Appalachians Tuesday morning. A narrow zone of deep layer moisture
(pwats around 1.25") hangs back across the interior tonight and
provides enough moisture to maintain a small chance of
showers/storms over the northwest zones late. Surface pattern
remains mostly unchanged. A ridge of surface high pressure off the
Mid-Atlantic, ridging southwest into the central Gulf maintains a
persistent, light southerly flow and dewpoints well into the 60s.
This synoptic pattern favors the development of overnight fog
which could become dense in some areas. Overnight lows in the
lower to mid 60s. Despite, short wave mid-level ridge moving
eastward across the forecast area Tuesday, east to west zone of
deep layer moisture holds which supports the potential of showers
and storms during daytime heating when instability is better.
Though, with the presence of the upper ridge and its associated
sinking vertical motions to act as a counter to convective
development, will only keep probability of showers and storms in
slight chance ranges Tuesday. Tuesday`s highs in the lower to mid
80s interior and mid to upper 70s beaches. /10

SHORT TERM /Tuesday night Through Thursday night/...A significant
weather event is forecast for Thursday and Thursday night as a
strong storm system approaching the region from the west moves
across the area. A 100-120 knot 300 mb jet max over the far
western conus will carve out a highly amplified upper level
trough over the southern Rockies and northern Mexico by Tuesday
evening, with an embedded closed low forming over the desert
southwest. This upper trough and low pressure area will then
advance eastward over the southern Great Plains midweek. The
closed low is forecast to be over eastern Oklahoma Thursday
morning, take a turn to the northeast, reaching extreme southern
Illinois by late Thursday night. Meanwhile the southern portion of
the trough is expected to swing quicker to the east, aided by a
90 knot jet streak, and possibly become slightly negatively tilted
over the region late Thursday night.

A surface low pressure area is forecast to develop near the north
Texas/southwest Oklahoma border Tuesday night ahead of the
advancing upper level trough, and move east over eastern Oklahoma
by Thursday morning. This surface low will also take a turn to
the northeast, also reaching extreme southern Illinois by late
Thursday night. An associated cold front approaching the region
from the west Thursday evening will sweep through the forecast
area after midnight.

A deep southerly wind flow ahead of the system will advect
moisture northward across the region, with precipitable water
values climbing to between 1.5 to 1.7 inches Thursday afternoon
and evening. Both the ECMWF and GFS models are advertising MLCAPE
values between 500 to 700 J/KG on Thursday, with SFC-1km storm
relative helicity values ranging from 100 to 200 m2/s2. A 30 to
40 knot low level jet is also expected to accompany this system.
Upper level lapse rates will be in the 7 to 7.5 c/km range. The
pre-storm environment will favor the development of strong to
severe thunderstorms throughout the day Thursday into Thursday
night as a line of numerous storms advances east through the
region. Damaging winds, large hail, and isolated tornadoes are all
possible with this system. Heavy rainfall will also occur with
widespread 1 to 2 inches likely.

Otherwise it will be dry Tuesday night through midweek as an
upper level ridge moves east over region, along with a surface
high pressure ridge across the southeastern states and eastern
gulf remain largely intact.

Low temperatures each night will range from 58 to 62 degrees
inland areas, with mid 60s along the beaches. Warm temperatures
will continue on Tuesday and Wednesday with highs ranging from 80
to 85 degrees inland areas, with mid to upper 70s along the
coastal sections. /22

Low temperatures each night will range from 58 to 63 degrees
inland areas, with mid 60s along the beaches. Warm temperatures
will continue Wednesday with highs ranging from 81 to 86 degrees
inland areas, and from 75 to 80 degrees along the coast. /22

LONG TERM /Friday Through Monday/...The precipitation will then
taper off from west to east late Thursday night and early Friday
in the wake of the cold front, followed by high pressure building
in from the west. The dry period will then persist through the
remainder of the week as an upper ridge and surface high pressure
dominate the southeast states. Precipitation chances will increase
once again early next week as yet another upper level trough
develops to our west and advances east over the southern plains,
with scattered showers and thunderstorms forecast for Sunday
afternoon through Sunday night. Above normal temperatures will
continue through the long term. /22

MARINE...High pressure to the east maintains a light onshore
component of flow through the middle of the week with seas 2 to 4
feet in range. A stronger onshore flow and resultant fetch brings
higher seas Thursday ahead of a frontal boundary approaching from
the west. Increased coverage of showers and storms are expected
as this weather system approaches, with a few strong to severe
storms possible late Thursday. Winds decrease and seas show a slow
subsiding trend going into the upcoming weekend. /10

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...None.
FL...None.
MS...None.
GM...None.
&&

$$

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