Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 242317 AAA

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
617 PM CDT Mon Apr 24 2017

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


00Z issuance...VFR conditions forecast the next 24 hrs. Winds
becoming light to calm tonight. /10


.PREV DISCUSSION... /issued 325 PM CDT Mon Apr 24 2017/

NEAR TERM /Now Through Tuesday/...Deep upper low over the se
conus is progged to move near the GA/SC coast this afternoon,
then shift ne and skirt the Atlantic seaboard through midweek.
This system is followed by a sharp upper ridge moving over the
north central gulf coast region by Tue afternoon. At the sfc high
pressure becomes better developed over the eastern and north
central gulf leading to a light west to southwest flow at the
surface early Tue backing to the southwest and building by mid to
late afternoon. With good ridging aloft and at the surface skies
will be mostly clear tonight followed by plenty of sunshine during
the day on Tue. For tonight winds become mostly light over most
inland areas leading to good radiational cooling by late evening
continuing through around sunrise Tue. With this expect a cool
night again tonight with warmer conditions on Tue compared to
today. Lows tonight will range from the upper 40s to lower 50s for
most inland areas and the mid 50s to around 60 near the immediate
coast. Highs Tue will be in the lower to middle 80s for most
inland areas and the upper 70s to near 80 along the immediate
coast. 32/ee

SHORT TERM /Tuesday night Through Thursday night/...An upper
trof over the Plains takes on a negative tilt while advancing
across the lower/mid Mississippi river valley Wednesday night into
Thursday morning. An associated surface low treks across the
Great Lakes region and brings a cold front across the lower
Mississippi river valley Wednesday night, which stalls over or
just west of the forecast area on Thursday. While relatively dry
deep layer air will be in place over the forecast area Tuesday
night into Wednesday with precipitable water values near 0.75
inches (about 75% of normal), a strengthening low to mid level
southerly flow ushers in abundant Gulf moisture with precipitable
water values approaching 2.0 inches (about 200% of normal) late
Wednesday night. A region of initially strong deep layer lift
shifts into the western half of the forecast area late Wednesday
night then weakens while continuing through the remainder of the
area Thursday morning, in response to the right entrance region of
a 125 knot upper jet coupled with a weakening low level
convergent flow ahead of the decelerating front.

While surface based CAPE values will be fairly high despite the
primarily nocturnal timing of the event, model soundings show near
moist adiabatic lapse rates below 725 mb, with the bulk of the
CAPE located above 725 mb where more favorable lapse rates exist.
Some favorable low level shear is present late Wednesday night
into early Thursday morning, but until lower level lapse rates
improve, this event continues to look more like one capable of
some strong storms with abundant lightning. The greatest potential
for strong storms also looks to be west of I-65 from late
Wednesday night into early Thursday morning, and will highlight
this in the Hazardous Weather Outlook. May also see some locally
heavy rainfall west of I-65 where the best deep layer forcing will
be present along with the anomalously high deep layer moisture.
Dry conditions through Wednesday will be followed by good chance
to likely pops Wednesday night into Thursday, then dry conditions
follow for Thursday night as the stalled frontal boundary lifts
northward as a warm front. Lows Tuesday night range from the upper
50s inland to the lower 60s closer to the coast, then trend much
warmer through Thursday night to range from the mid to upper 60s
well inland to the lower 70s closer to the coast. Highs on
Wednesday and Thursday will be mostly in the mid 80s except for
near 80 at the coast. /29

LONG TERM /Friday Through Monday/...An upper trof over the
central states amplifies significantly through Sunday, then takes
on a negative tilt while advancing into the eastern states on
Monday. An associated surface low over the southern/central Plains
moves off to the Great Lakes region during the period, and brings
a trailing cold front through the forecast area late Sunday night
into Monday. The 850 mb flow could strengthen significantly late
in the period ahead of the approaching front, with speeds of 50-70
knots possible over the forecast area Sunday night into early
Monday morning, although there is plenty of uncertainty regarding
the strength of the low level jet. Should this strong low level
flow materialize, this would result in high shear values, and
surface based CAPE values are currently indicated to range from at
least 1000-1500 J/kg, possibly up to 2500 J/kg, with the highest
values near the coast. The only limiting factor apparent at this
time is that there is the potential for a mid level cap to be
present. After consulting SPC, have included wording in the HWO
for strong to possibly severe storms Sunday night into Monday
morning. Dry conditions on Friday will be followed by slight
chance pops for Saturday then mostly chance pops for Sunday as the
front approaches. Good chance to likely pops follow for Sunday
night, with chance pops on Monday, tempered by concerns regarding
timing and coverage at this point. Highs on Friday and Saturday
will be in the mid to upper 80s, slightly cooler for Sunday, then
trend much cooler for Monday with highs in the mid to upper 70s.
Lows Friday night and Saturday night range from around 70 inland
to the lower 70s closer to the coast, then trend cooler Sunday
night to range from around 60 well inland to the upper 60s at the
coast. /29

MARINE...High pressure will continue to build se over the north
central and eastern gulf through Tue evening then shift east to the
eastern seaboard and western Atlantic later in the week. With this
pattern a moderate northwest flow will continue over the marine area
through early this evening followed by a light north northeast flow
tonight, then a light west to southwest flow early tue. Winds then
shift mostly south and build throughout the week in response to a
weakening cold front approaching from the west, stalling and
becoming diffuse mostly to the west on Thu, followed by a stronger
cold front slowly approaching from the west later in the week and
over the weekend. By late Fri into Sat a strong southerly flow is
expected ahead of the front with seas gradually building to 5 to 7
feet from 0 to 60 nm out Sun into Mon. Showers and thunderstorms are
also expected with this pattern with mostly scattered coverage on
Thu followed by slightly better coverage late Sun into Mon. Small
craft conditions will be likely required by the weekend. 32/ee




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