Scientific Forecaster Discussion
fxus64 kmob 291145
Area forecast discussion
National Weather Service Mobile Alabama
645 am CDT Sat Apr 29 2017
Discussion...updated for latest aviation discussion below.
12z issuance...IFR cigs and localized MVFR visibility reductions
start off this forecast. MVFR cigs possible at times through the
rest of the day, though scattering may occur later this morning
into this afternoon. Strong southeasterly to southerly flow
develops later this morning into the afternoon with sustained wind
speeds around 20 knots with some gusts up to 30 knots. /21
Previous discussion... /issued 453 am CDT Sat Apr 29 2017/
Near term /now through Saturday night/...
a deep layer southerly flow is in place across our forecast area
early this morning between a ridge of high pressure positioned
across the southeastern states and the adjacent western Atlantic
Ocean and a deep low pressure system over the southwestern U.S.
Satellite/radar/lightning data show an impressive expansive complex
of deep moist convection along a frontal zone extending from West Texas
and northeastward across OK and into the Missouri, mid-Mississippi
and Ohio River valley regions. Weather conditions are quiet locally
across our forecast area early this morning, with warm and moist
southerly low level flow bringing expansive low stratus to the
region, along with some patches of fog over our eastern zones.
The deep upper level low in the vicinity of The Four Corners region
early this morning is forecast to translate eastward toward the
Texas/OK panhandles by this evening, while the persistent upper level
ridge of high pressure holds from the western Atlantic through the
Florida Peninsula and northwest Caribbean. A southwesterly mid level flow
pattern will remain over our forecast area today between these
features. A dry deep layer airmass will initially be in place across
much of our area this morning, but precipitable water values quickly
improve to around 1.5-1.6 inches over southeast MS and adjacent
southwest Alabama by this afternoon. Weak ascent may result in the
development of an isolated shower or two over southeast MS or
adjacent southwest Alabama today, but rainfall amounts of any would be
very light. The biggest story for today will be increasing
southeasterly to southerly winds across our region as the mslp
gradient tightens between the ridge of high pressure over the
western Atlantic and the deepening low pressure system well to our
west. Sustained wind speeds between 15-25 mph, with frequent gusts
over 30 mph look likely over much of the cwa, particularly from mid
morning through the late afternoon or early evening, and will expand
the Wind Advisory to include the entire area today. A Wind Advisory
will be allowed to continue along the coast tonight as gusty winds
may continue through the overnight hours. Highs today will be quite
warm with readings in the mid 80s to around 90 inland, with upper
70s to lower 80s expected along the immediate coast and beaches.
Lows tonight should range from around 70 to the mid 70s. /21
Short term /Sunday through Monday night/...
a heavy rainfall and severe weather event still looks on track to
impact much of our forecast area during the Sunday afternoon though
Sunday night time frame. The upper level low is forecast to eject
northeastward toward the upper midwestern states through Sunday
night, while an associated cold front advances eastward from western
Arkansas through the southeast Texas coast early Sunday morning and eastward
through the Tennessee Valley and central Gulf Coast region by late
Sunday night and early Monday morning. Synoptic scale lift/deep
layer forcing along and ahead of the frontal zone will result in the
development of an organized qlcs that will advance eastward from the
Mississippi Valley early Sunday morning and through our forecast
area from late Sunday afternoon through overnight Sunday night/early
Monday morning. In terms of timing, we expect the line to initially
impact southeast MS by Sunday afternoon, with a gradual advancement
into adjacent southwest Alabama Sunday evening, and then generally along
and east of the I-65 corridor through late Sunday night. A deeply
moist airmass with precipitable water values locally enhanced over
2" along the line will bring potential for heavy rainfall with
amounts between 2-4" with locally higher totals still looking
possible especially along and west of I-65. We will continue to
mention the heavy rain and marginal flooding threat in the severe weather potential statement.
Locations generally along and west of the Interstate 65 corridor
remain most favored for severe weather potential Sunday afternoon
and 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 will also be
possible. The severe risk may decrease with time late Sunday night
as instability wanes and 850 mb flow gradually veers and decreases,
but we will still need to monitor for a few embedded strong to
marginally severe storms in addition to locally heavy rainfall
In terms of other hazards, another Wind Advisory may become
necessary over all inland areas again Sunday as strong southerly
flow redevelops ahead of the approaching front. 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 mean sea level range near times of
high tide). /21
The cold front should move east of the region by Monday night as the
upper low ejects into the Great Lakes region. Expect showers and
thunderstorms to decrease in coverage and intensity from the west
Monday afternoon. /Butts/
Long term /Tuesday through Friday/...
quiet weather conditions are expected Tuesday, after the passage of
monday's cold front. That should be short-lived, though, as another
shortwave trough 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. In addition, we also expect the development of another frontal
system to our north/west. All-in-all, it looks like wet weather
conditions are again expected for mid- to late week.
Medium-range ensembles appear to be in decent agreement with regard
to the overall solution, with mainly temporal discrepancies noted
within the individual members. Showers/thunderstorms will likely
begin during the day Wednesday, as isentropic forcing at the 310k
level increases over the region. 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
the small possibility of lingering showers Friday morning before
ending. Cooler temperatures will then be seen heading into next
weekend after the passage of this storm system. /Butts/
now through tonight...small craft advisories remain in effect.
Adverse conditions will continue over the marine area, as a strong
onshore flow continues ahead of an approaching storm system. Daytime
heating of a moist airmass may aid in the development of a few
showers or thunderstorms over (mainly) Mobile Bay and the
Sunday through Wednesday...a strong storm system will result in a
continuation of adverse marine conditions through Sunday, with the
possibility of strong to severe thunderstorms. 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
Alabama...Wind Advisory until 7 PM CDT this evening for alz051>060-261-262.
Wind Advisory until midnight CDT Sunday night for alz263>266.
High rip current risk through late Sunday night for alz265-266.
Coastal flood advisory until 4 am CDT Monday for alz265-266.
High surf advisory from noon today to 6 am CDT Monday for alz265-
Florida...Wind Advisory until midnight CDT Sunday night for flz201>206.
High rip current risk through late Sunday night for flz202-204-
Coastal flood advisory until 4 am CDT Monday for flz202-204-206.
High surf advisory from noon today to 6 am CDT Monday for flz202-
MS...Wind Advisory until 7 PM CDT this evening for msz067-075-076-078-
GM...Small Craft Advisory until 7 am CDT Monday for gmz630>635-650-