Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 230015

Area Forecast Discussion
National Weather Service Mobile AL
715 PM CDT Sat Jul 22 2017

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


23/06Z issuance...Isolated showers and lingering thunderstorms
over interior portions of southeast MS and southwest AL early
this evening, dissipating by mid evening. Will likely see some
redevelopment of isolated showers and storms near the coast late
tonight, primarily after around 23/09z Sunday. Scattered showers
and storms then expected to develop/spread inland during the
course of the day on Sunday. Localized MVFR or briefly lower
ceiling/visibility conditions, along with gusty winds can be
expected in and near any of the stronger storms that develop.
Otherwise, VFR conditions expected through most of the period for
most locations. Light and variable winds tonight, expect light
south near the coast, becoming light predominate southerly up to
around 10 knots Sunday afternoon. 12/DS


.PREV DISCUSSION... /issued 401 PM CDT Sat Jul 22 2017/

NEAR TERM /Now Through Sunday/...An upper level low continues to
drift westward across southern LA and the adjacent northern Gulf of
Mexico on the southern fringe of an upper level ridge of high
pressure stretching eastward from the Plains to the Tennessee
Valley. The combination of plentiful moisture (precipitable water
values between 1.7" and 2.0") and typical summertime instability
(MLCAPE values ranging between 1500-2500 J/KG) has resulted in
development of scattered showers and thunderstorms mostly along the
advancing seabreeze this afternoon. A few storms have occasionally
pulsed up strong and have been capable of producing localized wind
gusts over 40 mph, frequent lightning strikes and very heavy
downpours. There will still be opportunity for additional isolated
to scattered storms to develop farther inland through the remainder
of the afternoon. Otherwise, typically hot and humid conditions
prevail outside of convection, with temperatures in the lower 90s
and dewpoints in the lower to mid 70s producing heat indices between

Convection should gradually diminish in coverage over inland areas
this evening with loss of daytime heating, though isolated storms
may persist through 9-10 PM. A lull in activity could then occur for
a few hours before additional isolated to scattered showers and
storms develop near the coast again sometime after midnight. Will
maintain 20-30% POPs over southern areas, with the highest coverage
drawn near the immediate coast and beaches after 06Z. Overnight lows
will range in the lower to mid 70s inland and 75-80 near the
immediate coast and beaches.

Our forecast area will remain between the upper low/inverted trough
extending from the western Gulf of Mexico to southern LA and another
upper trough digging toward the eastern CONUS Sunday. Plentiful
moisture and instability over our area will support another round of
scattered showers and storms developing from south to north through
the day along the seabreeze. Will have POPs remaining in the high
end scattered coverage range (40-50%) over most of the area. A few
storms may occasionally pulse strong with gusty winds, frequent
lightning and locally heavy rainfall given the moist and unstable
environment. Highs Sunday should range from the lower to mid 90s
inland, and around 90 near the immediate coast and beaches. Max heat
indices generally range between 100-103, perhaps locally as high as
around 105. /21

SHORT TERM /Sunday night Through Tuesday night/...Sunday night
through Monday, a combination of an upper ridge that has built
west over the Gulf of Mexico and an upper high over the NM/Tx
border keep an upper trough stationary and tilted over the Lower
Mississippi River Valley to the southern Appalachians. A surface
ridge that has developed over the central Gulf of Mexico keeps a
steady south to southwesterly flow over the Lower Miss river
Valley and Southeast, keeping deep layer moisture high (precip h20
levels around 2"). In combination with upper dynamics from the
upper trough and a daily Gulf breeze expected to move inland, rain
chances around (near the coast) to a bit above (inland towards
the Miss River Valley closer to the better upper support) are
expected. With a bit of upper subsidence from the Gulf ridge
counter balancing the better than seasonal rain chances, temps a
bit above seasonal expected at night (general low to mid 70s) ,
around seasonal during the day (upper 80s to low 90s).

Monday night through Tuesday night...A shortwave trough swinging
east over the northeastern Conus flattens and pushes south the
Gulf ridge, allowing the upper trough to the west of the forecast
area to tilt a bit more (GFS) or shift east a bit (ECMWF).
Consensus leans the current package towards the GFS in this time
frame. With the upper trough tilting, but remaining north of the
forecast area, a cooler/wetter to warmer/drier gradient sets up
over the Southeast. For our little piece of the Southeast, this
means a tighter diurnal temp range over the northern half of the
forecast area with better than seasonal rain chances, warmer and
a bit drier over the southern half (around seasonal temps and


LONG TERM /Wednesday Through Saturday/...In the extended, the
disparity amongst the guidance continues. The GFS is advertising
the upper trough moving off to the east quickly Wednesday into
Wednesday night, allowing the upper ridge centered over the NM/Tx
border to build east over the Southern Plains to the Lower
Mississippi River Valley. The ECMWF is advertising the upper
trough continuing to shift east, to the East coast. Both are
advertising more shortwave energy diving into the upper trough
from over the Plains, helping to maintain it. Both are advertising
a decrease in PoPs through the latter half of the week, the GFS
especially so. The GFS also is advertising a warmer scenario due
to the better eastward extent of the wester upper high. Have went
with a blend in the extended, with PoPs tapering downward to below
seasonal and temps moderating upward to a bit above seasonal,
with Thursday/Friday being the driest and warmest. Saturday, will
see a return of better rain chances along with around to a bit
below seasonal temps as shortwave energy moves towards the
forecast area from the northwest.

MARINE...The persistent pattern of showers and thunderstorms
forming over the marine area mainly during overnight and morning
hours before moving inland through the afternoon will continue into
early next week. A few storms will be capable of producing gusty
straight line winds and frequent lightning, and locally higher waves
and seas. Otherwise, expect a light to moderate southwesterly to
westerly wind flow to prevail over the marine area into next week as
a ridge of high pressure stretches westward over the central Gulf of
Mexico. /21




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