Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 221736

Area Forecast Discussion
National Weather Service Mobile AL
1236 PM CDT Sat Jul 22 2017

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


18Z issuance...Scattered SHRA/TSRA are beginning to develop near
the coast, within the vicinity of KMOB/KBFM/KPNS. We have started
off the forecast with VCTS mention and will monitor trends closely
for potential amendments if convection approaches the local
terminals. Additional development will continue farther inland
through the afternoon. Localized MVFR or lower conditions and
gusty winds can be expected with the stronger storms. VFR
otherwise expected this afternoon with south winds gradually
increasing to 5-10 knots. /21


.PREV DISCUSSION... /issued 1133 AM CDT Sat Jul 22 2017/

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

UPDATE...The overall forecast is in good shape with only minor
adjustments to high temperatures today.

A mid to upper level low continues to slowly move westward across
southern LA and the adjacent northern Gulf of Mexico this morning
on the southern periphery of an upper level ridge of high
pressure stretching eastward from the central Plains states to the
Mississippi and Tennessee Valley regions. Most of the convection
has remained well offshore the past few hours, but visible
satellite imagery is beginning to show a nice cumulus field
developing in the warm/moist airmass over inland areas as of 16Z,
along with radar beginning to show isolated convective cells over
a few spots. We expect storms to become more scattered in coverage
over inland areas as the day progresses, and the current POP
forecast handles this well with 30% coverage indicated near the
immediate coast, and 40-50% inland by this afternoon. We will
continue to monitor for locally strong to marginally severe storms
capable of producing strong wind gusts, prolific lightning, and
very heavy downpours this afternoon, as MLCAPE values locally
range as high as 2000-3000 J/KG.

In addition to the storm potential, heat will be a continued
impact over the region today. Observations at 11 AM show
temperatures in the upper 80s to lower 90s, with dewpoints in the
lower to mid 70s. With temperatures already at or a degree or two
above forecast max readings, we have adjusted the high temperature
forecast upward slightly near the coast. Max heat indices will
range between 100-105 this afternoon, so those spending periods of
time outdoors today should exercise caution. /21

PREV DISCUSSION... /issued 622 AM CDT Sat Jul 22 2017/

DISCUSSION...Updated for latest aviation discussion below.

12Z issuance...Ongoing showers and thunderstorms near the coast
are creating locally MVFR cigs early this morning. Convective
activity is expected to increase in coverage and intensity as it
spreads inland through the remainder of the morning and afternoon
hours, before tapering off after sunset. Expect locally lower
cigs/vis, gusty winds, and frequent lightning in and around the
stronger storms. Outside of convection, VFR conditions and light
south to southwest winds prevail through the period. /49

PREV DISCUSSION... /issued 350 AM CDT Sat Jul 22 2017/

NEAR TERM /Now Through Saturday night/...A weak mid-level low
retrograding along the southern periphery of a broad ridge over the
central Plains is progged to settle over the Texas/Louisiana border
through the near term period. With the low now to our west, weak
upper-level ridging builds over the local area from the north, with
the associated subsidence maintaining a layer of relatively dry air
aloft. All the while, a surface ridge extending from the western
Atlantic into the central Gulf will keep surface and boundary layer
flow generally out of the south and southwest. Given this setup,
we`re once again expecting scattered shower and thunderstorm
activity to develop across the area today, with storms initially
firing offshore and along the coast in the morning hours before
spreading inland as the day progresses. Ample instability (MLCAPE
values around 2500 J/kg), coupled with the aforementioned layer of
dry air aloft and lingering mid-level height falls from the
retrograding low, could allow for a few storms to become strong to
marginally severe this afternoon. These stronger storms will be
capable of producing gusty downburst winds, on top of the typical
summertime threats of frequent cloud-to-ground lightning and locally
heavy rainfall. With decent steering flow still in place overhead,
not expecting much of a flooding threat aside from some possible
minor nuisance flooding in urban areas. Convective activity over
inland areas will once again taper after sunset, with a few
lingering showers and storms possible along the immediate coast and
offshore overnight tonight.

In terms of heat, still expecting to see afternoon temps in the low
90`s inland and upper 80`s to around 90 along the coast, with heat
indices generally reaching 100 to 103 degrees. Muggy low temps
continue tonight, with lows in the mid 70`s inland to upper 70`s
along the immediate coast. /49

SHORT TERM /Sunday Through Monday night/...An upper trof over the
Great Lakes region advances into the interior eastern states
through Sunday night, and while this feature moves off into the
extreme northeast states later in the period, shortwave energy
phases to form a shortwave trof over the southeast states on
Monday night. The shortwave trof will have a favorable impact on
convective development potential later in the long term period,
while in the meantime expect a series of modest shortwaves to move
across the region in the light westerly flow aloft as a surface
ridge over the northern/central Gulf maintains a light south to
southwest flow over the area. This pattern will continue to
promote scattered convection developing each day and lingering
into the evening hours, and some strong storms will be possible,
producing gusty winds, frequent lightning and heavy downpours.
Highs will be in the lower 90s with lows mostly in the mid 70s
except for upper 70s at the coast. /29

LONG TERM /Tuesday Through Friday/...The shortwave trof over
the southeast states shifts slowly eastward through Wednesday then
is absorbed into a larger scale upper trof which evolves over the
eastern states on Thursday into Friday. A surface low well to the
northeast brings a weak surface trof through the area on Tuesday,
and with the presence of the shortwave trof, expect scattered to
numerous showers and storms to develop.  Scattered convective
development is expected for each following day due to daytime
heating and the sea breeze as the axis of the shortwave trof
moves east of the area and is absorbed into the evolving larger
scale upper trof.  Highs will be predominately in the lower 90s
with lows ranging from the mid 70s inland to the upper 70s at the
coast. /29

MARINE...Expect showers and thunderstorms to continue forming over
the marine area during overnight and morning hours before moving
inland through the afternoon. These storms will be capable of
producing gusty straight line winds and frequent cloud to water
lightning. Expect locally higher waves in and around storms as well.
Outside of these storms, south to southwest winds around 8 to 13
knots and seas around a foot prevail through tonight. Winds then
shift to more westerly around 10 to 15 knots Sunday through early
next week, with seas building to around 2 to 3 feet during that
period. /49




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