Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 231127 AAA

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
627 AM CDT Tue Jul 23 2019

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


12Z issuance...Convection this morning has been confined to the
coast with an easterly storm motion of 10 to 15 kts. Given radar
coverage/trends, have vcty of tsra for KMOB/KBFM this morning
while KPNS will include a tempo group thru 23.16Z. As cold front
sinks southward to the coast thru the day, convective coverage is
expected to increase going into the afternoon. Wind shift follows
the frontal passage by evening with coverage of shra/tsra
decreasing. /10


.PREV DISCUSSION... /issued 423 AM CDT Tue Jul 23 2019/

NEAR TERM /Now Through Tuesday night/...Amplifying long-wave upper
trof over the upper Mid-West and OH River Valley dives southeastward
over the eastern US in near term. At the surface, cold front was
draped from off the New England coast, southwest over the southern
Appalachians to the upper TX coast. Leading edge of frontal boundary
was approaching the northwest zones where scattered showers were
dropping southeast out of central MS. Closer to the coast extending
southward offshore, showers have increased in coverage, along with a
few storms. Storm motion generally east northeast around 10 kts.
Considering mesoscale analysis showing an upward trend in normalized
values of low level instability, limited convective inhibition, weak
deep-layer shear and large values of surface cyclonic vorticity,
combined, favor the development of waterspouts this morning. Some
could move onshore before dissipating. The high resolution ensembles
of convective allowing models indicate that the coastal and offshore
zones will see the better coverage initially through at least mid
morning then an increased coverage over the land zones through the
remainder of the day as southward advancing front brings an
enhancement to lift. There is an indication that some areas will see
locally heavy rains, especially along and southeast of I-65 thru the
course of the day and a marginal risk of excessive rain is
outlooked. This results in a limited threat of mainly urban/nuisance
type flooding of lower lying areas subject to poor drainage. In
addition to this, a few storms could become strong producing brief
strong wind gusts and frequent lightning activity. The front is
progged to drop off the coast tonight with chances of wet weather
lowering from north to south.

With thickening cloud cover, increased coverage of showers/storms
and southward moving front, daytime highs well below climatology by
some 5 to 8 degrees, ranging mostly 82 to 86. Coolest lows tonight
over the northwest zones near 65. Elsewhere, values range mostly
from 67 to 72. /10

SHORT TERM /Wednesday Through Thursday night/...A large upper
trof extending from the eastern states into the north central Gulf
steadily weakens during the period while an upper high remains
anchored over the 4 Corners region. Another upper ridge over the
western Atlantic gradually begins to build into the eastern Gulf
meanwhile as the upper trof weakens. At the beginning of the
period, a cold front will be located just off the mid Atlantic
coast, across northern Florida, and extending westward across the
northern Gulf. The frontal boundary lingers generally in place
through Thursday night and gradually weakens. That said, there is
the potential for a surface low to develop somewhere along the
weakening frontal boundary, and this development would occur in a
rather weakly sheared environment (generally <10 knots 1000-400
mb). While nearly all guidance shows no significant development
of a surface low along the boundary, weak boundaries like this
that stall in the Gulf within a weakly sheared environment have
later on sometimes spawned tropical systems. For that reason, the
National Hurricane Center has introduced a 20 percent chance of
tropical cyclone development mainly within the time period
beginning Thursday and lasting into Saturday. Note that this
potential system is not associated with Tropical Depression Three,
currently located between the Bahamas and the Florida peninsula,
which is expected to meanwhile drift northward. For the forecast,
have gone with mostly dry conditions through the period except for
small pops near the coast and will need to closely monitor for
the possibility of a system developing in the Gulf and how this
might affect the area. Highs on Wednesday will be mostly in the
upper 80s then around 90 on Thursday. Lows Wednesday night range
from the mid 60s inland to around 70 at the coast, with a modest
warming trend for Thursday night when lows range from the mid 60s
well inland to the lower 70s closer to the coast. A low risk of
rip currents is expected through the period. /29

LONG TERM /Friday Through Monday/...A weak upper trof over the
central Gulf coast region (the remnant portion of the upper trof
mentioned in the SHORT TERM section) lingers roughly over the
area through the period. Will need to continue to monitor the
northern Gulf for a system possibly developing along a remnant
frontal boundary for which NHC has a 20% chance of tropical
cyclone development through Saturday (end of the 5 day window).
Otherwise, a surface ridge over the southeast states becomes
oriented along the northern Gulf coast with light easterly winds
on Friday becoming southerly on Saturday and continuing through
Monday. Will have slight chance to chance pops for most of the
area on Friday then chance pops follow for the entire area each
following day as the upper level weakness in place over the
region makes for a generally favorable convective environment.
Highs each day will be around 90 while lows initially range from
the upper 60s inland to the lower 70s at the coast then trend
warmer by Sunday night with lower 70s inland and mid 70s at the
coast. /29

MARINE...Cold front moves southward to the coast into the
afternoon then into the marine area by this evening. As the front
approaches, periods of showers and storms along and south of it
will bring the potential of visibility reducing heavy rains at
times. Brief strong wind gusts and frequent lightning will be
common in and near stronger marine storms. Waterspouts also
possible this morning. Wind shift occurs tonight with wind speeds
light to perhaps moderate at times. Offshore flow continues into
Wednesday with front stalling over the central Gulf. A light east
to southeast flow evolves late Thursday and continues into the
weekend with high pressure extending from the Appalachians into
the southeast US. Seas look to range from 1 to 2 feet through most
of the period. /10




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