Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 211239

Area Forecast Discussion
National Weather Service Mobile AL
739 AM CDT Mon Aug 21 2017

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


21/12Z issuance...A VFR forecast will generally prevail across the
forecast area through the period. Expect isolated to scattered
SHRA/TSRA to develop primarily after the 21.18Z time frame today.
Localized MVFR or lower conditions and gusty winds will be
possible near storms. Calm to light/variable winds early this
morning, becoming southeast to south and increasing to 5-10 knots
late this morning into the afternoon. The isolated to scattered
showers and storms will diminish shortly after sunset this
evening. Possibly some light patchy fog after 22/06Z. 12/DS


.PREV DISCUSSION... /issued 433 AM CDT Mon Aug 21 2017/

NEAR TERM /Now Through Monday night/...As has been discussed in
previous discussions, moisture will gradually begin to increase
from southeast to northwest as the TUTT (Tropical Upper
Tropospheric Trough) over the central Gulf of Mexico continues to
move slowly but surely to the west. Current IR satellite imagery
is already indicating some thin cirrus clouds streaming across the
area, and water vapor imagery is indicating a noticeable moisture
increase over the area. Some patchy light fog developing across
the region this morning is yet another indicator of this
increasing moisture trend. Obviously, there is a lot of interest
in the cloud forecast for today in association with the occurrence
of the solar eclipse. Given the expected increase in moisture, we
are likely to see a mix of high level clouds early this morning
with increasing cumulus development beginning late in the morning
and continuing through the afternoon, especially across the
southern half of the area. This will result in periods of mostly
cloudy skies. In addition, most of our meso-scale models indicate
that we will see isolated to scattered convection develop through
the course of the day, likely beginning around noon to 1 pm, with
the best coverage likely to be in the mid to late afternoon hours
across coastal areas due to the seabreeze aiding in pooling
moisture and increasing MLCAPES to 1500-2500 J/kg. Therefore,
areas north of Hwy 84 likely will be the least likely to have
eclipse viewing interruption from passing clouds, with the
southern half of the area having to deal with a greater coverage
of clouds. High temperatures today will top out in the low to mid
90s. Interestingly, we will likely see temperatures drop a few
degrees early in the afternoon due to the loss of incoming solar
radiation during the eclipse. Low temperatures tonight ranging
from the low to mid 70s over interior areas to upper 70s and even
near 80 along the coast. 12/DS

SHORT TERM /Tuesday Through Wednesday night/...Will start the
short term period with a mid-level ridge axis extending from the
western Atlantic to northern Texas. South of the ridge, an upper
tropospheric trof will be positioned over the western Gulf and
moves little thru mid week, while to the north of the ridge, trof
axis causes geo-potential heights to lower over the upper Mid-
West. The northern periphery of the southern US mid level ridge
begins to break down by Wednesday as long-wave high level trof
begins to dig into the Appalachians. Deep layer moisture continues
to modify area-wide with pwat values looking to average around
2.0 inches. With upper trof digging into the Appalachians, there
is support for a surface front to begin moving southward across
the Mid Mississippi River Valley Tuesday and Wednesday. With front
still well to the north this period, the higher chance of
convective initiation will be driven mostly by daily instability,
coupled by ascent provided by the passage of mid-level impulses
on the eastern periphery of western Gulf upper trof/low Tuesday
and Wednesday. Storms likely to be strong at times with the main
hazards being brief strong wind gusts, frequent lightning and
localized heavy rains that may contribute to mainly nuisance type
flooding at times.

Daytime highs mostly in the lower half of the 90s. Highest heat
indices look to range from 101 to 105 degrees. Overnight lows in
the lower to mid 70s interior to mid to upper 70s beaches. /10

LONG TERM /Thursday Through Sunday/...Frontal boundary settles
southward over the forecast area on Thursday and will provide for
an added focus for storm initiation. There are indications in the
latest global spectral physical solutions that the front will
begin to stall close to the coast by Friday and remain near the
coast on Saturday as well. In the other camp, the ensemble low
level wind fields favor the front slipping more off the coast into
the weekend. Either way, the front looks to be close enough as to
maintain a modest chance of showers and storms thru the period
and is consistent with the blended guidance. Daytime highs look to
be close to seasonal values on average, ranging from the lower
90s interior to upper 80s beach areas. Overnight lows may dip into
the upper 60s over the interior zones by the end of the week with
the front to the south. Otherwise, little overall change in
overnight mins. /10

MARINE...A ridge of surface high pressure will persist over the
interior southeastern states, just to the northeast of the marine
area through early Wednesday. This will result in a weak,
predominately onshore flow through Wednesday, but somewhat
influenced by the diurnal land breeze-sea breeze circulation. A weak
front is expected to move down from the north and stall near or just
north of the marine area by late Wednesday and then remain nearly
stationary through Friday. This will maintain a light and somewhat
variable wind flow across the marine area during the latter part of
the week. Little change in seas is expected through the period.
Winds and seas will be higher near scattered showers and
thunderstorms. 12/DS




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