Area Forecast Discussion
Issued by NWS Mobile, AL

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000
FXUS64 KMOB 211745 AAD
AFDMOB

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1145 AM CST Sat Jan 21 2017

.DISCUSSION...See updated information for land areas and aviation
discussion below.

&&

.UPDATE...
Did a quick update to the forecast to remove severe thunderstorm
wording in the weather grids for the rest of the afternoon after the
expiration of Tornado Watch #11. Also cancelled the remaining parts
of the Wind Advisory based on observational data. No other changes
were made at this time.

ISC grids have been sent and should be posted to the internet.
Updated text products have already been sent.  /02/

&&

.AVIATION...
Line of SHRA/TSRA continues moving E of forecast area late this
morning. Visible satellite imagery showed the back edge of the cloud
line was located near a KMGM-KPRN-KPNS line. Will maintain MVFR with
isolated IFR ceilings east of this line for another couple of hours,
with flight categories largely becoming VFR everywhere through the
afternoon.

Flight categories should deteriorate after 22/0000 UTC, as a second
round of convection develops. Probabilistic guidance suggests MVFR
ceilings should develop across SE MS/SW AL between 22/0000-0300 UTC.
SHRA/TSRA will be possible through the early morning hours before
ending as a cold front moves W to E across the region. Expect
improving conditions Sunday morning, with a gradual return to VFR
flight categories expected.  /02/

&&

.PREV DISCUSSION... /issued 1002 AM CST Sat Jan 21 2017/

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

UPDATE...
First things first -- Line of strong to severe thunderstorms
continues to advance eastward as an initial shortwave trough ejects
northeast over the region. An analysis of the near-storm environment
shows 500-1000 J/kg of MLCAPE ahead of the line, with slightly higher
values closer to the coast. Effective bulk shear values (35-40 kts) remain
sufficient for deep organized convection in a highly diffluent flow
aloft.

Think the bigger severe risk in our area through late morning
would primarily come in the form of damaging wind gusts since steeper
mid-level lapse rates are to the west of the squall line. A tornado
risk remains, although 0-3 km bulk shear vector orientation doesn`t
appear to be that favorable for tornado generation with any
mesovortices. The Tornado Watch will remain in effect through 11 AM for
those areas along, and ahead of the thunderstorm line.

Observational data continues to show gusty gradient winds ahead of
the squall line. A Wind Advisory remains in effect for those areas. I
have cancelled the advisory as the line moves through, and will
continue to do so. Current gut feeling (based in a solid foundation
of meteorological data assimilation) is that we will be able to
cancel the Wind Advisory for all areas prior to the 2 PM expiration
time.

While the immediate severe weather risk may be over after the passage
of this line of thunderstorms, another round will be possible for our
area this evening.  Latest subjective analysis shows a surface
boundary near a Mobile to Auburn line. Expect this boundary to sink
southward this afternoon, leaving much of the area "prime for
recharging."

The next (main?) shortwave trough, currently seen spinning over
northern New Mexico in the water vapor imagery, is expected to dive
toward North Texas by this evening. Upper-level diffluence will again
increase over the forecast area ahead of this system as the the left
exit region of the jet moves overhead. This increase in synoptic
scale forcing should push the surface boundary back northward over
the forecast area.

Forecast soundings indicate a near-storm environment primed for
another round of severe thunderstorms. MLCAPE values between 1500 and
2000 J/kg will be possible, with surface-3km bulk shear magnitudes
approaching 50 knots toward sunset with some mid-level dry air.
Given this environment, as well as the forecast location of the
surface boundary, I would not be surprised to see an increased
tornado and damaging wind threat again across parts of our area. In
addition, steep mid-level (700-500mb) lapse rates hovering around 8
degrees C/km suggests the potential for large hail, especially in any
of the stronger storms. There is still some debate as to the exact
timing of this round, but all indications suggest the threat ramps up
again toward 6 PM.

Other than an update to the hazard grids, I have not made any changes
to the ongoing forecast at this time. Will update a suite of text
products shortly due to the early cancellation of the Tornado Watch.
Thanks to the Storm Prediction Center for collaboration this morning.
/02/

PREV DISCUSSION... /issued 739 AM CST Sat Jan 21 2017/

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

UPDATE...Updated zones to indicate a Wind Advisory for coastal
counties out in advance of the squall line that is moving east across
the area. An increasingly strong low level jet has resulted in a
strong southerly surface wind flow, sustained at 15 to 25 mph with
frequent gusts to 35 mph. These southerly winds will diminish
somewhat this afternoon in the wake of the storms, so the Advisory
is only valid through 2 pm this afternoon. 12/DS

PREV DISCUSSION... /issued 715 AM CST Sat Jan 21 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...
21/12Z issuance...Strong southerly winds 15 to 25 knots, with ocnl
stronger gusts, out ahead of an approaching squall line. The line of
storms will pass west to east across the region this morning, with
southerly winds continuing but weakening. The showers and storms will
end for most areas this afternoon, but increase in coverage again
this evening and overnight. As the storms move through this morning,
IFR or LIFR ceilings/vsbys can be expected in and near storms. 12/DS

PREV DISCUSSION... /issued 516 AM CST Sat Jan 21 2017/

NEAR TERM /Now Through Saturday night/...Severe weather event
expected to be ongoing over the western portions of the forecast area
at the beginning of the forecast period, and HiRes models are in
agreement in moving the convective activity east across the remainder
of the forecast area through the morning and exit the region by early
afternoon. After a lull in activity this afternoon and into early
this evening, another potential round of severe storms is possible
late this evening and overnight (most likely in areas east of I-65 in
southwest Alabama and over the western Florida panhandle) Saturday
into early Sunday morning.

Latest RAP and HRRR models indicate an 850mb southwesterly jet of 50
to 60 knots working its way east across the forecast area (Mississippi
and into southwest Alabama and the western Florida panhandle) this
morning. This is also supported best by the 21/00Z ECMWF. At the
surface winds remain backed out of the south and southeast due to a
developing surface meso-low over southwest Mississippi early this
morning, that is expected to move to near Tuscaloosa Alabama by
around 7 am this morning, and then on east-northeast across central
Alabama and likely weakening through midday. With this, strongly
curved hodographs/low level shear parameters will favor the potential
for tornadoes this morning across southwest Alabama and the western
Florida panhandle, where RAP and HRRR both indicate 0-1 km Storm
Relative Helicity as high as 400-500 m2/s2. As a result,
strong/significant tornado (EF2+) threat will migrate out of
southeast Mississippi and into southwest Alabama through around 9 am.
Severe threat will then continue east across south central Alabama
and the western Florida panhandle through late morning and very early
afternoon as the upper shortwave ejects off to the northeast. A
Tornado Watch is in effect for our southeast Mississippi counties
through 7 am and may need to be extended in time if storms have not
exited east of that area by then, and a Tornado Watch is in effect
for all of southwest and south central Alabama and the western
Florida until 11 am. As the 850 mb jet works into the region from the
west, 65 degree plus dewpoints will advect inland across the region,
with MLCape approaching 1400 J/kg in some areas, so very unstable
airmass for this time of the year. With the increasing atmospheric
moisture, also looking at a flash flood threat increasing across
portions of the forecast area and will have to monitor for this
possibility.

Most of the morning activity will likely be east of the forecast
area by noon or early this afternoon, with a lull in convective
activity late this afternoon into early evening. As the upper low
ejects northeast away from the region late today into the evening,
shortwave energy aloft just in advance of the final approaching
front will possibly re-initiate convection east of I-65 in Alabama
and the western Florida panhandle. This scenario is a little more
iffy than the earlier weather, but if it does develop conditions
will still support another round of severe weather.

Highs today in the low to mid 70s, lows tonight in the upper 50s
inland to mid 60s coastal. 12/DS

SHORT TERM /Sunday Through Monday night/...Long-wave high level trof
axis from Oklahoma, southward to the upper Texas coast by daybreak
Sunday morning is forecast to translate eastward over the mid-south
to central Gulf coast by Sunday evening. This feature pivots east of
the local area by Monday morning. At the surface, a large occluded
frontal cyclone over Arkansas Sunday morning advances east during the
day, then lifts up to the northeast over the Appalachians Monday
morning. Extending southward from the low, cold front makes swift
passage Sunday morning with thunderstorms remaining likely along and
ahead of it. Into the afternoon, as the large occluded frontal low
begins to lift into Appalachians, forecasters anticipate wrap-around
cloudiness/moisture on the west and southwest side of the low`s
comma head to maintain a chance of showers Sunday afternoon.
Southwest winds breezy on Sunday. May see winds gusting to between 25
and 35 mph along the coastal zones. Daytime highs Sunday in the mid
to upper 60s northwest of I-65 with lower 70s southeast of the
interstate. Despite the frontal cyclone well to our northeast over
the Mid-Atlantic on Monday, the large expanse of its cyclonic
curvature will likely contribute to a shallow layer of wrap around
cloudiness holding tough for much of the day before clearing from
west to east late in the day and into Monday night. Rain chances
ending Monday and Monday night. Another potentially breezy day for
Monday with winds coming out of the northwest. With expectation of
clouds/lower deep layer thickness values will lean to the cooler side
of the guidance envelope for Monday`s highs (upper 50s interior to
lower 60s coast) which is closer to climatological means. Overnight
lows Monday night much cooler, near 40 interior to mid to upper 40s
coast. /10

LONG TERM /Tuesday Through Friday/...Surface high pressure moves
across the local area on Tuesday, bringing sunny skies and daytime
highs in the mid 60s. Next front approaches from the west on
Wednesday with chance of showers returning to the local area.
Daytime highs moderate back into the lower to mid 70s Wednesday
before frontal passage on Thursday causes another swing in
temperatures with highs lowering in the mid to upper 50s northwest
and lower 60s southeast of I-65. To close out the week, a cool day on
Friday with highs looking to not make it out of the 50s area-wide.
Low temperatures to dip back down into the mid to upper 30s north of
the coast Friday morning. /10

MARINE...Periods of strong to potentially severe storms will affect
the marine area through tonight...with winds and seas being higher
near storms. A strong cold front approaching from the west will move
across the marine area on Sunday, with showers and storms ending
from west to east. Increasing south to southwest flow in advance of
the front today and tonight, becoming west to northwest and
increasing drastically Sunday afternoon and Sunday night in the wake
of the front. Small craft conditions likely and gale conditions
possible Sunday night into Monday. The strong offshore flow subsides
along with diminishing seas late Monday into Monday night. 12/DS

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...High Rip Current Risk through late Sunday night for ALZ265-266.

FL...High Rip Current Risk through late Sunday night for FLZ202-204-
     206.

MS...None.
GM...Gale Watch from Sunday evening through late Sunday night for
     GMZ631-632-650-655.

     Small Craft Advisory from 6 PM this evening to 6 PM CST Sunday
     for GMZ631-632-650-655.

     Gale Watch from Sunday evening through Monday morning for GMZ670-
     675.

     Small Craft Advisory until 6 PM CST Sunday for GMZ670-675.

&&

$$

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