Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 211210
AFDMOB

Area Forecast Discussion
National Weather Service Mobile AL
710 AM CDT Wed Apr 21 2021

.AVIATION...
12Z issuance...VFR conditions prevail through the TAF period.
Winds will be northwesterly between 10 to 15 knots with gusts
between 20-23knots possible as a dry cold front continues moving
off to the southeast. JEH/88

&&


.MARINE UPDATE...Flow has begun transitioning to northwest as a
dry cold front moves over area waters. A Small Craft Advisory is
in now in effect for the MS Sound, Southern Mobile Bay, and the
AL Gulf waters through Thursday morning. Southern Mobile Bay and
the MS Sound could potentially be dropped earlier from the Small
Craft Advisory, however, Small Craft Exercise Caution will likely
still exist during this time. Otherwise, Small Craft Exercise
Caution conditions exist over Northern Mobile Bay, Pensacola Bay, and
the FL Gulf Waters. The FL Gulf waters remains in Exercise Caution
conditions till 4PM CDT this afternoon, and then become included
in the Small Craft Advisory until Thursday morning. Trends will
continue to be monitored and updated in the near future. By late
late Thursday morning, winds decrease and begin to transition to
more easterly by Thursday afternoon. Winds begin to increase to
moderate and become more southeasterly going into Friday, then
shift to be southerly and increase to moderate to strong going
into Friday night ahead of another cold front with seas also
beginning to increase during this time. This said, Small Craft
Exercise Caution conditions will likely be needed on Friday with
the potential of needing another Small Craft Advisory by Friday
night, and going into Saturday. Moderate northerly flow is then
expected in the wake of the FROPA by early Sunday. JEH/88

&&

.PREV DISCUSSION... /issued 502 AM CDT Wed Apr 21 2021/

NEAR TERM UPDATE /Now Through Thursday/...Wonderful dry and clear
weather continues through the near term. It`s not to hot and not
to cold, all you need is a light jacket. A mainly west-southwest
flow aloft begins to transition to northwesterly on early Thursday
as ridging aloft amplifies across the southern Plains and begins
moving into the region by late Thursday afternoon. At the surface,
a dry cold front continues to move into the region from the
northwest through the early morning hours and is expected to pass
over the entire region just after sunrise and cool dry air begins
to usher into the region. By Wednesday night, the front is well
off to our southeast with high pressure beginning to build into
the region from the west. Surface flow starts of northwesterly
today and transitions northeasterly as the surface high located
over the ArkLaMiss region moves into northern AL by early Thursday
morning. By the end of the period flow transitions to more
easterly as the high continues tracking east.

With the passing dry cold front, high temps today will be on the
cooler side with low to upper 60s northwest of I-65, and mid 60s to
low 70s southeast. Tonight, temperatures will drop to near, if not
reaching record going into Thursday morning. Lows during this time
are currently expected to be in the upper 30s for interior portions
of southeast MS, and southwestern/south-central AL, with low to mid
40s expected elsewhere for areas north of I-10. Closer to the coast
mid to upper 40s are expected. How low temps get will largely be
dependent on whether surface winds remain elevated given clear skies
on Wednesday night. Here are some notable record lows for Thursday
morning:

Mobile: 42 (1993)
Pensacola: 46 (1993)
Evergreen: 38 (1993)
Greenville: 38 (1993)
Andalusia: 35 (1993)
Crestview: 41 (1993/2000)
Niceville: 38 (1993)
Waynesboro: 36 (1993)
Wiggins: 40 (1993)

Moving forward, highs on Thursday range in the upper 60s to low 70s
across the region. Be sure to keep your jackets handy for the chilly
low temps and those sunglasses handy for Thursday during the day and
enjoy the nice quiet weather while it lasts. JEH/88

SHORT TERM /Thursday night Through Friday night/...An upper level
trough over the Desert Southwest and Baja California Thursday
night is forecast to move toward the southern Plains through noon
Friday and then will quickly translate toward the ArkLaTex region
by late Friday night. Clouds will gradually increase across our
area Thursday night into Friday as layer moisture trends upward
within the zonal mid level flow pattern. The lower levels will
remain quite dry Thursday night into Friday, so will keep a dry
forecast going through noon Friday. Southwesterly flow aloft and
deep layer moisture will increase Friday afternoon and evening
ahead of the approaching shortwave trough with precipitable water
values increasing to 1.6 to 1.8 inches. Ascent will increase over
western portions of our area Friday afternoon, with isolated to
scattered showers moving in from the west. Deep layer ascent will
increase over the entire area Friday night, especially across the
northern half of the CWA. Showers with embedded thunderstorms
will spread over our region during this time. We have POPs
increasing to 70-90% Friday evening northwest of I-65, and to
50-70% southeast of I-65. Pops will decrease across the southern
one-third of the CWA after midnight Friday night to 30-50% as a
warm front pushes further inland, but stay in the 60-90% range
across the northern two-thirds. Deep layer and low level shear
increases over the region after midnight with 45-55 kt of
southwesterly 850 mb flow expected to spread into locations west
of I-65 after midnight, along with 500-1000 J/KG of MLCAPE, which
could bring a low end threat of severe storms over northwestern
portions of our area overnight Friday night. Locally heavy
rainfall will also be possible, especially across the northern
two-thirds. Will watch trends closely over the next two days. /22

EXTENDED TERM /Saturday Through Tuesday/...Details on the system
Saturday continue to become clearer; however, there is still
plenty of time for changes and will have to continue to monitor
trends. For details on the severe potential and flooding rain
threat see the labeled sections below.

By Saturday morning, the stage has been set and the threat will be
ongoing for a potential round of severe weather and heavy rainfall.
A positive to slightly neutral tilt shortwave trough will be
powering across the area. Strong upper level diffluence will be
centered over southeastern Mississippi during the start of the
period and this region of strong synoptic ascent will continue to
move east. This will overspreading a likely rapidly destabilizing
warm sector over southern Alabama and the Western Florida
Panhandle bounded by a warm front to the north likely near the
I-20 corridor and a crashing "cool front" extending southeast from
a surface low over east- central Mississippi. Ongoing convection
will likely be focused along the warm front north of our area and
extending back into southeastern Mississippi during the start of
the period Saturday morning as a stout 50 to 60 knot southerly
850mb jet over spreads the warm frontal zone. Given this low level
shear, any surface based storms could pose a severe threat mainly
along the highway 84 corridor and north. This convection will
move northeast across the northern part of our area during the
mid-morning hours. Along with the potential for strong storms, the
strong low level jet will likely lead to gusty gradient winds and
a wind advisory might be needed especially as the warm sector
destabilizes and stronger winds will mix down during the mid-day
hours. By mid-day the main synoptic ascent will be quickly lifting
out of the area; however, there should be adequate ascent as the
trailing surface boundary drops south into the area. This boundary
will likely be the focus of more storms mainly east of I-65
during the afternoon as peak destabilization occurs. A line of
strong to potentially severe storms could quickly progress
eastward during the mid afternoon across the western Florida
Panhandle and south Central Alabama. Rain will likely linger
offshore through Saturday night as the weak boundary washes out
over the Gulf. Al

Drier air will move in behind the trough as high pressure quickly
builds over the central US. This is in response to a very large
broad longwave trough entering the West coast which will become
our next weather maker by the end of next week. Until then, high
pressure is the name of the game through Tuesday. Expect dry
conditions Sunday, Monday and Tuesday before return flow, well,
returns late Tuesday. Moisture will begin to increase on Wednesday
as the broad upper trough slowly pushes eastward. The trough will
arrive on Wednesday with plenty of deep moisture ahead of it.
Models are in fairly good agreement of this system and the overall
timing and given the amount of return flow and deep neutral to
negative tilt of this trough, we will have to monitor this system
closely as the week moves on.

Temperatures will be mild throughout the period with mainly
southerly return flow dominating the period. Highs will pretty
much be in the 80s. Lows will start in the mid 50s and slowly
climb throughout the period likely into the low 60s as our
dewpoints slowly return to the soupy summer feeling we are so use
to. Lastly, I would likely expect rip currents to be on the rise
with the southerly persistent flow and would not be shocked to see
a HIGH risk of Rip currents by the end of this week and again
next week.

Severe...Alright this is a rather complicated severe setup that
probably has two separate potentials. The first potential will be
along the warm frontal zone over our northern counties during the
early morning hours and into the mid-morning. This will be where
the best synoptic overlap with some instability and the best
shear will exist as the stronger 850 jet rotates over the area.
The big issue with this period will be the degree of
destabilization and the location of the best overlap of
ascent/shear/CAPE given the likely ongoing convection along and
north of the boundary and the noticeable capping inversion over
the area. This cap will likely be overcome by the strong synoptic
ascent and some destabilization should occur across the area;
however, the best ascent appears to be over the I- 20 corridor
north of our area leaving us a bit on the southern end.
Nonetheless, surface winds will likely remain backed during this
period leading to long curved hodographs supportive of rotating
updrafts and all severe modes would be possible. This threat would
mainly be along and north of highway 84.

The second threat will be more during the mid-day hours and into
the early afternoon. As the upper trough pivots over and
temperatures cool aloft, lapse rates will steepen during the
afternoon and surface heating over the eastern half of the area
will allow for appreciable cape to likely develop along and east
of I-65. Cape values will likely climb to 1500 to 2000 J/KG and
with deep layer shear still in the 50 to 60 knot range organized
convection would be favored. Surface winds will actually veer out
of the southwest leading to a straighter hodograph which will
likely support more upscale growth into a QLCS or potentially
splitting cells. Given the steep lapse rates, damaging winds and
maybe some hail will be possible. The biggest question is with
rather weak forcing and the best dynamics lifting north, will
convection re-fire along the weakening boundary while it is still
over our are or if it makes it out of the area before re-
initiating.

Heavy rainfall...Luckily rainfall totals have gone down as models
have pushed the heavier axis of rain north of our area with the
warm front. This will not exclude us from the potential for heavy
rainfall given high PWATS and the potential for training
convection and storms moving over the same area that has seen a
lot of rain over the last week or so. We will have to monitor the
progression of the early morning convection as this will have the
potential to train and heavy rain rates could drive an increased
flash flooding potential near the boundary. With the weaker
forcing and rather progressive nature of the mid afternoon storms,
not as worried about heavy rainfall but some localized flooding
cannot be ruled out and will have to be monitored. BB/03

MARINE...Westerly flow early this morning begins to transition to
more north-northwest as a dry cold front moves over area waters. A
Small Craft Advisory is in effect beginning at 5AM CDT this
morning for the MS sound, Southern Mobile Bay, and the AL Gulf
waters till this afternoon. Trends are still being evaluated to
determine if the Small Craft Advisory will need to be extended
going into Wednesday night and through early Thursday morning due
to elevated winds from cold air drainage. Small Craft Exercise
Caution exists over Northern Mobile Bay, and the FL Gulf Waters.
Trends will continue to be monitored and updated in the near
future. By late Thursday morning, winds decrease and begin to
transition to more easterly by Thursday afternoon. Winds begin to
increase to moderate and become more southeasterly going into
Friday, then shift to be southerly and increase to moderate to
strong going into Friday night ahead of another cold front with
seas also beginning to increase during this time. This said Small
Craft Exercise Caution conditions will likely be needed on Friday
with the potential of needing another Small Craft Advisory by
Friday night, and going into Saturday. Moderate northerly flow is
then expected in the wake of the FROPA by early Sunday. JEH/88

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...None.
FL...None.
MS...None.
GM...Small Craft Advisory until 10 AM CDT Thursday for GMZ631-632-650-
     670.

     Small Craft Advisory from 4 PM this afternoon to 10 AM CDT
     Thursday for GMZ655-675.

&&

$$

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