Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 281431 AAB

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
931 AM CDT Sun May 28 2017

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


.UPDATE...Outflow boundary from last night`s MCS activity to the
north continues to march south and has now entered our far
northern forecast area. As it moves south, it will encounter an
even more unstable atmosphere with MLCAPES around 3000 J/kg. In
addition, isolated coastal showers and thunderstorms indicates
capping aloft may not be as pronounced over the southern half of
the area. Given all of this, have increased rain chances for the
rest of the morning for areas south of the boundary as additional
development appears likely. Maintain better rain chances along
coastal counties through the afternoon with a diminishing trend
inland. The last several runs of the HRRR show increasing
convection this afternoon inland, but we are not certain of this
scenario as it is struggling with evolution of the outflow
boundary. We will watch radar trends and see if we need to
increase rain chances further inland for the afternoon.

A few strong storms are certainly possible, especially over the
more unstable southern half of the area. However, with some
warming around 850mb noted in soundings, it is uncertain how
vigorous thunderstorms will become. Will mention the potential for
a few strong storms with gusty winds/small hail in the hazardous
weather outlook. 34/JFB


.PREV DISCUSSION... /issued 618 AM CDT Sun May 28 2017/

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

UPDATE...Isolated showers have formed quickly this morning across
parts of the area. Meanwhile, an outflow boundary from convection
to north is working its way southward this morning. This boundary
continues to generate light showers as it moves southward.
Therefore, have added isolated showers for the morning hours
across the entire area. Updates have been sent. /13

12Z issuance...MVFR ceilings this morning will improve to VFR by
the afternoon hours. An isolated shower or thunderstorm is
possible through the day, however the chance is not high enough
to include at this time. /13

PREV DISCUSSION... /issued 425 AM CDT Sun May 28 2017/

NEAR TERM /Now Through Sunday night/...
Surface and upper ridge remain in place and slowly move east this
afternoon and tonight as the next upper trough approaches. This will
keep conditions mostly dry as deep layer subsidence continues.
However, rain chances will begin to increase this afternoon and
especially tonight from the west as the upper ridge slides east and
the trough nears. Temperatures continue to run 3 to 5 degrees above
normal, with highs in the upper 80s and low 90s away from the coast.
Highs near the coast will be in the low to mid 80s. Lows tonight
will also be above normal, in the low 70s inland to mid 70s along
the coast. /13

SHORT TERM /Monday Through Tuesday night/...
The Memorial Day forecast has resulted in lots of deliberation,
discussion, and consternation by this forecaster. In short, the
finer details will likely be dependent upon the convective evolution
over the next 18 to 24 hours. And, as such, will be subject to
change. Here`s how things appear to evolve for the time being,

Broad upper-level troughiness is expected to stretch from the Upper
Midwest and across the Ohio Valley for much of Monday. The nature of
this synoptic setup will likely result in rather weak perturbations
traversing the region until the main trough heads east. With weak
(if any) cross isobaric flow, the associated cold front will be
sluggish to move into our area and probably do so during the
afternoon. However, if convection develops along the front earlier
than expected, don`t expect much southward movement.

To pour salt into the uncertainty wound, synoptic scale kinematics
and thermodynamics are out of phase with this system. The main upper
jet axis remains stretched from southwestern Missouri into southern
Pennsylvania through the day, with any upper-level divergence
remaining closer to the jet. Surface to 6-km effective bulk shear
values remain focused along the front and *may* reach 35 knots along
and north of the US-84 corridor by late afternoon if the front sags
south/eastward into our area.

Abundant boundary layer moisture will be present, with precipitable
water values close to 3 standard deviations possible just inland
from the coast. There will be sufficient instability present, with
forecast soundings indicating MLCAPE values between 2000 and 2400
J/kg possible. This combined with the possibility of an approaching
front (and any mesoscale boundary interactions), will lead to the
possibility of strong thunderstorms Monday afternoon. Areas along
and north of the US-84 corridor will likely see the greatest threat
for 50 MPH wind gusts from the stronger thunderstorms. Right now,
though, it appears the greatest severe weather threat will be
north/east of our area.

Another piece of upper-level energy is expected to move into the
upper trough Tuesday afternoon, which may aid in pushing the front a
little farther south. With abundant low-level moisture, especially
in vicinity of the weakening front, shower and thunderstorm
probabilities will remain elevated through the rest of the short
term period.

Thanks to WFO Birmingham for collaboration this morning. /02/

LONG TERM /Wednesday Through Saturday/...
A weakening surface boundary is expected drift offshore Wednesday as
a shortwave rounds the base of the upper trough over the eastern
U.S. Even with its probable location just offshore, the boundary
will likely continue serving as a focus for shower and thunderstorm
development across the region. With the loss of stronger synoptic
scale forcing, the convective nature should become increasingly
diurnal in nature, with the greatest coverage expected during the
afternoon and early evening. With uncertainties in the location and
timing of the front, this diurnal nature was not reflected
precipitation probability forecasts for Wednesday.

Forecast confidence lowers thereafter given temporal and spatial
discrepancies between the deterministic output of the major medium
range models. Based on signals noted in the NAEFS, I still have
confidence that ridging will attempt to reassert itself over the
Southeast toward the end of the week/heading into next weekend. With
some indication being noted that the diffuse boundary *may* try and
waffle back onshore, shower and thunderstorm probabilities will
remain above climatological values. However, I suspect the presence
of ridging should allow the diurnal trend to largely be maintained.

A light to moderate south to southwest wind flow continues through
Monday as high pressure slowly shifts east. A weak frontal boundary
stalls near the coast Monday night into Tuesday morning leading to
decreased winds and seas through midweek. An increased coverage of
showers and thunderstorms can also be expected through midweek. A
generally light southerly flow develops late in the week into the
weekend. /13




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