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Area forecast discussion
National Weather Service Mobile Alabama
1258 PM CDT Mon may 29 2017

Discussion...updated for latest aviation discussion below.

&&

Aviation...
18z issuance...
MVFR ceilings and southerly winds expected for most of the
forecast period. Ceilings are associated with low level cloud
remnants from the storms earlier today. Overnight showers will be
possible but should have minimal effects on visibilities. Low
ceilings are the main concerns. 17/dj

&&

Previous discussion... /issued 1127 am CDT Mon may 29 2017/

Discussion...see updated information for land and marine areas below.

Update...did a quick update to the current zone forecast mainly
to lower pops a tad for this afternoon and this evening. Current
hi-res data and radar trends to the west and south support this
reasoning showing less coverage than earlier anticipated. We will
keep high end chance pops in the forecast for all land areas for
the rest of today and this evening followed by better coverage
over northern sections of the forecast area after midnight tonight
continuing through Tue morning. 32/ee

Marine...lowered rain chances over the marine area for this
afternoon and this evening. Also made a few tweaks to the winds
through Tuesday afternoon mostly for better consistency with
surrounding wfos and afternoon seabreeze effects. 32/ee

Previous discussion... /issued 749 am CDT Mon may 29 2017/

Discussion...see updated information for land and marine areas below.

Update...
initial convective complex continues moving east-northeastward
across parts of south-central Alabama and the Florida Panhandle.
Mesoanalysis data indicates decreasing amounts of parcel buoyancy
available, with 1000-1500 j/kg now noted near the coastal counties
in our forecast area. Precipitation probabilities have been
increased downstream to account for this initial convective
complex.

Area radars show another convective complex moving toward our area
from southeast Louisiana. Infrared satellite imagery indicates
warming cloud tops, which suggests a decreasing convective trend.
Radar trends show this as well. This trend is expected to
generally continue through mid-morning, although I made no
adjustments to precipitation probabilities past 10 am at this
time.

Updated text products will be issued shortly. /Butts/

Marine...
scattered showers and thunderstorms continue moving across the
marine area this morning. The stronger thunderstorms will be
capable of producing wind gusts around 30 knots along with locally
choppy seas and torrential rainfall.

Observational platforms over the northern Gulf indicate
significant wave heights over the Gulf of Mexico waters are
running a skosh higher than what's indicated in the current
forecast products. Current wind speed forecast appears to be in
decent agreement with data from available observational platforms.

Updated text product will be issued shortly. /Butts/

Previous discussion... /issued 651 am CDT Mon may 29 2017/

Discussion...updated for latest aviation discussion below.

Aviation...
12z issuance...MVFR to VFR conditions can be expected through
much of the day scattered showers and storms near the terminals by
the afternoon. IFR to MVFR conditions can be expected in and
around storms. /13

Previous discussion... /issued 542 am CDT Mon may 29 2017/

Near term /now through Monday night/...
a large upper-level storm system spinning over southern Ontario this
morning continues to result in broad troughiness over much of the
eastern U.S. The associated cold front remains analyzed from middle
Tennessee into the arklatex early this morning.

Showers and thunderstorms have already been seen this morning close
to home, initializing along an outflow boundary (or boundaries) from
previous convection. Mesoscale analysis indicates a very unstable
airmass in place across the northern Gulf Coast, with MLCAPE values
between 1500 and 2000 j/kg common. Mid-level lapse rates and
effective bulk shear values are marginal, however, with values 6.5
degrees c/km and 25 knots, respectively. Some stabilization is
expected across the region as this complex of storms traverses the
region. The big question is to what degree can we recover for the
afternoon.

I think convection today will likely be driven by largely mesoscale
processes. Sure, synoptic scale forcing is enhanced given the
proximity of the surface front. The upper jet, though, is forecast
to remain to our north with a minimum of cross isobaric flow between
the upper trough and the surface front. That said, I think the front
(and the best source of large scale forcing/shear) will likely
remain to our north/west tonight.

What we will have is a "soupy" airmass, with precipitable water
values between 1.75 and 2 inches over the region. Forecast soundings
indicate some airmass recovery is possible this afternoon, with
maximum MLCAPE values between 1500 and 2000 j/kg possible. Short
version - ample moisture and instability will be present, as will be
plenty of mesoscale boundaries and disturbances in the upper
atmosphere moving through the region.

The ongoing convection should take full advantage of the
thermodynamic environment through mid-morning, with additional
development possible across southeast Mississippi and adjacent
areas of interior southwest Alabama. Expect the areal coverage of
this convection to diminish by late morning as instability is
used. There may be a break in the convection late morning through
early to mid- afternoon, before more showers and thunderstorms
develop across southeast Mississippi with the approach of the next
upper atmosphere disturbance. Timing of precipitation
probabilities have been based on this train of thought.

There are lots of flies hanging out in the ointment today, so
updates will likely have to be made as the event evolves. Locally
heavy downpours will be possible in the stronger thunderstorms.

Short term /Tuesday through Wednesday night/...
minimal cross isobaric flow should be provided by the broad upper-
level trough through Tuesday. This should result in the surface
front remaining nearly stationary, generally waffling somewhere
either side of the ih-20 corridor.

The atmosphere across our region will remain quite moist.
Precipitable water values should remain between 2 and 3 Standard
deviations above climatological means, with amounts above 1.75
inches expected. Instability should not be as great as today, but
MLCAPE values around 1000 j/kg may be seen. Weaker perturbations
moving across our area will interact with this airmass to result in
additional periods of showers and thunderstorms over the region
through Tuesday. Expect the greatest convective coverage to remain
generally along and north/west of ih-65, where better moisture
transport will be seen.

Additional upper-level energy is expected to rotate southward across
the upper Midwest Tuesday afternoon and evening, helping to amplify
the upper-level trough. This should provide enough momentum to help
push the surface front southward into our area by Wednesday. Despite
the loss of daytime heating, increased synoptic scale forcing should
help maintain a scattered coverage of showers and thunderstorms over
much of the region Tuesday night.

The front should gradually weaken as it sags toward the coast
sometime Wednesday night. I've kept convective probabilities above
seasonal values through mid-week due to the presence of the
weakening front. We'll likely see a stronger diurnal signal by
Wednesday as well, although i've refrained from indicating this in
the forecast at this time.

Long term /Thursday through Sunday/...
broad upper trough axis is forecast to shift east across Quebec
Thursday. In its wake, weak ridging both at the surface and aloft
will attempt to rebuild across the region as the old surface front
gradually dissipates. With the loss of significant synoptic scale
forcing, expect scattered afternoon convection to exhibit a strong
diurnal signal with the greatest coverage in the afternoon and early
evening.

Low-level flow veers to a more southerly direction Friday night-
Saturday, just in time for another shortwave trough to move through
the plains. This feature will likely amplify over the eastern U.S.
By the end of the weekend and aid in bringing another cold front
toward the mid-south. Boundary layer moisture increases to nearly 3
Standard deviations above climatological values, and when combined
with ample instability, will likely result in an increase in
convective coverage for the upcoming weekend.

Marine...
light to moderate onshore flow is expected over the marine area
through tonight, along with scattered showers and thunderstorms. A
weakening cold front is forecast to enter the marine area by Tuesday
night and become stationary through mid-week. As the boundary
becomes more diffuse, a decreased shower/thunderstorm coverage will
likely be seen for the end of the week before showers and
thunderstorms increasing in coverage again next weekend. Winds and
seas will be higher in vicinity of any convection.

&&

Mob watches/warnings/advisories...
Alabama...none.
Florida...none.
MS...none.
GM...none.
&&

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