Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 311152 AAA

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
652 AM CDT TUE MAY 31 2016

For 12Z TAFs.


.SHORT TERM (Today through Thursday)...
 (Issued 302 AM CDT TUE MAY 31 2016)
00Z soundings from BMX and OHX had two very different profiles. To
our south moisture is a bit more abundant and is reflected with PWATs
around 1.3 in., while to our north PWATs were below 1 inch. Based on
surface conditions and RAP analysis believe the environment over the
TN Valley is more representative of that over OHX. Surface analysis
tonight indicated a convergence boundary near Jackson MS and this
coupled with a weak shortwave lifting NE through the MS River valley
has sparked a few showers and thunderstorms along the boundary. These
are being carried north by a subtle 850mb jet. Short-term models
begin to veer the 850mb winds to more of a SW direction after 12Z and
some of the activity may approach our western zones. With that said
though still think the dry air mentioned above will become entrained
in the storms as they approach W AL and may only end up with a few
light showers.

The clouds associated with those storms may keep temps down a degree
or two across the west but most of the area will see sunny skies
today, allowing afternoons highs to reach into the lower 90s. The
high pressure over the OH Valley begins to shift east tonight into
Wednesday as the low pressure system over the northern CONUS moves
into the Great Lakes region. Winds shift to the south and we will see
a return of mid to upper 60 degree dewpoints into the area on
Wednesday. As a result of this, diurnally driven thunderstorms are
possible across the entire area but coverage remains isolated as we
lack any real forcing.

The system in the Great Lakes continues east Wednesday into Thursday
and will push a front towards the MS/TN Valleys. There are some
differences in latest guidance with the handling of the front and
because of this have kept PoPs on the lower end of guidance. In
either case the additional forcing from the front and the good
moisture in place should be able to allow for more scattered
thunderstorms on Thursday.


.LONG TERM (Thursday night through Monday)...
A polar jet trough will be shifting east across the Northern Plains
on Tuesday/Wednesday. As it does so, it will push a Canadian Cp
airmass towards the Southeast. This cold front will be one of the
primary lifting mechanisms for showers and thunderstorms for this
weekend into early next week.

The polar trough will be moving atop an Omega blocking pattern
within the subtropical jet stream over the southern half of North
America. Within this subtropical jet stream, the med range model
guidance shows a cutoff low developing over the already saturated TX
plains on Thursday while ridging amplifies on either side of this
cutoff low. A convergence axis within the southern fetch over MS/W AL
on Thursday night may lead to additional thunderstorm development,
or thunderstorms could develop from southward propagating outflow
boundaries from upstream convection along front. Given the
uncertainties with these mesoscale features will adjust pops to
reflect a NW (higher) to SE (lower) gradient.

As the surface cold front moves (slowly) closer to the TN valley on
Friday, have included higher pops. In addition, it appears that
sb instability is moderate with CAPE values around 1500-2000 J/kg by
the afternoon. Coupled with little wind shear, there could be the
possibility for microbursts with any deep convective updrafts that
form. Then, on Saturday and continuing through Sunday, the cutoff low
over the TX plains appears to shift southward as a polar jet trough
expands/deepens southward. This will act to push the slow moving
front south over the central portions of AL/MS initially with
prevailing mid-upper southerly flow on Saturday. In addition, mid-
upper flow convergence axis over TN/Ozarks may enhance lift for
showers and storms parallel to the front. As a result, heavy rainfall
could be a threat on Saturday/Saturday night as well. A similar setup
may occur on Sunday with the frontal boundary shifted further south
and have left the higher chance pops for Sunday to account for this

Finally, on Monday, have included a gradual downward trend in
pops as drier and `cooler` weather accompanies mean trough over the
eastern half of the CONUS. Have kept daytime highs cooler in the low
80s for Sunday and Monday. With this particular pattern change,
MCS` originating from the Midwest will likely become more prevalent
to track towards the Southeast later in June and will keep monitoring
for this possibility.



For 12Z TAFs:

VFR conditions are expected to continue across N AL/S Middle TN for
the next 24 hours. Isolated -TSRA are possible over NW AL, but have
not included in KMSL TAF due to uncertainty with storms over N MS and
further development later this afternoon.



.HUN Watches/Warnings/Advisories...


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