Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 261945

Area Forecast Discussion
National Weather Service Huntsville AL
245 PM CDT Fri May 26 2017

.NEAR TERM...(Tonight)
Issued at 244 PM CDT Fri May 26 2017

Satellite this afternoon has a good depiction of what is going on
with the large cu-field along and west of the Mississippi River
representing the strong low level moisture return. Further to the NW
over MO a surface low is forming and convection is expected to
develop in another hour or two. Thankfully, winds aloft are out of
the W/SW and will help to keep the storms to our north. A few models
are trying to suggest some S/SE outflow from the storms moving into
western TN tonight, with some potential redevelopment along the
outflow. However, we still have a large CAP in place and this should
prevent any storms from making it into our area. So, have kept the
forecast dry for tonight but have started to increase cloud cover
late as the better moisture begins to creep eastward overnight. The
higher dewpoints and increasing cloud cover should keep temps a bit
on the warmer side with values only dropping into the mid to upper

.SHORT TERM...(Saturday through Sunday)
Issued at 244 PM CDT Fri May 26 2017

There are two ways this forecast can go this weekend. The first is
that the high to our south strengthens and keeps a majority of the
storms in KY/TN for Saturday into Sunday with the only real trigger
being the likely outflow driven storms dropping into the area and
it`s not until the cold front Sunday into Monday that we actually get
significant storms. Or, the high weakens and the boundary and
systems that are the focus for convection sneaks a bit further south
and could make for a very active weekend with wind, hail, and
flooding all possible. Needless to say, the forecast is still
uncertain and guidance is not much help with models showing various
solutions for this weekend. With some showing a majority of the
storms staying north and others showing the main storm axis over
middle TN, which again would be active for the TN Valley.

In either case, models are showing steep lapse rates aloft, which
occur with surface dewpoints ranging from the mid 60s Saturday to
lower 70s Sunday into Monday. The result of the two, is very unstable
conditions with CAPE of 2,500-4,000 J/kg both Saturday and Sunday.
The caveat is that with the high lingering to our south we should
have a CAP over the area Saturday and could prevent some activity
from forming. Additionally, dry air continues to show up in the mid-
levels via latest forecast soundings and could cause some storms to
weaken as they move into the area. Much of the CAPE mentioned above
will be in the hail growth zone and the shear of 30-50kts this
weekend is more than enough to support the development large hail.
The only limiting factor for large hail is the amount of warm air
beneath the freezing/wet-bulb zero level with a height of
11,000-13,000ft, any large hail will be falling through a deep warm
layer. But with those CAPE values could still see severe hail make
it to the surface. Damaging winds is the other major impact with the
storms this weekend. The combination of the melting hail and dry air
in the mid-levels would suggest microbursts are possible. Looking at
a few different microburst parameters and they all suggest the
possibility of damaging winds.

Again the main question is timing and at this point with the
uncertainty in the models have gone with a blend of guidance. At
this point believe that most of the activity that we see will be the
result of convection that forms to our N/NW and come in the form of
linear/MCS type of systems that are propagating mainly E/SE across
the TN/KY area and outflow from these storms could drive some S/SE
shift in storms into the TN Valley. Winds aloft are mainly zonal and
result in any MCSs moving predominantly eastward. However, there is
a slight SE turn in the Corfidi vectors for Saturday and could lead
to a few storms dropping into the area during the afternoon hours.

The more widespread storms won`t arrive until late Sunday into Monday
as a cold front drops S/SE through the area. There are still some
timing issues and overall severity concerns with the front. Models
show the potential for storms to develop in the afternoon along a
quick moving shortwave, well ahead of the main front and those could
be the highest impact and then when the front arrives convection
won`t be strong to severe but could morph into more of a heavy
rain/flooding threat.

It will be a very unsettled weekend and with all the outdoor events
it is important for folks to keep an eye on the weather. An
additional concern is tied to the humidity, as it will be very humid
both Saturday and Sunday and Heat Index values in some areas could be
in the low to mid 90s. We will be keeping a close eye on the activity
to our NW and future model runs hoping for a little better
consistency in guidance. But based on the parameters expected, a bit

.LONG TERM...(Sunday night through Thursday)
Issued at 244 PM CDT Fri May 26 2017

Monday starts off with the broad upper trough and surface low north
of the Great Lakes and a cold front sitting in the vicinity of N.
AL/S. Middle TN, although exact location can`t be determined this
far out. A line of thunderstorms will be accompanying this front
from the overnight period and most likely hold together as it moves
through. Although instability will be lessening, steep mid level
lapse rates and bulk shear of 40-50kts should be able to sustain a
strong storm or two through the morning. It really depends what
happens during the afternoon convection on Sunday in regards to how
worked over we will be once the front comes through. Unfortunately,
this front will slow down and almost stall as it moves through.
Training of cells is possible and with PW values of 1.25 to 1.5
inches, heavy rain is also possible. Eventually, an upper wave will
kick the front out to the south.

A pretty messy and uncertain pattern exists after that. The upper
low will meander north of the Great Lakes before slowly moving east
and opening up by Thursday. In the meantime, southerly flow at the
surface will return and westerly flow aloft will help bring multiple
shortwaves across the area. Tuesday will likely be dry but isolated
to scattered thunderstorms return for Wednesday through Thursday,
with highest coverages each afternoon. Won`t say much else about
that given how far away it is. Friday looks more complicated with a
bit of an Omega Block trying to set up. At the surface, a cold front
will be pushed south by a strong surface high Friday into Friday
night, bringing an additional round of thunderstorms. All in all, we
probably won`t see any sunny skies through the week, maybe a few
breaks here and there.

Temps through the long term will be near normal with highs in the
lower 80s and the lows in the lower 60s.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1204 PM CDT Fri May 26 2017

A storm system moving through the central US today will result in
breezy SW winds through the early evening hours with gusts of 20kts
possible, especially in NW AL. VFR conditions are forecast for much
of the period. However, starting around 12Z Saturday the low clouds
along the MS River will begin to stream into the area. Expect cigs to
drop to around 2,000-3,000 feet after 12Z and could remain there
through 16-18Z. There is a chance for a few showers and storms to
move through the area in the morning but confidence is low.




NEAR TERM...Stumpf

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