Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 261742

Area Forecast Discussion
National Weather Service Huntsville AL
1242 PM CDT Wed Apr 26 2017

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 1005 AM CDT Wed Apr 26 2017

We are warming up quickly today with temps as of 10am running about
8-10 degrees above yesterdays values. 12Z soundings from around the
region showed about what we were expecting for today with a sizable
warm layer between 850-700mb with temps of 17-20C. Further to the
west the edge of the surface and deeper moisture plume is delineated
well based on latest satellite imagery by the cloudy/cloud-free areas
over the ArkLaMiss. Dewpoints over the TN Valley have remained in
the upper 50s to around 60 degrees but based on the soundings and
expected mixing of the drier air aloft downward, these values should
drop a bit this afternoon. The only exception may be NW AL where some
of the higher dewpoints in NW MS/SE AR could filter in.

The good mixing mentioned above is the result of a tightening
pressure gradient and LLJ that will edge into the area later today.
Winds to our west are already increasing with gusts of 20-25mph noted
around Memphis. These gusts will build eastward and by this afternoon
the drier air combined with the stronger winds and warm temps aloft
should result in temps in the mid to potentially upper 80s this
afternoon. Tweaked a few temps but as previous shift mentioned have
gone above raw model guidance as they are a bit on the low end for

A cold front now located over AR/OK/TX is generating thunderstorms
currently and more are forecast to develop this afternoon. These will
congeal into a line and shift quickly east towards the TN Valley late
this evening. More discussion of these storms and the potential for
strong to severe storms will be discussed in the short-term section

.SHORT TERM...(Tonight through Friday)
Issued at 307 AM CDT Wed Apr 26 2017

By the start of the tonight period/00z tonight, the front should be
in eastern Arkansas and will quickly move east thanks to strong mid
and upper level forcing as well as upper jet dynamics. The squall
line is forecast to get to NW AL around 06z and this is fairly well
forecast by the range of model guidance. Thunderstorms are likely as
this squall line moves east but the severity of those storms are
questionable. One quick thing to note- with the split jet stream,
the GFS/ECMWF are showing the local area getting "split" with the
highest QPF to the north and to the south.

With or without the split, there is a lot working against the threat
for severe weather. For one, the southeast flow that builds back in
this afternoon should remain until the frontal passage which will
make it difficult to get dew points much higher than the mid to upper
50s. Guidance is showing a surge of moisture ahead of the front with
the S-SW LLJ but only briefly. This combined with steep mid level
lapse rates allows some instability to be present. There is a range
of ideas for how much CAPE but SREF plumes showed an average of
600-800 J/KG of MUCAPE. Note- this is all elevated instability with
little to no surface based. This sits well with the fact we are well
capped through the night with the NAM trying to show us breaking the
cap for an hour or two which would be possible but we would still be
too saturated to get anything surface based. Speaking of saturation,
we are going to be very dry today so it`s going to take a lot to
overcome the dry air as well.

Shear is the only thing we would need to watch as 0-1km shear is
around 35-40kts and 0-6km shear around 70 kts. Low level helicity
will be in the 200-300 m2/s2 range with the front as it moves
through. This leads me to believe the squall line will be able to
sustain itself as it moves east and a few cells could become strong
if the elevated instability is realized. Wet bulb zero heights are
high but given the elevated instability, small hail is possible.
Thinking strong winds would be the greatest threat given the mid
level dry air and the ongoing gusty surface winds to around 30mph.
Similar to what the day shift was forecasting, there could be a brief
window in NE AL from 9-12z where we could see a longer time for the
dew points to rise as well as the LLJ passing over then and the upper
jet finally nosing in. However, the upper lift passes well to the
north at this time. I don`t think this is a high possibility but it`s
worth a mention.

The line should move through fairly rapidly and guidance isn`t
showing the front getting held up over the area anymore. Dry air
moves in pretty quickly so skies should clear by the late afternoon.
The front will bring a brief relief to the warmer temps with highs
in the mid to upper 70s. Clouds will start to increase again Thu
night ahead of the next upper system with lows in the middle 50s.

A warm front starts to track north towards the area on Friday with a
warm, moist southerly flow returning, with the help of another
developing LLJ. Dew points are forecast to rise rapidly on Friday
with some guidance, such as the GFS, suggesting they get into the
lower 70s by the afternoon. This seems a bit aggressive and quick to
me so will continue with blended values in the middle 60s.
Instability is quick to increase but with little moisture aloft and
no upper lift, just have slight chance for thunderstorms for Friday.
Highs will be back into the upper 80s.

.LONG TERM...(Friday Night through Tuesday)
Issued at 307 AM CDT Wed Apr 26 2017

As the warm front from Friday afternoon/evening continues to move
north, a few isolated lingering storms could meander or develop over
central TN and backbuild over portions of southern TN/northeast AL
overnight Friday night into early Saturday morning. Meanwhile, the
ridge over the western Atlantic will continue to build over the
Southeast which will act to advect the maritime tropical airmass
further north on Saturday. The result, will be a conditionally
unstable environment with hot sfc daytime highs near 90 and sfc
dewpoints in the mid to upper 60s by Saturday afternoon (hot/humid
conditions). Mesoscale outflow boundaries from early convective
activity along with orographic lift on Saturday afternoon may cause
isolated afternoon/evening thunderstorms--especially over northeast
AL/southern middle TN. Current model runs have between 1500-2500 J/kg
of SBCAPE which could lead to a small hail threat with freezing
levels around 11-12kft. Any storm activity should then diminish but
may not completely end with the elevated instability and outflow
boundaries moving over the area. As a result, have left thunderstorms
in the forecast for Saturday night. QPF may be a bit overdone for
both Saturday and Saturday night, and further adjustments may be
necessary as we approach the day of.

Further to the west a strong LLJ and dynamical amplifying trough
will be bringing severe weather across the Plains to the MS river
Valley on Saturday. The progression of this trough has been
fascinating to watch as the model depictions of the downstream ridge
has changed at times. At this point, the lead shortwave trough
within the amplifying longwave trough axis looks to become
negatively tilted over the Corn belt region as it moves northeast on
the NW flank of the ridge. As it does so, an even stronger LLJ will
back to the south which combined with some cooling aloft will create
a modestly unstable, but high shear environment. In terms of the 850
mb jet streak, the latest model runs have between 40-55 kts overlaid
across the area. In addition, forecast soundings depict a slight
veering profile and curved hodographs on Saturday afternoon as what
looks like a strong to severe QLCS with embedded supercells moves
towards NW AL ahead of a cold front. The arrival timing is still a
question mark, but still looks to be at least arriving as a severe
line perhaps during the afternoon over NW AL. Then, the QLCS should
continue to move east. Right now, given the high amount of shear and
enough instability, all impacts could be possible (damaging winds,
large hail, and tornadoes) especially over NW AL. However, this
overall severe trend could change with the models as the jet streak
responsible for the formation of this trough is still over the
Pacific. An additional impact that is more related to the dynamical
nature of this system is that non-thunderstorm winds could gust up to
25 MPH in some areas.

Convective activity could continue until at least the early morning
hours on Monday morning. After the front and trough axis moves
across on Monday, expecting moderated temperatures with highs close
to 70 degrees. This cooler weather will be short-lived as westerly
flow aloft and sunny conditions should modify the airmass quickly on
Tuesday with highs back close to 80.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1242 PM CDT Wed Apr 26 2017

Southerly winds have ramped up this afternoon with gusts of 15 to
20kts. These are expected to continue through the afternoon and
evening hours. Thunderstorms are developing to our west and are
expected to spread eastward and potentially arrive into AL between
05-07Z. There is uncertainty in the timing of the line and if the
line will be able to hold together. Have kept the VCTS in the TAFs
for tonight to handle the uncertainty. Low clouds linger behind the
line frontal passage tonight and MVFR conditions are forecast to
continue through mid morning Thursday.




NEAR TERM...Stumpf

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