Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 270209

Area Forecast Discussion
National Weather Service Huntsville AL
809 PM CST Sun Feb 26 2017

.NEAR TERM...(Tonight)
Issued at 808 PM CST Sun Feb 26 2017

Mid and high level clouds that were spreading over the TN Valley have
dissipated, but more cloud cover will begin to overspread much of
the TN Valley as a weakly amplified mid-level shortwave trough over
the Southern Plains slowly moves east. Made some tweaks to the cloud
cover forecast to slow the progression of mostly cloudy conditions.
Meanwhile, over the TN Valley, there is an antecedent dry airmass
that will be slow to saturate as this trough moves east overnight.

Hi-res model output continues to indicate that precipitation will be
slow to saturate the low-levels. As a result, have adjusted the
forecast for on-set of precipitation to a later timeframe--later on
Monday morning--with POPs gradually increasing from west to east
during the morning on Monday. With no instability present, due to
warm air aloft, rain showers should prevail on Monday.

Looking at the air and dewpoint temperature forecast: the air
temperature trend tonight looks to be on track with overnight lows
dropping into the upper 30s to low 40s as cloud cover gradually
hampers radiational cooling. Meanwhile, dewpoints have been slow to
`recover` tonight and will likely take the rest of the night to rise
to the low 30s. This is mainly due to weak southerly to ESE flow
does little to mitigate as upstream dewpoints are quite low (low 40s
on the Gulf Coast and low 30s over central GA/AL).

On another sky obstruction note: due to a prescribed burn today in
Bankhead National Forest, there may be some smoke/haze lingering over
northwest Alabama at around 5-10 kft for at least a few hours before
prevailing SW flow slowly disperses it.

.SHORT TERM...(Monday through Wednesday night)
Issued at 310 PM CST Sun Feb 26 2017

By Monday morning we should see a few returns on radar across the
western half of the area but due to the dry air mentioned above, this
will likely be mainly virga. As the system shifts towards the TN
Valley Monday morning, the lower levels quickly become saturated and
should see scattered to numerous showers. Forecast soundings show
little, if any, CAPE during the day Monday. So have opted to keep
thunder out of the forecast for Monday. However, a rumble or two will
certainly be possible. The cloudy and rainy conditions tomorrow
should keep temps on the cool side and values won`t differ too much
from today. Models are all in agreement that as the upper level
system and main forcing quickly lift off to the NE and the axis of
moderate to heavy rain begins to dive SE. If this is the case we
could see higher rainfall totals over the western half of the area,
where forcing will be highest and lower amounts over NE AL and TN.

Although the near surface temps/dewpoints will be slow to respond,
good WAA begins in earnest early Tuesday morning. Models are all in
pretty good agreement in ejecting a weak trough out of SW TX Monday
night and bringing it through the area Tuesday morning. This should
result in showers spreading SW to NE with the passing of the
shortwave. Forecast soundings do show some elevated instability
(<300 J/kg) and have added thunder to the forecast.

The forecast for the rest of Tuesday is a little uncertain in terms
of thunderstorm potential. If you were to look strictly at forecast
soundings the afternoon period would be of a concern due to CAPE
increasing to around 1,000 J/kg, shear 40-45kts, and curved
hodographs, suggesting the potential for strong to severe storms.
However, there are a few limiting factors and missing puzzle pieces
for Tuesday. The first being that absent of the morning shortwave,
there isn`t really any lift/forcing mechanism to speak of during the
day/afternoon period. The second is that surface based instability
would require temps in the upper 70s and the only location for that
is well to our south or over western TN. The third is that as we
progress throughout the day, both model analyses and forecast
soundings indicate that an EML/CAP originating from SW TX/Mexico
building into the area during the afternoon hours, which is the
result of increasing SWWLY winds aloft. So, based on all this the
threat for storms Tuesday is limited at this time and will ultimately
be dependent on any potential lingering boundaries.

A stronger and potentially more potent trough is forecast to eject
out of the desert SW on Tuesday. As it begins to lift NE through the
central US it does take on a very subtle negative tilt. Unlike past
events where the lift has either been well to our north or convection
along the Gulf has cut us off, we should remain in a location of
good lift/forcing. In addition to this, we will be under the
favorable right entrance region of the jet aloft. In terms of
parameters for Wednesday, CAPE will be on the order of 500-800 J/kg
and with the strong jet, deep layer shear increases to around 60 kts.
The question then becomes, what mode will storms take, there is a
brief window in the morning where storms could take on some supercell
characteristics. But as the shear becomes more unidirectional
throughout the day believe the mode will quickly transition to a QLCS
right ahead of the front.

Now with that in mind though, models the past few runs have varied on
the timing of the system and eventual cold front through the area.
The ECMWF remains a bit slower while the GFS is a bit faster and
earlier with the storms. In either case severe storms are likely to
impact the TN Valley Wednesday with the main threat at this time
being damaging winds and hail. Due to the lingering differences in
the models will continue to watch the next few runs to see if models
can come to slightly better agreement on the timing of storms through
the area. An FYI though, SPC has put the area under a Day 4 30% risk
with their main concern being damaging winds. So, we will keep a
close eye on the forecast.

.LONG TERM...(Thursday through Saturday)
Issued at 310 PM CST Sun Feb 26 2017

Precip should be completely out of the area by Thursday morning with
cooler and drier air quickly arriving behind the front. A 20 degree
difference in temps is expected between Wednesday and Thursday for
highs with values Thursday in the mid 50s. A system moving through
the Great Lakes Thursday into Friday will bring a reinforcing shot
of cold air to the TN Valley. Low temps Friday morning will be in the
low to mid 30s and in the upper 20s to lower 30s on Saturday
morning. The forecast for the remainder of the weekend and into next
week will be mild and rain chances return by Monday of next week.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 520 PM CST Sun Feb 26 2017

VFR conditions are expected to prevail across northern AL/southern
Middle TN for most of the forecast period. An approaching low
pressure system will gradually increase moisture and result in cloud
cover overspreading the area over the coming 6-12 hours. Then, -RA
is expected to develop after 27/13Z-27/16Z with the potential for
MVFR cigs late in period. Given some uncertainty with the evolution
of an antecedent dry airmass in place, have not included MVFR cigs
during the last 6 hours of the forecast (between 27/18Z-28/00Z).




LONG TERM...Stumpf

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