Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 231138 AAA

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
638 AM CDT Thu Mar 23 2017

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 402 AM CDT Thu Mar 23 2017

Quiet weather prevailed over and near the Tennessee Valley, courtesy
of surface high pressure situated across the Great Lakes. An
anticyclonic flow around this high was bringing light northeast to
east winds across the forecast area. Given this high`s Canadian
origin, it has produced seasonably cool night time low temperatures
so far across the area, ranging in the upper 30s to mid 40s. Normal
lows today in Huntsville and Muscle Shoals are 44. The GOES-13 and
GOES-R satellite view indicated some low/mid clouds ~5-10kft forming
across the region. Higher altitude clouds were noted to our north and
west, moving this direction.

Upper level ridging in place across the southern CONUS was
progressing across the region, as troughing amplifying along the
southern Continental Divide, and a few other systems waiting in the
wings across the northern Pacific head eastward.

A somewhat warmer day is expected for the Tennessee Valley, as a
southeasterly flow becomes more established. This flow and warm air
advection noted by increasing thickness values will make for highs
rising a good 5-10 degrees warmer, ranging from the mid 60s the
Cumberland Plateau and southern Appalachians of our eastern areas, to
the lower 70s over northwest Alabama. The surface high should be
moving across the Mid Atlantic coast by the afternoon; with a
modified cold air damming effect making for cooler conditions to the
east. The warm up will occur, despite more clouds than sun. The
majority of the clouds today should be of the mid/high altitude
variety, allowing the sun to help in warming temperatures today.

.SHORT TERM...(Tonight through Saturday)
Issued at 402 AM CDT Thu Mar 23 2017

A southerly flow will become more established across the area
tonight as the high moves a bit further to the east. This will start
the process of bringing deeper Gulf moisture towards the forecast
area. Along with lower level moisture, clouds preceding the weather
system to our west will also overspread the region, resulting in
mostly cloudy skies. Despite the clouds, a milder and dry night is
expected with lows ranging from the upper 40s east (with cold air
damming fading there) to the mid 50s west.

Stayed with a dry forecast Friday and Friday evening, while the
system to our west continues to develop. Output from the various
models have the surface low "bottoming out" in the upper 990s mb
surface pressure Friday morning east of the Colorado front range,
before it gradually weakens as it moves further eastward. With a
deeper southerly flow, high temperatures on Friday should warm into
the lower 70s most locations under mostly cloudy skies.

The upper trough associated with the system progged to affect the
forecast area during Saturday will take on a neutral, to maybe a
slight negative tilt as it progresses eastward. Showers and
thunderstorms accompanying this system should start overspreading the
area from west to east, beginning after midnight. Given rather
consistent output from the models, stayed with a slower timing
regarding how fast the shower activity advances Friday night into

Saturday appears to be a wet day, with numerous showers and
thunderstorms in the forecast, especially during the afternoon and
evening. Model differences change a bit during Sat, with the ECMWF`s
depiction of the surface low a bit further to the south compared to
the NAM/GFS/Canadian. It was also a little slower bringing showers
eastward, with the GFS/Canadian the quickest. The NAM/GFS and
Canadian at this time were hinting at possible development ahead of,
or south of the forecast area. The first scenario especially from the
NAM perspective; could result in storms forming east of the main
line in the afternoon. Those storms and ones with the main line could
become strong to severe. Storms forming to our south would intercept
some of the moisture and instability, resulting in weaker storms
north of them. In any case, the main threats at this time appear to
be strong to damaging wind gusts and marginally severe hail (in the
stronger storms). A tornado threat still looks to be low, but would
increase if more discrete storms are realized Saturday afternoon.

.LONG TERM...(Saturday night through Wednesday)
Issued at 402 AM CDT Thu Mar 23 2017

The medium to long range is marked by an active pattern: one
shortwave trough departing, one moving through early next week, and
one potentially affecting the region at the very end of the period.
Given the amplitude of these features, confidence is moderate.

The main trough axis from System #1 will cross the area at the start
of the period. Depending on the outcome of convection earlier
Saturday, this could be the main event for the system. There is
sufficient instability and shear for a severe event; at this stage
this looks like primarily a damaging wind threat. Tornadoes can`t be
ruled out, particularly as one moves west towards Mississippi where
low-level wind shear is greater. Indeed, models/ensembles diminish
the storm coverage and intensity as the low moves into the mid-

Little will change behind this first system, with S-SW flow
continuing. That sets things up for System #2, which will already be
moving into the central Plains as System #1 exits. There is some
amplitude spread here, but the consensus points to weaker wind and
wind shear, but perhaps greater instability. There are some timing
differences here as well but the main timing looks to be Monday
night. Timing the associated frontal passage is problematic but
NAEFS/etc suggest a slower progression is better. The post-frontal
airmass will not be that much cooler (maybe ~5 degrees) than pre-
frontal anyway.

As System #2 slowly departs the stage, System #3 will be digging
deeply over the Desert SW. Model/ensemble solutions begin to diverge
here, as there is greater uncertainty regarding the overall pattern
and its blockiness. Given the pattern, the forecast will reflect a
slower solution more similar to the GFS than the ECMWF, which will
favor lower PoPs through Thursday. This could lead to a warmer and
drier forecast than what is indicated, but the forecast will stick
closely to default model blends for the time being.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 638 AM CDT Thu Mar 23 2017

VFR weather will continue into the evening. Surface high pressure off
of the east coast will provide a SE-S flow across the region. A storm
system slowly approaching from the west will result in clouds
increasing, especially this evening. This system, and deeper lower
level moisture should produce MVFR CIGs late in the TAF. Light E-SE
winds this morning will become SE 10-15kt, with some gusts near 20kt
in the late morning. SE winds should subside to around 10kt after
sunset this evening.





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