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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 241630

Area Forecast Discussion
National Weather Service Huntsville AL
1030 AM CST Sat Feb 24 2018

.NEAR TERM...(Rest of Today and Tonight)
Issued at 1030 AM CST Sat Feb 24 2018

Lingering light rain continues to push to the northeast across
areas east of I-65 this morning. This activity is expected to exit 
the area by noon, leaving mostly cloudy conditions in its wake. There
may be a few isolated showers elsewhere, but for the most part, 
expect mostly dry conditions across the area through mid afternoon. 
An upper level trough is slowly making its way east of the Rockies 
this morning, as the subtropical ridge remains anchored over southern
FL and the Bahamas. This setup continues to place the TN Valley 
under southwest flow through much of the vertical column, keeping a 
warm and moist influx of air across the region. At the surface, a 
cold front extends from its parent low over central OK, southward 
through TX. Meanwhile a warm front stretches eastward from the low 
through central Arkansas and into Kentucky. Several areas of showers 
and thunderstorms were observed on area radars to our west, within 
the warm sector of this system. 

Looking through the remainder of the day, the aforementioned surface
low is expected to lift to the northeast as the upper trough
continues its eastward progression. The trough is expected to take on
a more negative tilt as it moves into the eastern Plains and
Mississippi Valley this afternoon. Ahead of this, several waves may
move northeast and through the TN Valley, enhancing the lift across
the area. This may generate some elevated convection later this
afternoon. One caveat that we will have to monitor is the extensive
cloud cover across the area. Over the last hour or so, some breaks in
the clouds have been observed, but skies generally remain broken to
overcast across the area. Forecast soundings suggest enough mixing to
erode these clouds through the afternoon, with a mid level inversion
also eroding as we get better insolation. This will put our forecast
temps this afternoon right at or slightly above forecast convective
temperatures, so if we can get any lift in the area, isolated to
scattered thunderstorms will be possible. The better chance for this
would generally be after 21Z and continuing through sunset. Despite
the ample instability expected this afternoon, the low level jet will
still be displaced to the west, so organized convection is not an
huge concern this afternoon. However, we could see an isolated 
strong storm that could produce gusty winds and hail. 

As has been highlighted over the last few days, a line of severe
thunderstorms will develop this afternoon west of the Mississippi
River and slowly move eastward through the day. Forecast guidance is
still highlighting the time of arrival into the TN Valley around
midnight and then quickly moving out of the area around daybreak
Sunday. The upper trough will continue to push to the east-northeast
through the evening hours, with the low level jet reaching the TN
Valley between 03Z and 06Z, right as the QLCS is entering the
forecast area. Deep layer shear will be on the order of 50 to 60
kts, maintaining the organization of the QLCS as it moves into NW AL.
The question remains how much instability will still be in place and
if we are able to realize any surface based storms. At this point, 
it still appears likely that areas along and west of I-65 will still 
maintain some instability, despite it being overnight. This will 
combine with the decent low level shear to give us an isolated weak 
tornado threat as the line moves in, but the main threat continues to
be damaging straight-line winds. The threat diminishes further east,
as the upper jet races off to the northwest and the area stabilizes 
during the early morning hours on Sunday. Overall, the flash flooding
threat will be dependent on rainfall rates, as the quick movement of
the line will limit the threat for flash flooding. However, with the
saturated conditions, some of our lower lying areas may see some 
ponding of water. 

.SHORT TERM...(Sunday and Sunday night)
Issued at 335 AM CST Sat Feb 24 2018

The front is then expected to pass to the southeast by late Sunday
morning into the early afternoon. Models are showing that it could
stall along the I-20 to I-85 corridors before another embedded
shortwave trough (in the subtropical jet oriented across the region) 
could bring additional showers northward towards north Alabama on
Sunday afternoon and evening. Ongoing POPs/Wx only needed modest
adjustments to account for the latest model trends. One note about
the front: the extent of the cool air behind the front that arrives
here is modified and will only cause a modest decrease in the 
daytime highs (compared to early cold front passages) to the low 60s.
The more substantial temperature change will be noticed on the 
overnight lows (in the 40s) as drier air arrives.

.LONG TERM...(Monday through Friday)
Issued at 335 AM CST Sat Feb 24 2018

Decided to keep a very small chance of rain in on Monday morning for
portions of NE AL. Models disagree on how quickly the rain will be 
out of the forecast area as a shortwave moves into the TN Valley. The
front that passed through on Sunday will be located to our south and
will be where the axis of heavier rainfall will be. The ECMWF is the
wetter and slower solution so tended to lean more towards the GFS 
and NAM for this system. 

A sfc high building into the region on Monday will push the front 
down to the coast and filter in "cooler" and drier air. Using cooler 
in quotes as temperatures will still remain above normal. Dry 
conditions will persist into Tuesday. Monday will see highs in the 
low/mid 60s with overnight lows in the 40s, while Tuesday will see 
the mid/upper 60s and lows in the 50s. 

Models agree that an upper low will swing out of the Southwest U.S 
on Wednesday. They diverge from there on how quickly it moves east 
and if it phases with a northern stream of energy or not. The GFS 
does phase and develops a deeper sfc low which lifts into the Great 
Lakes region. The ECMWF tries to phase but ends up developing a 
secondary sfc low in the MS Valley, weakening the sfc low heading 
into the Great Lakes. This solution is much slower and brings more 
rainfall. Between the Canadian clearing out the fastest and the ECMWF
so slow, will again follow close to guidance in the extended. So 
overall, we are looking at rain chances increasing Tuesday night as a
warm front lifts into the area, with ample moisture return in the 
warm sector on Wednesday. Kept in thunder for Wednesday and Wednesday
night as there is a little bit of instability in the soundings. 
Soundings also show a very good wind field Wednesday night into 
Thursday ahead of the cold front, so will keep an eye on additional 
model runs. Precip will taper off Thursday night with drier 
conditions expected on Friday. 


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 558 AM CST Sat Feb 24 2018

MVFR cigs ~1500ft have already moved into KMSL and will enter into 
KHSV by 14Z. Cigs will temporarily return to VFR this afternoon. 
Light showers this morning, then thunderstorms this afternoon 
increasing in coverage overnight with an approaching cold front. SSW 
winds will be breezy through the TAF period 12-14kt with higher 
gusts. Thunderstorm wind gusts are possible but difficult to time but
should be between 03-10Z. Precipitation will start to decrease in 
coverage and cigs will start to improve by the end of the TAF period.





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