Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 231136

Area Forecast Discussion
National Weather Service Huntsville AL
536 AM CST Tue Jan 23 2018

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 237 AM CST Tue Jan 23 2018

GOES RGB imagery early this morning showed a few lingering clouds
clearing our eastern counties. But back to the W/NW a rather
expansive cloud shield is moving towards the TN Valley. This feature
is tied to the upper level system that is moving across IL/IN this
morning. Models are a bit mixed in just how far south the clouds make
it today. Due to the system starting to pivot more N/NE through the
day think clouds should be limited to location north of the TN River
and more likely along the AL/TN border.

The clouds will have an impact on temps and have nudged values down a
degree or so for our southern middle TN counties (mid 40s). The rest
of the will only be a few degrees warmer even with some sun due to
strong CAA spreading into the area. It`s kind of interesting looking
at current observations and MSL pressure fields hinting at 3-4
"fronts" associated with this system. The one that moved through
earlier this evening, another is moving across AR/OK, and another
over the northern plains. With the main upper level trough axis
swinging across the area today expect winds to remain on the breezy
side with gusts of 20 to occasionally 25 mph possible.

.SHORT TERM...(Tonight through Thursday night)
Issued at 237 AM CST Tue Jan 23 2018

A front will move across the area this evening switching winds to the
NW and sending somewhat drier air into the region. Dewpoints should
drop into the mid to upper 20s tonight. With the upper level system
peeling off to the east, a surface high settles in over the ArkLaMiss
region. Winds drop off some but a quick moving shortwave dropping
into the area on Wednesday morning will keep some slight breezes
around. Lows Wed fall into the upper 20s but a few of the more wind
sheltered valley locations could see temps drop into the low to mid
20s. Sunny skies in the afternoon and a break down in the CAA will
bring temps to normal for this time of year (lower 50s).

Thursday morning should be the coldest for the week as high pressure
is solidly over the area and with little in the way of winds. Expect
good radiational cooling sending temps into the low to mid 20s.
Guidance is on the higher side but models struggle with these good
radiational cooling events and have nudged values down a few degrees.
Winds shift to the south during the day and with upper level heights
increasing surface temps are going to be able to warm into the mid
50s. For Thursday night into Friday morning, increasing southerly
winds should keep temps right around freezing for much of the area.

.LONG TERM...(Friday through Monday)
Issued at 237 AM CST Tue Jan 23 2018

A deepening upper-level trough over the Great Plains will swing into
the Ohio and Tennessee Valleys this weekend, bringing with it a cold
front and the best opportunity (potentially) for wetting rains we`ve
seen in a while. Guidance continues to suggest very little to no
appreciable instability will be available during the passage of the
front and it`s associated convection -- so have kept thunder out of
the forecast.

Ahead of this system on Friday, winds will gradually veer from the
SSE to the SSW/SW, helping to advect richer Gulf moisture into the
region. The southerly flow and earlier sunshine will help to warm us
to near 60 degrees across much of the Valley. Think that models may
be underdoing temperatures a hare on Friday for these reasons and
have trended slightly above guidance.

Attention quickly turns to this weekend and the aforementioned low
pressure system. A weak shortwave will streak NE from the lower
Mississippi Valley into the Mid South Friday night, providing some
weak lift to generate a few light showers, especially west of I-65
after midnight. This feature will also help to veer winds to the SW
and enhance low-level moisture transport as PWATS increase to over 1
inch by 12z Saturday.

Some substantial differences exist with respect to the timing and
intensity of this system which will have an impact on the exact
details of this forecast. The GFS solution shows a broader and more
progressive longwave trough that will quickly move from the Central
Plains into the Ohio Valley Saturday afternoon and evening. This
would bring about a faster start (and end) time for the accompanying
rain showers and the initial frontal passage would clear the area
late Saturday night into Sunday morning. Something else of note with
the GFS solution would be the MCS feature it wants to develop along
the MS/AL/FL Gulf Coast. Should this occur, moisture (and any
potential instability) will be robbed, cutting back on shower
coverage and QPF further to the north over the Tennessee Valley.

On the contrary, the ECMWF/CFC solutions continue to show a slower
moving, but deeper, more magnified upper-trough developing over the
Great Plains Saturday evening. That timing would move the cold front
east through the Tennessee Valley during the day on Sunday. This
stronger trough and subsequent cold front (as well as the slower
forward movement) would allow for a better chance for shower
coverage and a high QPF potential, especially from late Saturday
night into Sunday morning. Given some of these differences, have
opted to stick with the model blend. However, slightly favored the
slower ECMWF/CFC solution, highlighting the higher PoPs and QPF
during the Saturday night/Sunday morning period.

By Sunday evening/night, a colder air mass with much drier air will
quickly fill in behind the front, bringing a swift end to
precipitation from west to east. The colder air will bring low
temperatures to near or below the freezing mark once again Sunday and
Monday night. However, with high pressure quickly building in by
early next week, highs will not fall off too much -- generally
remaining at seasonable levels in the lower 50s.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 536 AM CST Tue Jan 23 2018

MVFR cigs associated with an upper level system remain to our NW and
could spread into the area by 18z. Satellite imagery has shown that
these clouds are becoming more SCT as they move south. So, MSL and
HSV terminals may end up with just SCT to briefly broken clouds.
Tonight cigs begin to lower as winds turn to the N/NW and may fall
below 2,500 feet. As of 1130z winds are below 10kts but these should
increase through the morning hours. SW gusts up to 20kts are
possible. Winds slowly shift from the SW to NW through the day.




NEAR TERM...Stumpf

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