Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 102340
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
640 PM EST Sun Dec 10 2017

.UPDATE...
Issued at 640 PM EST SUN DEC 10 2017

The evening surface analysis shows an area of high pressure off
to our southwest and a weak surface low to our northwest. We
remain more mixed here with winds out of the southwest this
evening. The issue tonight will be the valleys decoupling, and
for now have continued with the current trends of lower 20s in the
deeper valley locations. The next tricky item that will also have
a impact on temperatures is a mid to lower deck of clouds
skirting the far eastern portions of Kentucky. This will lead to
some difficult temperature curves and have adjusted toward the
latest obs and trends to account for this.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 324 PM EST SUN DEC 10 2017

As of mid afternoon, an upper level trough was in place from
eastern Canada south into the Southeastern Conus with ridge in
place across the western Conus. Upper level winds across the
region were out of the west northwest to northwest with weak
shortwaves moving through from time to time. A band of mid level
clouds associated with a mid level shortwave is working across
northern and far eastern KY while cirrus is also passing through
the OH Valley. At the surface, high pressure extends from the
Southern Plains across the southeastern Conus with ridging north
into the Appalachian region while an area of low pressure is well
north of the Great Lakes.

Mid level height rises rises are expected on average through early
on Monday with the upper level flow becoming more westerly while
sfc high pressure is expected to remain in place across the
southeastern states. Periodic bands of clouds should moving
through the area tonight and models due indicate an increase in
lower level moisture, mainly late. This might be sufficient for at
least some scattered stratus or stratocu toward dawn. This
possibility leads to a low confidence low temperatures forecast,
however, due to mostly clear skies or mainly cirrus anticipated
during the evening points toward the potential of a quick evening
temperature drop in the valleys and at least a small to moderate
ridge/valley temperature split. COOP MOS guidance hints at some
upper 20s for the normally colder spots, which is possible if
skies remain generally free of low and mid level clouds
considering current dewpoints. For now opted for some low to mid
20s for the valleys and upper 20s for more open terrain areas and
ridgetops.

With stronger return flow as the axis of surface high departs
further to the south and east and the next clipper system
approaches as it drops into the mean eastern Conus/eastern NOAM
trough, temperatures will moderate to near normal if not slightly
above normal for highs on Monday. Low clouds will be possible
early with mid and high clouds expected from time to time in
advance of the next clipper.

Clouds with the approaching clipper system are not expected to
thicken and lower much before late on Monday evening again leading
to the possibility for another quick evening drop in temperatures
and lows in eastern valleys near midnight possibly in the upper
20s. Temperatures in those locations would rise toward dawn if
they make the evening dip as the pressure gradient increase and
clouds lower. Moisture increases for late Monday night and may be
sufficient for light precipitation prior to dawn. The model
guidance hints at some potential for a minimal amount of ice in
the clouds for late Monday night or marginal for ice in the clouds
though guidance indicates this moisture should begin to deepen
before the end of the period. At this point, have isolated to
scattered pops for snow showers late in the period.

.LONG TERM...(Tuesday through Sunday)
Issued at 245 PM EST SUN DEC 10 2017

The models are in generally good agreement with an amplified long
wave pattern to remain in place through the end of the work week,
before transitioning to more zonal-like flow for next weekend. For the
Ohio and Tennessee valleys, this will mean a continuation of well
below normal temperatures through Friday,  well as a few bouts of
light snow/rain at times. Temperatures will then rebound to the 50s
for highs by next weekend, with at least a small chance of rain
threatening the region.

On Tuesday, cold air advection will be ongoing, as an upper level
trough is reinforced across the Eastern Conus. Some upslope snow
showers will be on tap for eastern Kentucky through the day, and
given the available forcing and favorable wind direction, have gone
slightly higher than the blended POPs. POPs will taper off into
Tuesday evening, with middle teens expected for lows by Wednesday
morning. Have favored the valleys west of I-75 for the coldest
temperatures, given the thinner clouds.

Another front will approach Thursday into Friday, bringing
additional small chances of rain/snow, depending on the timing and
temperature profile. This system looks weaker, and the ECMWF and GFS
differ on the evolution of the small scale features. Temperatures
will continue to average below normal through Friday, before a warm
up ensues for the weekend, as heights recover across the area.
Another front will approach sometime this weekend, but given the
model disagreement, have undercut the POPs a bit until the
guidance can gel better.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
ISSUED AT 626 PM EST SUN DEC 10 2017

We see a surface high to our southwest and a weak surface low to
our northwest to begin the period. This is keeping us VFR with
the mix of mid to high level clouds across eastern Kentucky. Some
of the models continue to hint at some low level moisture and
potential stratus overnight. The blended guidance and MOS tend to
keep this more northeast of the region. This as a weak shortwave
moves southeast through the larger upper level trough in the
eastern CONUS this evening. Given this did add a lower FEW
coverage deck to the more eastern TAF sites of SJS/SYM, but did
cap this at 5 kft given the lack of confidence for a lower deck.
Winds have decreased this evening, but will remain light out of
the south and west.

&&

.JKL WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...DJ
SHORT TERM...JP
LONG TERM...GEOGERIAN
AVIATION...DJ



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