Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 182040
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
340 PM EST Mon Dec 18 2017

.SHORT TERM...(This evening through Tuesday night)
Issued at 340 PM EST MON DEC 18 2017

20z sfc analysis shows high pressure to the south with low
pressure to the north. In this pattern the air mass has not been
able to sufficiently change enough to clear out the ample low
level moisture trapped under an inversion that is keeping the
clouds in tight. In addition, patchy fog and drizzle still plague
the area - primarily on the ridge tops. Temperatures are still
rather mild despite the clouds with readings now up in the upper
40s most places with low 50s noted in many of the valley spots.
Winds, meanwhile, continue to blow from the southwest at 5 to 10
kts.

The models are in good agreement aloft through the short term
portion of the forecast. They all depict a northern stream wave of
weak energy passing by to the north this evening while a
southwestern trough bottoms out in northern Mexico. This wave
will slowly move east into Tuesday pushing downstream ridging up
into Kentucky. However, fast flow will persist over Kentucky with
energy from the southern stream moving through the Tennessee
Valley late Tuesday and into Tuesday night. As the wave opens up
and approaches Kentucky more energy moves in by Wednesday
morning. The trend has been for this wave and its impacts to shift
further north with each run. So have followed suit with increasing
confidence. With the models in good agreement handling these key
features will favor a model blend along with a healthy lean toward
the higher resolution NAM12 and HRRR tonight and into Tuesday.

Sensible weather will feature another mild but damp night. Low
stratus and patchy drizzle/fog will continue to affect most of the
area - especially on the ridges and in the higher terrain. With
sunup on Tuesday we should see the stratus start to break up.
There are indications in the models that this would occur from
east to west. Expect a fairly warm day with only limited sunshine
on Tuesday as high clouds from the next system move in around the
same time as the low ones break up. Southwest winds will continue
to support above normal temperatures and a fair amount of
moisture in this air mass. The center of the next low pressure
system will pass to the south of Kentucky with a well defined
comma head crossing through the southern half of the CWA start
Tuesday night. Warm conditions ahead of this system will prevent
any mix concern on the leading edge - it is possible later on in
the wake of the low passing by to the south and east that more
cold air gets pulled in and we will have to look as a mixed pcpn
potential on the northern fringe of the departing rain shield
- but that is just beyond the short term portion of the forecast.

Again with the clouds and moisture tonight into Tuesday did not
deviate too far from the ShortBlend temperatures. Did raise PoPs
a notch through Tuesday morning to keep up with the drizzle
threat. Also brought the higher PoPs in quicker on Tuesday night
with that inbound southern stream low.

.LONG TERM...(Wednesday through Monday)
Issued at 320 PM EST MON DEC 18 2017

A bit of a complex forecast in store for the extended. The latest
model data is suggesting an active pattern that will feature three
periods of active weather. The first would be Wednesday and
Wednesday night, followed by Friday through early Saturday evening,
with a third active period from Sunday morning through Monday
morning. An area of low pressure is forecast to move out of the
southern Plains and across the lower Mississippi and Tennessee
valleys Wednesday and Wednesday night. This system will bring
scattered to numerous rain showers to eastern Kentucky to begin the
period. Temperatures should remain warm enough to keep any
precipitation we get in liquid form. The models have been pretty
consistent with the evolution of this system and the timing, extent,
and amounts of its associated precipitation, so confidence is pretty
good with this initial wave of rain. The second period of
precipitation is forecast for Friday through Saturday evening. The
models differ here slightly with regards to timing of the system,
with the GFS being slightly faster than the ECWMF. The forecast will
reflect a slightly modified model blend that incorporates the timing
differences mentioned above. Once again we expect temperatures to
remain warm enough to keep any precipitation with this second
weather system in liquid form.

A good deal of uncertainty comes into play with the third period of
active weather in the extended. Timing differences exist amongst the
various models, with one model having a wide swath of precipitation
moving into eastern Kentucky by early Sunday morning, while another
model has no precipitation moving into the area until Sunday evening
at the earliest. The blended model was adjusted toward the typically
reliable ECMWF model for the Sunday through Monday time frame. The
onset of precipitation on Sunday was slowed down more than what the
stand along ECMWF model was showing and slightly faster than the
blended model data had. The new forecast will show some rain and
snow making their way into our southern counties between 10 and 12Z
on Sunday, with a gradual northward progression of the mixed precip
through out the morning. By early Sunday afternoon any ongoing
precipitation should be in the form of all rain. The model data is
currently trying to producing a decent snow event for Christmas eve
night and early Christmas morning along the Ohio River, north of the
river in particular. At this time the forecast will be calling for a
mix of rain and snow for Sunday night, with mostly snow ongoing
across the area by early Monday morning. There is alot of
uncertainty in the model that far out, however, so the very end of
the extended will be taken with a grain of salt at this time.

Based on the latest model data, and trends therein, temperatures in
the extended will be well above normal across the area. Particularly
toward the end of the week, when persistent southerly and
southwesterly winds become established across the Tennessee and Ohio
valleys. These winds will set up in the region between an area of
high pressure that will be parked off the southeastern CONUS, and an
area of low pressure aloft that is currently progged to be taking
shape over the central Plains and moving eastward Thursday night
into Friday. Highs on Thursday are expected to top out from the low
50s in our far north to the upper 50s along the TN border. On Friday
conditions may be even warmer, as by then we will be firmly
entrenched in the warm sector of an approaching cyclone. Highs on
Friday may climb as high as the upper 50s and lower 60s across the
area. Temperatures will then become progressively cooler, as the
upper trough exits the area, and cooler air moves in from the north
and northwest. Sundays highs should only make it into the 40s for
most of the area.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
ISSUED AT 105 PM EST MON DEC 18 2017

Most sites remain at IFR or lower flight categories this
afternoon mostly due to low CIGs. This remains the case thanks to
a low level inversion trapping moisture closer to the surface.
These conditions will hold for much of this TAF cycle with the
potential of some improvement after sunup on Tuesday. The drizzle
remains patchy and will continue that way through the rest of the
day and into the night - but am not expecting any sustained vis
restrictions for TAF purposes. Look for The winds will remain
rather light this period generally out of the southwest with the
highest speeds - up to 10 kts - during the rest of the afternoon.

&&

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

$$

SHORT TERM...GREIF
LONG TERM...AR
AVIATION...GREIF



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