Area Forecast Discussion
Issued by NWS Jackson, KY

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343
FXUS63 KJKL 230244 AAA
AFDJKL

AREA FORECAST DISCUSSION...UPDATED
National Weather Service Jackson KY
1044 PM EDT Sun Oct 22 2017

.UPDATE...
Issued at 1044 PM EDT SUN OCT 22 2017

Temperatures in the eastern valleys have dropped into the mid and
upper 50s. A few more degrees of drop is expected, before the
combination of cloud cover and an increasing surface pressure
gradient keep readings more steady through the rest of the night.
The forecast remains on track, and have mainly freshened up the
hourly temperatures and dew points over the next few hours to
capture the latest trends in observations.

UPDATE Issued at 753 PM EDT SUN OCT 22 2017

Thicker high clouds are streaming in across eastern Kentucky from
the southwest. Despite the this, eastern valleys are decoupling,
with a few spots reporting in the upper 50s already. Have included
a ridge/valley split through most of the night, as precipitation
will be slow to move in thanks to persistent downsloped flow from
the surface to 850 mb. Have lowered POPs in the east through dawn
given the pattern, and then bumped everything up to 100 percent
through the day as the cold front eventually moves through the
area. Updates have been sent.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 300 PM EDT SUN OCT 22 2017

After yet another day of dry weather and well above normal
temperatures across eastern Kentucky, a pattern change will
finally begin to occur late tonight and Monday. A large trough of
low pressure aloft will be moving across the eastern quarter of
the CONUS tonight through mid week. A surface cold front
associated with the upper level system will begin moving across
the area on Monday. This boundary will bring widespread rain
showers to the area, along with an eventual wind shift from
southerly to westerly Monday night and especially Tuesday.
Rainfall amounts may vary from around three quarters of an inch to
as much as one and a quarter inches across the area, with around
an inch on tap area wide on average. This rainfall should do nothing
more than provide a good soaking of the ground. The rain should
taper off quickly late Monday night, with only isolated to
scattered showers left over by Tuesday morning. Tonights lows
should still be quite warm, with min readings in the upper 50s
expected for most locations. Temperatures will be much cooler on
Monday than the past few days, with highs generally in the upper
60s expected. This will be due mostly to the expected widespread
cloud cover and precipitation. Once the front passes through
Monday night, a much cooler air mass will begin to spill into the
region. This will bring low temperatures in the mid 40s to the
eastern Kentucky for Tuesday morning.

.LONG TERM...(Tuesday through Sunday)
Issued at 336 PM EDT SUN OCT 22 2017

The period is expected to begin with an upper low over the Great
Lakes and a trough extending south into the MS and OH Valley
regions. Meanwhile, an upper level ridge should be centered over
the Western Conus. At the surface, a deep surface low should be
tracking toward Ontario with the trailing cold front having moved
east of the Appalachians before the start of the period. A
shortwave trough is progged to rotate around the upper low into
the OH Valley late Tuesday into Tuesday night with another
tracking into the Great Lakes on Wednesday, before reaching the
Central Appalachians on Wednesday night. Behind this second
shortwave, upper heights across the OH Valley will rise with
surface and upper ridging working across the Southeast Conus and
Ohio Valley. Also through midweek, a trough is expected to move
over the Western Conus/Eastern Pacific ridge through midweek and
into Canada.

The ridge along the west coast is expected to amplify from the
middle through the end of the week while a downstream trough is
expected to develop over the High Plains/Central U.S. Models vary
in the evolution of and timing of this trough. This leads to
timing uncertainties late in the period with a cold front moving
east which would affect precipitation chances and temperatures
from later Friday through the end of the period. Most recent ECMWF
runs have generally been further west initially in developing and
then slower with moving the trough east. This scenario would
result in the best chances for precipitation later in the weekend
and perhaps more prolonged chances per 12Z/22nd ECMWF. The colder
12Z GFS had more in the way of instability driven showers at the
end of the period so keeping pops into Sunday seems reasonable
attm.

Unsettled weather with clouds and a small chance for showers or
sprinkles is anticipated on Tuesday with the upper low working
across the Great Lakes and OH Valley. Temperatures should average
at least 5 degrees below normal. The unsettled weather will
continue into Wednesday evening before the 500 mb trough axis
moves east. Periods of low clouds and possibly a shower or
sprinkle cannot be ruled out during this time as several
shortwaves rotate through the region though moisture will be
limited and shallow. On Wednesday, with the 0C 850 mb isotherm
dropping south of the area per model consensus, temperatures may
struggle to reach 50. The degree of clearing on Wednesday night is
uncertain, which would affect low temperatures and the chances
for any frost. At this time, continued with frost potential in the
weather grids for Wednesday night though confidence is below
average as the center of sfc high pressure will be well south of
the area.

Temperatures will then moderate from Thursday into Friday ahead
of the next cold front and upper trough. At this time, it appears
that that system should bring another decent chance of at least a
wetting rain to the area when it arrives toward the weekend.
Behind that front, substantially colder air may follow especially
if the GFS verifies. However, the recent ECMWF runs generally
continue to give the OH Valley region more of a glancing blow of
colder air by the end of the period or after. Again, the GFS runs
remain substantially colder by a week from now.

Generally did not stray far from the model blend during the
period as it offered a reasonable compromise between the guidance.
If the GFS were to verify, temperatures would end up colder than
the current forecast by next weekend.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
ISSUED AT 753 PM EDT SUN OCT 22 2017

VFR conditions will prevail through the night, as mainly high
clouds move in from the southwest. Towards dawn, ceilings will
lower from west to east, with MVFR conditions and widespread
showers gradually overspreading the area during the daytime. A
low level jet will strengthen overnight and persist into the mid-
morning hours on Monday. This will provide a threat of low level
wind shear across eastern Kentucky, generally from 09 to 15z.
Light southeast winds will veer to the south and increase to 5 to
10 kts Monday morning. Surface winds will then become southwest by
late Monday afternoon as a cold front pushes east across the
area.

&&

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

$$

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



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