Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 040824
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
324 AM EST Fri Dec 4 2020

.UPDATE...
Issued at 1111 PM EST THU DEC 3 2020

As warm and moist advection continues across the region, the
potential for any additional sleet mixing in across northern
sections is waning. However, rain is reaching the ground from the
returns across the region (a trace of rain has fallen so far at
JKL) and some adjustments were made to pops over the next few
hours to include slight chances a bit further southeast and
increase pops a bit in the Lake Cumberland region to near the I 75
corridor. KY Mesonet sites indicate rain over south central KY has
measured and now has recently measured at KY Mesonet sites in
Pulaski and Wayne counties. Otherwise, adjustments were made
based on temperature and dewpoint trends. Steadier rain is still
expected to overspread the region later tonight.

UPDATE Issued at 840 PM EST THU DEC 3 2020

Showers that have moved across portions of central KY have had
some sleet reported to have mixed in. These showers are moving
northeast into East KY at this time. These will continue moving
northeast over the next couple of hours, though the chances for
any sleet mixed in will be greatest along or north of the Mtn
Parkway over the next couple of hours. Temperatures are several
degrees above freezing so there will be no accumulation where any
sleet falls. Steadier rain should overspread the region from the
southwest later tonight into early on Friday. Pops have been added
to the grids along with a bit of sleet mixed with rain as P type
over the next couple of hours with hourly temperature grids also
freshened up

&&

.SHORT TERM...(This evening through Friday night)
Issued at 503 PM EST THU DEC 3 2020

The latest upper level pattern features highly amplified flow.
Deep troughing is found across southeastern Canada, with an upper
level low located just east of Hudson Bay. Cyclonic flow from
this system extends into the Great Lakes/New England regions. The
trough then aligns to the southwest, becoming more positively
tilted, as it stretches through the Middle Mississippi Valley and
continues through the Baja California. An elongated cutoff low is
showing up nicely on the latest water vapor satellite imagery,
currently positioned from central Missouri down through
northeastern Oklahoma. Sharp ridging remains aligned from the
Eastern Pacific through the northern Great Basin/Rockies and
into central Canada, while modest ridging is hanging in across
the southeastern CONUS. At the surface, high pressure is currently
centered near the Carolinas. Low pressure is seen across the Gulf
of Mexico, with an inverted surface trough nosed into the Lower
Mississippi Valley. Mainly middle and high clouds have been on the
increase across Eastern Kentucky this afternoon, with some weak
radar returns currently brushing our Bluegrass counties. Plenty of
cloud thinness earlier allowed for temperatures to rebound into
the lower and middle 50s for many locations south of the Mountain
Parkway.

The models have come into better agreement regarding the
evolution of the cutoff low to move from the Middle Mississippi
Valley to the Tennessee Valley tonight and Friday, before
swinging towards the Middle Atlantic region by early Saturday. The
majority of model guidance, including the latest GFS, is
depicting a more progressive solution. At the surface, the
inverted surface trough will develop to the northeast into the
Tennessee Valley tonight, with low pressure evolving across the
Southern Appalachians by late Friday, before continuing to deepen
as it heads towards the Middle Atlantic region by early Saturday.
For Eastern Kentucky, this will result in a period of wet weather
through the short term.

Clouds will continue to increase and lower tonight, with an 850
mb low level jet transporting moisture up and over the inverted
surface trough as it develops to our southwest. Rain will
overspread the area from southwest to northeast generally after
midnight. This first surge of moisture looks to setup more so
across the northwestern half of our area, and have favored a
sharper gradient with the PoPs. Lows tonight will be tricky, with
some of the cooler eastern and northeastern valleys likely
dipping into the lower to middle 30s. Still, following highs in
the 50s, the ground will be quite warm, and too many clouds may
very well thwart this more typical quick diurnal drop off usually
seen for this kind of setup. Elsewhere, temperatures will cool off
into the upper 30s to around 40. The rain will overspread the
entire area on Friday. This should keep the temperatures down, and
have favored more of a blended RAW guidance, with highs mainly in
the mid 40s. Precipitation will gradually taper off from west to
east Friday night, as the system develops further to our east. Ice
looks limited on the backside of this system, so do not expect
much in the way of snow, with the exception of the higher
elevations across southeastern Kentucky, where there may just
enough lingering moisture to allow for a few flakes. Either way,
there won`t be impacts from this, given the warm ground
temperatures. Lows will range from the lower 30s to the northwest,
to the mid and upper 30s across the lower elevations in far
eastern Kentucky.

.LONG TERM...(Saturday night through Thursday)
Issued at 323 AM EST FRI DEC 4 2020

A rather complicated pattern is setting up through Monday, making
this a challenging forecast at best. We`ll be watching several
shortwave vort maxes, two in the northern stream, and a third in
the southern stream, as they come together across the region late
in the weekend. Ahead of these approaching features, we should see
heights quickly climbing across the area for Saturday night into
Sunday, with a surface high nudging into the area from the north.
This should allow any remaining cloud cover to erode, with
temperatures falling well into the 20s in the valleys Saturday
night. A few upper teens cannot be completely ruled out.

By Sunday, two of the vort maxes will move into the region. How
much phasing that occurs between these two features remains in
question with the GFS keeping the waves separate through Sunday,
and the ECMWF/Canadian merging these two waves. Then, we will be
watching a the third vort max push in from the north, which may
absorb the other two features allowing for a deeper shortwave
trough to develop and strengthen a surface low off the coast of
the mid Atlantic states. This low would then move up the eastern
seaboard, but models presently keep this system off shore. So,
what does that mean for our area. Well, right now, forecast
remains very uncertain. The degree of phasing on Sunday, could
really change what happens with the third wave. More phasing on
Sunday, would likely lead to a faster increase in moisture across
the region, and better potential for precipitation Sunday night
into Monday with a weak surface low likely forming and track
through the region. But a less phased solution, may keep
precipitation chances at bay or prevent them altogether and delay
or prevent a low from forming. The real question is how the third
vort max interacts with the first two. Models are certainly
trending towards a more phased system, with better precipitation
chances for the area on Monday. Just about all the 00z models show
some chance for precipitation on Monday along with strong cold
advection. If precipitation does occur, temperatures may be below
freezing long enough Sunday night to allow for colder ground
temperatures, and thus, better potential for snow to stick to
surfaces. Thus, if precipitation does materialize, it would be
reasonable to assume some impacts on Monday, even though
precipitation looks to be mainly light. For now, it does look
reasonable to include some low pops during the period until the
models can work out the details. With strong cold advection
anticipated on Monday (independent of whether we see precipitation
or not), plan to also go with a non- diurnal temperature forecast
with temperatures holding steady or falling through the day.

The upper air pattern flattens out next week and turns more
zonal. This would provide a quieter period of weather along with
warming temperatures from mid to late next week. Strong surface
riding over the southeast should hold through the remainder of the
week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
ISSUED AT 1234 AM EST FRI DEC 4 2020

Mid and lower level clouds have spread into eastern Kentucky this
evening. It will lower through the night into the MVFR and
eventually IFR as we saturate through the overnight into the day
Friday. Along with the lower CIGs we will see rain overspread the
region through the night and this will lead to periods of lower
Vis. Winds will average around 5 kts or less through the period.

&&

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

$$

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


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