Area Forecast Discussion
Issued by NWS Jackson, KY

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571
FXUS63 KJKL 280053
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
753 PM EST Mon Feb 27 2017

.UPDATE...
Issued at 753 PM EST MON FEB 27 2017

Area of clearing is passing across the area, more significant
clearing than what we had advertised. Models have trended in kind
with less cloud cover through the overnight period. Accordingly
made adjustments to sky cover and lowered temps a bit, especially
in our eastern valleys. Increasing gradient wind with return flow
and expected return of cloud cover in our southwest during the
pre dawn time frame may be enough to keep our western valleys from
decoupling as much and thus help keep temperatures up a bit more.
However, am concerned that lower adjustments to overnight may not
be enough overall, especially if clouds do not return in our
southwest. Will reevaluate and update as necessary. With cooler
temperatures and a stronger nocturnal inversion have lowered winds
in our more sheltered valleys as well. This will also set up the
potential of some valley fog as well. The forecast package has
been updated, including grids and zones.

&&

.SHORT TERM...(This evening through Tuesday night)
Issued at 417 PM EST MON FEB 27 2017

Showers have exited eastern Kentucky this afternoon as a surface
trough slides across the Appalachians. Abundant cloud cover has
only allowed temperatures to warm into the low-mid 50s, although a
few late day breaks may still allow a few locales to warm into the
upper 50s. May see a minor ridge/valley temperature split
materialize this evening as 20-25 degree dewpoint depressions
remain in place. Have seen these dwindle over the past couple of
hours as passage of the aforementioned surface trough has allowed
winds to veer more south to slightly southwesterly, negating the
downslope effect.

Warm air/moist advection will materialize later this evening and
tonight as a surface low ejects out of the central Plains into the
Midwest, subsequently lifting a warm front through the Tennessee
Valley and eastern Kentucky Tuesday morning. This will take place
downstream of an upper level trough moving across the
Intermountain West, increasing 0-6 km shear to on the order of 50
knots. Following early morning showers moving in across the
Bluegrass and Lake Cumberland regions, showers and thunderstorms
will increase across the remainder of eastern Kentucky throughout
the late morning and afternoon. Instability does look marginal as
temperatures warm into the mid 60s to near 70, with surface-based
CAPE values perhaps approaching 500 J/kg as a stubborn elevated
mixed layer remains in place, stemming from earlier day warm air
advection aloft. Regardless, enough low level veering, leading to
0-1 km storm-relative helicity values of 250-400 m2/s2 as winds
remain backed near the warm front, will exist for updrafts to
remain discrete, given they can grow tall enough to become
organized throughout a significant depth in a highly sheared
environment. Although any stronger storms are expected to be
isolated in nature, gusty southwest winds will become moreso
underneath any storms that develop. Isolated tornadoes can also
not be ruled out given rotating updrafts and sufficiently low LCLs
as PWATS of near 1.2 inches advect in.

Winds aloft will increase further Tuesday night as the upper
trough draws nearer. While an overall lull in shower/thunderstorm
activity is expected late Tuesday afternoon/evening, gusty winds
will continue through the night. Higher ridges may gust to upwards
of 35 mph or greater as deep layer shear further intensifies.
Shower and thunderstorm chances will increase later in the night
toward dawn Wednesday ahead of the approaching cold front, but
more widespread storms should hold off until after sunup. Any
storms that do develop will have the potential to produce severe
wind gusts as higher momentum air aloft will not have far to mix
down. Additionally, some drying aloft may lead to an increased
threat for hail through the night. Will also have to monitor the
potential for isolated flooding given the degree of moisture in
place, but the overall progressive nature of this system should
keep widespread flooding a low risk.

.LONG TERM...(Wednesday through Monday)
Issued at 309 PM EST MON FEB 27 2017

The primary forecast issue in the extended period will be the
potential for strong to severe thunderstorms on Wednesday. A
fairly well defined cold front is forecast to move across the area
on Wednesday, which will bring widespread showers and some
thunderstorms to eastern Kentucky. The best potential for strong
to severe thunderstorms will likely be between 18 and 23Z on
Wednesday, when the model data is suggesting the best instability
will combine with the lift along the front the strong low and
midlevel wind shear. The primary threat with the strongest storms
will be damaging wind gusts. The rain should taper off quickly as
the front moves east of the area early Wednesday night, with the
last rain forecast to exit the area by 6Z. We should see a brief
break from the precipitation Wednesday night and Thursday, as a
ridge of high pressure sets up just to our south. The models are
still suggesting that weak area of low pressure aloft will move
across the Ohio Valley region Thursday night into Friday morning.
The temperatures during that time would allow for rain and snow
showers to occur, should the model data be correct. Since the
models have been forecasting the passing upper low toward the end
of the week fairly consistently, decided to keep precipitation in
the forecast for the northern half of the forecast area for
Thursday night into early Friday. The system will be starved for
moisture, so only slight chance to chances PoPs were used once
again. Any snow accumulation that would occur would likely be
confined to our far northwestern counties, and would be a tenth of
an inch or less on grassy areas.

Once this fast moving systems moves past us, the weather should be
dry Friday through Sunday. A bit better developed weather system
is being forecast to affect the area Sunday night into Monday.
However, the models are not in the best agreement to end the
period, so only slight chance to chances PoPs were used in the
forecast for Sunday night into Monday morning. The blended model
PoPs seemed way overdone, so the lower PoPs were used to account
for the model uncertainty.

Temperatures during the period will be above normal yet again,
with daily highs varying from the 40s to the 60s across the area.
Nightly lows will be cold during the first half of the period,
with minimum values in the 20s and 30s expected. Low temperatures
Saturday night are forecast to fall into the upper 30s and lower
40s, with minimum values expected to only fall into the mid to
upper 40s Sunday night.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
ISSUED AT 753 PM EST MON FEB 27 2017

Skies are clearing across the area this evening. This presents
some problems, especially with respect to the potential of fog
through the overnight. Models are a bit slow picking up on the
clearing and consequently may not be picking up on the potential
for valley fog either. Models do suggest a return of mainly mid
level cloud cover through the pre dawn hours at our southwestern
terminals. In addition, southerly gradient winds are expected to
increase some through the overnight which should help keep our
western valleys from decoupling. Concern is that these may not
materialize, leading to a stronger potential for some early
morning fog Tuesday. With these recent and quickly changing
trends will evaluate further and update the forecast as
necessary.

Winds will continue to increase Tuesday ahead of a disturbance
passing through the region. This feature will generate rain
showers which will approach from the southwest by Tuesday
morning. Ceilings will lower in response, eventually dropping
into MVFR territory. In general model guidance is suggesting only
an isolated to low end chance type threat for thunder Tuesday. In
addition, am not overly impressed with surface based instability
due to lack of heating though there is some elevated instability
showing up in some of the forecast soundings. For now decided to
leave out any mention of thunder in the vicinity of terminals and
instead may reintroduce the threat of thunder as a general cloud
element with future TAF issuances.

&&

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

&&

$$

UPDATE...RAY
SHORT TERM...GUSEMAN
LONG TERM...AR
AVIATION...RAY



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