Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 211638
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
1238 PM EDT Mon Aug 21 2017

.UPDATE...
Issued at 1238 PM EDT MON AUG 21 2017

Updated NDFD for latest observational trends. With the partial
eclipse to begin within the next hour the main focus was to
update sky cover based on satellite trends. Satellite shows a
thicker patch of cirrus moving into the Cumberland Valley and
another area of thicker cirrus moving across the Bluegrass region
into the northern part of our forecast area. At least for the
southwest part of the forecast area where the partial eclipse
will have the highest percentage the thicker patch of cirrus will
have moved east of the area by the height of the partial solar eclipse.

UPDATE Issued at 1039 AM EDT MON AUG 21 2017

Updated NDFD for latest observational trends. There was very
little change in the ongoing forecast. Sky cover still looks to
be minimal during the eastern KY partial eclipse this afternoon.
Some thin cirrus will move across the area from the thunderstorms
over IL, but it appears that the thicker cirrus will remain north
of our area.

UPDATE Issued at 745 AM EDT MON AUG 21 2017

Freshened up the hourly temperatures, dew points, and sky cover
through this morning. Patchy fog will burn off by around 9 am,
with some thin cirrus moving in from the northwest out ahead of a
weakening thunderstorm complex located across the Midwest.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 358 AM EDT MON AUG 21 2017

Surface high pressure is centered near the mid-Atlantic, with a
quasi-stationary boundary aligned from the Great Lakes to the
central Plains, with a secondary warm front extending southeast
into the Ohio Valley. Aloft, an upper level ridge rules across the
southeast CONUS, with flatter flow across the majority of the
Mississippi Valley. A more vigorous short wave trough is moving
southeast out of the southwest Canada, currently moving through
the British Columbia/Alberta border.

Eastern Kentucky is enjoying another most clear night, with areas
of valley fog. Some debris high clouds have been making it into
the area from the west, but these remain pretty thin. The next 24
hours will remain dry, as the upper level ridge to our southeast
remains in control. Today still looks like an overall good viewing
day for the full eclipse. Highs will peak in the upper 80s to near
90, with some cumulus and thinner high clouds to contend with
during the day, but nothing extensive.

Tonight, skies will remain mostly clear initially, as afternoon
cumulus dries up. Meanwhile, the Canadian short wave trough will
deepen and continue to move southeast, reaching the Great Lakes
region by dawn Tuesday. Some high clouds will move in by early
Tuesday morning, keeping fog a bit less extensive. Lows will
generally range from 65 to 70 degrees.

As the short wave trough continues to amplify across the
U.S./Canadian border on Tuesday, low pressure will organize and
deepen rapidly along the surface boundary to the east. The cold
front will move southeast across the Ohio Valley Tuesday
afternoon, with showers and thunderstorms breaking out along and
ahead of the boundary. The blended POPs yielded likely values from
the northwest during the afternoon, and given the forcing and
instability, this seems reasonable. Additionally, stronger 0-6km
bulk shear values than we have seen in a while will be on tap,
supporting a severe thunderstorm threat, especially across the
northwest half of the area. Will highlight this threat in the HWO.

.LONG TERM...(Tuesday night through Sunday)
Issued at 358 AM EDT MON AUG 21 2017

Forecast will start out with a cold front approaching the region
Tuesday evening, and traversing from NW to SE Tuesday night and
during the first half of the day Wednesday. Showers and
thunderstorms will be possible throughout the night, but will peak
during the day Wednesday just ahead of the frontal passage. That
being said, once the front passes, winds will quickly shift to a
more northerly direction and instability will drop off. Lingering
precip on the back side of the frontal passage through the day
Wednesday will therefore likely be in the form of rain, and should
come to an end by Wednesday evening as the frontal boundary shifts
southeast of the state.

A large area of high pressure will take hold Wednesday night and
through the rest of the work week. A strong troughing pattern will
still be in place aloft, promoting NW flow across the state. This,
in conjunction with northerly winds at the surface, will help to keep
temperatures well below normal. Highs will generally be in the upper
70s with low humidity through Friday.

By Saturday and Sunday, models are still in fair agreement showing
the upper level troughing moving east of the region but some
shortwaves developing and moving into the Ohio Valley under
continued NW flow. That being said, they are not in good agreement
as far as the exact timing and magnitude of these waves across the
state. Surface winds also suggest a weak surface low passing across
the region during the day Sunday. However, not confident on the
exact location and timing given the uncertainty in models aloft.
Regardless, there is the possibility for some unsettled weather
during the weekend. Right now model consensus is for mainly
diurnally driven isolated to scattered showers and thunderstorms,
with best coverage during the day Sunday. Surprisingly both the GFS
and ECMWF are both putting similar areas of coverage across far SE
KY during the day Sunday as of now, so confidence was a bit higher
here. But expect this forecast to continue evolving over the next
few days.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
ISSUED AT 745 AM EDT MON AUG 21 2017

IFR or worse fog will burn off through 13z. Some cumulus at 5-6k
feet agl will develop during the day, along with some passing high
clouds. Patchy fog will develop once again after 06z, mainly along
the deeper river valleys. Winds will average less than 5 kts
through the period.

&&

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

$$

UPDATE...SBH
SHORT TERM...GEOGERIAN
LONG TERM...JMW
AVIATION...GEOGERIAN



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