Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 140249

National Weather Service Jackson KY
1049 PM EDT Fri Oct 13 2017

Issued at 1049 PM EDT FRI OCT 13 2017

Valley temperatures have fallen off quickly this evening under
clear skies, but they are also reaching saturation, and satellite
imagery shows fog developing. This will greatly slow the rate of
fall overnight, and forecast morning lows have not been changed
since the previous update. Models show some potential for a low
cloud deck to develop over southern and southeastern portions of
the forecast area toward dawn, but confidence in this is shaky.

UPDATE Issued at 707 PM EDT FRI OCT 13 2017

Skies have cleared for the most part, and ridge/valley
temperatures differences have already become apparent in the
eastern portion of the area. Have updated the forecast to account
for less sky cover this evening, and greater temperature
variation. At this early stage, have not made any significant
changes to overnight lows.


.SHORT TERM...(This evening through Saturday night)
Issued at 337 PM EDT FRI OCT 13 2017

High pressure is currently in control across the CWA and will
remain so throughout the duration of the forecast period as it
drifts slowly east of the area. A large mass of dry air is
accompanying this system, resulting in mostly clear skies. As was
the case last night, as temperatures fall overnight, a llvl
inversion will set up and trap moisture near the surface. This
will likely lead to good fog development across the CWA,
especially in the valleys and near bodies of water. The one
concern will be return southerly flow on the west side of the
slowly departing high, which may impact the western CWA early in
the morning and deter the fog. But confidence in this is low, as
this wind shift/increase will likely take place during the day
tomorrow instead and fog will be the result. Did not include any
dense fog wording in the grids at this time since confidence on
where and to what extent it would set up was somewhat low, though
do expect there to be some localized dense fog across the CWA.
Lows will generally be in the 50s, coolest in the valleys.

Mostly clear skies will remain intact through the day tomorrow
once fog lifts and clears out, save some diurnal CU that may
develop in the afternoon. Under deeper southerly flow, afternoon
temperatures are forecast to rise back to around 80 in many
locations. Lowering heights are expected in the upper levels as
we head into Saturday night as a shortwave continues to dig in the
northern Plains/northern Mississippi Valley. At the surface, a
low pressure system will be moving NE into the Upper Great Lakes,
pulling a cold front eastward into KY. While the front will not
reach us Saturday night, it will bring with it an increase in
cloud cover as well as precip potential as we head into the day
Sunday (start of mid term). This will deter from any fog
development Saturday night and will also keep temps mild,
generally in the low 60s.

.LONG TERM...(Sunday through Friday)
Issued at 250 PM EDT FRI OCT 13 2017

The focus of the Sunday through Friday period will be the passage of
a cold front on Sunday and a period of more autumn like weather next
week. Model trends point towards a quicker passage of the cold front
on Sunday, with the front clearing the area by early Sunday evening.
Instability on Sunday still appears limited, but there should be
enough instability in a narrow ribbon in advance of the front to
support at least the mention of a slight chance of thunder. We will
continue with the forecast of a slight chance of thunder on Sunday
which is in agreement with previous NDFD forecasts, and the latest
SPC Day 3 Outlook which places our entire area in the general
thunder outlook. Even if there end up being no thunderstorms on
Sunday, which certainly is possible, it will still be rather breezy
with some gusts to near 30 mph especially in the Bluegrass counties
and other more open areas.

A surface high will build into the area early next week in the wake
of the cold front, and bring drier and cooler conditions. The
superblend and model guidance support widespread mid to upper 30
degree minimum temperatures Tuesday morning and scattered mid and
upper 30 degree temperatures on Wednesday morning. Use of the
forecast builder generates areas of frost, especially on Tuesday
morning. However, as is often the case, fog formation in river and
stream valleys will inhibit frost formation. Despite that, some
areas should see their first frost of the season. We will continue
to highlight the frost possibility in the HWO. As the week
progresses the surface high will drift east and upper level ridging
with rising heights will bring a warming trend to the area.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)

The period started with mainly clear skies and no visibility
restrictions. Relative humidity during the afternoon and at the
start of the 00z TAF period was lower than at the same time
yesterday. This would argue for less in the way of fog tonight.
There will also be strengthening warm air advection. While low
level winds should not be strong enough to mix the valleys
tonight, the ridges and some mid-slope locations will see a bit
of stirring at times, which will also help to restrict the extent
of fog. With that said, the normally colder and foggier valleys
should still be IFR or worse by dawn, but TAF sites should not
fair as bad. Restrictions are still forecast at KSME and KLOZ,
and temporarily at KSYM. Models also suggest a low cloud deck
trying to develop in the south and southeast portions of the
forecast area toward dawn and lasting into the morning. Have
accounted for this with MVFR ceilings in the forecast at KSME and
KLOZ, but there is not good confidence in this aspect of the




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