Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 170550 AAC

National Weather Service Jackson KY
150 AM EDT Tue Oct 17 2017

Issued at 147 AM EDT TUE OCT 17 2017

Hourly grids have been freshened up a bit to reflect the valley
fog that has already developed per GOES 16 satellite imagery.
Hourly temps and dewpoints were also freshened up based on recent
observations and to be consistent with the fog already present.

UPDATE Issued at 1020 PM EDT MON OCT 16 2017

Did a quick update to the grids to reflect the current obs and
trends, otherwise the forecast is on track. These have been sent
to the NDFD and web servers.

UPDATE Issued at 750 PM EDT MON OCT 16 2017

23z sfc analysis shows high pressure building over the state. This
is in the process of clearing out the lingering mid level clouds
with winds now settling. Temperatures are running in the mid and
upper 50s with dewpoints in the upper 30s to lower 40s. Look for
clearing skies and light winds to support the development of an
inversion overnight under good radiational cooling conditions to
yield a decent ridge to valley temperature split later tonight.
This will prompt valley temperatures to fall into the upper 30s by
dawn with a few locations touching the mid 30s. Given the
dewpoints and clear conditions we expect to see some patchy frost
form in in the deeper valleys. However, most locations will
experience fog development instead keeping the frost at bay.
Locations along the rivers and near lakes will most likely see
dense fog formation. An SPS is out for these concerns along with
highlights in the HWO. Have updated the forecast to tweak some
valley spot locations low temperatures and also sky cover per the
latest satellite images. Also, modified the T and Td grids per
the current obs/trends. These updated grids have been sent to the
NDFD and web servers.


.SHORT TERM...(This evening through Tuesday night)
Issued at 234 PM EDT MON OCT 16 2017

A sprawling area of surface high pressure from the southern plains
into the OH valley will control our weather through Tuesday

The forecast issue of the day is the frost possibility tonight and
Tuesday night. Once the broken cloud deck currently over the area
scatters out and winds diminish, tonight will be a good night for
radiational cooling. However dewpoints are currently in the upper
30s to lower 40s, as are dewpoints well upstream over OH and IN.
Based on latest guidance, blended data and upstream dewpoints it
doesn`t appear dewpoints will be able to come down much more as we
head into evening. Would really like to see lower dewpoints at
this point, as it appears there may end up being more fog than
frost especially where more rain occurred yesterday into last
night. While there may be a few spots that see some very light
frost, most of the area will not see any frost. After
collaboration with neighboring offices have decided to not issue a
frost advisory for any of our counties. Will issue a Special
Weather Statement to cover isolated frost possibility and the
locally dense fog possibility for tonight. Even as we head into
Tuesday night, with another day of dry weather it still appears
dewpoints will not be much lower than today, so only some patchy
light frost will again be possible Tuesday night.

.LONG TERM...(Wednesday through Monday)
Issued at 304 PM EDT MON OCT 16 2017

The period will begin with an area of high pressure just to our
east and weak trough aloft. This area of surface high pressure
will meander near and just to the east of eastern Kentucky through
Sunday, with upper level ridge building east by Thursday. The
biggest issue through the period will be overnight lows and how
this will be affected by afternoon mixing. Given this leaned
toward the previous forecast which had the valley temperatures
covered well, however, struggled to undercut the usually coldest
COOP MOS guidance. Overall the period will be quite dry with
afternoon MIN RH values on the 25 to 30 percent range through late

The models become more convoluted moving toward day 7, with
respect to approaching cold front and upper level features. The
GFS brings the front through and leads to a couple of shots of
showers along and near the front, and in contrast the ECMWF wants
to close a upper low across the lower Mississippi River Valley
which could cut some moisture off based on the 00Z solution.
However, the latest 12Z ECMWF cutoff the low further west allowing
more moisture to filter into the region. Given the large amounts
of spread and less continuity will lean toward chance POPs given
by the blended model data.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)

High pressure is in place across the OH Valley this morning. Some
valley fog has already developed in East KY and this should
become dense in a few valley locations during the next few hours.
At this point, the TAF sites are expected to remain MVFR or
better. The exceptions may be KSYM where fog is evident in the
nearby Licking River Valley and possibly KSME. Guidance has
generally trended toward less fog for both sites, however, so
confidence was not high enough to go with IFR at this time. Valley
fog will lift and dissipate by 14Z, with VFR conditions in all
locations from then until the end of the period. Winds will be
light and variable through the period with high pressure in place.




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