Area Forecast Discussion
Issued by NWS Jackson, KY

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477
FXUS63 KJKL 210803
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
403 AM EDT Fri Jul 21 2017

.UPDATE...
Issued at 340 AM EDT FRI JUL 21 2017

An obs blend into the forecast was done. A more thorough update
will follow shortly.

UPDATE Issued at 1029 PM EDT THU JUL 20 2017

Forecast lows continue to look on track. The cooler valleys have
dropped off into the mid 70s, with ridges holding at around 80
degrees. Cirrus has thinned somewhat this evening, but will
continue to stick with partly cloudy skies, given the coverage
still out there. Updates have been sent.

UPDATE Issued at 807 PM EDT THU JUL 20 2017

Isolated showers have dissipated into the early evening, with
mainly just some thicker cirrus hanging around. This should
gradually dissipate over the next few hours, but did beef up the
cloud cover initially to account for the latest satellite trends.
Forecast lows look on target thus far, and have mainly adjusted
the diurnal drop off over the next few hours to account for the
slightly warmer readings still in place.

&&

.SHORT TERM...(This evening through Friday night)
Issued at 420 PM EDT THU JUL 20 2017

As of mid afternoon, a ridge of surface high pressure extended
into East KY from the southern MS Valley region. A weak surface
trough is moving southeast across the area and along it some
enhanced cumulus developed along it as well as some showers near
the Daniel Boone Forest/escarpment. These showers have weakened
over the past half an hour. At mid and upper levels, a ridge
remains in place, centered over the Plains to MS Valley region.
Meanwhile, south of an upper level low over the Hudson Bay region.
A series of shortwave troughs are moving through the northern
stream, the first is moving into the St Lawrence Valley and
Upstate NY at this time and is helping send a cold front south of
the Great Lakes.

The upper level ridge will weaken from tonight through tomorrow
with 594 dm heights or higher decreasing in areal extent though
the center of the ridge should migrate to closer to the confluence
of the OH and MS Rivers. At the same time, disturbances moving
around the ridge will pass into parts of the OH Valley and
Appalachian. Weak southerly flow between higher pressure over the
southeast US and the nearly stalled front leading to a slight
increase in surface dewpoints by Friday. Some of the model
guidance brings some convective activity toward northeast KY late
toward dawn including the HRRR. Confidence on convection that far
south is not too high, though given the warm and moist airmass a
couple of stray showers cannot be completely ruled out. Otherwise,
with high pressure dominating and some passing clouds combined
with generally higher dewpoints and crossover temps compared to
yesterday afternoon and night, overnight lows should be a couple
of degrees warmer on average for valley locations. At least patchy
river valley fog if not greater coverage is also anticipated with
it becoming dense in a few spots overnight.

High pressure will again dominate on Friday though dewpoints
should creep up another degree or two. This combined with similar
max T to today and yesterday will yield greater coverage of the
area with heat indices peaking at 100 or slightly above. In
general, the current forecast has heat indices in the 88 to 103
range for most locations, but if dewpoints are a degree or two
higher Heat Advisory Criteria will be approached. At this point,
the heat is highlighted in the HWO and an SPS and later shifts
will evaluate any need that might arise for a headline for a
portion of the area. With higher dewpoints and the ridge
weakening, and the potential for a cluster of convection to move
into the area or send and outflow boundary into the area, the
chances for convection during peak heating should be higher on
Friday as compared to today. The warm and moist airmass lingering
into Friday night with the boundary to the north and weak
disturbances moving around the ridge, the threat for at least
isolated convection will linger into Friday night.

.LONG TERM...(Saturday night through Thursday)
Issued at 403 AM EDT FRI JUL 21 2017

Based on the latest model data, the extended will be active and will
feature periods of showers and storms and cooler weather. It appears
that a frontal boundary will meander about the lower Ohio Valley and
Upper Tennessee valley during the extended. There should be rain
chances each day, except Wednesday, as the front meanders about the
area. The front may move far enough south on Wednesday, to keep
precipitation out of eastern Kentucky. A second weather system
moving from the south may bring showers and storms back to the area
from Thursday onward, after only a brief respite. The best chances
for showers and storms will be the afternoon and evening hours, with
little if any thunder expected during the night time periods.
Temperatures during the period should be around normal, or even
slightly below normal at times, due to persistent cloud cover and
repeated episodes of showers and storms. Daily highs are expected to
average in the low to mid 80s most days for most locations. Nightly
lows will start out in the lower 70s, but should generally be in the
60s for most of of the period.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
ISSUED AT 340 AM EDT FRI JUL 21 2017

Fog is developing in valleys and will spread overnight. Most TAF
sites should drop to at least MVFR conditions. VLIFR can be
expected in the most fog prone valleys. Fog will dissipate on
Friday morning, leaving VFR. A few thunderstorms are expected to
develop on Friday afternoon, but overall coverage should still be
sparse. With that in mind, have not included it in the TAFs.

&&

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

$$

UPDATE...HAL
SHORT TERM...JP
LONG TERM...AR
AVIATION...HAL



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