Area Forecast Discussion
Issued by NWS Jackson, KY

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595
FXUS63 KJKL 200800
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
400 AM EDT Thu Apr 20 2017

.UPDATE...
Issued at 127 AM EDT THU APR 20 2017

WSR-88D radar showing a few left over showers are exiting far
eastern KY this hour, but the thunder has now subsided with VII
nill on the MRMS, cloud top warming, and no recent reports from
the various lightning detection networks. Updated to deal with
left over rain showers and blend in the latest obs and trends.

UPDATE Issued at 1133 PM EDT WED APR 19 2017

The last of the showers and storms look like they are finally
winding down. Will hold onto some isolated POPs for another hour
or so matching the radar trends, with the rest of the night
looking quiet. Clouds are much less prevalent tonight, compared to
last several nights, so have played up the fog more, especially
with pockets of locally heavy rainfall having occurred earlier.
Lows will range from the mid 50s in the cooler spots, to around
60, where light southerly winds keep things a bit more mixed.
Updates have been sent.

UPDATE Issued at 755 PM EDT WED APR 19 2017

Mainly freshened up the hourly temperatures and dew points over
the next few hours, as localized storms have cooled off places,
including JKL. POPs were already in good shape, with a gradual
diminishment expected through midnight. Only made a few minor
adjustments to these based on the current radar trends.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 433 PM EDT WED APR 19 2017

Scattered showers and thunderstorms have fired across the area
this afternoon. These storms are not quick builders, the likely
result of slight height rises and relatively weak instability to
feed off of. Activity is expected to gradually die down through
the late afternoon and early evening as sunset approaches. We will
probably see some patchy fog develop through the overnight where
precipitation has fallen late this afternoon and/or this evening.
Added a mention of fog to the zones, particularly in the valleys.

Otherwise, focus turns to a cold front that will drop into the
region by late Thursday night into Friday. There is still
considerable uncertainty with the details of this system. The GFS
and NAM manage to take the boundary considerably further south
than the ECMWF. This lowers confidence a bit with respect to
sensible weather. Ramped up pops Thursday night as the front
approaches but stayed close to model blends and guidance.

For sensible weather, expect showers and thunderstorms to
gradually die off through sunset. Skies will partially clear
allowing for some reasonable radiative cooling. Preferred the
inherited cooler valley temps over the warmer blends and model
guidance for tonight. This will favor some patchy valley fog,
which might tend to become more dense at times where late
afternoon and evening precip occurs. Clouds will be on the
increase late Thursday afternoon and gradually brought pops back
into the picture late in the day. Showers and thunderstorms
increase in coverage Thursday night with the cold front dropping
into the area. Temperatures will climb to around 80 for much of
the area Thursday.

.LONG TERM...(Friday night through Wednesday)
Issued at 400 AM EDT THU APR 20 2017

The models are in decent agreement with the long wave pattern
aloft for the bulk of the extended. They all depict an amplifying
trough moving into the Ohio and Tennessee Valleys to start the
weekend with the GFS and ECMWF quite similar, but the Canadian is
lagging. This trough slows and further deepens overhead on Sunday
- most prominently in the latest ECMWF. However, it does appear to
remain progressive enough to bring a rising height regime back
into the area at the start of the new work week. While weak
ridging sets up over Kentucky on Tuesday the northern stream will
be active and may bring more troughing and energy into the Ohio
Valley by evening - particularly seen in the ECMWF as the GFS is
now out of step with its overseas cousin. Likewise, confidence
falls for Wednesday as the ECMWF quickly presses a trough through
Kentucky while the GFS holds on to ridging. With the agreement
early - particularly among the GFS and ECMWF - will favor a blend
having a good deal of confidence while further out as confidence
flags a blend splits the difference effectively.

Sensible weather will feature a wet and cool weekend as a nearly
closed low descends over the region and slowly passes through.
This will mean cloudy and wet conditions with a chance of thunder
- mainly for our southern counties on Saturday. As the sfc low
starts to pull away so will our thunder chances and better chances
for measurable rain. There is a potential for showers to linger
in the far east in Sunday night as the sfc low`s movement east is
hampered by high pressure off shore. In time though, it will pull
far enough away for drier conditions to prevail followed by more
sunshine during the days ahead and temperatures back above normal.
This will be the case through Tuesday before more uncertainty
crops up as to whether a low passing through the Great Lakes will
swing a cold front into Kentucky on Wednesday (ECMWF) or if sfc
high pressure holds on for another day at least (GFS). For now
have kept it dry, but confidence in this is lower than the rest of
the forecast.

Did make some adjustments to the temperatures each night with
minor/spot specific ones early and then larger adjustments for
Monday and Tuesday nights as ridge to valley splits should
develop. With the PoPs, did not make many changes as the blend
came in pretty good, but will caution that Wednesday may
eventually need some PoPs if future model runs trend toward the
ECMWF.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
ISSUED AT 120 AM EDT THU APR 20 2017

TAF sites are starting off the period VFR and should generally
remain that way through the TAF period. The caveat will be the
more valley sites could see fog, but right now will lean toward
LOZ for MVFR VIS potential and leave out at other sites. Outside
of TAF sites would think anywhere that saw convection and places
that saw showers would have the chance of seeing fog through mid
morning. While the convection this afternoon will be isolated in
a weakly forced environment will mention VCTS for all the TAF
sites. Winds will remain light out of the south to start the
period, but deeper mixing into a modest LLJ could provide gusts of
15 to 20 knots through the afternoon. The better gusts would
likely be in the Bluegrass and Lake Cumberland regions.

&&

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

$$

UPDATE...DJ
SHORT TERM...RAY
LONG TERM...GREIF
AVIATION...DJ



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