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Area forecast discussion 
National Weather Service Jackson Kentucky
357 PM EDT Friday Mar 27 2015

Short term...(this evening through Saturday night)
issued at 355 PM EDT Friday Mar 27 2015

The axis of an upper level longwave trough is currently moving
across the Ohio River valley. Along this trough axis...some weak
instability has formed and is leading to some light scattered precipitation
across western and central Kentucky...as well as points to the north and
south. As the trough continues to shift eastward...expect this
precipitation to make it into our County Warning Area over the next few hours...and then
taper off in the evening as the trough exits and we lose any daytime
heating influence.

A couple things to note with this system. First...returns thus far
today are quite light...and the best returns /where precipitation is likely
actually hitting the ground/ are currently over northern Tennessee and
shifting sewrd. If some of the models hold true...the longwave
trough may have dug far enough southward that the best precipitation will
develop and move just to our south and then eastward...only touching
our far southern counties. Went ahead and included slight chances
across a good chunk of the County Warning Area though...just in case some of the
other returns actually do start producing something as they shift
eastward into our region.

The second thing to note is the precipitation type. With such a deep
upper level trough...temperatures near the surface are only around
40 degrees...but aloft are cooling off at a much faster
rate...thereby creating a strong lapse rate. While the threat of
thunder is probably not going to happen at this time...still agree
with previous forecast thinking that it wouldn/T be out of the
question that some ice pellets from aloft could make it to the
surface and mix with the rain. As such...have the mention of both
rain and snow through this evening...before temperatures finally drop
enough that some locations /mainly the high terrain/ will see all
snow for a brief period of time. Still not expecting this snow to
amount to much...only put in 0.1 inches in the high terrain with
this forecast package.

After discussion with iln and lmk...there is some concern about
the potential cloud cover overnight. If this longwave trough sinks
any farther south...we could get the shot of drier air to our NE.
However...it is likely that this drier air will make it into iln
and ind...and clouds will shift southward into our area into the
evening. While many of these clouds are diurnally driven and will
dissipate overnight...there is enough clouds that this process
may take some time. And with incoming high pressure and a
subsidence inversion developing overnight...there could be some
clouds which get trapped below the inversion and linger throughout
much of the night...at least until we get enough sunlight/heat in
the morning to quickly mix them out. As a result...held on to
clouds a bit longer throughout the night compared to previous
forecast...though most locations should be clear shortly after
sunrise tomorrow. Regardless...these clouds will do little to
hamper the strong pull of northwest air into the region...as temperatures are
still expected to drop well into the low to middle 20s.

For tomorrow...high pressure and dry weather will be the story as we
continue under dry northwest flow. In fact...it won't be out of the
question for some of the higher elevations to tap into some
impressively low dew points by 12z tomorrow while the low level inversion
is still in place. As the inversion lifts and mixes out...these dew
points are likely to recover some...but overall the theme through
the day will be dry. Relative humidity values in the upper teens and low 20s are
expected by the afternoon. This dry air will have an effect on
the temperature forecast for overnight. Northwest flow aloft...weak winds
near the surface with high pressure nearly overhead...and clear
skies will all be in play. We could see record low temperatures
across much of the region by 12z Sunday...upper teens to low 20s.
Expect coldest temperatures in the valleys...with some locally
cooler spots possible.

Long term...(sunday through friday)
issued at 314 PM EDT Friday Mar 27 2015

There is general model agreement on the overall mean pattern next
week...with fairly fast zonal flow...and a series of short wave
disturbances and associated surface fronts bringing periodic chances
of rain showers. With this type of pattern it is not surprising
there are timing differences among the models with the short waves
that will be affecting the area next week.

At the start of the forecast period surface high pressure will be
shifting off to the southeast...providing for a south to southwest
low level flow of warmer air. This will allow temperatures to
rebound into the lower to middle 50s Sunday afternoon after a very
cold start to the day Sunday morning. A short wave trough moving out
of the MS valley and an associated surface cold front will be making
their way towards Kentucky on Sunday. The 12z GFS has come in faster with
this system...and points towards rain chances for the northern part
of the forecast area by late Sunday afternoon. However...have
followed a model consensus which will keep even slight chances of
rain out of the entire area until after 6 PM on Sunday. Rain chances
will increase from northwest to southeast Sunday night...with rain showers
becoming likely almost area wide Sunday night. Rain chances will
then decrease from northwest to southeast during the day Monday.

After the Sunday night into Monday system...the next short wave to
affect the area looks to be during the middle to latter part of the
week...followed by yet another system as we head towards next
weekend. There are again significant timing differences...and thus
questions on when to reintroduce rain chances back in the
forecast. Will follow a blended model approach for the middle to late
week systems...which will yield slight rain chances most areas
Wednesday...and then chance or likely probabilities for Thursday
and Friday.

With the zonal flow a fairly mild week will be in store for the
area...with highs in the 60s forecast for Tuesday through Friday.

&&

Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 140 PM EDT Friday Mar 27 2015

Despite developing afternoon cloud cover...most of eastern Kentucky
remains under VFR conditions. Light rain showers will likely
develop over central Kentucky and move into eastern Kentucky over the next few
hours. According to the latest forecast soundings...ceilings could
drop down to 3kt feet around the same time...hovering the
threshold between MVFR and VFR. Otherwise...showers will be light
and cause little in the way of visible restrictions. Just included vcsh
at this time as it is still unclear exactly where and when these
showers will develop in relation to the Airport sites. The upper
level trough driving these showers will begin pushing east of the
region this evening causing any lingering showers to subside. Ceilings
should bounce back up to VFR conditions and scatter out
overnight...with mostly clear conditions expected into tomorrow
under the influence of high pressure. Winds should generally be
out of the northwest and less than 10 knots.



&&

Jkl watches/warnings/advisories...
none.
&&

$$

Short term...jmw
long term...sbh
aviation...jmw

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