Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 261147
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
747 AM EDT Fri May 26 2017

.UPDATE...
Issued at 747 AM EDT FRI MAY 26 2017

Low clouds have just about cleared out of eastern Kentucky this
morning. Fog has mostly mixed out and dissipated everywhere but
down inside deeper valley locations, with this expected to also
mix out within the next hour or so given abundant sunshine. Next
notable feature of interest will be storms approaching from the
northwest later this evening into tonight following a dry and
mostly sunny Friday.

&&

.SHORT TERM...(Today through Saturday)
Issued at 359 AM EDT FRI MAY 26 2017

Cloud cover continues to push east and dissipate early this morning
as a surface ridge axis, extending from Florida to Lake Superior,
shifts overhead. This will continue to bring areas of fog to
portions of eastern Kentucky through mid morning, including some of
this becoming dense in river valleys. Increasing heights aloft
downstream of shortwave upper ridging building into the Great Lakes
and Ohio Valley will provide for further subsidence, leading to a
dry Friday across eastern Kentucky. Stout warm air advection will
result in a drastic warmup compared to Thursday, with highs climbing
into the upper 70s to right around 80 degrees this afternoon.

Shower and thunderstorm chances will increase late this evening and
tonight as surface low pressure ejects out of the Missouri Valley
into the Ohio Valley, lifting a warm front through Kentucky toward
the Ohio River. This along with the first round of shortwave
impulses, stemming from an upper low across Manitoba, will serve as
focus for convection through tonight. Best chances across eastern
Kentucky will thus reside north of the Interstate 64 corridor nearer
the greater forcing for ascent.

The threat for showers and storms will continue Saturday morning.
Elevated bases will begin to become rooted nearer the surface by mid-
late morning through the afternoon as diurnal heating allows for
steepening low level lapse rates. Not expecting more than scattered
shallow-top storms through early afternoon as upper heights will be
near-neutral with the core of the upper low still well back to the
northwest. Greater heating through the mid-late afternoon combined
with arrival of additional shortwave energy and a frontal boundary
drawing nearer should support high-end scattered to perhaps
numerous coverage. Deep layer shear of 30-40 knots in tandem with
surface-based CAPE values approaching 2-3 kJ/kg and dewpoints in
the upper 60s should support sustained updrafts, capable of a few
robust multicellular storms. Southwest surface winds will provide
for weak directional shear and thus a rather low threat for any
defined supercells. Hail should be limited given rising freezing
levels and greater moistening negating the effect of any wet bulb
cooling. Strong to severe winds will certainly be possible
underneath collapsing downdrafts, especially by mid-late
afternoon.

.LONG TERM...(Saturday night through Thursday)
Issued at 336 AM EDT FRI MAY 26 2017

The period will start with some weak shortwave ridging over the
area Saturday evening. However, convection upstream fueled by a
vort max pushing across the area will bring a complex of showers
and storms into the Ohio valley region sometime late Saturday
night or early Sunday. Timing and areal extent of this activity
remain in question. With PW values increasing to just above 1.40
inches, locally heavy rainfall would be a possibility wherever
storms do track. Models have all kinds of convective feedback, but
the actual vort max aloft should move through early Sunday
morning, so that will likely be a period of higher rain chances.
While mid level lapse rates are somewhat steep early Sunday, shear
is not tremendous, so would not anticipate a big severe weather
threat with the early day activity.

This all changes as we head through the afternoon and evening
hours on Sunday. Models have just above 3000J/KG of SBCAPE
developing across the area by mid afternoon on Sunday with LI`s
down around -10C. Thus, instability will not be lacking. Shear
will also increase, but will be mainly speed shear as very little
turning of the winds through the column. There is also quite a
bit of dry air in the mid levels which may help to aid in the
threat for damaging winds. DCAPE in fact is near 1000J/KG, which
is right about what you need for a good damaging wind threat. We
also have steep mid level lapse rates supporting a modest hail
threat. Now the question is how will convection evolve or
materialize? Well, this is not an easy question at this point as
early day convection may help to lay out potential boundaries for
later convection in the afternoon. The potential for training
storms and continued high PW values will lead to a continued
threat for flash flooding through at least Sunday evening. Will
hold off on any watch right now, but may eventually need to
consider one for some of the area. By late in the day, a cold
front will bear down on the area and assuming airmass remains
untapped, we could see showers and storms develop along this
boundary. Either way, it looks like a decent threat of severe
weather/flooding at some point on Sunday. Its just a matter of
fine tuning the details in the upcoming forecast. For now, given
its still 3 days out, highlighting the threat in the HWO and
weather stories is the way to go right now.

Front sweeps on through by early Monday morning leaving behind a
few days of dry weather through Tuesday and perhaps even lasting
into Wednesday. Weather may turn more unsettled again by late
Wednesday or Thursday of next week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
ISSUED AT 747 AM EDT FRI MAY 26 2017

VFR conditions expected to hold strong today and tonight as an
approaching ridge of high pressure builds in. May still see some
fog inside deeper valleys through mid this morning, but all TAF
sites should remain fog free. Southwest winds will increase to
5-10 knots by mid-late morning through this afternoon before
diminishing somewhat this evening. Will have to watch a potential
round of showers and thunderstorms this evening near the Ohio
River, but have kept any SH/TS or CB mention silent for now until
trends in this development become better defined. Southwesterly
low level wind shear may also warrant watching for this evening,
but this currently looks too weak, around 30 knots, to be
concerned with at this time.

&&

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

$$

UPDATE...GUSEMAN
SHORT TERM...GUSEMAN
LONG TERM...KAS
AVIATION...GUSEMAN


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