Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 301929

Area Forecast Discussion
National Weather Service Louisville KY
329 PM EDT SAT APR 30 2016

.SHORT TERM (Now through Sunday Night)...
Issued at 310 PM EDT Sat Apr 30 2016

The synoptic pattern this afternoon features a large trough churning
across the central CONUS.  This trough will shoot a couple of PV
anomalies out of it through the period, which will bring a couple of
chances for showers/thunderstorms through tomorrow night.

A complex situation unfolding for this evening as lingering showers
and cloud cover are throwing quite a bit of uncertainty into the
forecast.  The latest hi-res guidance continues to be adamant that
renewed convection will develop in response to an approaching PV
anomaly (now across AR) late this afternoon over western
KY/northwest TN, spreading into our region this evening.  However,
guidance has been underdoing the shower activity and overdoing the
resultant thermodynamic fields all day, so have doubts as to how
much convection will develop as a result.  Will leave pops in the 40-
60 percent range for now, but these will likely be adjusted as
storms develop (or don`t) to the SSW.  Even if convection does
develop, the latest guidance suggests we never do get into the
richer warm sector airmass.  Therefore, convection will likely be
slightly elevated as it moves into our region.  As a result, agree
with SPC`s decision to downgrade to the Ohio Valley to a Marginal
risk. Threats with any storms that can get going will be some small
hail and locally gusty winds.

As the first PV anomaly lifts off to the northeast tonight, think we
will reside in a region of large-scale subsidence (briefly).
Therefore, have dried out the forecast early Sunday morning into
early Sunday afternoon.  However, the exit region of an 110-knot
upper-level jet streak will overspread the Ohio Valley during the
afternoon hours.  Given the expected dry start to the day, plenty of
instability should build during the afternoon hours as 1500-2500 of
MLCAPE J/kg develop (agreed upon by NAM/GFS/ARW/NMM/NSSL-WRF). The
main limiting factor to a more robust severe threat will be veered
low-level flow and generally weak convergence in the warm sector.
Nonetheless, deep-layer shear of around 50 knots and long, straight
hodographs will favor a mixture of potential supercells (possibly
splitting supercells) and bowing multicell clusters capable of
locally damaging winds and hail to the size of quarters.  If trends
in guidance continue, think an upgrade to a Slight Risk will be
warranted by SPC.

Convection will decrease in coverage Sunday night as the cold front
sags south through the region.  Overnight lows will be cooler,
dipping into the low to mid 50s.

.LONG TERM (Monday through Saturday)...
Issued at 327 PM EDT Sat Apr 30 2016

The big story in the long term is a deep upper trof that has
consistently been forecast to dig into the eastern United States. As
it does so, another trough will dive into the west, pumping up an
omega blocking ridge in between. Shortwave trofs dropping down into
the mean eastern trof will need to be watched for shower chances.
The long term models generally do not handle these subtle
disturbances very well.

Monday afternoon/evening a wave dropping southward through the
Midwest will interact with a broad surface trof stretched from the
Gulf Coast to the mid-Atlantic. There may also be a weak area of low
pressure riding up the trof. So, will need to include some PoPs for
the Monday/Monday night period. Will keep the numbers low until
confidence can increase.

After a possible respite Tuesday, the next system of interest will
arrive Wednesday/Thursday. The models have actually been relatively
consistent with this time period over the last few days. It appears
that a cold front will drop south out of the Great Lakes Wednesday
and cross the Ohio/Tennessee Valleys Wednesday night. Showery
weather will be possible ahead of, coincident with, and, to a lesser
extent, behind this system.

For now will continue to be cautiously optimistic for Friday and
Saturday. GFS and GEM, which generally agree with each other and are
less extreme than the ECMWF, are pointing toward high pressure and
mostly dry conditions. Even the ECMWF, which spins up a huge surface
low, keeps the low and its associated precipitation mostly to our
east. Having said that, though, with the deep upper trof overhead we
can`t confidently say for sure that there won`t be shower chances.
Some climate signals have also suggested shower chances, especially

The chilly weather we`ve been expecting is still in the cards late
in the week, with morning lows in the 40s each day Thursday through


.AVIATION (18Z TAF Update)...
Updated at 119 PM EDT Sat Apr 30 2016

Deep moisture continues to pump into the region, resulting in a band
of showers and a few thunderstorms.  The showers and isolated storms
should begin to taper off at BWG and SDF in the next couple of
hours, lasting at LEX until later this afternoon.  However,
additional shower/storm activity is expected to develop over western
KY late this afternoon, moving into all sites this evening into the
early overnight hours.  Confidence is not very high in this scenario
given the widespread rain/clouds limiting instability currently,
thus will go continue with just VCTS/TEMPO wording.  Winds will be
out of the ESE through this evening, veering to more SSW overnight.

Any storms should clear out by Sunday morning, leaving VFR
conditions.  However, additional thunderstorms will be possible
Sunday afternoon in the SDF planning period.  Winds will increase out
of the SSW through the day to around 10-15 knots.


.LMK Watches/Warnings/Advisories...


Short Term........KJD
Long Term.........13
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