Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 010536
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
136 AM EDT SUN MAY 1 2016

...Updated Aviation Discussion...

.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
Friday.

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
Saturday.

&&

.AVIATION (06Z TAF Update)...
Updated at 136 AM EDT Sun May 1 2016

Expecting the TAF sites to be in a relative lull in precipitation
activity through the remainder of the overnight and into the late
morning. The exception will be another hour or two of showers and
storm in the vicinity of LEX. The rest of the activity will be to
the east of the TAF sites. Will continue with the thoughts that
convection to our west will diminish as it approaches.

Main concern overnight now switches to potential for some vis or
ceiling restrictions. With some clearing possible as we near dawn
combined with small T/Td spreads think fog or low stratus is a
possibility. Models have been overdone with low ceilings recently so
will not go as pessimistic as suggested. Think BWG will have the
best shot at low MVFR/IFR toward dawn, with LEX possibly seeing
MVFR. Will keep SDF just on the VFR side of things, but going close
to MVFR.

Otherwise, expect steady and generally south winds overnight. SW
winds prevail later today with another disturbance passing just
north of the region. This will spark afternoon and evening
thunderstorm chances under otherwise VFR conditions.

&&

.LMK Watches/Warnings/Advisories...
KY...NONE.
IN...NONE.
&&

$$

Short Term........KJD
Long Term.........13
Aviation..........BJS


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