Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 211043
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
643 AM EDT Mon Apr 21 2014

...Updated Aviation Discussion...

.SHORT TERM (Now through Tuesday)...
Issued at 305 AM EDT Mon Apr 21 2014

The synoptic pattern early this morning features a cutoff low just
off the southeast coast.  In its wake, an upper-level ridge has
established itself over the Ohio Valley.  However, an upper-level
trough will approach and push through the region during the short
term period, bringing chances for showers and isolated thunderstorms.

Upper-level clouds continue to increase this morning out ahead of
the aforementioned system.  These clouds will continue to increase
in coverage and thicken throughout the day.  Despite the increasing
cloud cover, still think there will be some sunshine from time to
time today, especially across the eastern CWA.  Additionally,
mechanical mixing will be much better than yesterday, which should
help to make up a bit for less sunshine.  Therefore, will go with
temperatures very similar to Sunday`s values, which puts highs in
the upper 70s and lower 80s.

The main forecast challenge will come with convection potential late
this afternoon into tonight.  Overall thinking is that this system
may a bit of an underachiever due to a few factors.  First,
low-level moisture is lacking, and we once again have the potential
to mix down drier air during peak heating today (especially east of
I-65 where surface moisture advection will not be as robust).  This
will tend to limit overall instability late this afternoon.
Additionally, a very dry layer exists between 600-800mb, which will
be hard to overcome even with a slug of isentropic lift progged to
slide across the region late this afternoon.  Therefore, will
continue with previous forecast thinking and keep low-end pops going
along and west of I-65 by 00Z Tuesday.  If an isolated shower or
storm were to develop this afternoon in peak heating, it could be
capable of some gusty winds given large surface dewpoint depressions
and the potential for mid-level dry air entrainment.

By tonight, synoptic forcing for ascent increases which will help
pull deeper moisture into the region.  This will enhance the
precipitation coverage, but will still forecast pops in the 60-70%
range.  Given very weak convergence along the surface front and
mediocre dynamics, can`t see going any higher with pops.  The
latest hi-res solutions support this thinking, showing a few gaps in
coverage as the precipitation pushes through tonight.  Given the
region will be ahead of the surface front tonight, will go on the
high end of guidance with lows, which puts them in the upper 50s and
lower 60s.

Precipitation will push to the southeast of the region by early
Tuesday afternoon as the surface front slides across the region.
Northwesterly winds behind the front will usher in a cooler and
drier airmass as temperatures struggle in the 60s and lower 70s.

.LONG TERM (Tuesday Night through Sunday)...
Issued at 300 AM EDT Mon Apr 21 2014

Main forecast challenges in this period include just how cold it
will get Tuesday night, and timing of precip associated with a
late-week closed low in the Great Lakes.

Respectable 1022mb surface high will nose southward from the Great
Lakes, with the ridge axis near I-65 by 12Z Wednesday. Min temp
forecast is fairly well in line with latest MOS guidance, with lows
around 40 for most of the area, but plenty of areas in the Bluegrass
region in the mid/upper 30s. Light NNE breeze staying up through the
night would keep temps from bottoming out and mitigate any frost
concerns. However, the GFS is fairly bullish with the ridging into
the Ohio Valley, and if it verifies we could see quite a few of the
sheltered valleys and typical cold spots flirting with the freezing
mark. Next couple of forecast cycles will need a close look at the
frost/freeze potential Wednesday morning, so stay tuned.

Expect a fairly quick recovery in temps starting on Wednesday, going
back above normal for Thursday and Friday. Next system is on tap for
Thursday night, as a series of impulses will pinwheel around a
"bowling ball" upper low making its way across the Great Lakes.
Consensus seems to be building for the best rain chances to occur
Thursday night as a negatively tilted impulse moves through, so will
go with high-end chance/low-end likely POPs. Another impulse will
reinforce the trofiness aloft Friday night and push a surface front
through, but at this time it looks like that system will be
moisture-starved.

Below-normal temps are expected over the weekend, with rain chances
re-entering the picture on Sunday ahead of an upper low developing
over the Plains. `Tis the season for cutoff lows, and it looks like
we will have the opportunity to become very familiar with this
particular system as April comes to a close. That said, it is mostly
beyond the scope of this forecast, and there are still details to be
worked out yet.

&&

.AVIATION (12Z TAF Update)...
Issued at 640 AM EDT Mon Apr 21 2014

An area of low pressure will approach the Ohio Valley today.  Out
ahead of this feature, high and mid-level clouds will continue to
increase through the day, but all cigs will remain VFR.  Winds will
increase out of the SW to around 10 knots today.

By tonight, the system will spread showers and a few embedded
thunderstorms across the region.  Specific timing of these showers
still is a bit uncertain, given coverage is only expected to be
about 60%.  Therefore, will continue with VCSH wording, although
prevailing -SHRA will likely be needed at some point tonight.  The
moderate showers will be capable of dropping cigs/vsbys briefly
to MVFR thresholds.  In addition, guidance suggests the potential
for some MVFR cigs toward the end of the TAF period and into the
planning period for KSDF.  These ceilings will largely hinge on
precipitation coverage overnight, thus will leave cigs at VFR for
now until trends can be better ascertained.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...NONE.
IN...NONE.
&&

$$

Short Term........KJD
Long Term.........RAS
Aviation..........KJD




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