Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 201403
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1003 AM EDT Wed May 20 2015

...Forecast Update...
Updated 955 AM EDT Tue May 19 2015

Tweaked temps/dewpts/sky cover to match obs with this update.
Precipitation still looks to arrive west of I-65 this evening with
most locations likely receiving some scattered showers tonight.
Isolated t-storm chances still look possible over south central KY
this evening.  High temps should range through the 60s to around 70
over southern IN and north central KY.  Portions of central KY and
south central KY should warm into the mid to upper 70s.

.SHORT TERM (Now through Thursday)...
Issued at 255 AM EDT Wed May 20 2015

Surface high pressure over the Great Lakes continues to ridge into
the Ohio Valley, maintaining cool and dry conditions. Upper
shortwave trof working its way through the Plains has triggered
precip all along its axis. Main challenge is how this system will
impact the Ohio Valley late this afternoon and tonight.

Models converging on a solution that yields at least light QPF for
the entire forecast area, even though this disturbance will be
shearing out as it approaches. Another major source of uncertainty
is the dry low-level air mass, and how much precip will be lost to
evaporation. Will stay middle of the road with timing, ramping up
chances west of I-65 late in the afternoon. POPs tonight will be
limited to around 50% due to low-level dry air, and the very narrow
window in which the soundings actually saturate. By 06Z the deeper
saturation is lost and POPs should tail off from there. More
moisture available along the TN border where a front could wave
northward, therefore we will continue to include a slight chance for
embedded thunder roughly along and south of the Cumberland Parkway.

Could see quite a spread in temps this afternoon, with southern
Indiana just reaching the mid 60s while south-central Kentucky could
be pushing 80. Cloud cover is the most likely factor that would
dictate a sharp cutoff in between.

Once this system scoots past after midnight tonight, it will leave
behind a trapping inversion and just enough low-level moisture that
stratus will linger well into Thursday morning. This will keep temps
unseasonably cool on Thursday afternoon with highs struggling to get
out of the 60s.

.LONG TERM (Thursday night through Tuesday)...
Issued at 252 AM EDT Wed May 20 2015

The synoptic pattern at the start of the long term period will
feature northwesterly flow aloft.  However, this regime will quickly
transition to southwesterly flow aloft by the middle of the upcoming
weekend as a ridge amplifies over the Ohio Valley and pushes toward
the eastern seaboard.  The southwesterly flow will bring a more
active weather pattern to the region as several weak waves eject
through the flow into the Commonwealth through the end of the period.

The first part of the long term period will feature seasonably cool
and dry conditions.  High pressure will build into western and
central Kentucky by Friday morning, helping winds go calm.  The main
limiting factor to ideal radiational cooling conditions may be some
lingering cloud cover across the Lake Cumberland and northern
Bluegrass regions Thursday night.  If these clouds hang on longer
than expected, current forecast temps will be too cool.  However,
assuming these clouds do clear out, overnight lows will drop into
the 40s.  Even a few upper 30s readings cannot be ruled out in the
usual cold spots!

850mb temps of around 7C into Friday afternoon will support
continued cool conditions, despite the expected sunshine.  High sun
angle this time of year will help compensate somewhat, but still
think highs will be cooler than average with readings expected in
the low to mid 70s.

The aforementioned surface high pushes east of the region Saturday
into Sunday, as upper-level ridging amplifies over the Ohio Valley.
This will begin a stretch of more active weather, as deep moisture
returns to the Ohio Valley on southwesterly flow aloft, along with
warmer temperatures.  Unfortunately, timing individual waves within
this regime is difficult at this timeframe, so will continue with
generally broad-brushed pops of 30-50% Sunday through at least
Tuesday of next week.  There should be diurnal minima overnight with
this convection, so have lowered pops a bit in the overnight
periods.  Any organized severe weather threat continues to look low,
as weak tropospheric flow should limit the organization of
updrafts.  However, depending on the timing of the individual waves,
moderate instability may build in the afternoons which could support
some robust pulse cells.  It appears this better chance for higher
amounts of instability will come from Monday onward next week as
low-level moisture pools into the region.  Temperatures will be
highly dependent on convective timing/coverage, but a general
warming trend from the 70s on Friday into the low/middle 80s late
this weekend into early next week seems likely.

&&

.AVIATION (12Z TAF Update)...
Updated at 645 AM EDT Wed May 20 2015

Main questions involve timing and impact of late-day and evening
precip, and just how low ceilings will go in the stratus that
follows.

For most of the day, expect thickening cirrus and light NE breezes.
Shortwave trof will get sheared out as it approaches from the
Plains, but will still at least pull low-level moisture into
Kentucky by late afternoon. Looks like a narrow window with deep
enough saturation to support a decent chance for rain from roughly
21Z until 03Z Thursday, so will include VCSH during that time frame.
This may seem optimistic given the high POPs in model guidance, but
this is still a weakening system that will be fighting a lot of dry
air. Ceilings will be high-end MVFR in BWG, while SDF and LEX remain
barely VFR.

After the forcing for precip departs late this evening, we`ll still
have lingering low-level moisture beneath a strengthening inversion
near 850mb. Stratus deck will lower as the night goes on, and models
are hitting IFR ceilings hard. Climatology does not favor these
types of stratus events so late in May, and the trapping inversion
doesn`t appear overly strong or low, so will carry fuel-alternate
MVFR conditions for most of the overnight. SDF could start to see
some gradual improvement in the planning period, but maintained
status quo as this TAF does not have ceilings tanking as much as
guidance would suggest.

&&

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

$$

Update.........AMS
Short Term.....RAS
Long Term......KJD
Aviation.......RAS





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