Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 262251

551 PM EST Thu Feb 26 2015

...Forecast Update...
Issued at 545 PM EST Thu Feb 26 2015

Updated the forecast to change precip wording to scattered snow
showers/flurries.  Radar has really blossomed over southern
IN/northern KY early this evening so feel that snow shower wording
is appropriate in addition to flurries.  Still not expecting any
accumulation more than a dusting from these snow showers/flurries
which should diminish across the area by midnight.

.SHORT TERM (Now through Friday Night)...
Issued at 300 PM EST Thu Feb 26 2015

The synoptic pattern this afternoon is characterized by a broad
trough across the central and eastern CONUS. At the surface, low
pressure is quickly moving off the mid-Atlantic coast while Arctic
high pressure is pushing into the northern Plains. This has put the
Ohio Valley under weak northerly flow. A weak upper level
disturbance has been working in from the west but hasn`t amounted to
much thanks to some drier air and overall weak dynamics. A fair
amount of sunshine allowed mid afternoon readings to rise into the
low to upper 30s, even top 40 degrees across the south. Clouds are
now quickly spreading east across the area, capping our temperature
rise for the day.

Plan on the Arctic high pressure to nose in from the northwest this
evening into tonight bringing another unseasonably cold night. As
colder air begins to surge southward, the cloud layer will become
more within the DGZ. Thus, scattered light snow showers or flurries
will remain possible through the evening. Some of this activity will
be diurnally driven, so expecting it to wane somewhat after sunset.
Nothing more than a dusting, if at all, is expected.

The air mass will become increasingly drier overnight, so expecting
mostly cloudy to cloudy skies early then clearing toward sunrise
Friday. Look for lows to range from around 10 degrees across
southern Indiana to mid/upper teens closer to the TN border. This is
contingent on clouds clearing out by late tonight, so if cloud
coverage is higher than anticipated, forecast lows are too cool.

High pressure will be the rule for Friday, bringing a very cold but
mostly to partly sunny day to southern Indiana and central Kentucky.
Zonal to southwesterly flow aloft will spill high level clouds
across the area, especially south of the KY parkways. Plan on
readings to top out in the low 20s across the north to the upper 20s
across south-central Kentucky. For comparison, normal highs are in
the low to mid 50s.

By Friday night, the high is expected to be centered over southern
Ohio which would be an ideal setup for radiational cooling. While
we`ll have light winds, forecast models are suggesting increasing
mid clouds from the west could prevent lows from bottoming out.
There is a good agreement between the 26.12z guidance in this, so
raised lows several degrees. This agrees with some of the gridded
guidance as well. Overall, still plenty cold with lows in the upper
single digits to the north to mid/upper teens across the Lake
Cumberland region.

.LONG TERM (Saturday through Thursday)...
Issued at 310 PM EST Thu Feb 26 2015

High pressure should be centered just northeast of the region at the
start of the period, making for a cold start to the day Saturday.
That high will shift east of the region during the day, allowing for
a warmup to begin. With that warmup however also will come more
moisture, making for better rain chances for most of this forecast
period. Deep-layered isentropic lift will begin Saturday night, with
models focusing the best lift and moisture over our northwest
forecast area. Sounding profiles are mixed around the freezing mark,
so have a wintry mix of precip in the forecast as well, mainly over
the northern half of the area Saturday night. Models are trending
toward that focus staying over the northern half of the region
Sunday, closer to a weakening warm frontal boundary and the forcing
it provides. Have kept pops in everywhere, but have the likelies
restricted to along and north of the I-64 corridor.

Then Sunday night a cold front will move through the region. This
front will not clean the region`s moisture out however, nor really
change airmasses much. After a fast-moving high pressure system
scoots by to our north Monday we will return to a southwest flow
aloft and another warming trend. Monday night and Tuesday we will
return to a warm air advection pattern with that moisture already in
place. Deterministic models are in pretty good agreement with this
pattern, with perhaps a break Tuesday ahead of a cold front to sweep
through here Tuesday night. 12Z Euro just came in with a much later
solution for rains. Ensemble models are not in as much of an
agreement as well. Means are focused in the correct area, but
standard deviations almost are equal to the mean value, indicating
still quite a bit of uncertainty. Given this new Euro solution and
because of this spread in the ensembles have backed off some on pops
for Monday night and will hope for more unity in the next few model
runs. The GFS and GEM solutions would bring a shot at some thunder
as well and cannot rule out severe weather as well. Have added in
some slight chance thunder for the Tuesday night and Wednesday
period (with the latter giving some credence to the Euro solution).

Latest GFS has a third system moving across our region in the colder
air Wednesday and Wednesday night, ahead of the deep trough well
behind the cold front. 00Z ECMWF is similar but also has even
another vortmax bringing precip Thursday. The 12Z Euro backed off on
this thinking, though again it has slower timing on the event as a
whole. Given the uncertainty, cannot call for a dry period at this
point Wednesday or Thursday. Will go for a blend of model and
guidance temperatures as well that period.

With all this rain in the forecast, have some concerns for flooding.
First is the present snow pack, which has roughly half an inch to
two inches already on the ground waiting to melt. Second, the ground
has a thin layer of frost just underneath the surface and even if
this goes away by next week the ground is still pretty saturated.
This forecast package will have storm total areal average rains of
2-3 inches. As these are averages, there is potential for greater
rains. This much rain likely would call some flooding across the
region. Stay tuned for updates as we fine tune this forecast over
the coming days.


.AVIATION (18Z TAF Update)...
Issued at 1210 PM EST Thu Feb 26 2015

Broken strato-cu with bases mainly VFR will slowly move over the TAF
sites this afternoon. However, drier air and overall weakening of
the system will limit coverage somewhat. As a result, removed VCSH
from TAF but still could not rule a brief snow shower or flurry
though impacts to aviation should be minimal. Confidence in sub-VFR
clouds increases this evening as better/deeper moisture pivots into
central Kentucky associated with fetch from the Great Lakes. Plan on
winds 5 to 10 kts to remain out of the north through the period.
Overnight, stratus could be stuck across the area beneath the low
level inversion, with the highest probability at LEX. Look for
scattering of the clouds between 12-18z Friday as high pressure
moves in.




Short Term.....ZBT
Long Term......RJS
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