Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 082332
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
632 PM EST Fri Dec 8 2017

...Updated Aviation and Forecast Discussion...

.Forecast Update...
Issued at 630 PM EST Fri Dec 8 2017

Below freezing at all ASOS/AWOS/KY Mesonet sites this hour.
Dewpoints also show a very dry low level in the atmosphere. As a
cold front approaches the region, any snow formed aloft will have to
overcome this dry air before we see anything making it down to the
surface. Still looks like we could start seeing a dusting in Dubois
and Orange counties by just after daybreak and a couple hours later
into the Louisville metro, before that main synoptic band starts to
dissipate. Then we should transition to more showery activity as low-
topped cells develop. It will be interesting to see how being in the
wake of the first trough passing us by will affect this shower
activity. Still some models are spitting out QPF and with how cold
it is aloft, we should see higher snow ratios. For now have average
anowfall totals around an inch in Jefferson/Scott counties in
Indiana, around a half inch along the I-64 corridor and less to the
south, but again with any showers with those higher snow ratios, we
could see some narrow swaths of totals near an inch.

Made some minor adjustments to pop timing, but the zones look good
and no update is necessary.

&&

.Short Term...(This evening through Saturday night)
Issued at 335 PM EST Fri Dec 8 2017

...Light Snow Accumulations Possible Saturday...

It`s been a chilly day, with skies ranging from partly to mostly
cloudy. Temperatures were in the 30-35 degree range and won`t make
it much higher than that. Cirrus spreading overhead from a southern
stream system will gradually move off to the east during the evening
hours, allowing a quick drop in temps tonight. Lows in the upper
teens to low 20s are likely.

The main weather concern in the short term is the potential for
light snow accumulations on Saturday. This is a quick-hitting
clipper system, with the surface low swinging through the southern
Great Lakes on Saturday. Overall, it`s a very light QPF event, but
could afford many their measurable snowfall of the season.

As the primary upper trough swings overhead on Saturday, a compact
mid-level shortwave will dive southeast out of the Midwest and
provide synoptic support. The relatively weak system`s cold front
will move in from the northwest in the 12-15z time frame. A light
synoptic snow band is expected to move into southern Indiana between
12-14z and then gradually spread southeast. A consensus of the GFS
and mesoscale models suggests light snow beginning in the Louisville
metro around 15z and in Lexington by 16-17z.

We`ll likely see plenty of virga on the leading edge as the snow
battles low-level dry air. This will act to reduce accumulations. As
the column saturates from top to bottom, wet bulb
effects/evaporational cooling is expected to keep the precip type as
all snow. As we move into the afternoon hours, colder air advects in
aloft and steepens the lapse rates quite a bit. So the snow will
almost have a convective nature to it. However, moisture depth
diminishes steadily beyond 17z as well, with all of that moisture
below 10 kft. The dendritic growth zone does remain saturated
through the afternoon, helping to boost snow ratios.

Overall, this is likely to be a light synoptic band of snow in the
middle of the day with very spotty snow showers behind it. The
isolated snow showers should wind down by mid-evening. Total
accumulation will range from nothing or a dusting to a half inch.
Isolated amounts up to three-quarters of an inch will be possible in
southern Indiana. Impacts should be relatively limited, but a quick
dusting on some roads is possible. With highs in the low to mid 30s,
road conditions should improve quickly once the snowfall has ended.

Winds will become breezy Saturday afternoon as the lapse rates
steepen, with gusts to around 25 mph. Saturday night will be cold
and dry, with lows in the upper teens to around 20.

.Long Term...(Sunday through Friday)
Issued at 330 PM EST Fri Dec 8 2017

The current pattern is expected to continue with a large trough over
the eastern CONUS. This will place the Ohio Valley in a northwest
flow pattern through the end of the coming week. The trough will
funnel cool air from Canada as well as some small disturbances from
the north that could bring some light precipitation through the week.

(Sunday through Monday)

As the cloud cover from Saturday`s system moves to the east, clouds
will give way to clearing skies and we`ll see a return to sunshine.
Winds will advect warmer temperatures from the southwest bringing
highs back to around 50 on Monday.

(Tuesday through Wednesday)

Tuesday morning a cold front will work its way south bringing cloud
cover and a scattered chance of light snow to areas east of I-65.
Southern Kentucky counties could see light rain instead of snow.
This front will help lower high temperatures back to the 30s.

(Thursday through Friday)

Thursday could see another shortwave work through the area bringing
a chance of light snow or flurries to southern Indiana and northern
Kentucky counties. The timing of these systems have low confidence
between the models. Moisture is going to be limited. Better chances
of precipitation remain northeast of our area.

&&

.Aviation...(00Z TAF Issuance)
Issued at 625 PM EST Fri Dec 8 2017

A cold front will cross the region Saturday, bringing a chance for
some light snows in the morning, mainly in the HNB/SDF corridor and
then afternoon snow showers in the SDF/LEX corridor. At this point
not forecasting more than MVFR conditions with these snows, but the
afternoon instability may lead to worse conditions at LEX.
Otherwise, expect gusty winds ahead of and behind the frontal
passage.

&&

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

$$

Update...RJS
Short Term...EBW
Long Term...KDW
Aviation...RJS



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