Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 281709
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1209 PM EST Wed Jan 28 2015

...Updated Aviation Discussion...

.SHORT TERM (Now through Thursday)...
Issued at 234 AM EST Wed Jan 28 2015

High pressure over the upper Ohio Valley today will provide us with
mostly sunny skies, light breezes, and temperatures just a few
degrees cooler than normal.

Tonight the high will advance to the East Coast as developing low
pressure moves from Omaha to Chicago.  Temperatures will likely
bottom out by midnight and then begin to creep back up as winds turn
southwesterly and increase to 10 to 15 mph.  Clouds will increase
but little if any measurable precipitation is expected as dry air in
the low levels will take some time to overcome.  Nevertheless, if
any precipitation does make it to the surface, it could be in the
form of a very light wintry mix over the Blue Grass with very light
rain elsewhere.  The best chance for precipitation will be after 4am.

Light rain chances will increase on Thursday as the Chicago low
heads for Toronto and swings its trailing cold front through the
Ohio and Tennessee Valleys.  Moisture is shallow and rainfall
amounts are expected to be on the order of a tenth of an inch.

Southwest winds could get rather gusty Thursday morning and early
afternoon, with gusts of 20 to 25 mph possible.  These winds will
help pull temperatures into the mid 40s to low 50s by midday, but
then temps will fall back into the low and mid 40s by evening behind
the cold front.

.LONG TERM (Thursday night through Tuesday)...
Issued at 300 AM EST Wed Jan 28 2015

...Wintry Mix Possible Late This Weekend...

The main focus in the long term period is on the potential wintry
mix late this weekend across parts of the Ohio Valley.

Thursday night the weather pattern is expected to feature the
departing clipper strengthening across the Northeast US while strong
Canadian high pressure builds into the Ohio Valley from the
northwest. Lingering moisture trapped within a low level inversion
will likely keep clouds in place east of I-65 Thursday night. The
north/northeast flow will act to anchor the moisture in place Friday
morning, so there is some question on how much clearing will take
place across the northeastern portions of the forecast area during
the day. The 28.00z NAM soundings are the most aggressive with this
inversion/moisture setup, holding a stratus deck in place east/north
of Louisville metro. For now, incorporated a gradual but delayed
clearing trend Friday afternoon. Other areas likely to see more
sunshine as the surface high continues to build southeastward. It`ll
be a seasonably cool day with high temperatures a couple degrees
either side of 32F for most. The exception will be across
south-central KY where mid/upper 30s are expected.

For Saturday, zonal to weak ridging flow moves overhead as the
southwest US trough digs all the way south of the US border. We`ll
likely start off seasonably cold with good radiational cooling
conditions Friday night but rebound into the upper 30s to lower 40s
with a fair amount of sunshine and light winds.

Now onto the late weekend system. The 28.00z guidance wasn`t much of
a confidence booster as models have yet to stabilize on a particular
solution or placement of synoptic features. The lack of run to run
and model to model consistency leaves a lot to be desired in regard
to precipitation type late Saturday night through early Monday
morning. The 28.00z GFS/GEM trended toward a similar path compared
to previous runs and at this time take the surface low on a Memphis
to Louisville to Columbus, Ohio track. Meanwhile, the ECMWF solution
shows a weaker surface low but stronger, faster upper level wave
across the area. The GFS/GEM camp would be warmer /mostly rain/
solution while ECMWF doesn`t draw as much warm air northward thus
brings the wintry mix a bit further south into the forecast area.

At this time, precipitation is expected to spread into the area late
Saturday night into Sunday morning and could begin as light snow or
a wintry mix. There could be some impacts here as we`ll have a cold,
dry surface and road temperatures likely to be on the colder side
too. If precipitation begins close to or before sunrise, this could
be more problematic.  Then, as warmer air is drawn northward, a
changeover to all rain is likely for most if not all of the area. At
this point, the greatest chances for a period of accumulating snow
look to be across southern Indiana and points northward. While
Sunday into Sunday night would be the main timeframe for
precipitation to fall, once the surface low passes the area, much
colder air begins to quickly dive southward. Another changeover to
wet snow before precipitation tapers off is possible overnight into
Monday morning.

A blend approach was used for the forecast, showing a brief period
of light snow to mix to all rain Saturday night into Sunday morning.
With highs Sunday possibly well into the 40s, transitioned weather
to all rain for much of the day, then brought back a rain/snow mix
to snow late Sunday night into Monday morning.

Confidence is below average in regard to ptype, and the lack of
consistency in the models reduces our confidence even more. If the
thermal profiles are slightly cooler, then the accumulating snow
band could lie further south. This system has a long way to go
before being well sampled so changes and oscillations between the
models are expected. However, there is potential for some impacts to
portions of the area later this weekend into Monday morning, so stay
tuned to the latest.

In the wake of this system, there are signals that a shot of Arctic
cold will encompass much of the Upper Midwest through the Ohio
Valley. This could bring a period of unseasonably cold air with
highs in the lower 30s and overnight lows in the teens.

&&

.AVIATION (18Z TAF Update)...
Issued at 1209 PM EST Wed Jan 28 2015

High pressure will continue to drift through the region today and
provide the terminals with mainly clear skies this afternoon and
into the evening hours.  The high will push off to the east tonight
with surface winds shifting to the southwest.  High clouds will push
in from the west late this evening with ceilings lowering throughout
the night.  Surface winds will also pick up toward morning as the
pressure gradient increases.  Widely scattered light precipitation
will be possible toward the end of the period, but the low coverage
precludes a mention in the TAF at this time.

&&

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

$$

Short Term........13
Long Term.........ZBT
Aviation..........MJ




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