Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 300821
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
321 AM EST Fri Jan 30 2015

.SHORT TERM (Now through Saturday)...
Issued at 315 AM EST Fri Jan 30 2015

...A Few Slick Spots Possible this Morning...

An upper level disturbance pushing south within the broad upper
trough over the area is forecast to maintain and perhaps enhance
very light precip occurring over the region this morning.  This
precip was in the form of a very light rain/drizzle or snow as of
8Z.  Expect this light mix to continue and transition over to mostly
isld-sct light snow showers over southern IN/northern KY/eastern KY
during the pre-dawn hours.  Locations that do get a light snow
shower this morning may see a dusting of accumulation.  This light
snow accumulation and wet road sfcs from light precip overnight
combined with temps falling into the upper 20s and lower 30s this
morning may cause some slick spots.  Road temps as of 8Z were
lagging behind sfc temps by about 2-4 degrees with most roadways
well above freezing in the mid to upper 30s early this morning.
This may help minimize travel issues this morning.  However, with
good CAA in place and the light precip expected to continue through
around sunrise, feel an SPS is in order to highlight possible slick
spots developing especially on bridges and overpasses.  There is a
potential that travel impacts may worsen especially if road temps
can fall below freezing over a wider area creating a more widespread
glaze on roadways.  If this happens, an Advy product may be needed.
Will monitor closely this morning.

Precipitation should end from west to east by mid morning with skies
becoming partly cloudy this afternoon.  A good range of high temps
can be expected today with highs ranging throughout the 30s.  The
coolest locations should be southern Indiana and the Bluegrass, and
the warmest locations will be in the Bowling Green area and
southwest toward the TN border.

Sfc high pressure will settle over the region tonight bringing a
period of mostly clear skies and light or calm winds.  Temps should
fall into the mid teens to lower 20s with the coldest locations over
east central KY which will be better positioned within the colder
airmass.

Saturday clouds will be on the increase ahead of an approaching
storm system.  With southerly winds bringing in slightly warmer air,
temps should moderate into the upper 30s and lower 40s for highs on
Sat.

.LONG TERM (Saturday Night through Thursday)...
Issued at 305 AM EST Fri Jan 30 2015

...Accumulating Snow Possible Across Parts of the Area Sunday...

The focus for the long term period is on the possible winter storm
across parts of the Ohio Valley beginning late Saturday night
through early Monday morning.

The 30.00z model guidance lowered our confidence as models
continue to waffle on the surface low track. The GFS/NAM shifted its
low track a hundred miles northward, bringing it through central
Kentucky Sunday evening. This greatly impacts the low level
temperatures and subsequent snow banding potential across northern
KY into southern Indiana. If this solution verified, only the far
north could see a period of accumulating snow. However, the GEM
swung the other direction with a much further south low track,
resulting in a colder solution and the band of accumulating snow
across Kentucky. The GEM has been unstable the past few days and at
this time has been considered an outlier. Finally, the ECMWF also
trended more north and warmer, coming in line with the GFS/NAM
warmer solution. Despite the 30.00z guidance agreeing somewhat,
confidence remains low due to this shift from the 29.12z guidance,
and previous runs that have waffled the past 3 days. It would be
nice to see some model to model continuity plus run to run
consistency before latching onto a particular storm track.

Current thinking is that precipitation will break out across
Illinois, Indiana and Ohio initially late Saturday night, mainly in
response to upper level lift from the right entrance jet and
northern stream energy. This would be mainly in the form of snow as
thermal profiles are below 0C. As the surface low takes shape and
moves closer to the area, we`ll see precipitation spread across the
entire area and at the same time, warmer air drawn northward will
transition precip to a mix or all rain depending on low level
temperatures. This is where the greatest uncertainty lies. To avoid
a huge shift from the previous forecast, will trend temperatures
slightly warmer, creeping the rain/snow line north /close to the
Louisville and Lexington metros/ but not quite as aggressive as the
30.00z guidance suggests. Hopefully the 30.12z guidance will
increase forecast confidence as the northern and southern stream
energy should be sampled by the upper air network.

At this time, parts of our southern Indiana locations stand the
greatest probability to see accumulating snow, though this
probability isn`t necessarily high. There is the potential for 1 to
3 inches of snow for areas from Dubois to Jefferson County, Indiana.
The accumulating snow would fall from Sunday afternoon through the
early Monday morning hours. Some travel impacts here are possible.
All locations to the south have too much uncertainty to discuss
possible snowfall amounts at this time.

Days 4 - 7 Discussion:

In the wake of the weekend storm system, there`s a good consensus
that the upper trough deepens across the Great Lakes bringing an
unseasonably cold air mass into the Ohio Valley. If a healthy
snowpack is realized across portions of the area and to the north,
then Monday high temperatures may only top out in the upper teens
across southern Indiana to mid/upper 20s across southern Kentucky.
Combined with a brisk north wind, particularly Monday morning, and
wind chills may remain in the 0 to +10 range across the north. Our
best setup for strong radiational cooling conditions comes Monday
night into Tuesday morning as the Canadian high pressure slides
overhead. Model consensus puts lows in the lower to middle teens,
but again, a fresh snowpack could send readings well into the single
digits above zero. We`ll have to see how that plays out. Otherwise,
the surface high pushes to our east Tuesday bringing southerly to
southwesterly flow. We`ll begin to moderate some, but likely remain
below normal with highs in the 30s.

By midweek, another active southern stream system crosses the Gulf
coast and begins to lift northeast along the southeast US coast.
There is still considerable uncertainty in the track of this
system, potential phasing with northern stream energy and how far
northwest precipitation may spread back over the Ohio and TN
Valleys. Thermal profiles suggest this could fall as a wintry mix or
snow. Right snow, the 30.00z deterministic and ensembles suggest
this system stays to our southeast but it does bear watching since
it`s still 5 to 7 days out. For now, a model consensus of 20-30
percent POPs seems reasonable and highs/lows in the 30s/20s
respectively.

&&

.AVIATION (06Z TAF Update)...
Issued at 1230 AM EST Fri Jan 30 2015

A few upper level disturbances in NW flow will keep light patches of
precipitation going in the area through around sunrise.
Precipitation will likely remain a mix of rain/drizzle/snow at
SDF/LEX for the next few hours before switching primarily over to
-SHSN during the pre-dawn hours.  BWG will remain of mix of
rain/drizzle/snow throughout the morning hours.  As temps fall below
freezing before sunrise, some slick spots on runways could develop.
MVFR conditions with cigs below 2 kft will primarily be the rule
overnight with cigs expected to decline quickly over the next few
hours.  Flight conditions will improve after sunrise with the TAF
sites returning to VFR by early afternoon. Winds will remain out of
the NW-N for much of the day and then becoming light NE this evening
as sfc high pressure moves in.

&&

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

$$

Short Term........AMS
Long Term.........ZBT
Aviation..........AMS





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