Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 292053
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
353 PM EST Thu Jan 29 2015

.SHORT TERM (Now through Friday Night)...
Issued at 315 PM EST Thu Jan 29 2015

Latest analysis shows surface low pressure centered over west
central Illinois while a cold front stretched through central
Missouri into northwest Arkansas. Ahead of this feature, a
tightening pressure gradient and strong 50-60 kt jet between 900 and
800 mb has brought warming temperatures to the Ohio Valley
overnight. After bottoming out in the 20s to to lower 30s, readings
have warmed into the mid 40s in the west while some eastern
locations are holding in the lower 30s.

The sheltered and valley locations across the far eastern and
northeast forecast area have been slower to respond. Temperatures
are expected to be right around 32 degrees as precipitation moves in
early this morning and combined with the drier low levels, concerned
that for a brief period, light freezing rain could fall and coat
road surfaces. Road temperature sensors across that area are in the
30 to 32 degree range. This could cause some travel issues,
especially for the morning commute. After coordinating with ILN/JKL,
have opted to issue a SPS to cover the potential slick and icy spots
across the Bluegrass region.

Hi-res models are capturing the band of showers associated with the
initial warming and PV anomaly fairly well, bringing it through
central KY and southern IN early this morning through the
mid-morning hours. Will time the highest POPs during its passage.
The true cold front and upper trough will arrive early to mid
afternoon which could bring additional rain showers. High
temperatures are expected to warm into the upper 40s to mid 50s
before beginning to fall mid afternoon post frontal.

Wrap around moisture then arrives later this evening and through the
overnight hours. Cross sections and soundings show the moisture
mainly resides in the low levels. As colder air filters in while
moisture exits, there`s still the window for wintry precipitation.
Soundings show loss of ice and saturation only reaches up to about
-5C, so there is a concern for a period of drizzle or freezing
drizzle across the Bluegrass region before enough cold air supports
a changeover to snow flurries or showers. Have added a slight chance
of freezing drizzle to the forecast for the late evening and
overnight hours. Plan on low temperatures in the mid to upper 20s.

Wrap around stratus is expected to hold in place Thursday night
through a good portion of Friday, especially east of I-65, as
moisture remains trapped beneath an inversion. Plan on a seasonably
cold day with highs only topping out in the 30s. If clouds hang on
longer into the afternoon, it`s possible forecast high temperatures
may be a few degrees too warm.

.LONG TERM (Saturday through Thursday)...
Issued at 329 PM EST Thu Jan 29 2015

...Accumulating Snows Increasingly Likely Late This Weekend Across
Southern Indiana and Portions of Northern Kentucky...

==================================
Synoptic Scale Overview
==================================

A split flow pattern aloft will be found at the beginning of the
forecast period.  By Sunday, a moderately strong mid-level wave will
drop out of the northern Plains and amplify as it approaches the
Mid-Mississippi Valley.  As this occurs, surface cyclogenesis will
take place over Arkansas with the newly formed surface low moving
from SW Tennessee into the Mid-Atlantic region by Monday morning.
With cold air in place across the northern half of the forecast
area, a swath of accumulating snows looks increasingly likely.
Further south, near the surface low track, a wintry mix of rain and
snow appears likely...eventually transitioning to all snow by Sunday
night.  The upper air pattern is then forecast to relax a bit,
however, significant amplification is expected to occur by midweek
as waves in the northern and southern stream attempt to phase and
carve out a large/deep trough over the eastern third of the United
States.

==================================
Model Trends/Preference/Confidence
==================================

Model Trends...

Over the last 24 hours, we have started to see a more definitive
case of model convergence on the evolution of the late weekend storm
system.  All of the 29/12Z guidance has trended colder with regards
to the lower boundary layer temperatures.  The GFS and Euro have led
the trend in the colder solutions, while the NAM and Canadian GEM
have now started trending colder as well.  The NAM is still the
warmest of the models, while the GFS/GEM/Euro are generally the same
as their previous runs.  Noticeably, the Euro and GFS solutions have
continued to show remarkable continuity in their runs, which results
in increasing forecaster confidence on the evolution of this
system.  Given the current datasets and trends, it appears
increasingly likely that accumulating snows will be a threat to
southern Indiana and portions of northern Kentucky.

The model data continues to exhibit some spread regarding the
mid-late week system as the upper level flow and timing of features
remain uncertain.  However, there appears to be at least some hint
of agreement that another weather system will affect the region
Wednesday into Thursday.

Model Preference...

For this forecast cycle we will be utilizing a blend of the previous
forecast while incorporating the colder solutions from the new
datasets.  The forecast itself remains largely unchanged, but given
the relatively good model agreement, we will be increasing PoPs
for Saturday Night through Monday morning.  As for QPF, a blend of
the GFS/Euro/WPC Guidance will be incorporated.

From midweek onward, have generally stuck closer to the blend of the
29/12Z GFS Ensemble and the 29/00Z Euro ensembles along with
incorporating our previous forecast.

Forecast Confidence...

Forecast confidence on the occurrence of precipitation from Saturday
Night through Monday morning is high given the good model
agreement.  Forecast confidence on the precipitation type is much
lower given the uncertainty in the forecast of lower atmospheric
temperature profiles.  This is especially true across much of
southern Kentucky where low-level warm air will likely keep
precipitation mixed or in plain liquid.  Forecast confidence is
increasing however, that much of southern Indiana and portions far
northern KY will see mostly snow with the late weekend system.

Forecast confidence from midweek onward can be described as
low-moderate in both temperature and precipitation departments.

==================================
Sensible Weather Impacts
==================================

A quiet day of weather is expected on Saturday as we wait for the
approach of the late weekend weather system.  The day will likely
start off partly sunny, but clouds will be on the increase late in
the day.  Highs look to warm into the upper 30s in the north with
lower 40s across the south.

By Saturday night, we expect a band of precipitation to develop out
to our northwest within a large baroclinic zone from the central
Plains into the Midwest.  Moisture is expected to return to the
region during the overnight period with precipitation breaking out
across southern Indiana and central Kentucky very late Saturday
night.  Thermal profiles generally look cold enough for mostly snow,
but a mix of snow and rain may occur down across southern Kentucky.

Precipitation is expected to expand and increase in intensity during
the day on Sunday across the region.  Model proximity soundings and
thermal cross-sections suggest that mainly snow will occur across
southern Indiana and across far northern KY.  Just exactly how far
south the rain/snow line will be is not definitive at this time.
However, incorporating the multi-model consensus, it does appear
that the previous forecast of rain and snow across much of
south-central Kentucky still looks to be likely at this time.
However, should thermal profiles end up being slightly cooler, the
lower boundary layer may be sufficiently cool enough to have snow as
the pre-dominant precipitation type despite the fact that surface
temperatures may be several degrees above freezing.

As the surface low passes through the far southern portion of our
forecast area, colder air will be pulled in from the north resulting
in all areas seeing snow Sunday evening.  The GFS and Euro continue
to show a good deformation band signal within their data sets this
afternoon.  This deformation band looks to start off in central IN
Sunday afternoon and then many pivot southeastward into Kentucky
late Sunday night and early Sunday morning.  Based on the current
datasets, the best chances for accumulating snows looks to be from
Sunday afternoon through early Monday morning.

Considerable uncertainty remains in the evolution of this system.
It should be noted that the weather features forcing this system are
located quite a ways from the North American upper air network and
will not be properly sampled until late tonight or Friday morning.
Because of the numerical models sensitivity to initial datasets,
oscillations in the model solutions are still possible.  We
encourage people with travel plans this weekend to keep a close eye
on subsequent forecasts as travel may be negatively impacted late
this weekend.

Quieter weather is expected for Monday and Tuesday as we will be in
between weather systems.  High and low temperature forecasts are
particularly hard to nail down at this point as they may be greatly
impacted if we have snow cover.  For now, have stuck close to the
SuperBlend guidance with highs in the mid-upper 20s to the lower 30s.

From midweek on, we will need to keep an eye on another weather
system that will affect our region in the Wednesday/Thursday time
frame.  The track and evolution of this system is highly unknown and
may be modulated to an extent by the late weekend system.  For now,
have kept a rain/snow mix chance in for Wednesday with some light
snow possible Wednesday night.  Much colder conditions look likely
as we head to towards the end of the week.  Current guidance
generally gives highs in the 20s, but this could be too optimistic
if a snow pack exists.  Overnight lows in the upper single digits to
lower teens are expected.  However, sub-zero readings will be on the
table if a significant snow pack is realized.  Stay tuned.

&&

.AVIATION (18Z TAF Update)...
Issued at 1228 PM EST Thu Jan 29 2015

A cold front will quickly slide through the TAF sites this
afternoon, with SW winds veering to WNW then NW through the
afternoon and evening. Expect sustained winds between 10 and 20 mph,
occasionally gusting up around 35 mph during this time. As we move
into mid afternoon, an upper disturbance combined with low level
moisture may be enough to squeeze out a few showers. Any
precipitation should be light so doubt vis restrictions will occur.
Will put VCSH in TAFs just in case.

Otherwise, ceilings are expected to lower into the MVFR range very
quickly, with ceilings expected to be below fuel-alternate by late
afternoon at SDF/LEX.

Low ceilings should persist fur much of the night with showers
changing to snow showers at SDF/LEX. Winds will continue to veer to
NW and slacken. A return to higher MVFR or even VFR is possible by
late morning, although there is some question as to whether low
level moisture will still be trapped under the inversion.

&&

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

$$

Short Term........BJS
Long Term.........MJ
Aviation..........BJS






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