Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 270534
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1234 AM EST Tue Jan 27 2015

...Forecast Update...
Issued at 1234 AM EST Tue Jan 27 2015

A narrow band of slightly more significant snow has set up from
Salem to just west of Indianapolis, and is sliding to the SSE.  Have
upped snowfall amounts ahead of this band, including the Louisville
metro, to around half an inch for the pre-dawn hours.


Issued at 951 PM EST Mon Jan 26 2015

Most of the snow from earlier has dissipated with just some very
light returns across portions of central KY and southern IN
remaining. The next disturbance will dive through the northwest flow
overnight. The surface low associated with this is currently over
eastern Iowa. This low will move into western/central KY late
tonight. The 0Z NAM is very similar to the previous runs and brings
a swath of snow into portions of southern IN and west central
Kentucky between 06-12Z. Still think an addition half an inch to an
inch of snow will be possible with this activity, so have kept the
Travelers Advisory. Did expand it a couple of counties to the east
across south central KY based on the latest NAM. Also expanded the
other SPS to cover all our counties. Though confidence is less in
the Bluegrass for any accumulations, if some snow does fall temps
are cold enough a few slick spots could develop on untreated
roadways.

Issued at 634 PM EST Mon Jan 26 2015

The initial area of light to moderate snow that moved in this
afternoon has resulted in a light dusting of snow in portions of
southern IN and west central KY. This activity has become more
showery in nature over the last hour. Hi-res models suggest that
this area of snow showers will continue to diminish over the next
couple of hours. However, some additional light accumulations of a
tenth or two are possible with this activity. In addition, given the
showery nature of the snow, visibilities may drop quickly over
short distances, so travelers should use caution. Still think we
could see another round of snow after midnight as a second wave
drops through the NW flow.

.SHORT TERM (Now through Tuesday Night)...
Issued at 331 PM EST Mon Jan 26 2015

...A Round of Light Snow this Evening, and Again Overnight...

Light snow is currently overspreading the Wabash River Valley and
will slide into our CWA this evening. This snow is associated with
1000-700 mb moisture beneath the left exit region of an upper level
jet and its associated frontogenesis. This first wave will initially
spread through this evening with some very light accums possible
mainly west of I-65 into south central KY. Think any light returns
that get too far east will have more trouble overcoming the slightly
drier air to the east.

A second wave will then quickly drop through the amplified NNW flow
through the pre-dawn hours, with its associated surface reflection
passing just to our west across western KY. Conditions will be most
favorable for light snow during this time, where additional light
accumulations could occur. Overall, think areas west of I-65 could
see around a half an inch of snow, spreading more SE toward Lake
Cumberland in south central KY. Expect that there could be a narrow
band closer to an inch from Huntingburg, IN to Bowling Green, KY. Do
think enough snow could fall to coat some roadways, especially
untreated ones. So, will issue a Travelers Advisory across out
western CWA where we think the best chance for the slightly enhance
amounts will be. Further east, will go with a regular Special
Weather Statement where less, if any, impacts are anticipated from a
dusting of snow.

Snowfall rates are not expected to be heavy, in fact data supports a
fairly light snow as saturation only goes up to around -8C.
Additionally, there isn`t a whole lot of moisture to work with. So,
the main concern is that any untreated snow covered roads will be
hazardous during the Tuesday morning commute.

As I type this, the 18z data is coming in much more robust with a
narrow 1 to 2 county wide band that would likely support closer to
an Advisory if it verified. Can`t rule out an upgrade across a small
area tonight.

Should be a sharp cutoff between any heavier amounts in our west to
just a dusting east. Will call this line along and east of I-65.

Any lingering snow should quickly diminish by mid morning with
temperatures slowly rising up through the 30s during the day. Temps
will likely stay around freezing across our NE.

Surface high builds in Tuesday night with cold temperatures in the
mid teens to low 20s.

.LONG TERM (Wednesday through Monday)...
Issued at 326 PM EST Mon Jan 26 2015

...Significant Weather System May Affect The Ohio and Tennessee
Valleys Late In The Weekend...

==================================
Long Term Synoptic Overview
==================================

The long term period will start off rather dry across the forecast
area as we remain in northwesterly flow aloft.  A large mid-level
gyre will be located across northeastern New England with a deep
trough extending down the eastern coast of the US.  This trough will
lift out by late Wednesday.  A weak and quick moving upper level
wave will swing through the region on Thursday bringing a round of
precipitation to the region.

Late in the extended period, the models are suggestive of an active
split flow pattern setting up across the CONUS.  Within the southern
stream, the models continue to show a strong signal with a potent
mid-level wave moving out of the southwestern US and into southern
Plains by the weekend.  As this occurs, a large surge of arctic air
will be pushing southward into much of the northern US.  The
combination of these two features will set the stage for a large
baroclinic zone to develop from the southern Plains into the
Mid-Atlantic region.  A rather widespread precipitation shield looks
like a distinct possibility as we head into the late weekend.  The
northern edge of this precipitation field may include significant
wintry precipitation.

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

Overall model agreement can be considered relatively good in the
first half of the extended period.  The timing and spatial patterns
of the Thursday clipper system are relatively good in the GFS and
Euro data sets.  The overall strength of the clipper system is
slightly stronger on the model runs.  This may result in slightly
warmer temps on Thursday followed by much colder conditions Thursday
night and into Friday.

For the late weekend storm evolution, the models had the storm late
last week, lost it over the weekend, and now have brought it back. A
typical pattern we`ve seen within the extended period this winter.
The GFS/Euro/GEM all suggest that we`ll see precipitation from late
Saturday night through early Monday as we`ll be within the
aforementioned baroclinic zone.

The devil in the details here will be the low-level thermal
profiles.  The crux of the forecast will depend on how far south the
PV over Hudson Bay will drop.  The further south it drops, the
deeper the penetration of arctic air into the northern US and into
the OH/TN valleys.  The Euro and Canadian are signaling that it will
drop into south-central Quebec while the OP GFS keeps it a bit
further north.  The current runs of the Euro/GEM would argue for
more wintry precipitation while the GFS run would be more plain rain.

Needless to say, nothing is set in stone here and the models will
continue to oscillate on the pattern over the next few days. Given
the the spreads in the model data, have gone closer to the GFS and
Euro Ensembles here.  Confidence on precipitation for the weekend is
moderate, but temperature and p-ptype confidence remains low at this
time.

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

Dry/cold conditions are expected for Wednesday as we wait for the
approach of the clipper system.  Expect precipitation to move in
from the west late Wednesday and early Thursday.  There remains a
concern that precipitation may get in here while surface
temperatures remain at/below freezing. This could result in some
mixed precipitation at onset...especially in the Bluegrass region.
Strong warm air advection should kick in out ahead of the clipper
resulting in rain showers for the day on Thursday.  Colder air will
surge in behind the clipper resulting in precipitation changing back
over to snow before ending...perhaps yielding a minor accumulation
of snow for some areas.  Highs Wednesday will range from the upper
30s in the NE to the lower 40s in the SW.  Overnight lows Wednesday
night will cool into the lower 30s.  Highs on Thursday will be in
the 45-50 degree range, with lows Thursday night cooling back into
the upper 20s.

Dry and seasonal weather is expected for Friday and Saturday as the
upper level pattern becomes more zonal.  Highs should range from the
mid-upper 30s to the lower 40s.

The weather is expected to take a downhill turn by late Saturday
night and through the day on Sunday as precipitation overspreads the
region from the west.  As discussed above, the models are in good
agreement with a widespread band of precipitation extending from the
southern Plains eastward to the Mid-Atlantic states...with the Ohio
and Tennessee Valleys right in the middle of things.  Depending on
the depth and extent of the arctic air, a wintry mix of
precipitation looks to be in the cards for our region.  For now,
have left the forecast with a rain/snow mix in the forecast until
the low-level thermal data becomes more clear.  However, we should
mention that the overall synoptic setup for the this period fits the
conceptual model for significant wintry precipitation for the Ohio
Valley.  This will be something to watch closely over the coming
week...stay tuned!

&&

.AVIATION (06Z TAF Update)...
Issued at 1220 AM EST Tue Jan 27 2015

A small disturbance diving to the SSE will bring a brief period of
light snow to the region during the pre-dawn hours.  BWG will be the
most affected by the snow, with SDF on the eastern edge and LEX east
of the main axis.  IFR conditions are most likely at BWG, with
transient IFR ceilings possible at SDF for a couple of hours.

The system should be just south of the airports by the time the sun
comes up.  Low clouds will gradually lift today and clear out
tonight as high pressure enters the region from the north.

&&

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

$$

Update.........13/EER
Short Term.....BJS
Long Term......MJ
Aviation.......13





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