Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 131526
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
1026 AM EST SAT FEB 13 2016

...Forecast Update...
Updated at 1026 AM EST Sat Feb13 2016

It looks like the snow shower threat has decreased enough that we
can go back to a dry forecast. Radar returns have exited the CWA.
Visible satellite shows cloud streets forming over the Blue Grass,
but web cams and AMDAR soundings indicate that these clouds are very
shallow in depth. Also, surface air beneath them is very dry. So,
will go with dry weather for the rest of the day.

Updated at 801 AM EST Sat Feb 13 2016

A tiny snow shower recently passed right over Blue Grass Field and,
despite the shower`s diminutive nature, managed to take visibility
down to a half mile very briefly. The weather camera at Keeneland
also showed the shower passing through, and a quick call to the
observers at the airport verified what ASOS was telling us.

So, we have put some isolated snow showers into the forecast for the
next few hours for the Blue Grass. In reality, snow shower coverage
will likely be below 15% and the chance of actual measurable snow is
almost nil, but hate to have dry wx grids when we know there are
snow showers around that are able to reduce visibility that much.

.SHORT TERM (Now through Sunday Night)...
Issued at 324 AM EST Sat Feb 13 2016

...Accumulating snows likely Sunday into Sunday Night...

The synoptic pattern early this morning features a deep trough
across the eastern CONUS.  A strong PV anomaly was diving into the
northwestern CONUS, and will be the main focus of the short term
period as it will bring accumulating snows to the region Sunday into
Sunday night.

For today, expect an Arctic airmass to become entrenched over the
Ohio Valley as a Canadian ridge slides southeast into the Midwest.
Strong subsidence due to this feature will give way to mainly clear
skies today, but temperatures will really struggle as highs only
climb into the low to mid 20s!

==========================
Sunday/Sunday Night System
Confidence: High
==========================

The aforementioned PV anomaly will dive southeast into the southern
Great Lakes region on Sunday into Sunday night.  In response, a
rather impressive warm conveyor belt will develop over portions of
the MS River and spread east into the Ohio Valley Sunday afternoon
into Sunday night.  This warm air advection will ascend atop the
cold airmass in place, saturating the column from the top down.
Therefore, it will take some time to get flakes to the ground, but
think a good swath of snow will eventually move through from west to
east Sunday afternoon through Sunday evening.

The broad warm air advection regime should give a pretty consistent
2-4 inches across the region, with the lowest amounts likely in
southern IN.  A swath of 4+ inches is not out of the question across
portions of south-central KY (very near the track of the heaviest
snows from the big snowstorm earlier this year), where the better
overlap of QPF and enough cold air and lift within the DGZ will
coincide.

Once the main branch of the warm conveyor belt pushes into
southeastern KY, guidance does show the potential for the loss of
ice nuclei activation as saturation depth only goes up to around -4
or -5C.  This may cause some freezing drizzle to fall on top of the
snow.  Any impacts from this would likely be the same as the impacts
from the 2-4 inches of snow (slick roads), so will keep the message
simple and continue to advertise a mainly snow event.  Speaking of
message, do have fairly high confidence in Advisory level snows
across the entire region and discussed putting out a late 3rd period
Advisory. However, neighboring offices had some concerns on surface
temps, QPF amounts and saturation depth, so in coordination with
them will hold off on an Advisory for now and just issue a heads up
SPS.

Given the degree of warm air advection with this system, portions of
southern KY will likely see some rain begin to mix in towards 12Z
Monday.  This takes us into the next storm system, which is
discussed below.

.LONG TERM (Monday through Saturday)...
Issued at 356 AM EST Sat Feb 13 2016

The synoptic pattern at the start of the long term period will
feature a PV anomaly diving into this southern Plains within a broad
trough aloft.  This regime will transition to amplified ridging over
the Ohio Valley by the end of the period, bringing a substantial
warmup (!!) to the region by the end of the period.

============================
Monday/Monday Night System
Confidence: Low-Medium
============================

The aforementioned PV anomaly will eject across the TN Valley Monday
into Monday night, helping to deepen a surface low across portions
of the mid-South, sliding northeast into the mid-Atlantic region by
Tuesday morning.

Guidance continues to remain a bit split on the track of the low and
the strength/track of a potential deformation band that is likely to
set up northwest of the main surface low.  The GFS solutions over
the past couple of days have been absolutely all over the place, so
it has largely been ignored with this forecast.  The GEM is the
southern outlier with the track, while the ECMWF remains the
furthest north.  The NAM solution is in-between with the track.
Given the consistency of the ECMWF and the bias of the GFS to be too
strong/progressive with northern stream systems (thus shunting the
surface low too far south), have favored a general ECMWF/NAM blend
with this forecast.

Given these model preferences, it does appear eastern areas of the
LMK CWA will get glanced with the northwestern edge of this system.
The main question will be what wins out, the surface warm air
advection or the cool air likely to be in place over the fresh snow
from Sunday night`s system. Think it is too early to answer this
question with any certainty as of now, thus the low-medium
confidence.  The raw NAM soundings for LEX would support all snow
through the day on Monday (although the best QPF is shown just to
the south).  However, the other guidance is considerably warmer,
with even the raw ECMWF getting surface temps into the upper 30s.
Therefore, will go with all areas transitioning to rain through the
day on Monday, before going back to snow Monday evening into Monday
night as the system deepens and draws cold air into it.  Just how
fast this occurs and how much QPF there is left to work with will
determine any snowfall amounts with the developing def band on the
backside of this system. As of now, it appears any amounts would be
generally light and mainly east of I-65 (more along I-75).  But this
system has yet to be sampled by the upper-air network, so some
adjustments (possibly major) are likely.

Yet another clipper will bring a rain/snow mix to the region on
Tuesday, but will not get too specific with that right now given the
two current systems.  After that, it appears things will dry out and
temperatures will moderate through the week.  For those getting
cabin fever, it appears 60s (!!) are likely by the end of next week!

&&

.AVIATION (12Z TAF Update)...
Updated at 624 AM EST Sat Feb 13 2016

VFR conditions are expected at the terminals through the upcoming
TAF period.  Some scattered low clouds may affect KLEX early in the
period but are expected to mix out during the mid-morning hours.
Surface winds will remain out of the NW through the day with speeds
of 5-8kts.  Winds should shift to the north by late afternoon.
Clouds will be on the increase tonight as the next weather system
approaches from the northwest.  We expect to see snow move in from
the NW late in the TAF period and we`ll carry that in the SDF
planning period in this issuance.

&&

.LMK Watches/Warnings/Advisories...
KY...NONE.
IN...NONE.
&&

$$

Update.........13
Short Term.....KJD
Long Term......KJD
Aviation.......MJ


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