Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 121117
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
617 AM EST FRI FEB 12 2016

...Updated Aviation Discussion...

.SHORT TERM (Now through Saturday)...
Issued at 325 AM EST Fri Feb 12 2016

Early morning IR satellite imagery and observations reveal partly to
mostly cloudy conditions across the region.  Temperatures ranged
from the low-mid 20s across the Bluegrass region to the upper 20s
out near the heritage corridor (I-65 corridor) and out across our
southwest sections.

Regional radar imagery shows an area of snow showers out across
eastern MO and Central/southern IL.  This is associated with a mid-
level vorticity perturbation along with a jet streak pushing in from
the northwest.  Latest high-res short term models continue to
suggest that this activity will move east-southeast this morning
with an axis of snow pushing through western and southern KY.  We
still think a period of 2-4 hours of moderate snow will occur with
this system as we have a good period of lift through the DGZ.  The
current Winter Weather Advisory area still looks very good and no
changes will be made to the ongoing headline.  Within this advisory
area, 3/4-1 inch of snow looks likely though some locations that get
under a moderate-heavy period of precip could see isolated higher
amounts.  This snow occurring around or shortly after sunrise will
negatively impact the morning commute across southern KY.  Further
north, antecedent dry air in place will likely result in snow
flurries and lighter intensity snow showers.  A dusting to a half of
an inch will be possible but the major impacts will be south of the
WK and BG Parkways.  We plan on continuing to highlight the north
with a Special Weather Statement.

We`re likely to see a brief lull in precipitation by late morning as
the upper air perturbation and jet streak moves east-southeast and
away from the region.  Colder air will push into the region aloft as
a surface front pushes through the region.  The colder air aloft
will steepen lapse rates fairly well so convective snow showers are
likely to develop across the region.  The model trends though have
suggested lesser moisture available this afternoon, so areal
coverage may not be as widespread as previous forecasts indicated.
In any case, some light accumulations of snow will be possible
across the region this afternoon...with the far NE and Bluegrass
region being the most favored.

Temperatures this afternoon will likely spike ahead of the front
into the lower 30s but then temperatures will fall markedly late
this afternoon and into the evening hours with brisk NW winds.
Evening temps will drop quickly into the teens pushing wind chills
down near zero.  Overnight lows will likely drop into the upper
single digits across the north with lower teens across the central
and southern sections.  Dry and cold conditions are expected on
Saturday with high temperatures warming into the 15-20 degree range
across southern IN and northern KY. Slightly warmer readings will be
found across southern KY with most locations warming into the 19-24
degree range.

.LONG TERM (Saturday Night through Thursday)...
Issued at 345 AM EST Fri Feb 12 2016

...POTENTIAL FOR WINTER STORM TO AFFECT THE REGION SUNDAY THROUGH
TUESDAY...

=======================================
Long Term Synopsis
=======================================

Upper air pattern across the CONUS is expected to undergo
amplification early next week as the northern branch of the jet
stream strengthens and moves into the central US.  Initial long wave
trough over the eastern US will move out over the weekend with a
small window of ridging holding sway across the Ohio Valley.  As the
upper jet digs into the central CONUS a deepening trough axis will
result in surface cyclogenesis across the lower MS Valley by late
Monday.  The surface cyclone will trek across the TN Valley and then
up the eastern side of the Appalachians on Tuesday and then into New
England by midweek.  A long wave trough will remain in place across
the eastern half of the US by midweek as heights build across the
western CONUS.  The western CONUS ridge is expected to be short
lived as strong Pacific Jet will send a series of disturbances into
the Pacific northwest resulting a break down of the ridge.  A
largely zonal flow pattern is expected to develop with a flood of
milder Pacific air intruding into much of the US by the end of the
week.

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

In terms of model trends, there is increased forecast convergence
that indicates that we`ll see two weather systems affect the region
from late Saturday and through Tuesday.  The first of these will be
a fast moving perturbation moving through on Sunday followed by a
stronger/developing system for Monday into Tuesday.

Overall model trends with the first system remain generally the same
and overall forecast confidence is higher here than in previous
forecasts.  A blend of the Euro/GFS/GEM seems reasonable here
(Saturday night through Sunday night time frame).  This system will
bring a swath of light snow to the region Sunday and Sunday night.

With regards to the secondary system, the models continue to
struggle with the overall complexity of the upper level pattern.
There remains potential for the northern and southern branches of
the jet to interact and possibly phase.  In addition, the Ohio
Valley will be on the southern periphery of a pre-existing polar
airmass.  Return flow ahead of the developing system poises a
problem in terms of overrunning and the potential for mixed
precipitation. The system has yet to enter the North American upper
air network, so we expect run to run differences to continue for the
next 24 hours or so.

In terms of model trends, all of the global models have trended
slightly faster in terms of the system moving through the region.
The GFS and GEM are a bit more progressive than the Euro and have
higher QPF amounts across southern KY/northern TN through the
period.  The Euro has trended a bit more south with its QPF axis but
remains generally north of the GFS/GEM solutions.  All of the models
do show warmer air pushing northward into the region on Monday which
results in mixed precipitation issues.  Ensemble spread remains
large at this time with varying degrees of phasing and non-phasing
with the systems.  Corresponding surface tracks from the ensembles
vary as well resulting in a wide variety of p-type solutions from
the models.

For this forecast, we`re going to attempt to maintain forecast
continuity here.  We plan on utilizing a blend of the new
operational models with more emphasis from the Euro and its parallel
runs combined with the ongoing forecast.  We anticipate that the
model spreads will decrease a bit within the next 1-2 runs as the
storm system becomes properly sampled in the upper air network.
Therefore, further refinements (some of which may be significant)
are likely later today and tonight.

Forecast confidence remains low in the Tuesday through Thursday time
frame as timing issues remain with the potential for another clipper
system to impact the region Tue/Wed.  In addition, the near surface
temperature environment will be close to critical levels resulting
in the possibly of mixed precipitation continuing.

Forecast confidence does increase a bit toward the later portion of
the period given the relatively good model agreement in terms of a
moderating trend in temperatures.

=======================================
Sensible and Societal Weather Impacts
=======================================

Early in the forecast period we`ll see quiet weather conditions.
Conditions are expected to deteriorate early Sunday morning as the
first weather disturbance slides into the region from the northwest.
This will result in light snow moving through the region during the
day on Sunday with the potential for some light accumulations as
most locations will see temperatures remain at or below freezing.
Overall multi-model ensemble solutions suggest afternoon
temperatures warming into the upper 20s to around 30 in the north
with lower 30s in the south.

Secondary system will begin to affect the region late Sunday night
and into Monday morning.  Model proximity soundings indicate that
precipitation should remain in the form of snow Sunday night and into
the early morning hours on Monday.  However, the northward transport
of warmer air aloft ahead of the developing surface system poses a
risk of mixed precipitation across portions of the region.

The crux of the forecast will be on the depth of the cold air in the
near surface environment and the evolution and track of the surface
cyclone.  Given the current data, the most likely scenario still
looks to be a snow to wintry mix event during the day on Monday. Our
northern areas will likely stay snow for a bit longer as the depth
of the cold air looks to be larger across the north with a more
shallow layer down across the south.  While it is far too early to
speculate on snowfall amounts, the amount of moisture being
transported northward from the Gulf could result in significant
amounts of snowfall for portions of our region.

As the system departs to the east on Monday night, expect a change
over back to snow as colder air comes back in. Winds would be quite
gusty by this time.

For the Tuesday through Thursday time frame, we`ll see another
clipper system cruise through Tue/Wed.  Depending on the timing,
this system could bring a mix of rain/snow to the region.  The
models do agree that a moderating trend in temperatures will occur
by late week.

&&

.AVIATION (12Z TAF Update)...
Updated at 606 AM EST Fri Feb 12 2016

Snow will impact BWG this morning while SDF/LEX lie on the northern
edge of this system and likely will just see flurries or very light
snow showers. At BWG, plan on brief periods of IFR conditions
through 15z or so, then the snow will move east of the airport. In
its wake, MVFR to low-end VFR ceilings will remain. At SDF/LEX,
ceilings will lower to around 3-4 kft.

This afternoon a front will pass through, bringing gusty west winds
to near 20 kts at all sites. Scattered snow showers are possible
between 21 and 00z at SDF/LEX, some of which could bring short
periods of restrictions. Confidence in coverage isn`t high enough at
this time to prevail in the TAF.

Lingering mid to low level moisture will keep clouds around this
evening, but as high pressure builds in tonight, these should
scatter out. Winds will remain out of the west/northwest at 10 kts
or so tonight.

&&

.LMK Watches/Warnings/Advisories...
KY...Winter Weather Advisory until 11 AM EST /10 AM CST/ this morning
     for KYZ026-027-061>067-070>078-081-082.

IN...NONE.
&&

$$

Short Term........MJ
Long Term.........MJ
Aviation..........ZT



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