000
FXUS63 KLMK 042318
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
618 PM EST Wed Dec 4 2013

...Updated Aviation Discussion...

.SHORT TERM (Now - Thursday)...
Issued at 335 PM EST Wed Dec 4 2013

One quick note: After coordination with neighboring offices, decided
to add in Logan county for the winter storm watch.

As for the short term, low pressure area over northeast Iowa is
dragging a front through the central Missouri/Arkansas at this time.
This front will shift slowly eastward during this period, only
getting to the Ohio River around daybreak Thursday and into our
southeast zones by the end of the day. Southwest flow aloft through
the period will bring a steady increase in low-level moisture, with
precipitable water up to 2-3 standard deviations above normal, near
record levels for December.

That said, rain chances will go up to categorical Thursday, with
rainfall totals by the end of this period ranging from a quarter to
a half inch. A look at soundings indicates the precip type as all
rain, as we get warm air aloft into the region and surface temps
remain far enough above freezing. The only thing to mention is that
forecast soundings still have a sliver elevated cape that could
allow for an isolated rumble of thunder late tonight all locations,
then shifting to only ahead of the front during the day Thursday.

Low and high temps this period will depend on how quickly that front
moves through. Lows tomorrow morning likely will have a
twenty-degree range across the forecast area, with a similar spread
for highs.

.LONG TERM (Thursday Night - Wednesday)...
Issued at 346 PM EST Wed Dec 4 2013

Thursday Night - Saturday Morning...

...IMPACTFUL WINTER EVENT LIKELY EARLY FRIDAY MORNING THROUGH FRIDAY
NIGHT...

Arctic cold front will be south of the forecast area at the start of
the forecast period Thursday evening. Initial precipitation type
will be all rain across the CWA as warm boundary layer temperatures
will not support anything frozen. The warm layer aloft will begin
collapsing from northwest to southeast through the overnight, while
sub-freezing surface temps also begin to nudge in from the
northwest. As this occurs, a wintry mix characterized by initial
freezing rain and sleet, eventually transitioning to light or
moderate snow dependent on location. As we move through the day on
Friday, expect this freezing/transition line to slide gradually
southeast, changing areas south and east of the Ohio River to a mix
and then snow. Precipitation will then end mainly as light snow on
Friday evening from west to east. Will break down specific areas
where confidence is higher in the most dominant precipitation types
and talk about general amounts. Please keep in mind that these
initial numbers are based on a highly complex low level thermal
profile, and numbers locations will likely change as we near the
event.

...Southern Indiana and Hancock County Kentucky...

With the cold air arriving in this region quickest, expect light
accumulations of sleet and/or freezing rain before dawn to change to
light to moderate snow around dawn and after. Snow should last for a
good portion of the day on Friday, with mid level analysis
supporting strong frontogenesis underneath the right entrance region
of the strong upper level jet. Cross section analysis does indicate
some negative EPV values indicative of conditional symmetric
instability (slantwise instability) in this region. Banded snowfall
is expected with brief periods of heavy snowfall possible. Will go
with a range of 3 to 7 inches of snow for this area, with varying
amounts of light sleet and ice accumulations to go along with it. It
should be noted that if the transition takes longer, the lower
amounts on snow totals are more likely, with greater sleet accums.
The opposite is true if the change over is faster with the higher
snow totals (maybe underdone) and lesser sleet accums. As far as the
Winter Storm Watch goes, this area appears poised to be upgraded to
a Winter Storm Warning when confidence becomes a bit higher.

...West Central Kentucky...

The latest data suggests that this area has the highest potential to
see some significant (up to 0.25") ice accumulations due to a
slightly prolonged period of freezing rain from 10 AM to 4 PM EST on
Friday. Looking at the low level thermal profiles, this area will
see the most pronounced overlap between surface sub-freezing
temperatures and the warm nose aloft holding on. Depending on how
long this scenario holds before the transition to sleet and snow,
ice accumulations up to a quarter inch on roadways may be possible.
Trees and powerlines would likely see slightly lower accum ratios
due to the lighter winds and moderate precipitation rates. Once this
area transitions to snow, they could pick up a quick 1 to 3 inches
before deep moisture exits and ice crystals are lost Friday evening.
As far as the Winter Storm Watch goes in this area, an upgrade to a
Winter Storm Storm Warning may be needed when confidence in thermal
profiles goes up.

...South of the Ohio River and East of I-65...

The rest of the watch area is a going to be mix of rain, freezing
rain, sleet and snow. Some lighter accums are expected, although
still significant. The transition south of the Ohio River should
take place around 7 AM, gradually sliding southeast through the day.
Have gone with 1 to 4 inches of snowfall with a tenth of an inch or
less of ice. The higher totals will be closer to the Ohio River
southern part of the watch. Overall, this area is a bit more low
confidence as to whether an eventual Winter Storm Warning or Winter
Weather Advisory may be needed.

...South Central Kentucky...

This area will see mostly rain through the event, although will
transition late to a mix of freezing rain, sleet, and snow. Feel
that precipitation intensity will quickly taper as this transition
occurs, especially with the loss of ice crystals as deep moisture
exits.

...Other Notes and Thoughts through Saturday Morning...

Overall, went with the more conservative model data with the p-type
change over. There are more aggressive model solutions with the cold
air and if these pan out, totals in certain areas may go up with
respect to different p-types.

The temperature trend will basically be non-diurnal after Midnight
on Thursday night in this cold advective pattern. Could see a slight
rise in temperatures across the southeast during the day on Friday,
but overall temperatures will fall through the period. Look for
highs on Friday ranging from the upper 20s northwest to the lower
40s southeast. Temperatures will really crash on Saturday night,
especially if skies begin to clear across the northwest and radiate
out with a fresh snowpack. Some of the higher res models are hinting
at this bringing some sub-zero surface temps into our northwest by
dawn on Saturday. Will not go this cold, although cannot completely
rule it out. Look for lows of 10-20 over our northwestern counties.
With 20-30 from central Kentucky down to the Lake Cumberland region.

With the rapidly falling temperatures Sat. Night. Could see any
remaining wet spots or slush on roadways quickly freezing. This
could create travel problems through Saturday even though it will be
dry.

P-type could actually transition back to freezing drizzle on the
back side of the system Friday night as deep moisture exits and we
lose ice crystals. With temperatures rapidly falling at the surface,
more slick spots could develop.

Finally, don`t want to lose sight of the overall amount of rain we
are expecting, especially across southeastern portions of the CWA.
Have been considering a Flood Watch for the area, but the amount of
time that the precipitation (2-3 inches) is expected to fall should
be long enough to minimize any flooding impacts. Will continue to
analyze data and watch for a possible upgrade.

Saturday through Wednesday...

...SIGNIFICANT THREAT OF ICING LIKELY LATE SATURDAY NIGHT AND SUNDAY
ACROSS THE OHIO VALLEY...

Active weather pattern is expected to continue across the region as
cold low-level air continues to sink across the region while a broad
southwesterly flow continues aloft.  Saturday should be a rather
quiet day weatherwise as high pressure attempts to build into the
region from the northwest.  This will bring an influx of drier and
much colder air into the region.  Generally stuck close to the raw
multi-model consensus for temperatures as highs will only warm into
the lower-middle 20s across southern Indiana with middle-upper 20s
across northern Kentucky.  The warmest region will probably be down
around the Lake Cumberland region with highs topping out in the
lower-middle 30s.  It is possible that these temperatures may be a
little on the high side of the spectrum if significant snow/ice
cover occurs with the first weather system in the medium range
period (Thu Night-Friday).  Temperatures could easily be 5-10
degrees cooler if we have a decent snow/ice pack.

Next weather system will then start to affect the region late
Saturday night as a potent upper level wave ejects from the western
CONUS trough and rides northeastward around the southeastern ridge
axis.  As this wave approaches, we will see a combination of strong
isentropic lift along with frontogenetical lift bringing a round of
moderate to heavy precipitation across the region.  Low-level cold
air will already be firmly in place and a strong fetch of warm
mid-level air will overspread the region.  This will lead to a
thermal profile conducive for a bout of freezing rain across the
forecast area.

There continues to be a bit of spread within the medium range model
guidance with the overall thermal structure.  Comparing the 12Z GFS
and Euro runs, the GFS is a little more aggressive at scouring out
the low-level cold air, while the Euro runs continue to advertise a
colder and deeper cold layer near the surface.  The 12Z Canadian is
a bit colder than the Euro, but the GEM has shown a cold bias of
late...so we have to take that into consideration here.  Overall,
the models show precipitation initially starting off as a mix of
snow/sleet late Saturday night quickly changing over to freezing
rain as the warm layer aloft moves in.  The real challenge here is
how long will we see the freezing rain and how much.  For this
forecast, we have leaned closer to the Euro here with regards to the
thermal structure, but used a blend of the GFS and Euro for
precipitation amounts.  In general, it appears that we are looking
at the potential for 0.25 to 0.75 inches of freezing rain across the
region.  There could be areas that potentially see more than that,
but at this time range, that is simply impossible to pin down where
the heaviest precipitation will fall.  Nonetheless, this is a
potentially dangerous situation with the possibility of significant
icing accumulations.

As the warmer air aloft continues to invade on Sunday, we expect a
gradual change over to plain rain from south to north.  Depending on
the depth of the low-level air, it is possible that areas along and
north of the I-64 corridor may not reach freezing...thus increasing
the threat of significant icing.  The precipitation looks to
diminish late Sunday night as the wave moves off the east and colder
air works in from behind.  The colder air looks to change the
precipitation back over to snow before ending...though the column
looks to dry out within the -10 to -20 degree ice crystal layer
suggesting that light freezing drizzle could occur into the day on
Monday.

Drier and colder conditions look likely for Monday night through the
remainder of the period.  The main challenge for the Tuesday through
Wednesday period will be temperatures.  Some of the models suggest
clearing skies and good radiational cooling that may lead to single
digit and possibly some sub-zero readings.  At this time, the
probability of sub-zero readings looks fairly low, but if
significant icing takes place on Sunday, then the probabilities of
those kind of temperatures will increase significantly.  For now
have trended the forecast close the AllBlend guidance here which
suggests highs in the teens to the 20s and overnight lows in the
single digits to the teens.

&&

.HYDROLOGY...

Issued at 315 PM EDT Wed Dec 4 2013

A prolonged period of precipitation is forecast this weekend. The
first batch is expected Thursday morning through Friday night. A
stalled front south of Kentucky will allow several waves of
precipitation over the weekend. This first system should dump around
2 to 3 inches of rain across much of central Kentucky. This rain
will cause rises on rivers and streams but since this area is fairly
dry, there should be no flooding of major rivers. However, localized
flooding could still occur.

The second system Saturday night through Sunday could drop an
additional 1 to 2 inches of rain and freezing rain mix over the
saturated ground. This next shot could trigger widespread minor
flooding on parts of the Kentucky, Green, Licking, and Salt basins
with a few locations approaching moderate levels. If so, flooding
will extend into next week.
&&

.AVIATION (00Z TAF Update)...
Updated at 615 PM EST Wed Dec 4 2013

Low level moisture will again be on the increase tonight and a cold
front will approach and move through the region through the TAF
period. Ahead of the front rain will move in late tonight and
showers will continue off and on through tomorrow. The latest
soundings show a bit of elevated instability towards daybreak
tomorrow which could lead to a few thunderstorms across the area.
However, these look to be isolated enough at this point that they
are not worth mentioning in the TAFs.

Ceilings and visibilities will once again be an issue tonight.
Ceilings look to lower to MVFR at all sites. Confidence in IFR
conditions at SDF and LEX is lower tonight, especially given the
latest guidance has been coming in with higher ceilings. Therefore,
will keep SDF and LEX at low-end MVFR with the rain tomorrow morning
and only lower cigs to IFR at BWG.

Winds will be out of the south to SSW overnight. They will shift to
westerly and then northwesterly through the day tomorrow as the
front moves through the area.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...WINTER STORM WATCH from late Thursday night through Saturday
     morning FOR KYZ023>043-045>049-053>057-061-070.

IN...WINTER STORM WATCH from late Thursday night through Saturday
     morning FOR INZ076>079-083-084-089>092.

&&

$$

Short Term........RJS
Long Term.........BJS/MJ
Hydrology.........CMC
Aviation..........EER