Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 052256
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
556 PM EST Thu Dec 5 2013
Issued at 553 PM EST Thu Dec 5 2013
The freezing line has begun to move more quickly this evening across
southern Indiana with the temperature at Huntingburg down to 32
already. There is a bit of break in the precipitation across
portions of our southern Indiana counties. However, this should fill
back in through the evening hours. Given the slightly faster timing
of the freezing temperatures moving into southern Indiana, the
beginning time for the Winter Storm Warning has been moved up by two
hours to 8 PM EST/7 PM CST. The warning has also been updated to
reflect the current thinking for the ice, snow, and sleet
.SHORT TERM (Now through Friday Night)...
Issued at 418 PM EST Thu Dec 5 2013
...IMPACTFUL WINTER EVENT TONIGHT THROUGH FRIDAY NIGHT...
The Arctic frontal boundary has made its way through Kentucky and
will settle to our southeast. This frontal boundary will stall out
while the very shallow and cold Arctic air behind it will funnel
into the region from the northwest. The freezing line at the surface
has not moved much during this afternoon, but will begin to collapse
toward the Ohio River later this evening and overnight. As this
occurs, the cold air will undercut warmer and moist air aloft,
changing precipitation type over to a wintry mix through much of the
overnight across southern Indiana to along the Ohio River. By Friday
morning, the transition line will hover around and south of the Ohio
River, with a wintry mix falling at a moderate rate. Moderate to
heavy snow will take over across southern Indiana through the
morning. By later Friday afternoon and evening, the transition line
will start progressing rapidly southeast across the rest of central
Kentucky changing most of Kentucky over to a mix and then snow by
evening. Finally, as deep moisture exits, precipitation type may
change back over to flurries or freezing drizzle as we begin to lose
ice crystals. Temperatures will crash overnight on Friday night,
ranging between 10 and 28 degrees from NW to SE by Saturday
Let`s try to break down the specifics, keeping in mind that the 12Z
soundings still indicate a very complex low-level thermal profile
and remain slightly different between each model. So, these amounts
and locations could slightly change.
...Southern Indiana into Portions of North Central Kentucky...
Cold air arrives here first, with periods of freezing rain and sleet
starting as early as 7 to 10 PM tonight and lasting through the
overnight. Expect light to moderate accumulations of sleet, and very
minimal impacts from any freezing rain through dawn. After dawn,
periods of sleet will be heavy at times before the transition to
snow. Up to an inch of sleet could fall ahead of this transition.
Then we will transition to light to moderate snow in the morning
hours, lasting a good portion of the day Friday. Strong
frontogenesis underneath the right entrance region of the strong
upper-level jet, coupled with at least conditional symmetric
instability supports periods of moderate banded snowfall. Snowfall
amounts should range between 4 and 8 inches, with significant sleet
and minimal ice accumulations prior to the snowfall. The Winter
Storm Warning will continue for this area.
...Along and Just South of the Ohio River Along and north of I-64
Corridor, Including Louisville, Frankfort, and Lexington...
Much like the transition line has stalled across south central MO
and southern IL today, the transition line will stall along and just
south of the Ohio River tomorrow. This will set the stage for a long
period of wintry mix, most of which appears to fall as sleet through
much of the morning and early afternoon. Forecast sounding analysis
shows that the warm layer (800-600 mb) will be above freezing for
much of this time. Basing the mix/snow transition off of 850 mb
temps or 100-850 mb thicknesses would lead to snow totals too high.
Have elected to go lower on snow totals (1-4 inches), but with a
wide range based on uncertainty with the transition. If we do see
mostly sleet in the morning, would not be surprised to see an inch
or two of sleet accumulation before the snow sets in
afternoon/evening. Will also mention up to a tenth of an inch of
freezing rain accumulation, especially on elevated surfaces.
...West Central Kentucky (Ohio, Hancock, Meade, Breckinridge)...
This area continues to have the highest potential to see some
significant (up to 0.3") ice accumulations due to a slightly
prolonged period of freezing rain from 10 AM to 4 PM EST 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 accumulation
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. Sleet accumulations up to a half an inch may also accompany
the transition from freezing rain to sleet. This area was upgraded
to a Winter Storm Warning earlier today and will remain.
...The Rest of the Winter Storm Watch Area...
The rain/wintry mix line is expected to slide slowly southeast to
central Kentucky from late morning to mid to late afternoon, where
sizable sleet and freezing rain accumulations are possible. Again,
sleet accumulations from a half an inch to 1 inch will be possible
with up to 2 tenths of an inch of ice accumulation also possible.
The wintry mix will then transition to to light snow Friday evening
where a quick 1 to 3 inches will be possible. Decided to leave this
area in the watch for now as there is still uncertainty in how this
area will transition. Will say that this region will likely go into
some sort of headline (Winter Weather Advisory or Winter Storm
Warning) with coming forecast updates. We do have bit of time before
impacts will start being realized so will hold off to analyze more
data and monitor upstream obs.
...South Central Kentucky to Lake Cumberland...
This area will see mostly rain through the event, although will
transition to a mix of freezing rain, sleet, and perhaps light snow
Friday evening. Precipitation intensity will quickly taper off as
this transition occurs, due to the loss of deep moisture. Could
actually see some light freezing drizzle as this system ends. This
area will be considered for headlines over the coming forecasts as
travel impacts could also be realized in this region.
Confidence is high that impacts will be seen if you are located in
the Winter Storm Warning region. Don`t be in denial if you are in
the Warning area or plan on traveling through it. This storm is
coming! Drive carefully. If you look upstream, impacts are already
occurring with accumulating freezing rain and sleet over SE MO and
When you get into specific impacts, confidence is only
moderate. The reason for this is that there is just no way to truly
pin point exactly when change overs will occur in any given area.
Therefore, precipitation types and amounts cannot have my highest
confidence. Am confident that we have highlighted which areas will
have the most likely chance to see the most of each precipitation
type. Adding to the complexity of impacts will be that road
temperatures will start out above freezing and rainfall ahead of the
wintry mix will delay accumulations a bit.
Am concerned about the potential for freezing drizzle on the back
side of this system as deep moisture departs and we lose ice
crystals. If freezing drizzle occurs, it will be happening in
coincidence with rapidly falling temperatures Friday night. This
could cause more slick spots over areas and may need to be
considered when tossing the idea of headlines around for the rest of
Wind chills will drop into single digits by Saturday morning.
.LONG TERM (Saturday through Thursday)...
Issued at 405 PM EST Thu Dec 5 2013
...SIGNIFICANT WINTRY WEATHER EXPECTED LATE SATURDAY NIGHT AND
SUNDAY ACROSS THE REGION...
...Saturday through Monday...
Current model guidance continues to show good continuity for
Saturday with dry but cold conditions for the area...as we will be
between two winter storms. Partly cloudy skies look like a good bet
for much of the day as high pressure attempts to nose into the
region. Real challenge will be high temperatures which will be
highly dependent on snow/ice accumulations on the ground. Since we
are expecting a decent accumulation in our northwestern areas, plan
on undercutting the guidance a bit. Highs in the lower 20 across
our southern Indiana counties look pretty good at this point. Highs
south of the Ohio River look to warm into the middle to upper 20s
with upper 20s to lower 30s across southern Kentucky. Temperatures
look to fall off Saturday night into the mid teens across the far
north with lower-mid 20s across central and southern Kentucky.
Next winter storm will then take aim at the region late Saturday
night as next mid-level wave heads northeastward within the
southwesterly flow aloft. The recent model trends have slowed the
onset of the precipitation across our region. Initially thought
that precipitation may break out before midnight in previous
forecasts...but the models now suggest precipitation arriving
perhaps around 6-8 hours later. It appears that precipitation will
develop and spread northward in earnest after 06Z Sunday. As is the
case in these time ranges, precipitation types are still difficult
to pin down. The crux of this forecast will depend on the depth (in
the vertical) of the low-level cold airmass across the region. This
leads to one of two scenarios for the region.
Scenario 1: We see a decent accumulation of ice and snow from winter
storm number 1 on Friday. If ice and snow are on the ground, then
it is expected that surface temperatures will be colder and the
depth of the cold airmass will be sufficiently deep to support
mostly snow at the precipitation onset. The snow could be
particularly heavy early Sunday and a significant amount of snow
could occur across portions of central Kentucky. The warmer air
would eventually overwhelm the cold surface layer and result in a
change over to freezing rain during the afternoon and then over to
plain rain by Sunday evening.
Scenario 2: Snow/ice accumulations from winter storm 1 on Friday are
generally more limited. In this case, surface temperatures still
cool overnight Saturday, but the low-level cold air is not as deep.
In this case, the warmer air aloft would come into the region and
lead to more freezing rain developing Sunday morning. With the less
thick cold layer near the surface, the warmer air coming in would
scour out more quickly and we would see the freezing rain change
over to plain rain more quickly during the day...but not before a
significant amount of freezing rain falls over portions of central
At this time, the multi-model consensus and model trends are suggest
that scenario 1 are more likely to happen as of this writing. So we
plan on trending the forecast closer to that scenario. With that
said, expect precipitation to start off across the region as snow
and then change to freezing rain from south to north throughout the
day. Hard to pin down amounts, but significant amounts of snow
could fall early Sunday morning followed by a bout of freezing rain.
As mentioned above, we are concerned that a band of heavy
precipitation may develop...just NW of the inverted surface trough
axis. Right now, this band looks to develop from near Bowling Green
northeastward through Lexington. It should be noted that future
forecasts may result in a shift in this band...so it is less
important to focus on that aspect of the forecast...but rather the
impacts that this second storm will bring with the mixed bag of
As the wave rides up through the region we should see temperatures
continue to warm and model soundings suggest that a change over to
plain rain will occur by late Sunday and continue into the Sunday
night. As the wave departs, colder air rushes back in and will lead
to any leftover precipitation changing back over to snow before
Highs Sunday look to warm to around freezing in areas north of the
Ohio River late in the day...with readings in the mid-upper 30s
across much of KY. Lows Sunday night will cool back into the lower
30s. Highs on Monday are really tough to pin down because they will
be so dependent on snow/ice cover. Right now kept things close the
AllBlend raw guidance with highs in the lower 30s...but if
substantial snowfall/ice occurs, readings would be colder than that.
Monday Night through Thursday...
Weather during this period calms down as flow aloft becomes less
amplified with no significant weather systems to affect our area.
Will keep a slight chance of light snow over far eastern and
southeastern counties Monday evening as broad shortwave aloft moves
through. Otherwise, looks dry through the period at this time.
Temperatures will be quite cold with highs in the 20s and 30s
Tuesday though Thursday (coldest north), with lows generally in the
single numbers north and teens south. Whatever snow cover exists
north will affect these numbers.
Issued at 315 PM EDT Thu Dec 5 2013
A prolonged period of precipitation will continue into the weekend.
The first system has already dumped up to an inch and another 1 to 2
inches are expected before the precipitation ends Friday night. The
northern portions will see much of this precipitation change over to
freezing or frozen forms. Thus, rises will occur on rivers and
streams but no flooding on major rivers is expected. However,
localized flooding could still occur tonight into Friday.
The second system Saturday night through Sunday could drop an
additional 1 to 2 inches of precipitation is expected over southeast
Kentucky. Again, the precipitation will be a mixture of frozen and
freezing types and will runoff slower than pure rain. This next shot
could trigger some minor flooding on parts of the Green, Licking,
and Salt basins. If so, flooding will extend into next week.
.AVIATION (18Z TAF Update)...
Updated at 1230 PM EST Thu Dec 5 2013
Storm system could have major impacts on aviation interests during
and after the current forecast period, especially at SDF. Currently,
main area of precip has moved east of TAF sites, but scattered light
showers are expected this afternoon especially at SDF and LEX.
Visibilities should be or rise into VFR category for a short time in
rain free areas this afternoon, but MVFR ceilings should persist
much of the time, rising out of the IFR category. Surface winds
this afternoon will remain northwest to north around 10 mph.
The next surge of moisture begins to move in late today and this
evening over western Kentucky and southern Indiana, which will first
affect SDF tonight. This will be rain at all TAF sites, but with a
transition first to rain/freezing rain mix at SDF around 12z as
temperatures fall to around 31 or 32. The rain/freezing rain will
continue in the morning at SDF before mixing with ice pellets and
then snow by the end of the SDF valid forecast period(18z Friday).
Temperatures from 30 to 33 in the morning at SDF may not be cold
enough to create significant ground icing, especially if precip
rates are high, but could well cause problems on elevated surfaces,
especially aircraft wings which may be a little colder. Expect
visibilities and/or ceilings to be in the IFR category at that time.
At LEX, rain will be predominate precip type through much of
forecast period (18z Friday) as temperatures should hover just above
32. However, even so, elevated surfaces and aircraft wings will need
to be monitored after 12z Friday. Freezing rain may become more
prominent at the end of the forecast period (possibly slightly
earlier if surface temperatures are a little colder than expected),
with deteriorating conditions after 18z.
Finally at BWG, dry conditions for awhile this afternoon will be
replaced by scattered then numerous showers tonight and Friday with
ceilings again becoming IFR. However, it appears precip type should
remain liquid rain thru 18z Friday, again with slight icing possible
at end of period on a few elevated surfaces. Conditions may
deteriorate after the current valid time period.
KY...WINTER STORM WARNING until 1 AM EST /Midnight CST/ Saturday FOR
WINTER STORM WATCH from 1 AM EST /Midnight CST/ Friday through
late Friday night FOR KYZ027>029-034>043-045>049-053>057-
IN...WINTER STORM WARNING until 1 AM EST /Midnight CST/ Saturday FOR