Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 051532
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1032 AM EST Thu Dec 5 2013
Issued at 1015 AM EST Thu Dec 5 2013
Cold front continues to sag slowly southeast across central Kentucky
at this time. At 15z, front was knocking on LEX`s doorstep, just to
the west. It extends southwest to just past Glasgow. Temperatures
will not fall dramatically right with frontal passage, but will fall
steadily and surely through the afternoon. For example, at the NWS
office in Louisville, temperatures have fallen steadily from 63 at
12z to 48 at this time (1520z). Readings are around 40 in our
northwest southern Indiana counties now. So, the well advertised
return to winter-like temperatures is beginning.
Along the cold front, numerous showers and embedded thunderstorms
are occurring over our eastern and southern forecast area, with
heavy rain at times. Radar has shown periodic small bowing
structures, but highest winds are remaining elevated with these
structures. The passage of the front should shut off and push
convection to the southeast.
Behind the front, radar currently shows an area of showers near the
Ohio River moving east-northeast. There may be a relative lull in
precipitation this afternoon for parts of southern Indiana and
central Kentucky before additional showers develop and propagate
east-northeast over the area later this afternoon. Overall though,
showers remain likely this afternoon, but not as heavy as the
convection with the cold front.
No other forecast changes for tonight or Friday have been made at
this time, although new data for this period is currently being
evaluated by other forecasters.
.SHORT TERM (Now through Friday)...
Issued at 350 AM EST Thu Dec 5 2013
...IMPACTFUL WINTER EVENT LIKELY TONIGHT THROUGH FRIDAY NIGHT...
Low pressure north of Wisconsin is dragging a front through the
central Indiana/western Kentucky at this time. This front will
progress slowly eastward, getting to our portions of the Ohio River
around daybreak this morning and into our southeast zones by this
evening. South and southwest flow through the lower atmosphere
continues to pump in anomalous moisture from the Gulf and that will
continue through the day. Precipitable water values will be 2-3
standard deviations above normal for this time of year.
Precipitation has developed along and ahead of the front. Expect
that to continue and become more widespread through the day. Given
the available anomalous moisture, rain could be moderate to heavy at
times, especially across southern and east-central Kentucky. There
remains a sliver of CAPE within the soundings, so will keep isolated
thunder in the forecast. The precip along and ahead of the front
will move east, with a lull in the steady precip anticipated a
little later this afternoon before our next wave of precip arrives.
Temperatures have remained mild this morning, but as the front moves
through temperatures will decrease. High temperatures today will
likely occur this morning, then fall through the afternoon.
Currently have highs ranging from the middle 40s in the northwest to
middle 60s in the southeast, but by this evening we should have
middle 30s in the northwest to low 50s in the southeast. The colder
air seems to be arriving a little earlier than previously forecast,
which plays into the snow, sleet, and ice timing/amounts.
By this evening, our next weather system will begin spreading
moisture across the Middle Mississippi and Lower Ohio Valleys. The
cold front will be south of the forecast area and with temperature
profiles still above freezing, initial precip type will be all rain.
However, temperatures will continue to cool from northwest to
southeast through the night. There is still a warm layer aloft, but
that too will begin to diminish. This will set the stage for a
wintry mix of freezing rain and sleet across southern Indiana
starting around 10 PM, and eventually transitioning to light to
moderate snow closer to daybreak Friday. Locations generally south
of the Ohio River will experience a cold rain for most of the night.
The freezing/transition zone will slide to locations along the Ohio
River around or shortly after midnight. Then the mix zone will
continue to slide southeast changing areas south and east of the
Ohio River to a mix and finally snow. Precipitation will end mainly
as light snow Friday evening from west to east. However, as
mentioned in the previous discussion, ice crystals aloft will
diminish with the exit of deep moisture but we will be left with a
lower saturated layer. So, the system could end with light ice
pellets/sleet across the north to light freezing drizzle across the
Let`s try to break down the specifics, keeping in mind that the 00Z
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 through the next 24 hours or so.
...Southern Indiana into Portions of North Central Kentucky...
Cold air arrives here first, with light accumulations of sleet
and/or freezing rain starting around 10 PM. Then we will transition
to light to moderate snow around dawn and continue through 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 the some sleet and ice accumulations prior to the
snowfall. With this, have upgraded the Winter Storm Watch to a
Winter Storm Warning.
...West Central Kentucky to the Bowling Green Region...
This area continues to have 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 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. A little farther south into the Bowling Green area, icing
potential seems to be increasing with each model run yet there
remains some uncertainty. Have increased the freezing rain amounts
in that area. While there will be a transition to sleet and perhaps
very light snow, the big threat for this area will the accumulation
of freezing rain, which could approach 0.25" by the time it is all
over. Have extended the Winter Weather Watch to cover this area, and
since the accumulations will occur in forecast period 3, will let
another set of model runs fine tune the details down there.
...South of the Ohio River and East of I-65...
This area is a going to be a mix of rain, freezing rain, sleet and
snow. Will see some lighter accumulations of ice than points further
west-southwest, although still significant. The transition south of
the Ohio River should take place around shortly before daybreak,
gradually sliding southeast through the day. Have 1 to 4 inches of
snowfall with a tenth of an inch or less of ice for this area. This
continues to be a lower confidence forecast for this area. Currently
have Winter Weather Advisory criteria in the forecast, but some
areas could borderline Warning criteria. Given the accumulations
will mainly occur in forecast period 3, will continue the Winter
Storm Watch and let another set of model runs help define the
...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
late in the day Friday. 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.
.LONG TERM (Friday Night through Wednesday)...
Issued at 310 AM EST Thu Dec 5 2013
...SIGNIFICANT THREAT OF ICING LATE SATURDAY NIGHT AND SUNDAY ACROSS
THE OHIO VALLEY...
The first of a couple of winter weather systems will be ending at
the start of this period, with snow being the predominate weather
type and a brief wintry mix expected across our southeast before
they too change over to light snow or dry out, as the moisture
associated with this system shifts east. Northerly winds through the
night will drive in much colder air. Those areas receiving a snow
pack, over southern Indiana, likely will get down to the lower
teens, whereas points closer to the front will stay in the upper
We may see some sun Saturday, but the persistent northerly flow, and
again any snow pack will keep temperatures well below normal (at
least 20 degrees below). Then Saturday night another disturbance
traveling within our southwest flow aloft will bring a new round of
precipitation. Given the cold temperatures to start, this precip
likely will start off as snow, but then as warm air builds in
aloft, it quickly will become freezing rain, likely after midnight.
The next phase change will come as a low pressure system develops at
the surface and allows warmer air at the surface. Exact placement of
this low will make a big difference on where/when that transition
takes place. For now have kept freezing rain dominant over the north
during the day Sunday, but transition to rain across the south by
the afternoon. Still a bit early to be exact with ice accumulations,
but as a best guess with current QPF fields have values totaling
around a half inch across south central KY, about a quarter of an
inch along the I-64 corridor and a little less to the north, where
overall QPF looks lighter. The preceding snow combined with this ice
eventually should lead to another headline product, but will allow
the forecast to fine tune some more before issuing.
Another issue with all of this mess is the amount of precipitation
falling onto already soaked grounds from the first wave coming
through today and Friday. Current rainfall totals forecast for that
first wave across central KY ranges from two to three inches. For
Saturday through Sunday night, we have an addition 1-2 inches across
south central KY. For now, will allow one more forecast cycle (at
least) to pass before issuing or not. The rain being spread out over
some time should alleviate the flood threat.
This second system tries to get out of our area Monday afternoon,
but it may back build briefly into our eastern zones Monday night as
yet another vortmax pushes over us. Will throw in a slight chance
for precip in those areas to account for this possibility.
Thanks to all of these winter weather systems, temperatures look to
stay well below normal through this period.
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 (12Z TAF Update)...
Updated at 610 AM EST Thu Dec 5 2013
Showers have increased along a cold front that will drift east
across the region this morning, bringing a shift in winds from
southwesterly to westerly as well as lower ceilings. Flow aloft will
remain from the southwest behind the front and another slug of
moisture will move in, keeping rain chances up the rest of the
period. Given this moisture, have gone with IFR conditions by sunset
and continuing Thursday night. Some colder air moving in will bring
a chance for a rain/sleet mix at KSDF by the last part of this
KY...WINTER STORM WARNING from 10 PM this evening to 1 AM EST
Saturday FOR KYZ032.
WINTER STORM WATCH from late tonight through late Friday night
IN...WINTER STORM WARNING from 10 PM EST /9 PM CST/ this evening to 1
AM EST /Midnight CST/ Saturday FOR INZ076>079-083-084-