Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 02/16/21

...Updated Forecast Discussion Following the Mesoscale Updates...



 .Mesoscale Update...
Issued 739 PM EST Mon Feb 15 2021

AMDAR sounding out of Louisville at 2320Z showed a maximum warm
layer temperature of 0.5C...and then half an hour later the max warm
layer was -1C. Radar continues to show the precipitation changing to
snow in central Kentucky along and west of I-65, with snow
continuing throughout southern Indiana. Webcams and reports indicate
big, dendritic snowflakes. This isn`t over yet. Snow accumulations
are still expected, and will create additional travel hazards with
layers of ice, snow, sleet, ice, and snow on the ground.

Will freshen wording in WSW (to remove mention of "afternoon").

Issued 646 PM EST Mon Feb 15 2021

Still quite a bit of light freezing drizzle/freezing rain over
central Kentucky but a large area of snow extends from Ohio/Indiana
southwestward to west Tennessee. This area of snow has already
started to edge into the westernmost sections of central Kentucky
with big snowflakes being reported west of Bowling Green. As low
pressure near Cumberland Gap continues to pull to the northeast this
area of snow will gradually move into the region from the west while
simultaneously lifting northeastward. Even though folks in central
Kentucky have gotten more sleet and freezing precipitation thus far,
snow is still expected between now and midnight. The most
significant accumulations, another 2-4", still appear possible
northwest of a BWG-FFT line, with the highest amounts in southern
Indiana. Cross-sections show frontogenetical forcing and slightly
negative EPV over the next few hours as the cold air deepens, with
banding a possibility. Any bands that set up would drop higher
amounts of snow.

Issued 525 PM EST Mon Feb 15 2021

As lighter reflectivities passed over the NWS office our ptype
changed to -FZDZ. This also happened upstream at BWG in the past
half hour. However, there is some banding of higher reflectivities
developing from southwest of Bowling Green southward into northwest
Alabama ahead of a subtle 5H shortwave trough. There is likely more
frozen, rather than freezing, precipitation under these bands. They
are low-topped enough that the clouds tops on IR satellite aren`t
much colder than surrounding tops, but Clarksville, TN did change
over to snow as one area of brighter reflectivity moved overhead.

Mesoscale model projections indicate that these bands will continue
and may spread out a bit, along with the def band from Ohio and
Indiana into the lower Ohio Valley. It will be a race between ptype
changing to mainly snow and the system lifting out, taking its
moisture with it. Still can`t rule out a quick additional 2-4" west
of a BWG-FFT line through midnight as the tail of the snow to our
north/west swings through. Main event will be over by midnight.

Issued at 436 PM EST Mon Feb 15 2021

For the most part, though boundaries between ptype regimes are not
distinct, ptypes are generally ranging from snow in southern Indiana
to mostly sleet in central Kentucky to freezing rain in eastern
Kentucky. Road conditions have deteriorated quickly across the
region regardless of ptype.

Regional radar shows dry slot of mature cyclone heading from
Mississippi through west Tennessee into central and western Kentucky.
Typically this results in a precipitation change from sleet/snow to
freezing rain/freezing drizzle. Indeed, Cloud Top Phase satellite
product shows ice clouds across most of the Ohio Valley, including
Kentucky, but supercooled liquid in the dry slot from west Tennessee
south. Most reporting stations in west central KY are still
reporting snow as of this writing, but Clarksville, TN is reporting
freezing rain and that actually matches up well with what the
satellite is indicating.

At the same time, KLVX dual pol is showing the freezing level
lowering from northwest to southeast over southern Indiana northwest
of the surface low analyzed over east Tennessee. Feeling is that
the deeper cold air probably won`t make it much farther to the
southeast for the next few hours. Then after 00Z the surface low
will continue to head to the northeast through the central
Appalachians and will allow that cold air to work across the Ohio
River into central Kentucky, supporting the idea of snow as the
ptype this evening.

&&

.Forecast Update...
Issued at 944 PM EST Mon Feb 15 2021

::Precipitation::

Wintry mix has largely changed over to snow and is moving to the
northeast as the storm system lifts out of the region. Snow should
be exiting the northern Blue Grass by around 1 am EST. The remainder
of the overnight hours will be cloudy with a few flurries possible.

::Headlines::

After discussing with OHX and PAH, will likely trim back the WSW
within the next hour and replace with a Special Weather Statement
for continuing dangerous road conditions.

::Temperatures::

Given widespread clouds, snow and ice on the ground, cold air behind
the system, the latest guidance, and seeing where our forecast
compared with model guidance, have taken tomorrow`s high
temperatures forecast down three degrees. This results in highs
tomorrow ranging from the middle teens to lower 20s.

::Winds::

Upstream in the Wabash Valley winds have been gusting to around 20kt
this evening. Will increase the winds and wind gusts in the forecast
slightly, with gusts of 15-20kt. Wind chills will be quite cold
Tuesday morning...possibly in the 0 to -9 range especially west of I-
65.

&&

.Short Term...(This evening through Tuesday)
Issued at 245 PM EST Mon Feb 15 2021

...Significant Winter Storm Ongoing Across The Region...

The first wave of wintry weather has pushed out of the region and we
are now dealing with the second (more potent) wave. Regional radars
reveal widespread precipitation across southern Indiana and central
Kentucky. The transition zones of snow, sleet, and freezing rain
have wavered throughout the early afternoon, though for the
remainder of the afternoon we should see snow from a line primarily
west of Madison (IN) to Louisville (KY) to Rochester (KY). East of a
line from Frankfort (KY) to Glasgow (KY) we are expecting freezing
rain as the primary p-type. The corridor between those two areas
will likely be a transition zone of snow, sleet, and freezing rain
throughout much of the afternoon. Precipitation will slowly but
gradually transition to all sleet and/or snow tonight as colder air
wraps around the system as it departs. Areas east of a line from
Frankfort (KY) to Glasgow (KY) may only see sleet/snow showers for a
few hours, and accumulations will be limited. Ice will be the main p-
type for those areas.

Confidence is fairly good that there will be significant (+6")
snowfall accumulations across southern Indiana and portions of
central and western Kentucky. Snow rates will exceed 1" per hour for
periods this afternoon and evening.

There will be a sharp cut-off in snow/sleet accumulations near the
transition zone. It is completely feasible that western portions of
a county within our region could end up with +6" of snow while their
eastern counterparts only get an inch or two of sleet and snow.
AMDAR soundings out of SDF have recently shown a small but notable 1
degree C warm nose around 850-800mb that has resulted most
precipitation around the Louisville area to be a wintry mix of
freezing rain, sleet and snow. Model soundings in the near term show
this column cooling slightly over the next few hours, but if it does
not, then snowfall totals along and near the I-65 corridor will
likely be overdone.

Meanwhile, toward the east, areas within the ice storm warning will
likely receive a quarter to as much as a half inch of ice. There was
some consideration to expand the ice storm warning farther west, but
think sleet will mix in enough in those counties to keep ice
accumulations below a quarter inch. In many areas, there still
remains ice on trees and power lines from the previous winter storm
last week. This additional ice will likely result in additional
power outages.

Precipitation will begin to wind down across the region overnight
starting first in the southwest and ending last in the northeast.
Think that by about 08-09z most precipitation will be out of the
region. Clouds will linger through much of tomorrow and high
temperatures will be on the cold side (20s) with wind chills in the
single digits.

.Long Term...(Tuesday night through Monday)
Issued at 330 PM EST Mon Feb 15 2021

...POSSIBLE WINTER STORM WEDNESDAY NIGHT AND THURSDAY...

Tuesday night is expected to remain uneventful. Surface high
pressure, centered over southern Indiana, will weaken winds and help
skies go partly cloudy. This however is going to be short lived as
the next winter system begins to enter the picture on Wednesday.

On Wednesday, a longwave trough will become more amplified and dive
farther south across the Plains into Texas. Southwest flow, ahead of
the trough, will push a low pressure system from the northern Gulf
of Mexico to the northeast towards the Mid-Atlantic coast. This will
result in lifting system associated moisture towards southern
Indiana and central Kentucky. Relative humidity values will increase
through the mid-levels above 850mb.

Precipitation chances begin to increase through Wednesday, but
chances greatly increase Wednesday night and last through Thursday.
This is when the majority of precipitation is expected to fall. The
main focus with this system is going to be a common one, the
question of P-type. A warm nose is expected to change snow over to
freezing rain in the Lake Cumberland region and expand westward
towards Bowling Green and north into the southern Bluegrass during
the early morning hours on Thursday.

Around mid-day on Thursday, surface temperatures warm enough to
cause rain in the Lake Cumberland area, but as the shortwave passes,
cool temperatures, early Thursday night, will fall below freezing.
Any remaining light precipitation will fall as flurries. Any
remaining ground moisture will likely refreeze as Friday morning
temperatures are expected in the teens.

For the rest of the week, return flow will lift temperatures to near
normal by late weekend.

&&

.Aviation...(00Z TAF Issuance)
Issued at 641 PM EST Mon Feb 15 2021

KLVX radar showed a dry slot across the region this afternoon, but
heavier bands of precip are beginning to move in. Confidence remains
limited on precip types. IFR conditions across the terminals as
visibilities are mostly around or under 2SM. Expect to see these
types of conditions continue this evening as moisture and precip
work across the region. Precip looks to exit the region late
tonight, but OVC skies and MVFR cats will remain through the
tomorrow.


&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...Winter Storm Warning until 1 PM EST /noon CST/ Tuesday for
     INZ076>079-083-084-089>092.

KY...Winter Storm Warning until 1 PM EST /noon CST/ Tuesday for
     KYZ023>040-042-043-045>047-053-054-061>065-070>074-076.

     Ice Storm Warning until 1 PM EST /noon CST/ Tuesday for KYZ041-
     048-049-055>057-066-067-075-077-078-081-082.

&&

$$

Mesoscale...13
Update...13
Short Term...DM
Long Term...KDW
Aviation...CJP