Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 02/11/21

Area Forecast Discussion

National Weather Service Louisville KY

 816 PM EST Wed Feb 10 2021

.Mesoscale Update...
Issued at 812 PM EST Wed Feb 10 2021

...Mesoscale Update...

Regional radars continue to show a broad band of precipitation
stretching from southeastern MO into southern OH at this hour. As
expected, the main corridor/axis has not shifted much over the past
few hours, and this has resulted in ice accumulations ranging from
0.10-0.20" across southern IN and north-central KY. Latest model
trends do not show this axis moving much, though we may see some
breaks develop in the precipitation across southern IN and portions
of north-central KY closer to midnight.

AMDAR soundings out at SDF have shown some cooling in the 900-800mb
layer to where the warm nose is now between 1-1.5 C. However, the
depth of this warm air is about 4-5kft deep, which is enough to
transition snow over to liquid before it drops into sub-freezing air
again. As the night continues, cold air advection in the low levels
should gradually transition p-types from freezing rain to sleet
and/or snow across southern IN and portions of north-central KY.

Based on some of the latest model trends and upstream observations,
ice amounts have been upped across a corridor of counties in north-
central KY where freezing rain is expected to persist for the next
several hours. The Owensboro (KOWB) airport in Daviess County
reported close to a half inch of liquid equivalent (not all ice
accumulations!) in three hours time with some reports of power lines
down in the county. Expect some counties within our CWA to
experience similar results. In particular, am concerned about
Hancock, Breckinridge, Meade, Bullitt, Jefferson, Spencer, Shelby,
Franklin, and Scott counties seeing an additional 0.25" of ice
accumulations on top of what is already there. Additionally, based
on latest temperature trends, have started and upped ice
accumulations a little earlier for some of our central KY counties
along and south of the Parkways. These areas have transitioned over
to freezing rain a little ahead of schedule.

Issued at 525 PM EST Wed Feb 10 2021

Regional radars reveal a fairly broad band of precipitation
stretching from southeastern MO into southern OH at this hour. This
band of precipitation is located within a favorable zone of low-to-
mid level frontogenesis and moisture convergence. Over the next few
hours, this area of favorable lift will shift ever so slightly
southward, but in general, areas seeing wintry precipitation now
should see it continue well into the evening hours.

Latest AMDAR/aircraft data out of SDF has been pretty intriguing
over the last couple of hours. The warm nose between 900-800mb has
varied from 0.5 C to 3.5 C within that timeframe, and as a result,
we`ve seen observations go from freezing rain to sleet to snow or a
combination of one of the aforementioned p-types at SDF. A lot of
this appears to be influenced by precip-rates and pockets of deeper
moisture within some of the heavier bands that moved over the area.
This p-type behavior has also been observed at other sites generally
within a 25-50 mile radius of the Ohio River across southern IN and
north-central KY.

Despite these modulations, freezing rain has been the main p-type
for locations generally along and south of the Ohio River where
surface temperatures have been below freezing. Counties across
southern IN near the Ohio River have jumped around from sleet to
freezing rain, while the tier of counties to their north (Dubois,
Orange, Washington, Scott, Jefferson) have primarily been snow and
sleet.

The highest freezing rain accumulations over the next few hours look
to occur for counties south of the Ohio River but north of the
Bluegrass Parkway. Freezing rain rates as high as 0.10"/per hour are
possible, though based on visual observations here at the forecast
office, some of this has fallen a bit too quickly/`heavy` for it all
to freeze on contact. Still, the current ice accumulation forecasts
look in pretty good shape.

&&

.Short Term...(This evening through Thursday)
Issued at 323 PM EST Wed Feb 10 2021

...SIGNIFICANT WINTER STORM TO IMPACT THE REGION THROUGH THURSDAY...

...CONDITIONS EXPECTED TO DETERIORATE RAPIDLY THIS AFTERNOON AND
EVENING...

An impactful ice storm, with some sleet and snow along the northern
edge, is underway across north-central KY and southern IN. A mid-
level shortwave impulse will continue lift ENE over the region
through tonight. A notable 35-40 kt SW LLJ at 850 mb stretches from
Arkansas northeast across Kentucky, isentropically lifting plentiful
moisture on the north side of a sfc boundary. A strong baroclinic
zone is place across KY and IN on the north side of the front, which
currently extends from Middle TN northeast across the Appalachians.
Southerly flow in the 925-850 mb layer has indeed pushed
temperatures above freezing across the entire CWA, with the 0 degree
line at 850 mb currently draped through roughly Bedford IN and
Seymour IN. Moderate moisture transport has actually pushed PW
values above 0.8 inches along and south of the Ohio River, notably
high for early Feb. The rich moisture, combined with subfreezing sfc
temps, will lead to significant ice accumulations this afternoon
through Thursday.

Moderate sleet and freezing rain is ongoing now over southern IN and
northern KY. As the upper wave and sfc boundary slide east through
tonight, the heaviest/primary QPF axis will slide southeast with
time. By 7-10 pm EST this evening, an axis of moderate freezing rain
will likely be progressing across central KY, generally along the
WK/BG Parkway corridor. Our northernmost row of Indiana counties
will gradually dry out around this time, so based on the latest snow
and ice forecasts, kept that area going in a Winter Weather
Advisory. That product may be able to be dropped early (perhaps
overnight or Thursday morning). Closer to the Ohio River, light
freezing rain/sleet/snow will continue overnight while significant
icing takes places over central KY. On Thursday, the focus area
becomes central and southern KY with the greatest icing south of the
WK/BG Parkways.

The main change with this forecast package was to increase QPF this
afternoon into tonight, especially near and north of the WK/BG
parkways. Similar ice accumulations are forecast for much of central
KY, but ice accumulations have been increased along the I-64
corridor. At least 0.15-0.25 inches of ice looks possible in the
counties straddling the Ohio River, with a quarter to half inch of
ice expected south of I-64.  The heaviest ice accumulations (closer
to a half inch) are concentrated in a band across central KY,
especially near the WK/BG parkways. Decided to introduce a Winter
Storm Warning this afternoon near and just north of I-64 given the
greater potential for sleet. Significant ice accumulations combined
with moderate sleet at times will likely snarl traffic throughout
the evening commute. Travel is not recommended.

The potential for power outages increases after sunset as the weight
of the ice on trees increases. Scattered power outages are more
likely initially in north-central KY and far southern IN, with
impacts spreading south early Thursday. Widespread freezing rain is
likely on Thursday over central and southern KY, but temperatures in
the Lake Cumberland region could be warm enough to delay ice
accumulation until late in the event. For this reason, there is low
confidence on Cumberland and Clinton counties reaching Ice Storm
Warning criteria. But still have 0.10-0.30 inch ice accumulations in
the far southeastern CWA, so would like to hang on to the Ice Storm
Warning for now. Refinements to the Winter Storm Warning and Ice
Storm Warning on both the northern and southern edges are likely in
future updates this evening into tomorrow.

Bottom line...a significant winter storm is underway. Heavy icing is
expected across much of central KY and far southern IN now through
tomorrow. Expect impacts to only increase through the evening. Only
travel if you must between this afternoon and Thursday evening.

.Long Term...(Thursday night through Wednesday)
Issued at 200 PM EST Wed Feb 10 2021

...Active Winter Pattern Continues into the Weekend and Next Week...

The primary concern for the period will be a prolonged period of
frigid temperatures as strong CAA brings in Arctic air and some of
the coldest air of the winter along with another potential winter
storm early next week.  Leading up to that cold spell, a wintry mix
will be possible late Friday into early Saturday with a weak area of
850mb moisture convergence and lift associated with that
convergence. Coverage will be limited with an overall lack of
moisture in place in the aftermath of the significant system
Wednesday and Thursday.

The true blast of cold arrives Saturday night and continues into
early next week with low temperatures in the upper single digits to
low teens. A strong 500mb low will be crossing through southern
Canada during the day Saturday with CAA associate with the low
bringing the frigid air into southern Indiana and Kentucky. There is
some uncertainty about how deep the cold air will intrude into the
area, but will continue to monitor the forecast trends and stick
close to the NBM initialization which captures that uncertainty
well. Another potential caveat for realizing the low single digit
temperatures will be persistent low clouds with model guidance
suggesting an area of stratus across the area both Saturday and
Sunday nights.

The chilly temperatures will continue into early next week with
signals for another significant weather system arriving Monday into
Tuesday.  Model guidance is still quite varied on the potential
solutions, but the current thoughts are that the potential is
certainly there for another wintry mix with snow and ice
accumulations. This system currently looks to arrive late Monday
into Tuesday with a push of warmer air arriving as the system moves
through. Confidence remains low in this system with a wide variance
on how strong the warm air advection is associated with the low, but
significant snowfall numbers are possible across southern Indiana
and northern Kentucky with a more rain/mixed setup likely for
central and southern Kentucky.  We will continue to monitor the
forecast trends going forward as details become clearer. In the
aftermath of this system, warmer air finally looks to return as
surface flow finally gains a more southerly component.

&&

.Aviation...(00Z TAF Issuance)
Issued at 625 PM EST Wed Feb 10 2021

A mixed bag of precip continues this evening and into the overnight,
predominately as freezing rain, sleet, and snow. Conditions are
currently IFR across the region, and expect to see these conditions
continue as OVC skies remain under 1000ft. Brief periods of LIFR are
possible for BWG and LEX late tonight during waves of increased
precip rates that could drop visibilities. Will hold on to a PROB30
group through Thursday afternoon for any lingering wintry mix we
have at HNB, SDF, and LEX.


&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...Winter Weather Advisory until 7 PM EST Thursday for INZ076>079-
     083.

     Winter Storm Warning until 7 PM EST /6 PM CST/ Thursday for
     INZ084-089>092.

KY...Winter Weather Advisory until 7 PM EST Thursday for KYZ032.

     Ice Storm Warning until 7 PM EST /6 PM CST/ Thursday for
     KYZ023>029-038>043-045>049-053>057-061>067-070>078-081-082.

     Winter Storm Warning until 7 PM EST Thursday for KYZ030-031-
     033>037.

&&

$$

Mesoscale...DM
Short Term...EBW
Long Term...WFO IND
Aviation...CJP