Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 111044
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
544 AM EST Mon Dec 11 2017

.Short Term...(Today through Tuesday)
Issued at 234 AM EST Mon Dec 11 2017

...Wintry Precipitation Possible Tuesday Morning During Rush Hour...

Latest satellite imagery showed mostly clear skies across central
Kentucky and southern Indiana. Regional observations showed
temperatures generally in the mid 20s to lower 30s. An area of low
pressure was analyzed north of Grand Forks, North Dakota, which will
be the next weather system to pass through the lower Ohio Valley
late tonight into Tuesday morning.

For today, expect a warmer day with southwest winds 10 to 15 mph.
We`ll start off mostly sunny but plan on an increase in mid to high
clouds by afternoon associated with the next weather system,
especially across southern Indiana and north central Kentucky. Most
of the 11.00z temperature guidance trended warmer, and as a result,
bumped up highs a few degrees. Readings will top out in the upper
40s across southern Indiana to lower to middle 50s across central
Kentucky.

The aforementioned low pressure near Grand Forks will quickly dive
southeast toward Illinois and Indiana this afternoon, reaching
western Ohio by this evening. A cold front will pass through
southern Indiana and central Kentucky around midnight. Ahead of it,
dry conditions are expected. However, in its wake, there`s a thin
wedge of low level moisture likely. This moisture layer may be deep
enough and cold enough to support snow showers for a few hours,
especially along/north of the KY Parkways, during the overnight
hours into the first part of Tuesday morning (rush hour time). The
NAM, ECMWF, and a few of the hi-res models paint light QPF amounts
across the northern half of the forecast area. In addition, as drier
air works down in the area, saturation is potentially lost between -
8 and  -12C. This could spell a brief period of freezing drizzle if
surface temperatures drop below freezing. If this scenario
materializes and precipitation falls as all snow, a few tenths of an
inch may be possible through Tuesday morning along with a brief
period of light wintry mix. In coordination with IND, ILN, and JKL,
decided to add a few hours light freezing drizzle to the forecast
overnight into the morning rush hour time Tuesday, mainly across
southern Indiana and north central KY including the KY Bluegrass and
Lexington area.

To summarize, there is potential for light wintry precipitation
after midnight tonight into Tuesday morning for southern IN and
north central KY, especially across the Bluegrass region and
Lexington area. It appears that precipitation will be snow with a
possible transition to a wintry mix. Although overall amounts will
be light, there could be travel issues on roads given the time of
day it is expected to occur (rush hour), especially if the roads are
untreated.

Remainder of Tuesday: Very strong cold air advection will limit
diurnal rises in temperatures to less than 5 degrees. Many areas
will struggle to reach the freezing mark while southern Kentucky
stays in the mid 30s. Combined with brisk north to northwest winds,
wind chill readings will stay in the 20s for the entire day. With
another strong upper level shortwave passing overhead during the
afternoon hours, the steepening lapse rates along with colder
temperature profiles could support isolated to scattered flurries
for much of the day, especially across southeast Indiana and KY
Bluegrass.

Clear and cold conditions are expected Tuesday night with readings
bottoming out in the mid teens to lower 20s.

&&

.Long Term...(Tuesday night through Sunday)
Issued at 235 AM EST Mon Dec 11 2017

The main theme for the long term period is for a couple more clipper
systems through the end of the work week followed by a warming trend
into next weekend.

The next of our series of quick moving northwest flow systems
arrives Wednesday night into Thursday. The 11.00z guidance didn`t
provide any better forecast confidence as the various models have a
wide range of solutions with the surface low track and thermal
profiles. A consensus approach takes the surface low just north of
the area, which would limit the chances for snow across most of our
area. However, any deviation south could bring higher chances into
central Kentucky. For now, have continued the mention of flurries or
light snow, generally along/north of KY Parkways.

Eventually the global models agree that the longwave trough that has
persisted over the eastern US for the past week will begin to leave
and be replaced with more zonal to even weak ridging by this
weekend. This could provide warmer temperatures to at or even above
normal by this upcoming weekend. This is reflected well in the
latest 6-10 day outlook from CPC which shows higher chances of above
normal temperatures for the lower Ohio Valley.

&&

.Aviation...(12Z TAF Issuance)
Issued at 544 AM EST Mon Dec 11 2017

Wind will be the main concern for aviation during most of this TAF
period.

Low pressure moving into the Great Lakes will swing a cold front
through the Wabash Valley by this evening. Ahead of this system SSW
winds will increase and may gust to around 20kt by this afternoon.

The front will cross the region tonight, turning winds to the west
and eventually WNW. Winds will remain brisk behind the front with
sustained speeds of 12 to 17 knots expected.

Widely scattered light snow showers will be possible several hours
either side of dawn Tuesday morning as those winds bring moisture
down from the Lakes. Chances of measurable precipitation are low,
but went ahead and included a VCSH at SDF and LEX late in the
forecast. Depending on depth of moisture and amount of lift, some
freezing drizzle may occur as well, though ceilings are expected to
remain high-end MVFR at worst.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...None.
KY...None.
&&

$$

Short Term...ZT
Long Term...ZT
Aviation...13



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