Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 110538
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
1238 AM EST Mon Dec 11 2017

...Updated Aviation Discussion...

.Forecast Update...
Issued at 955 PM EST Sun Dec 10 2017

No concerns with the forecast this evening. Forecast lows are on
track for lower 20s in our cold spots and mid/upper 20s elsewhere.
Those cold spots have dropped a little quicker than forecast, but
that should slow down to our ongoing low forecast.

&&

.Short Term...(Today through Tuesday)
Issued at 301 PM EST Sun Dec 10 2017

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

Mid/high level clouds currently streaming in from the northwest
ahead of a weak shortwave at this hour. Very little moisture
associated with this wave so no precipitation expected, and clearing
skies are being observed upstream in central IL/IN. Skies should
remain clear overnight, and winds will be relatively light after
midnight, so temperatures will fall well into the 20s by tomorrow
morning.

Monday will be one of the warmest days for the upcoming week as weak
ridging and southwesterly flow around 850mb advects in warmer
temperatures. Warmest temperatures will be southwest toward Bowling
Green where highs will generally be in the low 50s, while southern
Indiana and northern Kentucky will remain in the mid to upper 40s.

A strong shortwave will come diving into the region Monday night. At
the surface, a weak area of low pressure and surface cold front will
pass through southern Indiana and central KY between 02-07z. Mid/low
level moisture will filter in behind the cold front. There remains
some uncertainty regarding the depth of the moisture, which will
have huge implications on the type of precipitation we observe. Most
models show a small window (on the order of 1-3 hours) where near-
saturation occurs in the DGZ and precipitation type would be all
snow with temperatures at or just above freezing at the surface.
After that, drier air quickly moves in aloft and dries out the DGZ,
but the low levels of the atmosphere (surface to ~800mb) remain
saturated. Temperatures within the low-level saturated column would
generally stay at or greater than -12 degrees C, and would support
snow and possibly some freezing drizzle as the precipitation type.
If there is enough lift to generate light precipitation, and surface
temperatures remain at or below freezing, there could be a period of
light snow and/or a wintry mix accumulations Tuesday morning during
the rush-hour commute. Most models show very minor precipitation
amounts, generally on the order of 0.01" to 0.03" liquid equivalent
QPF. If this all fell as snow, general snow accumulations would be
close to or under a half inch. The general consensus with the
highest QPF appears to be over the Kentucky Bluegrass region near
the Lexington metro, with lesser amounts as you work your way
southwest. Southwestern portions of the CWA, like Bowling Green,
could stay dry through the whole event. After collaborating with
neighboring offices, have decided to stick with an all-snow forecast
for now, and continue to monitor temperatures/moisture aloft in case
we need to add freezing drizzle to subsequent forecasts.

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

By late Tuesday morning into early afternoon, enough dry air will
work in to at least keep precipitation from falling. Low level
clouds will block out the sun for most of the day, with the
exception of southwestern portions of the CWA (like Bowling Green)
where the low level moisture may mix out with daytime heating. Winds
will be breezy out of the northwest during the daytime due to strong
CAA and pressure rises , and the combo of temperatures/winds will
keep the wind chill readings in the 20s for the afternoon.
Temperatures Tuesday night will be cold as the Canadian air settles
in, with readings in the upper teens to low 20s expected.

&&

.Long Term...(Tuesday night through Sunday)
Issued at 300 PM EST Sun Dec 10 2017

Wednesday - Friday...

Low forecast confidence exists in the long term period as mid to
long range models vary greatly.  We know that we`ll be in NW flow in
an upper trough over the Midwest Wed-Fri, but timing waves within
the flow is difficult and 12Z models offer a wide range of
solutions.

So overall Wed-Fri will be colder than normal with perhaps several
chances at light precipitation mainly in the form of light snow.
However, Thu temps may warm enough to produce a mix or plain rain in
some areas.

Saturday - Sunday...

By the weekend, the upper trough will lift with height rises across
the Midwest expected leading to moderating temps. Temps should be
back near normal with another precipitation producing weather system
possible Sat night/Sun.

&&

.Aviation...(06Z TAF Issuance)
Issued at 1238 AM EST Mon Dec 11 2017

After quiet pre-dawn hours, winds will pick up out of the SSW once
again today as a cold front approaches from the northwest, crossing
the Wabash River this evening. The front will push through the
airports during the 00Z-06Z time frame, shifting the winds to
westerly and maintaining their strength...possibly even increasing a
bit. Moisture from the Great Lakes will stream in behind the system
and will bring scattered showers to the region after midnight.

&&

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

$$

Update...RJS
Short Term...DM
Long Term...AMS
Aviation...RAS



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