Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 071720

Area Forecast Discussion
National Weather Service Louisville KY
1220 PM EST Thu Dec 7 2017

.Forecast Update...
Issued at 1220 PM EST Thu Dec 7 2017

Updated the forecast to beef sky cover up this afternoon. A broken
deck of stratocu is dropping down out of southern IL/IN. Expect this
deck to break up over the next few hours. Temps will likely drop
back toward the freezing mark as the new batch of clouds slides


.Short Term...(Today through Friday)
Issued at 245 AM EST Thu Dec 7 2017

It`s a seasonably chilly night across the Ohio Valley as light
southerly winds have developed ahead of an approaching weak wave,
and temps have not been able to bottom out. Upstream obs show a few
different bands of clouds associated with this impulse, and we have
seen a few reports of light snow or more accurately flurries.

This system should swing through this morning with little more
impact than a fairly high-based strato-cu ceiling, but we can`t rule
out a few flurries across southern Indiana. Chances for measurable
precip are slim and none.

Main weather impact today will be the reinforcing shot of cold air
that will really limit our temp recovery. Highs will only reach the
mid/upper 30s across most of the area as cold advection takes hold.
Heights continue to fall through Friday, so cold and dry weather
will continue with temps 10-15 degrees below climo both day and

.Long Term...(Friday night through Wednesday)
Issued at 245 AM EST Thu Dec 7 2017

Dominant feature for most of the upcoming week will be an eastern
CONUS upper trof, which will maintain cold and somewhat unsettled
conditions across the Ohio Valley. Several impulses will dive out of
Canada and swing through the base of this trof, bringing periodic
chances for light precip mainly in the form of snow.

First impulse drops out of Manitoba Fri night and the upper wave
axis crosses the Ohio Valley around midday Saturday. Most of the
forcing with this feature stays to our north and east, and there is
substantial low-level dry air to overcome, and both of these factors
will limit our QPF. However there is a narrow window Saturday
morning with steep lapse rates and saturation in the dendritic
growth layer, which would support a snow shower regime especially
along and north of Interstate 64. Will carry chance POPs and
accumulations of a half inch or less, further limited by the timing
during daylight. Expect little or no impact south of I-64 given less
precip, and temps just warm enough to kick over to a rain/snow mix.

Expect quiet weather Sat night through Monday as the pattern
reloads, with temps on Monday actually recovering to near normal
under some shortwave ridging.

Monday night through Wednesday is where the models diverge, as both
GFS and ECMWF have multiple disturbances diving into the Ohio Valley
in the NW flow, but some differences in timing. There will be precip
chances at some point, and will likely be cold enough for snow or
mixed precip, but for now will keep POPs low and paint with a broad
brush on precip type. Temps will be back below normal, but the
question remains by how much.


.Aviation...(12Z TAF Issuance)
Issued at 622 AM EST Thu Dec 7 2017

Plan on wind shift to the west/northwest over the next few hours as a
cold front passes through the region. Mid-level clouds will spread
over the TAF sites with bases ranging from 2500 ft in spots to
around 5000 ft. Snow flurries will be possible at HNB, SDF, and LEX
but should not have any aviation impact. This cloud deck should be
replaced by afternoon strato-cu with bases around 3500 ft. Northwest
winds will be sustained 10 to 13 kts into this afternoon then
subside toward sunset.




Short Term...RAS
Long Term...RAS
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