Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 030950
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
450 AM EST Tue Dec 3 2013
Issued at 450 AM EST Tue Dec 3 2013
Just enough near-surface moisture coming in from Tennessee, with
surface winds going calm across southern Kentucky, that areas of fog
have developed roughly along and south of a line from Fort Knox to
Danville. Have not yet seen anything too dense on local webcams, but
with visibilities down to 1 to 2 miles in a few spots, it is
noticeable enough to mention in forecast products. Updates are
.SHORT TERM (Now through Wednesday)...
Issued at 300 AM EST Tue Dec 3 2013
Expect unseasonably mild conditions to continue through Wednesday,
in spite of plenty of cloud cover. Low-level southerly flow is
keeping the boundary layer nearly saturated up through about 800mb,
with fairly dry zonal flow above.
A milder start and continued modest warm advection will push temps
into the 60s this afternoon, a solid 10-15 degrees above climo. Even
with the low-level moisture, a substantial inversion around 800mb
and the lack of any forcing mechanism will keep us dry today.
Low-level jet will crank up tonight, with winds increasing to
roughly 30 kt before daybreak Wednesday. Between the wind fields and
abundant cloud cover, expect some cooling in the evening followed by
steady or slowly rising temps after midnight.
Best chance of precip will be Wednesday morning as 850mb warm front
lifts NE across Kentucky and continues into the Great Lakes. MOS
guidance is not hitting this at all, but GFS, ECMWF, and NAM all
spit out some very modest QPF with this feature. Limiting factor
remains the lack of depth to any moisture, so will keep POPs just in
the chance category, highest over southern Indiana. Low-level wind
fields will increase and we will see some impressive warm advection
with 850mb temps climbing to +15C. Most of that warm air will skate
over the top of the trapped boundary-layer moisture, but it won`t
take much to push temps well into the 60s.
.LONG TERM (Wednesday Night through Monday)...
Issued at 300 AM EST Tue Dec 3 2013
...Heavy Rainfall and Wintry Weather Possible for the Latter Half of
At the start of the long-term period, surface high pressure will be
positioned off the east coast with a cold front approaching our area
from the northwest as an area of low pressure tracks northeast
across Lake Michigan. Very moist air will be pulled from the Gulf of
Mexico into southern and eastern Kentucky ahead of this boundary.
The front will slide southeast through the forecast area late
Wednesday night and through the first half of Thursday, clearing our
southeast counties likely by early Thursday afternoon. Rain showers
and perhaps a few rumbles of embedded thunder will overspread the
area Wednesday night into Thursday. Several shortwaves will ride
along this boundary Thursday night through Friday night, supplying
the potential for a wide range of mixed precip as the cold front
sinks southeast and colder air filters in late this week.
Late Wednesday night and into Thursday, PW values will be running
between 1.25 and 1.40 inches across southern Kentucky and between
1.15 and 1.25 inches across east-central Kentucky, with sub-cloud
layer relative humidity coming in at 70-80 percent and warm cloud
depth values ranging between 9000 and 11000 feet. This is very
anomalous for the first week in December. East-central Kentucky PWs
will be pushing 2+ standard deviations, while southern Kentucky will
be pushing, if not exceeding, 3+ standard deviations! Expect rain to
be moderate to heavy at times, with some localized flooding possible
as several waves of rain track across the area late Wednesday night
through Friday morning before the precip type changes. Anticipate
total rainfall amounts of 2-3 inches, with locally higher values
possible. This is also supported by analogs indicating chance
probabilities (30-50 percent) of exceeding 2 inches, generally
across south-central Kentucky. Locations further north/west will see
lighter rainfall amounts.
If that is not bad enough, now we move on to the precip type
forecast, which is more of a concern for our northern CWA, as this
front will be through the area when another shortwave rides
northeast along the boundary Thursday night and Friday. Latest
guidance indicates temperatures will gradually drop Thursday night.
Sounding analysis and SREF probabilities show it will be a cold rain
across southern Indiana and north-central Kentucky for much of the
evening hours. Should start to see some sleet and snow mixing in
across our southern Indiana counties after midnight and gradually
progress south making it to the Ohio River shortly before daybreak
Friday. The NAM has a much larger/warmer layer aloft, which can be
attributed to stronger winds, 10 knots stronger, than other
guidance. Not really buying into this right now, but if this were to
pan out, freezing rain could become a concern. The temp profile will
continue to cool, with the change-over line progressing southeast
across the forecast area through the day and evening hours Friday.
Currently, any wintry mix accumulation would likely be across
southern Indiana and north-central Kentucky, but amounts would just
be a guess at this point since precip type remains a question. The
lower atmosphere will be quite moist, with wetbulbing not really
changing the profile much at all. So, this changeover will rely on
temperature advection, which is always a tough forecast. At this
stage it certainly is a low confidence forecast.
Expect precip to exit the area late Friday night, with a dry
Saturday on tap. Cold air will continue to advect over the entire
region, with temperatures Saturday ranging from the upper 20s in the
north to lower 30s in the south. This cold air will be a player for
more wintry weather as another shortwave trough and associated
moisture/precip moves into the region Sunday. This one appears to
generate mixed precip in the opposite order from the previous system
as a warm air advection aloft will work into the area overtop colder
surface temperatures. Would expect this to start out as snow late
Saturday night, changing to sleet, then changing to freezing rain,
and finally changing to all rain Sunday afternoon as the profile
warms. However, temperatures will then fall behind this system as
well, with light snow possibly mixing in on the back edge Sunday
night into Monday. This is still a long way off and we have to
contend with our initial set of shortwaves Thursday through Friday
night. It will certainly be an active period though!
.AVIATION (06Z TAF Update)...
Updated at 1205 AM EST Tue Dec 3 2013
Ceiling and visibility forecasts will remain quite challenging
through about midday Tuesday. Abundant moisture from the surface up
to roughly 800mb means that clouds are a sure bet, but confidence in
ceiling heights is quite limited. Fog potential is also on the
table, mainly at BWG where we are starting to see the most breaks in
the cloud cover.
Expect the clouds to hang in through the night, with ceilings
lowering to MVFR by daybreak but mainly staying above fuel-alternate
criteria. The one caveat with all this low-level moisture is that if
the clouds should break, visibilities will quickly drop to IFR.
Higher-end MVFR conditions will persist through the morning, with
light SSE winds. Expect ceilings to lift to VFR again in the
afternoon. Diurnal lowering of cig/vis again Tuesday evening, but
again that should be MVFR with minimal operational impacts. Can`t
rule out precip just before daybreak Wednesday at SDF, but
confidence is too low to include it in the planning period just yet.