Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 010808
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
308 AM EST Sun Dec 1 2013
.SHORT TERM (Now through Monday)...
Issued at 306 AM EST Sun Dec 01 2013
A rather low impact but complex in store for the short term period
as low-level moisture and associated cloud cover increases today
into tonight. This is all in advance of a weak surface cold front
which will undergo frontolysis through the short term period.
Current satellite imagery reveals clouds on the increase across the
Ohio Valley. These clouds are in response to weak isentropic ascent
along the 290K surface. Have used this 290K surface to forecast the
evolution of these clouds throughout the day. It appears the lowest
pdefs and best ascent will slowly shift southeast throughout the
day, thus think central and south-central KY will likely see the
majority of the cloud cover throughout the day today. There also
could be some isolated drizzle out of the deeper clouds, but expect
this to remain isolated thus will leave any mention out of the
forecast for now. Southern Indiana and northern KY may see some
sunshine by this afternoon as ascent weakens and drier air works
in. Have tried to forecast high temperatures with the expected
cloud evolution and the recent cool bias in guidance all in mind.
This method came up with the warmest temperatures (mid 50s) in
southern IN and northern KY, where temperatures are starting much
warmer (SDF currently at 48 degrees) and where there is a chance for
some sunshine later. South-central KY will likely stay much cooler
due to the cloud cover through much of the day, with highs expected
in the upper 40s to lower 50s.
By tonight, forecast soundings depict low-level moisture increasing
even further. Isentropic analyses depict the best upglide and
lowest pdefs residing across southern KY, thus will continue with a
slight chance of measurable precip there overnight. This scenario
is looking more and more like areas of drizzle, with only a few
places that measure a hundredth of rainfall. With the expected
cloud cover across southern KY, have gone above guidance with lows
in the middle 40s. Upper 30s and lower 40s will be common across
southern IN and northern KY.
On Monday, an upper-level PV anomaly will slide across the region.
However, a dry layer will exist between 500 and 800mb, so this upper
forcing will have little affect on any precip potential.
Drizzle or light rain potential will continue to hinge solely on
low-level isentropic ascent, which appears to lift northeast and
wane by the the late morning hours. Thus, will go with decreasing
pops through the afternoon hours. Temperatures remain a bit of a
challenge due to cloud cover, but highs in the mid 50s seem
reasonable as clouds decrease in coverage throughout the day.
.LONG TERM (Monday Night through Saturday)...
Issued at 300 AM EST Sun Dec 1 2013
Low-confidence forecast as we get into a fairly prolonged period of
unsettled weather. Timing is quite uncertain and dependent on the
evolution of the western CONUS trough.
Flow aloft is largely zonal Monday night into Tuesday, but most
recent model runs are quicker to eject a piece of the Pacific
Northwest low eastward along the Canadian border. This could make
the developing return flow a bit more unsettled, or at least keep
light warm advection rains on the table. Will follow the drier GFS
solution and keep POPs out Monday night and Tuesday in spite of
decent low-level moisture, because it is not that deep. Have trended
temps a bit milder, actually above climo, but will stay a few
degrees below bullish MOS highs in the lower/mid 60s out of respect
for abundant cloud cover.
Cold front will move into the Ohio Valley Wednesday and hang up
either overhead or just to our south and east as it becomes parallel
to the deep WSW flow aloft. POPs ramp up beginning Tuesday night,
with solid chance POPs in play Wednesday night and likely POPs
Thursday into Friday. Will continue to carry thunder across the
south on Thursday, but chances are fairly slim. QPF over the latter
half of the week is still on the order of 2 to locally 3 inches and
could lead to some short-term nuisance flooding, which we have
highlighted the potential in the HWO.
Temps will be highly dependent on frontal position, and will also
lead to precip type issues, especially on the tail end of the system
late Friday and Friday night. Not enough confidence to hit any
specifics this early, so at this point will leave it as a rain/snow
potential. However this bears watch for mixed/wintry precip with the
last of the waves to ride along the front.
By Saturday a strong Arctic surface high will build into the Great
Lakes and plunge our temps well below normal. Extended GFS MOS is
already advertising temps a good 10-15 degrees below normal, so even
those figures could be optimistic given the typical bias toward
climo at these time ranges.
.AVIATION (06Z TAF Update)...
Updated at 1220 AM EST Sun Dec 1 2013
Low level moisture will steadily increase ahead of an approaching
weak cold front. Clouds have already begun to develop, ahead of
schedule, from Michigan to western Kentucky. These clouds will
likely increase in coverage and remain in the region throughout the
The clouds have formed at the low end of VFR, and it appears they
should stay that way at least for several hours. Models have backed
off a bit on the MVFR ceilings they had been predicting for this
afternoon, so have reflected that in the TAFs. Still, ceilings
won`t be very far from the VFR/MVFR border, and prevailing MVFR
ceilings may still develop, especially this evening. There may also
be some restrictions to vsby late tonight. Future TAF packages can
address that in more detail, but for now will trend that way by
introducing 6SM BR.
Winds will be steady from the SSW at 5 to 9 knots through this
afternoon, becoming light and variable tonight.