Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 040639
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
139 AM EST Wed Dec 4 2013
...Updated Aviation Discussion...
.SHORT TERM (Now through Wednesday Night)...
Issued at 335 PM EST Tue Dec 3 2013
Expect continued low clouds this evening with misty/light fog
developing around and after midnight. Late this evening we`ll see
an area of higher theta-e and isentropic lift associated with a warm
front enter the region around 9Z providing for light rain chances
after 9Z. High-res models have really backed off though on rain
chances for late tonight and tomorrow so will keep POPs in the
20-30% range through tomorrow. Low temps should remain mild in the
upper 40s and lower 50s.
For tomorrow it seems we`ll have a better chance of rain in the
morning with that wedge of better theta-e air and isentropic lift.
Tomorrow afternoon looks mostly dry although an isld shower can`t be
ruled out. Southerly winds will become breezy ahead of an
approaching cold front tomorrow afternoon. However, low clouds
should limit gusts to the 20 to 25 mph range. A strong surge of
warm air should push high temps into the upper 60s to around 70.
Went just under MOS guidance for highs tomorrow due to the cloud
For Wed night, winds will remain breezy ahead of the front. Solid
rains with an embedded rumble of thunder should arrive after
midnight. Low temps will range from the upper 40s over southwest
Indiana to lower 60s east of I-65 in Kentucky.
.LONG TERM (Thursday through Tuesday)...
Issued at 318 PM EST Tue Dec 3 2013
...Heavy Rainfall and Wintry Weather Possible for the Latter Half of
Arctic frontal boundary will be working through the CWA during the
day on Thursday, clearing our SE by Thursday evening. Meanwhile,
broad and southwest flow aloft will continue to pump moisture into
the area as several disturbance ride up along the boundary. Expect a
tight temperature gradient during the day on Thursday, although the
boundary layer will still be too warm to support anything but rain
across the entire CWA. Rain is pretty much a certainty south of the
Ohio River on Thursday, with between a quarter and half an inch
expected in most spots. Some higher amounts will be possible. Along
and north of the river, chances of rain will taper from likely to
chance across southern IN. Highs should range from the upper 40s
northwest to the lower 60s southeast and gradually fall through the
Thursday Night - Saturday...
The fun begins Thursday night and should last through at least
Friday evening. This will occur as the cold air behind the arctic
front undercuts the warm and moist flow aloft. Most areas will
continue to see all rain on Thursday night, however a changeover is
expected north of the Ohio River before dawn, where freezing rain,
sleet, and snow will be possible. Through the day on Friday, most
model data supports the rain line gradually dropping southeast,
bringing the transition zone (sleet, snow, freezing rain) into
central Kentucky by Friday afternoon, and the all snow line to
around the Ohio River. Colder air aloft is then supposed crash in
aloft on Friday night, quickly changing p-type to all snow, except
across the SE CWA where a mix will still be possible. This will all
occur as the deeper moisture exits the area and we start to lose
coverage intensity. So, expect the biggest impacts from any frozen
precipitation type along and north of the Ohio River, with lesser
impacts south of the river. SE Kentucky will see the most rainfall
where at least 2 to 3 inches of rain will fall. Higher amounts are
likely. Flooding problems may become a concern in this area as a
result. Refer to hydro discussion in this product.
Will not speculate on amounts in any given area at this time, as
there is still much uncertainty in the model data. The phase between
the polar and subtropical jets will not occur until tomorrow, so
expect the model data to continue to waffle until then. The biggest
outlier seems to be the warm NAM, which goes nuts with the warm
layer aloft (as much as 10 C difference at H85 compared to other
model consensus!) due to a much more pronounced LLJ around 60 knots.
This solution is also slower with the cold air at the surface and
would likely give us all rain for much of the event. Don`t want to
completely disregard this scenario as it does depict the best
temperature gradient at the surface and H85, which makes
meteorological sense in this scenario. However, we are at the end of
its forecast range and don`t usually have much faith in it beyond 48
hours. Plus, it is an outlier to the other operational models. At
this point, will lean toward the colder solution in line with the
previous forecast, and issue an SPS to highlight concerns with
wintery precip and flooding.
Use a blend of raw model data during the Thursday - Saturday time
frame as this usually does well with advective patterns, especially
arctic airmasses. This will yield highs only in the low and mid 30s
across much of the CWA, however the tight gradient will result in
highs in the 40s and 50s across our extreme east. Temperature
gradient for lows on Friday night will range from mid teens NW to
around 30 SE. Highs on Saturday will only range in the mid 20s NW to
mid 30s SE.
Saturday night through Tuesday...
Active weather regime will continue this period with two more
systems to ponder as deep southwest flow aloft continues to import
abundant moisture into the lower Ohio and Tennessee Valleys. The
lull in precipitation Saturday will end Saturday night as moisture
lifts northward quickly across much of central Kentucky by Sunday
morning. With the deep southwest flow and a nice upper jet streak
entrance region over the area, potential is there for significant
precipitation again, although this will be a faster moving system
than the ones this Thursday and Friday.
Along with moisture return will be warmer air aloft on Sunday.
However, the problem will be residual boundary layer cold air and
low dewpoints. As precipitation falls into this air mass, it will
fall as rain (possibly sleet at onset on northern edge), but should
then freeze as it encounters the cold ground. So, a hazardous icing
event is possible depending on actual temps Sunday morning and
precipitation rates. By afternoon, surface temps should rise into
the mid 30s to lower 40s over central Kentucky changing freezing
rain to rain, unless cold air does not scour out as much as expected
or if morning icing keeps the surface colder. Southern Indiana could
remain with more icing.
This system exists Sunday night, but GFS and especially ECMWF
quickly bring the final shortwave across the area Monday night which
could reinvigorate a quick shot of more precipitation. The more
aggressive ECMWF favors rain southeast sections of central Kentucky
transitioning to snow before ending, with snow possible across
northern areas. The GFS shunts moisture more off to the east, but
with still wintry precipitation quite possible over eastern forecast
area. This second system also has a good jet streak right entrance
region signal at this time. With several systems before this last
one, will not try to get too specific this far out in time.
Arctic air will move into the Ohio Valley by Tuesday and Wednesday
of next week. High temps should only be in the 20s (could be even
lower especially if there is any snow cover), with lows by Wednesday
morning in the single numbers to lower teens very possible.
Issued at 315 PM EDT Tue Dec 3 2013
A prolonged period of precipitation is forecast this weekend. The
first batch is expected starting Thursday morning through Friday
night. A stalled front south of Kentucky will allow several waves of
precipitation over the weekend. There will be another batch of
precipitation starting Saturday night through Sunday night as the
front finally pulls out of the region. In the north, the
precipitation will be a mixture of freezing rain, sleet, and snow.
However in the south, it will be mainly rain. This rain will
accumulate over the weekend to cause minor flooding on rivers and
streams in southern and central Kentucky.
Up to four inches of liquid precipitation is expected over this
period with a few spots in the south seeing over four inches. This
rain could cause minor flooding in parts of the Kentucky, Green, and
.AVIATION (06Z TAF Update)...
Updated at 1220 AM EST Wed Dec 4 2013
Plenty of strato-cu over SDF and LEX, but mainly staying VFR so far.
A lower stratus deck has pushed into BWG from the south within the
last hour, and looks like a decent bet to take both SDF and LEX into
fuel-alternate MVFR by 08-09Z.
With the abundant low-level moisture, expect the stratus to
gradually build down as we head toward daybreak, and all terminals
should go into IFR if not LIFR for a substantial part of the
morning. Warm front will lift through from SW to NE from dawn
through late morning, and could trigger some scattered precip. Weak
forcing and the lack of moisture above about 800mb means that any
precip will be light, and may not even serve to improve the IFR
visibilities much. Will carry VCSH as confidence is not very high
that any terminal will actually be impacted.
Expect gradual improvement through the morning, with MVFR by late
morning and finally VFR conditions for the afternoon as gusty south
winds bring better mixing. Could see gusts of 20-25 kt, but will
parallel the long runways at SDF.
Gusts will diminish during the evening, but any nocturnal stratus
build-down will be limited Wednesday night by stronger low-level