Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 091125
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
625 AM EST Mon Dec 9 2013
...Updated Forecast and Aviation Discussion...
Issued at 620 AM EST Mon Dec 9 2013
Tweaked grids some for current drizzle and factored in the 06Z
models for tonight`s snow. QPF came down a little, but that was made
up some by continuing to raise forecast snow ratios. Still have
totals under an inch. Will issue a special weather statement shortly
to highlight this system a little more than our current HWO.
.SHORT TERM (Now through Tuesday)...
Issued at 240 AM EST Mon Dec 9 2013
Coverage of light rain showers now more isolated than advertised,
but drizzle does look to be going on east of a KSDF/KBWG line. Temps
in these areas all are above freezing, so no concern for freezing
drizzle at this time. An interesting feature to our northwest may be
hinting at what could happen tomorrow at this time. The right
entrance region of a 160-knot upper jet is over southern IL/central
IN. A couple of hours ago a feature appeared on IR satellite imagery
in this zone, and now we have a thin band of light snow over west
That jet will intensify tonight and drift southward, bringing that
entrance region into our northern zones. Models the last couple of
nights have been advertising a narrow band of snow oriented along
the same axis as the Ohio River to move through early Tuesday. NAM
also enhances the uplift along this zone with some isentropic lift
indicated in its 305K analysis around 06Z Tue. There will be some
lift farther south as well, which could bring back the precip shield
now south and east of us. A look at forecast soundings indicates
this precip also would fall as light snow, or perhaps a sleet/snow
mixture across the far southeast. Will keep forecast simple for now
and just go snow. Have gone with two areas with a chance of snow
overnight, and the in between in the slight chance range, in case
the location of either area is off. Ratios for the northern band
could be in the 15-20 to 1 range, given the saturation forecast in
the dendritic growth zone. Even with this, QPF is low enough and
band should move fast enough to limit snow totals to under an inch.
The ratio should be lower over my southeast, but likewise totals are
under an inch.
Skies should clear quickly behind this band, though a steady west
wind should keep temperatures well below normal, in the upper 20s
over the north and mid 30s in the south.
As for the temperature forecast for today, clouds should linger for
most of the day, so readings should not change much from where they
are now, perhaps a degree or two warmer.
.LONG TERM (Tuesday Night through Sunday)...
Issued at 310 AM EST Mon Dec 9 2013
.Tuesday night through Sunday...
Upper air pattern by Tuesday, which is the beginning of the extended
portion of the forecast, will feature a split flow off the west
coast of North America. Both streams of the jet are then forecast to
come together within a confluent zonal flow across the northern Ohio
Valley. Within this slowly progressive regime, a cutoff low Tuesday
off of Southern California will migrate across the southern plains
Surface high pressure forecast over the southern plains Tuesday
evening will extend eastwards over the Lower Ohio Valley. Subsidence
associated with confluent upper flow will bring mostly clear skies
Tuesday evening through Friday.
A dry cold front will switch winds to the north late Wednesday. High
pressure of Canadian origin will then settle south over Missouri by
We will stay rather chilly until Friday, when southerly flow ahead
of the southern plains low will bring milder temperatures. Average
highs for mid-December generally range in the mid 40s. Under mostly
sunny skies, highs Wednesday will vary from the lower 40s near
Tennessee to the mid 30s over southern Indiana. Temperatures
Thursday afternoon will cool from 5 to 10 degrees from Wednesday`s
highs. Light winds and mostly clear skies will bring several chilly
mornings Wednesday through Friday with widespread lows in the mid to
Modified Gulf moisture will return northwards along the Mississippi
River late Friday ahead of the southern plains low. This 500mb
trough will weaken as it approaches the Commonwealth, but warm air
advection will bring increasing clouds late Friday and a chance of
rain or snow early Saturday morning, changing to rain Saturday.
Milder temperatures will arrive Friday afternoon and continue
through Saturday. Highs Friday and Saturday may reach the lower to
mid 40s, with low early Saturday and Sunday possibly staying above
.AVIATION (12Z TAF Update)...
Updated at 615 AM EST Mon Dec 9 2013
Low pressure deepening over northern Michigan is inducing a stronger
westerly flow this morning. This flow should help to slowly lift the
low stratus through the morning hours. SDF is improving the fastest,
with some drier air working in. As for winds, that westerly
flow will become more northwesterly during the day as a surface
ridge develops over the Ohio River. A second upper level system may
bring some light snow to the terminals toward the end of its
forecast period. Have cigs going back down to MVFR for now around
daybreak, but not confident enough yet to put in snow wording in the