Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 202052
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
352 PM EST Thu Nov 20 2014
.SHORT TERM (Now through Friday Night)...
Issued at 300 PM EST Thu Nov 20 2014
High pressure to our west today will move over us tonight, providing
us with another cold night. Should see lows range from the lower 20s
in our warm spots to the mid teens in our cold spots. That high will
be to our east Friday, but temps will just be a few degrees warmer
than today. Friday night we`ll see a zone of isentropic lift move
northeast across the region. This lift may be enough to generate
some light precip, possibly allowing a small band of freezing rain
on the leading edge of the precip as it crosses our northern
forecast area. Given the very light totals, not expecting much, if
any, impact. Temperatures should warm quickly a little after the
onset of the precipitation too.
.LONG TERM (Saturday through Thursday)...
Issued at 345 PM EST Thu Nov 20 2014
Saturday through Monday Night...
The medium range models continue to be in decent agreement with a
rather impressive low pressure system that will move from the Red
River Valley on Saturday, through the Ohio Valley on Sunday and into
the Great Lakes by Monday night. This feature will mainly affect
our region Saturday night and Sunday. Ahead of this system we`ll
see a modest, but short-lived warmup, for Saturday and Sunday as
we`ll be in the warm sector of this system. Scattered light rain
showers will be possible during the day on Saturday as the low
pressure system gets organized to our southwest. We`ll see an
increasing southwesterly flow with generally partly to mostly cloudy
Temperatures look to warm into the lower-mid 50s in the north with
upper 50s to the south. Given that we`ll be under southwesterly
flow, I suspect that these temperatures may be underdone slightly,
and upward adjustments may be required...especially if we see a
little less cloud cover. The best chances of any rain showers will
generally be across our far west and northwest sections...mainly
west of the I-65 corridor. Moisture will continue to increase as we
head into Saturday night and the southwesterly flow increases in
response to a tightening pressure gradient. Lows Saturday night
will likely only cool into the lower-middle 40s. Rain showers will
enter from the southwest late Saturday night. Thus, will plan on
ramping up showers from the SW to the NE late Saturday night.
Main impact from this system will be felt on Sunday as the surface
low passes to our west. An increasing pressure gradient will result
in windy conditions across the region. Model profiles still are
fairly aggressive with low-level wind fields increasing well above
60kts during the day on Sunday. This, along with increasing
moisture (dewpoints rising into the 50s) may result in some surface
based instability during the late-morning and afternoon. In any
case, it does not take a lot of instability in mid-late fall to
generate strong/severe storms in the Ohio Valley. Thus, feel that
there is still some risk of seeing some strong/severe storms on
Sunday with the primary threat being damaging winds. Even if
instability does not develop, we could have some issues with just
the gradient winds mixing down to the surface. So this will be
something to closely watch over the next few days. Heavy rainfall
will be an issue as well with 1-2 inches of rain being possible as
the system moves through. Some localized hydrologic issues are
certainly possible...especially in areas that see repeated rainfall
or training of storms. Highs Sunday look to top out in the upper
50s to the lower 60s...but again, these may be a little under done
with a strong southerly flow. Additional showers and storms will be
possible Sunday night with lows only dropping into the lower-middle
As the surface low reaches the Great Lakes on Monday, we should see
the surface cold front push through the region...probably in the
late-morning to early afternoon hours. May see some convection
redevelop along the boundary, but overall coverage looks less than
what we expect to see on Sunday. We`re not going to see much of a
cold push directly behind the front as the models keep a deeper
trough axis out to our west. The colder air will likely arrive
Monday night. Highs Monday should top out in the mid-upper 50s with
lows Tuesday morning dropping to around 30-35 in many places.
Tuesday through Thursday...
This portion of the forecast remains rather challenging as the long
range models start to diverge in their solutions with the upper
level pattern from Tuesday through Thanksgiving. The challenge here
is whether the western US ridge will build up like the Op GFS and
parallel GFS suggest...or if the flow is going to remain rather
progressive in nature and less amplified like the 12Z Euro and Euro
Ensembles. Given the overall teleconnection patterns, am not really
sold on the high amplitude ridge that is featured in the OP GFS and
parallel GFS versions...given the expected fast flow over the north
Pacific courtesy of the Pacific jet.
Our current thinking is that we`ll see some sort of baroclinic
boundary set up from the lee of the Rockies eastward into the Ohio
Valley by mid week as a another cold airmass drops southward out of
Canada. Speed max looks to drop out of the inter-mountain west and
will likely result in cyclogenesis along this frontal boundary.
This system will track NE into the Ohio Valley by late in the
period. Depending on the eventual track, it appears that another
rain to snow event may be in the offing around Thanksgiving and into
the holiday weekend. This will be something to watch closely over
the next week for sure.
.AVIATION (18Z TAF Update)...
Updated at 1215 PM EST Thu Nov 20 2014
High pressure to our west this afternoon will move over the region
tonight and Friday morning, shifting our winds from westerly this
afternoon to easterly by Friday afternoon. Aside from a few strato
cumulus this afternoon along the SDF/LEX corridor, skies should
clear out this evening and remain that way the rest of the period.
Bases may dip into the MVFR range at LEX during the afternoon.
Pushed the timing back a little based on latest visible satellite
showing a break for at least an hour or two.