Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 021727
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1227 PM EST Mon Dec 2 2013
Issued at 1226 PM EST Mon Dec 2 2013
Temperatures have warmed into the low and mid 50s in most spots
under variably cloudy skies this afternoon. Have tweaked high
temperatures a bit in some locations, and should generally end up
with a range in the mid and upper 50s for highs. A few locations
that see the most sun could touch 60 degrees.
.SHORT TERM (Now through Tuesday)...
Issued at 305 AM EST Mon Dec 02 2013
The synoptic pattern this morning features zonal to slight northwest
flow aloft, with two PV anomalies of note. The first was located
across northern IL, with the second down across the Southeast.
In response to this northern feature, analysis reveals weak
isentropic ascent along the 290K surface continuing mainly across
southern KY. Further north, ascent has decreased (with actually
some subsidence noted in cross sections) which has begun to poke
some holes in the cloud cover over the past hour or two. This trend
will likely continue through the morning hours, thus temperatures
may fall off rather rapidly as skies go partly cloudy at times.
Here at WFO LMK, skies went clear and we quickly dropped 5 to 7
degrees into the middle 30s over the span of about an hour.
As was the case yesterday, the forecast remains rather tricky due to
lingering cloud cover and its effect on temperatures today. Over
the past 12-16 hours, the operational GFS has really struggled in
its low-level moisture/thermal fields, thus will largely ignore it
for this forecast package. The NAM has performed much better and
has a good handle on current conditions, thus will trend this
forecast toward the NAM. Forecast 290K isentropic charts show
weakening ascent through the morning hours and forecast soundings
also show clouds mixing out this afternoon mainly across the western
CWA. Condensation pdefs will continue to remain low across
southeastern KY (albeit with weakening upglide), so think clouds
will likely hang on there for a bit longer today. The
aforementioned system passing to the north will also spread clouds
into our far northern sections this afternoon as it streams across
IN/OH. With all this in mind, think the warmest temperatures will
be across the western CWA, where sunshine will be the most prevalent
(clouds passing to the north and southeast). Therefore, have bumped
highs up several degrees in these locations, more in line with MET
guidance. Have removed all precip from the forecast today, as
saturation will not be deep enough to produce any drizzle/light rain.
A stronger PV anomaly will slide across IN/OH tonight in the
northwest flow aloft. In response, a weak LLJ/isentropic ascent
will develop, spreading additional low-level moisture northeastward
into the region. Therefore, think low-level clouds will increase
markedly from south to north once again overnight. Forecast
soundings depict enough low-level saturation and lift to keep
drizzle mention going. With the widespread cloud cover moving back
in, temperatures will be tricky. They will likely reach their mins
this evening before the cloud cover arrives, then remain steady or
even rise a bit overnight. Southern KY will see the clouds first so
lows in the upper 40s/lower 50s seem likely there, with low to mid
40s across central KY/southern IN.
These lower clouds look as if they may scour out a bit by Tuesday
afternoon as the main wave of isentropic ascent pushes to the north,
especially across southern and eastern KY. Given the way
temperatures have overachieved the past several days, will continue
to go on the higher end of guidance. However, low-level moisture
will be on the increase thus solar insolation may not be as
efficient in warming us up as it has been the past few days. Will
go just a degree or two shy of the very warm MET guidance, which
puts highs in the upper 50s north and middle 60s south.
.LONG TERM (Tuesday Night through Sunday)...
Issued at 315 AM EST Mon Dec 2 2013
Low-confidence forecast for most of the upcoming week as the one
sure bet seems to be that it will be an unsettled weather pattern.
Flat upper ridge developing over the Gulf of Mexico will direct an
active southern stream our way, and allow a parade of systems to
affect the Ohio Valley.
Tuesday night and Wednesday will be unseasonably mild in the
southerly flow ahead of a northern stream system cranking up
somewhere over the northern Plains. This southerly flow will be just
juicy enough to warrant a 20-30 POP, mainly west of I-65. Biggest
question is just how warm we get on Wednesday, with a 10-15 degree
difference between the GFS MOS and the much warmer NAM MOS. Guidance
has not been warm enough lately, but will split this difference out
of respect for cloud cover and possible precip. That will still be
good enough to push highs well into the 60s.
GFS is much faster with the cold front than the NAM or ECMWF, and we
have largely disregarded its timing on the front end of this system.
Will keep POPs in the high-end chance category Wednesday night, as
that is the best shot at cold frontal precip, but not a slam-dunk if
the GFS solution works out, blasting the front through. Will see a
sharp temp gradient with the front, with a 15 degree spread from
southern Indiana to Lake Cumberland a pretty safe bet.
This front will then hang up to our south, with a series of southern
stream disturbances riding ENE along it. Will keep likely POPs
through Friday, with 2-3 inch QPF amounts still in play south of the
Western Kentucky and Bluegrass Parkways.
The other complicating factor will be precip type as ever colder
low-level air tries to push south into the Ohio Valley. Forecast
soundings are too marginal to forecast precip type with any
confidence, so will just go with a rain/snow mix generally west of
I-65 and north of the Bluegrass Parkway for Thursday night/Friday.
Best shot at a dry period will be Saturday and Saturday night, but
that will be unseasonably cold under the Canadian high pressure
center. Locations along and north of the Parkways will struggle to
crack the freezing mark, and if skies remain clear enough, lows in
the teens are quite possible over southern Indiana.
Next southern stream disturbance comes into play on Sunday. At this
point it certainly looks cold enough for precip type issues but will
not get too cute with it this far ahead of time, with so many other
disturbances yet to play out ahead of it.
.AVIATION (18Z TAF Update)...
Updated at 1216 PM EST Mon Dec 2 2013
Conditions will stay VFR for the remainder of the afternoon and
evening with variable cloudiness between Sct and Bkn for much of the
day. Any of the lower ceilings should range between 4-5 K feet.
Winds will be light and variable with any noticeable gradient out of
the S or SE.
The forecast is low confidence as we head into the late evening and
overnight. The main problem has the been model guidance handling of
low level moisture. It does appear that we will see an increase in
low level moisture after Midnight as weak ascent commences from SW
to NE. This is expected to lead to at least high MVFR ceilings,
although potential exists for lower ceilings and some visibility
restrictions in BR. Additionally, some patches of drizzle cannot be
ruled out although impacts would be minimal to non-existent.
A more noticeable southerly gradient up to around 10 mph will set up
on Tuesday and sky cover will likely vary between Sct-Bkn between