Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 210236
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
936 PM EST Fri Dec 20 2013
Issued at 935 PM EST Fri Dec 20 2013
Rain showers have been fairly scattered across the northern forecast
area, with little to nothing across the south this evening. That
will change as midnight approaches. Showers are filling in to our
north, west and even across central Tennessee. The showers across
southern Illinois, southeast Missouri and eastern Arkansas will be
the steady precip that heads in our direction overnight as
isentropic lift increases. There have been a few lightning strikes
through that region as well. Given the sounding profile that will
advect into our area, anticipate a few rumbles of thunder overnight
associated with some elevated instability. The forecast handled all
this quite well. Only made a few minor adjustments to temperatures
in the near-term period as readings remain a couple degrees either
side of 60. So far, SDF has a low temperature of 52 degrees today
and I find it very hard for them to drop below that value from now
through midnight. So, this will likely tie the record warm minimum
of 52 degrees last set in 1988.
.SHORT TERM (Now through Saturday Night)...
Issued at 305 PM EST Fri Dec 20 2013
Upper pattern today features a deep western CONUS trough with a
nearly closed low over the Sonoran Desert, and flat ridging over the
southeastern CONUS. First wave in the WSW flow produced a smattering
of rain showers over the Ohio Valley this morning, which have
largely waned this afternoon. Elongated surface low from just north
of STL to near SBN marks a strong baroclinic zone, with temps in the
30s just to the north, and 50s and 60s just to the south.
The upper low over the Desert Southwest will open up tonight and
lift NE on Saturday, with surface waves developing over the southern
Plains and riding NE along the sharp baroclinic zone. Strong and
dynamic southerly flow ahead of this system will draw an
unseasonably moist air mass into the Ohio Valley, with PWATs
increasing to 1.5 to 1.8 inches, challenging if not shattering the
highest observed values on record for the month of December.
Expect the current lull to continue into this evening, but the
boundary over IL/IN will edge southeast behind the first departing
wave, and POPs will ramp up substantially after midnight as the next
wave moves NE along the front, and the nocturnal low-level jet
cranks up. Will lower POPs to start, then ramp up to likely or
categorical after midnight, with tonight`s QPF tapering from a half
inch over southern Indiana to a tenth or less near Lake Cumberland.
Based on the overnight temp forecast, Saturday`s record high minimum
temps could be in jeopardy. Record warm minimums for December 21 are:
Bowling Green: 58 1934
Louisville: 58 1949
Lexington: 56 1923
Frankfort: 54 1923
It is quite possible that Saturday will not be an all-day washout,
especially across Kentucky, as clusters of showers move northeast
along the front. Our forecast leans heavily on the GFS solution,
which keeps the axis of heaviest rain across our southern Indiana
counties. If the NAM works out, the heavy rain threat will be
farther north in Indiana, and a good chunk of the day would be dry
south of the Bluegrass Parkway.
Max temp forecast Saturday will be highly dependent on cloud cover
and rainfall. If the front gets too far into southern Indiana, highs
near 60 in Dubois County will be too warm, while the NAM solution
could allow a lot of 70s across central Kentucky. This forecast
undercuts model guidance by a few degrees, especially along and west
For reference, record highs are:
Bowling Green: 71 1967
Louisville: 69 1967
Lexington: 71 1967
Frankfort: 70 1949
Most active weather will be Saturday night. Winds will really crank
up on Saturday afternoon, with sustained 20-30 mph and 40+ mph gusts
quite possible anywhere in the warm sector. Will post a Wind
Advisory at this time, and at this point will just leave out our
westernmost Indiana counties which will be closer to the front.
Nearly steady temps in the 60s overnight will keep us just unstable
enough that severe weather is on the table. SPC has extended the
slight risk well northeast into the Ohio Valley, with damaging winds
as the primary threat. The NAM is the most aggressive with the
instability, and points to the main severe potential (albeit a
marginal one) just either side of midnight.
Total QPF is around 3 inches over southern Indiana, and tapers
quickly to 1.5 inches or less east of I-65 and south of the
Bluegrass Parkway. Still not inclined to alter the Flood Watch, but
confidence is decreasing that the southern counties will see
.LONG TERM (Sunday through Friday)...
Issued at 255 PM EST Fri Dec 20 2013
The long term will start out with the low pressure system moving
quickly from the Ohio Valley to the northeast. A dry slot will build
in across the region bringing an end to the bulk of the rain through
the morning hours. As the low moves off to the northeast it will
drag a cold front through Sunday as well. With the front moving
through, temperatures will follow a non-diurnal trend on Sunday.
Highs will likely be early in the morning in the mid 50s to lower
60s, with temps then holding steady or dropping through the day.
They will continue to drop through the night into the upper 20s to
low 30s on Monday morning. Winds on Sunday will be breezy with gusts
of 25 to 30 mph out of the west.
High pressure will then build across the area bringing dry
conditions through Christmas. Temperatures Monday and Tuesday will
be quite chilly with highs in the 30s and lows in the teens to 20s.
We will see a slight warm up on Christmas with highs into the lower
A few showers will be possible on Thursday as a front swings across
the area. Temperatures will drop back into the teens to lower 20s
once again Thursday night in the wake of this front.
.AVIATION (00Z TAF Update)...
Updated at 630 PM EST Fri Dec 20 2013
Moist and gusty south-southwest flow will continue through the TAF
period. Winds will remain breezy overnight and ramp up to gusts
around 30 knots by Saturday afternoon. Scattered showers through the
evening will give way to steady rain between 07Z-09Z. Will continue
VCSH for the scattered evening showers, as any of this activity will
generally be light enough not to further restrict ceiling or vis.
However, once the steady rain arrives, ceilings and vis will be
reduced. Expect the worst conditions to occur at BWG and SDF, where
conditions could go IFR before daybreak and last through about
midday Saturday. LEX should stay fuel-alternate MVFR.
Guidance continues to hint at a dry slot getting into Kentucky
around midday Saturday. Still not sure on the northward and westward
extent of this dry slot. For now will indicate some improvement at
all sites, with a loss of steady showers and a slight improvement in
cigs and vis, although still remaining fuel-alternate MVFR. The
final round of precip will move across SDF during the planning
period. Will not include in TAFs just yet due to timing
uncertainties, but reduced cigs and vis will return.
KY...FLOOD WATCH from Saturday afternoon through Sunday afternoon FOR
WIND ADVISORY from Noon EST /11 AM CST/ Saturday to 4 AM EST /3
AM CST/ Sunday FOR KYZ024>043-045>049-053>057-061>067-
IN...FLOOD WATCH from Saturday afternoon through Sunday afternoon FOR
WIND ADVISORY from Noon Saturday to 4 AM EST Sunday FOR INZ078-