Scientific Forecaster Discussion
fxus63 klmk 170555
Area forecast discussion
National Weather Service Louisville Kentucky
1255 am EST Wed Jan 17 2018
..updated aviation discussion...
updated 1040 PM EST Tue Jan 16 2018
Updated the forecast to lower temps overnight. Portions of south
central Kentucky that have been cloud free after sunset have already
dropped below zero. However, low level clouds are on the way and
should limit temps from dropping too much after their arrival. Low
temps over the region should range from -5 to +4 tonight.
Flurries and snow showers are present in the low clouds and are
expected to overspread the area overnight into tomorrow morning.
Did lower winds a bit overnight based on the latest observations and
model data. This should still yield wind chills in the -10 to 0
range tonight (when there is a little wind overnight).
Given the above factors, the Special Weather Statement in place for sub-zero wind chills,
light snow showers, and potentially icy conditions on roadways looks
good so will continue it through tomorrow morning.
Updated 410 PM EST Tue Jan 16 2018
Updated products to expire the last portion of the Winter Storm
Warning as accumulating snow has moved east of the area. Flurries
and light snow showers may still occur tonight. Also roadways may
remain hazardous through the overnight hours.
Short term...(this evening through Wednesday night)
issued at 252 PM EST Tue Jan 16 2018
The snow is nearly out of the forecast area this afternoon and
should be out of here by 21z or shortly thereafter. The current
headlines expire at 21z and at this time it looks like we should be
able to let them go by then.
Skies have been clearing across the region in the wake of the snow
this afternoon. However, satellite and observations show a stratus
deck beginning to sink in from the north across in. These clouds
will spread across the area late this afternoon into this evening.
Looking at soundings and time/height sections, the low level
moisture tonight will stretch into the dendritic growth zone. This
will result in flurries and/or snow showers from this evening into
the overnight hours. Some light snow accumulations up to a few
tenths of an inch will be possible in areas that do see the snow
showers. Snow and flurries should taper off through the morning
tomorrow with dry conditions expected into tomorrow night.
Temperatures tonight will fall back into the single digits. Winds
will be much lighter than they were last night, but will be enough
that wind chill values will fall into the 0 to -10 degree range.
Highs tomorrow will be warmer than today, but still top out in the
lower 20s at the most. Lows tomorrow night will once again fall back
to the single digits again.
Given the potential for some light snow accumulations tonight and
the continued very temperatures and wind chills, will be issuing an
Special Weather Statement this afternoon. Roads may worsen overnight as well as the
sunshine has likely allowed some of the snow to melt and this could
refreeze as the sun GOES down.
Long term...(thursday through tuesday)
issued at 303 PM EST Tue Jan 16 2018
Main theme for the long term will be a warming trend as surface high
pressure slowly slips off to the southeastern US and warm, southerly
flow returns to the lower Ohio Valley. Temperature forecasts will be
tricky during the long term period as we're not anticipating getting
above the freezing mark until Thursday, and the deep snowpack will
act to deflect much of the sun's energy until it is melted down.
Will knock temperatures down a few degrees from what most guidance
has for Thursday and Friday to account for this potential.
Lee cyclogenesis will begin Saturday across the Central Plains in
advance of a strong shortwave trough. Meanwhile, across the lower
Ohio Valley, models show warm air advection and weak isentropic lift along
295k/300k surfaces starting Saturday ahead of a warm front. This
will result in increased cloudiness as well as some isolated showers
across portions of the region.
The trough and associated surface low will deepen and begin to wrap
up on Sunday over the plains and Midwest. Long range models show a
nice "comma head" signature associated with a mature midlatitude
cyclone. The surface low will track off to the northeast and take
aim toward the Great Lakes region, leaving our region well within
the warm sector.
By late Sunday night, a cold front with the aforementioned surface
low will be knocking on the front door of our western-most counties.
Models indicate a healthy line of precipitation along the front, and
while there are some timing differences, it looks like the best
chance for precipitation will be Sunday after midnight for western
portions of the cwa, with it ending Monday afternoon near the
Bluegrass/Lake Cumberland Plateau areas. While deep layer shear will
be strong with this system, models show hardly any MUCAPE, and this
should limit any severe weather potential. Given the strong forcing,
can't rule out a some isolated thunderstorms, though confidence to
include them this far out is too low.
Unlike the past few cold fronts the lower Ohio Valley has had to
deal with, the air mass behind the fropa Monday will not be of
Arctic origins. High temperatures Monday and beyond will generally
range in the upper 40s to low 50s.
Aviation...(06z taf issuance)
issued at 1255 am EST Wed Jan 17 2018
A period of MVFR is likely overnight and Wednesday morning, with a
broad area of stratus over the lower Ohio Valley. Flurries are also
ongoing and are expected to continue, and an occasional light snow
shower is possible as well. The highest/deepest low-level relative humidity looks
to be around 12z at sdf and perhaps between sdf/Lex from 12-15z, so
we may see some light snow showers around that time.
As surface ridging slowly builds east tomorrow and the low-level
inversion weakens, MVFR clouds are expected to scatter out from west
to east. Hnb will see an improvement first, with prevailing VFR
after 12z. Right now, have sdf/bwg scattering out by mid to late
morning, with Lex improving during the afternoon. Confidence is only
medium on this timing, however.