Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 170541
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1241 AM EST Tue Dec 17 2013
...Updated Aviation Discussion...
Issued at 958 PM EST Mon Dec 16 2013
Made a few updates to the overnight forecast. The clipper system to
our north continues to track across the Great Lakes, with clouds on
the increase across the forecast area. A few radar returns across
our northern CWA are tracking east-southeast and may contain a few
flurries. So, have added this to the forecast for a few hours,
tracking from southeast IN into the northern Blue Grass region of
KY. Otherwise, the surface front, as noted in the short-term
forecast discussion, will settle just south of the Ohio River
overnight. Low-level moisture below about 2Kft will accompany this
boundary. All signs point to low clouds building in around or
shortly after 4 AM EST and lingering through daybreak, coincident
with the front. Not expecting precip with this boundary given the
very shallow/thin moisture, but cannot rule out some very light,
patchy drizzle. The rest of the forecast remains on track.
.SHORT TERM (Now through Tuesday Night)...
Issued at 257 PM EST Mon Dec 16 2013
A tight temperature gradient, augmented by differences in snowpack,
currently lies across southern Indiana. At 2 pm, temperatures ranged
from 45 at Louisville to 24 degrees at Indianapolis. Low clouds and
light fog will continue right along a line from Dubois through
Jefferson County, Indiana.
Farther south, clear skies and mild temperatures will continue
through this evening, as light southwest winds continue.
The synoptic pattern across the CONUS features a broad longwave
500mb trough over the Great Lakes. Two weak shortwaves will move
across Indiana over the next couple of days. The first will push a
weak cold front south of the Ohio River this evening and overnight.
No precipitation is expected with no mid-level moisture. However,
winds will shift to the west overnight, and temperatures will lower
a few degrees from our lows earlier this morning.
The second front will move across during the day Tuesday. A tight
pressure gradient will develop during the afternoon as high
pressure builds south across Kansas. West winds will pick up
during the afternoon and peak around 15 mph with some gusts over
Clouds will increase by late this evening as cooler air filters
in. Expect that cloudy skies will continue through the morning
hours Tuesday, especially north of Interstate 64. Lows tonight
will range from the upper 20s north to the lower 30s towards
Tuesday will remain cooler than earlier this afternoon, even over
our southern counties, with highs ranging from the mid 30s to the
mid 40s. High pressure will build east over Tennessee Tuesday
night. Expect mostly clear skies and lows from the upper 20s to
the lower 30s.
.LONG TERM (Wednesday through Monday)...
Issued at 250 PM EST Mon Dec 16 2013
A couple of noticeable pattern changes are on the way for the
extended forecast. The first will come as the persistent upper
trough over our region flattens out and allows warmer temperatures
into the region. High temperatures have been below normal at SDF
since the snowfall of 12/6, and they should flip back above by
Wednesday with highs going up into the 60s possibly by Friday. This
warm air will come from a southerly flow that becomes established as
high pressure to our southwest makes it way east of the area
The next pattern change will come from an upper low currently along
the Alaskan coast. A piece of this energy will drop to the southern
California coast and stall for a bit. Despite apparent unity among
the deterministic models on this forecast, ensemble spreads still
show a bit of uncertainty. Still the rough location of this feature
will tilt the flow aloft over us to southwesterly, which should
increase our moisture. Combined with a cold front forecast to move
in Friday with a northern stream trough will mean a good shot at
That front still looks to stall somewhere in the area for the
weekend and with continued southwesterly flow will mean good rain
chances likely through the weekend. As with the agreement on what
the upper low will do Friday, the GFS/GEM/Euro also are in
general agreement with that low opening up and moving into the
southern Plains Sunday. In response a surface low should develop
along the stalled frontal boundary. As with any system this far out,
there will be timing/location differences for several model runs to
come on where this surface low tracks/how deep it gets. Thus we have
a low confidence forecast in temps and potentially precip type over
our northern forecast area.
Will keep in good rain chances through the weekend, and then leave
in lesser chances for Monday, with the upper trough passage.
Rainfall totals from Thursday night through Sunday night with this
package will range from 2.5-3.5 inches across the region. Depending
on how quick this rain comes at any one time, we may see some water
issues, and will keep that wording in our HWO. In addition, should
the deepening low track close to our area, we will have to watch for
severe weather potential as well. Stay tuned.
Issued at 300 PM EST Mon Dec 16 2013
A winter storm will move across the Ohio Valley this weekend. A cold
front will stall and a series low pressure systems will move along
the front. Several waves of precipitation will begin Thursday night
and continue through Sunday. In southern Indiana and Kentucky these
systems are forecast to bring a large amount of rain. This area is
recovering from heavy rain and snowmelt from the previous storm so
the rivers and streams will respond faster and rise higher than
during the previous storm. More specific forecasts will be released
as the event comes clearer.
.AVIATION (06Z TAF Update)...
Updated at 1240 AM EST Tue Dec 17 2013
An upper level disturbance along the Iowa/Missouri border will slide
to the southeast early this morning and cross northern Kentucky
around dawn. Radar returns with this feature have been rather
impressive across southern Iowa and northern Missouri over the past
few hours. However surface obs show no precipitation reaching the
ground. It appears that the hydrometeors are falling from clouds
primarily between 8000 and 12000 feet into a layer of very dry air
centered around 5000 feet AGL. Despite additional high RH at the
surface, the dry layer is deep enough to sublimate the snowflakes
falling from the higher clouds before they can make it all the way
So, will keep the TAFs dry even as this system moves in. However,
ceilings are very much in question. Ceilings AOB 800 feet and MVFR
vsbys have been prevalent across Illinois since yesterday. However,
this is primarily from increased low level moisture over the snow
pack. The presence of this moisture may be contaminating mesoscale
models` depictions of ceilings in Kentucky this morning in
association with the incoming wave. Satellite loops have shown some
slight southeastward advection of the low cloud field on WNW winds
behind a very weak surface boundary stretched out roughly along the
Ohio River. There is some concern that the incoming upper wave may
pick up some of this low level moisture as it moves above the cloud
field and eventually interacts with the surface boundary. So, will
continue with MVFR cigs at SDF/LEX for several hours as the upper
disturbance, surface boundary, and low level moisture interact. BWG
should be too far south and should stay VFR. Could see a few
flurries at SDF/LEX with the low clouds, but likely nothing
impactful so will leave out of the TAFs.
An upper trof will then swing through this afternoon. Will account
for this with a broken low end VFR stratocu ceiling. Again we could
see some very light precipitation with this feature but likely
Could see a few gusts to around 20 knots this afternoon out of the
Tonight skies will clear and winds will drop off as high pressure
moves in from the west.