Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 211737 AAA

1137 AM CST Sun Dec 21 2014

Issued at 1137 AM CST SUN DEC 21 2014

The AVIATION section has been updated for the 18Z TAF issuance.


.SHORT TERM...(Today through Tuesday night)
Issued at 332 AM CST SUN DEC 21 2014

Strong jet energy coming into the Pacific Northwest early this
morning will quickly form a occluded storm system over the
northern Plains and upper Midwest by 12Z Monday. This system will
then wobble eastward while more energy drops southward along the
Rockies through Monday night. This energy will bottom out along
the Rio Grande in Texas, carving out an intense full latitude
trough by 00Z Wednesday.

While the 00Z GFS, ECMWF and NAM all show this same evolution
there are differences in the finer details. With multiple systems
within the larger-scale trough, it is unrealistic to expect the
models to be locked in at this point. Interactions between the 2
systems will have a significant impact on the cold frontal
movement, which will be the focus for best PoPs in our region.

With the original occluded system, the cold front will be ill-
defined, and really not much of a factor through Monday night. It
should sharpen up, as low pressure rides northeast toward the area
Tuesday, once the secondary jet energy bottoms out. We should have
general warm, moist advection in the low-levels beginning late
tonight and continuing until the cold front moves through the
region presumably sometime Tuesday.

It looks like light rain or drizzle with a saturated near surface
layer will be the primary precipitation mode Monday and Monday
night. Chance PoPs on Monday may be more realistic for Monday
night, too, compared to the likelies that are forecast. With the
delayed frontal passage, there will be more time for moisture to
increase across the area, so increased PoPs to categorical levels
in the east Tuesday.

The 00Z NAM actually develops an environment supportive of QLCS-type
tornadic activity, according to a local research project, mainly
over west Kentucky very late Monday night and into Tuesday.
Precipitable water values around 1.25" are near the 99th
percentile with respect to climatology, and 0-1km and 0-6km bulk
shear values of 40kts and 65kts, respectively, would be
supportive, if elevated instability can develop. The 00Z NAM did
have 200-300J/kg of MUCAPE which could be enough given the shear.

This is one run of one model, and others are not as moist or
unstable, so there is no need to get too excited at this time. We
just need be conscious of the possibilities, as it approaches. For
now will add thunder to the eastern third of the area Tuesday.

.LONG TERM...(Tuesday Night through Saturday)
Issued at 332 AM CST SUN DEC 21 2014

The primary feature of interest in the long term period is still the
storm system that will exit our region on Christmas Eve. While all
models agree that colder air will arrive Tuesday night and
Wednesday, they differ on the timing of the ending of precipitation.
If the precipitation lingers into Wednesday evening as the 00z gfs
indicates, there is a small potential for minor snow accumulations
on Christmas Eve. This appears unlikely, but there has been too much
model variability to rule it out.

The differences in the models revolve around their handling of
individual 500 mb shortwaves rotating through the high-amplitude
trough. The deep trough will extend from the western Great Lakes to
northeast Mexico Tuesday night. Various past runs of the gfs have
separated out two shortwaves within the large-scale trough. The
shortwave of interest digs south into Texas Tuesday night, then
lifts northeast across the Lower Ohio Valley on Christmas Eve. The
gfs develops a secondary area of precipitation with this mid-level
low, which lingers long enough for a changeover to wet snow before
ending Wednesday afternoon/evening.

The 00z ecmwf does not show the 500 mb shortwave indicated by the
gfs. The result is a faster end to the precipitation and faster
arrival of cold air late Tuesday night and Wednesday morning, but no
measurable snow. The gfs is slower with the arrival of colder air in
the lower levels. Even if precipitation lingers as long as the gfs
indicates, the changeover to snow across most of the area would be
too late for any appreciable accumulation. The possible exception
would be in far northwest counties along and northwest of a kcgi to
kmvn line, mainly on Wednesday. The 00z nam does seem to support the
00z gfs.

All the precipitation will exit our region Wednesday night, followed
by high pressure Christmas morning. The high will move quickly east,
causing winds to become southerly on Christmas afternoon and night.
Temperatures will recover into the 40s on Christmas after a cold
start to the day. The ecmwf has been indicating highs in the 50s on
Christmas, but it is a warmer outlier.

Another cold front will move southeast across the Lower Ohio Valley
on Friday afternoon and evening. This front will be starved for
moisture, and upper-level troughing will be much weaker with this
system. Therefore, pops and qpf will be much lower with this system.
Only slight chance pops are forecast in parts of southern IL and
southeast MO.

Colder high pressure will arrive on Saturday, with highs struggling
to reach 40.


Issued at 1137 AM CST SUN DEC 21 2014

MVFR cigs at KCGI should lift to the north by 00-01Z, otherwise
as a system moves out of the plains, generally speaking,
cigs/vsbys will deteriorate with time from west to east through
the rest of the period. MVFR cigs/vsbys and -RA expected to make
it into KCGI/KPAH around 08-09Z and into KEVV/KOWB around 10-11Z.
IFR cigs and/or vsbys possible at KCGI/KPAH toward the end of the
period. Variable winds AOB 5 knots through the first half of the
period will gradually increase out of the southeast AOB 10 knots.




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