Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 181146
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
546 AM CST Mon Dec 18 2017

.UPDATE...
Issued at 543 AM CST Mon Dec 18 2017

For aviation section only.

&&

.SHORT TERM...(Today through Wednesday night)
Issued at 330 AM CST Mon Dec 18 2017

Areas of drizzle should should diminish this morning, but low
clouds will hang on much of the day as time height cross sections
indicate a fairly thick layer of moisture trapped beneath a low
level temperature inversion. Still, with a decent southwest flow,
temperatures should creep into the lower half of the 50s this
afternoon.

Another split flow southern stream low will move into the
southern plains later Tuesday and Tuesday night. This will help
to generate a surface low over the Arklatex region. The low is
now progged to trek a bit further north than yesterday`s model
runs. Therefore, needed to increase pops significantly over
southern portions of the forecast area Tuesday night into Wed.
The mid/upper feature looks to be dampening with time, but some
locations down near the TN border could stand to pick up a half
inch or so of rain before the system moves off to the east later
Wed and Wednesday night.

.LONG TERM...(Thursday through Sunday)
Issued at 330 AM CST Mon Dec 18 2017

The extended forecast period begins with low level ridging and warm
air advection in the wake of the departing upper level mesoscale low
that is forecast to move to the mid-Atlantic coast by 6 am CST (12z)
Thursday. Although temperature forecasts may vary somewhat, highs on
Thursday will be some 10-15 degrees above normal as increasing
southwesterly flow/warm air advection takes place into Thursday
night with the approach of a closed low through the Central Rockies.
This low is somewhat further north than previous model runs. The
ECMWF and the Canadian (more pronounced) 00z Monday model runs
develop distinct zone of isentropic lift (warm conveyor belt)
showers late Thursday night, with a hint of a pre-frontal surface
trough reflection across the WFO PAH forecast area.

The low over the intermountain region early Thursday is forecast by
most of the model guidance to elongate and become progressive along
the eastern half of the broad upper trough in the western 2/3rds of
the U.S.  The ECMWF remains progressive, but the Canadian shows some
suggestion of enhanced lift with the elongated trough becoming
slightly negative tilted with time. At this point, blended the
influence of the ECMWF/Canadian Guidance to support some moderate
rainfall (increased QPF (rainfall amounts) and a differential
mention of mainly elevated thunderstorms. There may be some timing
issues for the passage of the cold front into the WFO PAH forecast
afternoon and evening. This may play havoc with the temperature
forecast Friday and Friday night. However, between midnight and 6 am
Saturday morning, sufficiently sub-freezing air should move through
the WFO PAH forecast area to change any lingering precipitation
within the departing moisture layer within the peak dendritic
production zone to produce a brief shot of snow near the I-64
corridor in Illinois. Given the antecedent ground/road temperatures,
any snow should dissipate quickly.

The lead shortwave mentioned on on Thursday will set the stage to
broaden and lower the leading edge of the large wave trough across
the northern 1/2 of the U.S.

Tightening of the baroclinic zone and channeling of vorticity in the
southwest flow immediately south of the mean, positively tilted,
major trough axis will help to left moisture back into the WFO PAH
forecast area late Saturday night into Sunday morning. The ECMWF and
Canadian model both suggest that the shallow cold post-frontal
boundary layer will be cold enough to support a precipitation type
as snow early Sunday morning, before warming the layer to rain
during the day on Sunday as the baroclinic zone moves from southeast
to northwest.

The biggest challenge for snow potential will be the degree of lift
within the dendritic growth zone for enhanced snow production and
the ground/road level surface temperature time at or below freezing
Saturday into Sunday morning.  This will determine whether there
will be any travel impact at all. At this time, projected snowfall
is expected to be 1/2 inch or less.

By Sunday night into Monday morning (Christmas Day), forecast
enhancement and southeastward translation (advection) of vorticity
will continue to sharpen the baroclinic zone over the WFO PAH
forecast area, lowering temperatures aloft within the Arctic airmass
establishing itself across the area and increasing moisture
advection. Although there will be some timing and location
uncertainty, there may be the potential for minor snow accumulation
late Sunday night into Monday.

As mentioned in previous forecast discussions, there remains high
uncertainty in the position of the baroclinic zone, as well as
precipitation type going from the 24th into the 25th of December.
The signal with the passage of the cold front on Friday and the
morning wintry mix potential on Sunday is becoming more consistent.

Regardless, weather conditions for travel will be interesting
starting late Friday night and again into the latter half of the
holiday weekend. Stay tuned.

&&

.AVIATION...
Issued at 543 AM CST Mon Dec 18 2017

LIFR cigs and MVFR to IFR vsbys with fog/drizzle expected to
continue through 14-15z, then gradually rise to aoa 1 kft by late
morning. MVFR cigs likely to continue the rest of the day, before
expected clearing late.

&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...None.
MO...None.
IN...None.
KY...None.
&&

$$



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