Area Forecast Discussion
Issued by NWS Paducah, KY

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000
FXUS63 KPAH 131249
AFDPAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PADUCAH KY
649 AM CST Sat Dec 13 2014

.MESOSCALE DISCUSSION...
Issued at 649 AM CST SAT DEC 13 2014

Given the short time resolution of this feature and it impact on
the WFO PAH forecast area, the forecast adjustment will be
primarily restricted to a gridded forecast update.

Within the last two hours, the KPAH radar returns showed a
developing zone of very light (generally 10 dBZ or less)
reflectivity banding across parts of extreme Southern Illinois and
adjacent parts of Southeast Missouri and the Purchase Area of West
Kentucky.

These returns have been been coincident with minor reductions,
albeit still VFR category, in visibility. Direct observations show
that this was indeed light drizzle.

Checking the 11-3.9 micron difference channel of GOES IR Channel,
as well as the GOES Sounder 7.0 and 7.4 micron Water Vapor
Channels, there is an evident southwest moving (around 16kts)
Shortwave moving toward the western limb of of a mid-level low
over Southern AR.

This area of light trace precipitation also corresponds with a
narrow 285-290K moisture and differential local pressure
derivatives (lift within the isentropic layer).

This feature has been hinted at by the 12km NAM-WRF during the
last 48 hours, but has been so subtle in time/space to consider as
a mention in the forecast package. However, given its presence,
adjusted the forecast grids to account for its movement across
extreme Southeast Missouri and the Purchase area of West Kentucky
through 9 am CST today.

&&

.SHORT TERM...(Today through Monday night)
Issued at 343 AM CST SAT DEC 13 2014

Two forecast challenges in the short term.

First concern will be the degree of lift and instability to
generate trace precipitation in the form of drizzle late tonight
into Sunday morning across a majority the WFO PAH forecast area.
Theta-e lapse rates within the 0-1km layer are almost super-
adiabatic to auto-convective in nature...so any parcel movement in
the saturated layer should generate at least some patchy drizzle.
There is also upward lift within the 285-290K layer, which should
maintain widespread lift across the area between midnight and noon
on Sunday.

The second concern is the degree of instability and lift as the
northwestward tilted low over Kansas rotates toward Lake Michigan
Monday night. Moisture associated with this low appears outrun the
main mid-upper level forcing and instability (both surface and
aloft). This may effectively diminish the coverage and intensity
of the rainfall Monday and Monday night. In addition, the vast
majority of the differential vorticity/thermal gradients will be
either with the upper low (northwest of the WFO PAH forecast area)
or much further south of the region. With that in mind, left out
any mention of thunderstorms. Cannot rule out an isolated
thunderstorms over part of Southeast Missouri or Southwest
Illinois, but will not mention in the gridded forecast.

When this feature was first introduced into the 10 day medium
range guidance, the instability was much greater. Amazing how
things change in a week.

Had to be more aggressive on the degree of cloud cover today,
since model guidance was leaning toward lesser opaque cloud cover
for Saturday.

.LONG TERM...(Tuesday through Friday)
Issued at 343 AM CST SAT DEC 13 2014

Confidence remains fairly low in the extended time frame. As was the
case yesterday, by mid week the models tend diverge with their
solutions. The 00z GFS wants to suppress the ridging over the plains
and into the Mississippi Valley Wednesday into Thursday. This would
allow Gulf moisture to flow up into the area as a surface low forms
out in the Texas Panhandle. The 00z ECMWF and Canadian runs indicate
a much stronger amplitude mid level ridge that holds the moisture to
its west over the plains until the long wave trough arrives late in
the week.

Out if respect for the GFS, will go with a least small precip
chances Wed night into Thursday. If it does arrive that early, there
may be enough low level cold air left over to support some light
snow or sleet, esp over southwest IN, srn IL and nw KY. It would
probably be warm enough for just rain there after.

&&

.AVIATION...
Issued at 649 AM CST SAT DEC 13 2014

Minor update to reflect temporary addition of VFR visibility
adjustments due to transitory drizzle associated with a passing
shortwave at the KCGI and KPAH TAF sites through at least 14z Saturday.

Kept persistance with previous TAF package, utilizing a long term
MVFR ceilings through at least 06z Sunday for each TAF site. Main
adjustment to was to add MVFR visibility with the onset of drizzle
after midnight. Was difficult to separate out any particular
period for a separate forecast group, given the uniformity of the
low level cloud deck through the forecast period.


&&

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

$$

MESOSCALE...Smith
SHORT TERM...Smith
LONG TERM...GM
AVIATION...Smith





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