Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 131740

1140 AM CST Sat Dec 13 2014

Issued at 1140 AM CST SAT DEC 13 2014

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


.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 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.


Issued at 1140 AM CST SAT DEC 13 2014

Low VFR ceilings cover the entire area late this morning, and
satellite imagery shows no signs of the overcast breaking. Would
not be surprised to see any location drop to MVFR levels through
the afternoon, but that seems most likely at KCGI. The latest
guidance continues to indicate at least IFR conditons with drizzle
and fog late tonight. Not sure how quickly or gradual ceilings
will lower through the night, but certainly should see MVFR
ceilings at all locations by 06Z. Some clearing or decent lifting
at KCGI and KPAH is possible in the last few hours of the
forecast, but conditions are not likely to improve at all through
midday in the northeast.




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