Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 171750

Area Forecast Discussion
National Weather Service Paducah KY
1250 PM CDT Thu Aug 17 2017

Issued at 1250 PM CDT Thu Aug 17 2017

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

UPDATE Issued at 1039 AM CDT Thu Aug 17 2017

The convective system has mostly dissipated with only scattered
mainly light showers streaming northeast across southeast Missouri
into southwest Illinois. With plentiful cloud cover over much of
the area, temperatures are running a bit behind forecast warming.
Expect the cloud cover to diminish in the west enough to allow
most of that area to catch up to the high temperature forecast. In
the east, the opposite is expected, as temperatures approach 90 in
the next hour or two over Pennyrile, look for scattered convection
to develop and knock temperatures down.

12Z NAM soundings over west Kentucky for this afternoon indicate
that scattered convection is a decent bet, but lapse rates are not
that steep. In addition, the HRRR output keeps convective
development at widely scattered levels, mainly over west
Kentucky. Cannot rule out a strong to severe storm with damaging
winds, but locally heavy rainfall and lightning will be the
primary concerns this afternoon, primarily over the Pennyrile.

15Z observed winds are right near Lake Wind Advisory criteria over
the southeast half of the region, but the short term guidance
keeps them right at or just below criteria. With the cloud cover
and convective potential do not plan on issuing an Advisory, but
will keep an eye on observational trends.

In this same area, dewpoints are holding in the mid to upper 70s.
Where temperatures can reach the lower 90s, heat indices may
touch 100 in a few locations, most likely near the Tennessee
border. Given the limited area and duration expected, will not
mention in the HWO at this time.


.SHORT TERM...(Today through Saturday night)
Issued at 328 AM CDT Thu Aug 17 2017

Band of convection forecast to move across the area from west to
east through the day, departing our east counties this evening.
Will use likely category PoPs early this a.m. into SEMO, southern
IL, then scattered terminology with isolated storms rest of the
time, given the trend around here lately is for convection overall
to be somewhat underwhelming. Will monitor for a couple of
stronger storm events. Right now, parameters not looking too
favorable though. There is some modest MLCAPE forecast. But
1000-850mb lapse rates, and DCAPE values are marginal. Mid level
lapse rates are not impressive, and thermal profiles suggests hail
threat minimal to none. Dry after midnight tonight, and on
through Friday and Friday night with weak high pressure moving
across the area. Saturday, kept PoPs slight chance for convection,
as energy aloft moves across the area from NW to SE. Dry Saturday
night, though may have to monitor southern sections in the
evening for a slight chance.

.LONG TERM...(Sunday through Wednesday)
Issued at 328 AM CDT Thu Aug 17 2017

From the regionally blended model initialization, some of the
emphasis of the PoPs seems to reflect the influence of the Canadian,
more so than the GFS/GFS Ensemble and European (ECMWF).

The retrograding west to east oriented high pressure over the
southern Plains and southeastern U.S. early Friday appears to shift
east with time Sunday through Tuesday until a broad shortwave in
Canada, sharpens over the WFO PAH forecast area later on Tuesday.
This places the WFO PAH forecast area into a strong northwest flow
through the middle of the week.

At this point, only have medium confidence in PoP/Weather forecast,
mainly due to the timing, degree, and trajectory of deep layer
moisture into the WFO PAH forecast area. The Canadian is the
wettest, followed by the ECMWF, then the GFS/GFS Ensemble.

Any convective activity during the Sunday through Tuesday time
period across the WFO PAH forecast area will be diurnal in
character, with the mid-week convective activity dynamically tied to
the passage of the shortwaves in the northwest flow over the area.

...Discussion for Solar Eclipse - WFO PAH Forecast Area...

The model trends of the GFS/GFS Ensemble/ECMWF/Canadian all seem to
place the WFO PAH forecast area on the western end of the low and
middle level ridge axis, with a minor vorticity lobe moving through
Missouri around 18z (1 pm CDT) Monday. The trend suggests some form
of an MCS developing over KS and OK overnight, then moving east
through southern Missouri during the morning.  Depending on the
maturity and initiation time of the MCS, there may still be
lingering showers and a few thunderstorms Monday morning, especially
over southeast Missouri. The question will arise on how much
convection will be maintained into the early afternoon. Right now,
the current forecast package will have greater than 25% rain chances
along a line from Mayfield and Paducah Kentucky, onward to Eddyville
and Pinckneyville Illinois. Given the low PoP through the remainder
of Southeast Missouri, we may only see pockets of showers and
thunderstorms. However, given the flow, this could still spread a
measurable opaque cirrus cloud deck downstream across the rest of
the WFO PAH forecast area. With the cloud cover and precipitation
potential, kept temperatures in the middle 80s through the partial
and total eclipse time periods.

There is still quite a bit of wiggle room, especially on the
mesoscale, so forecast confidence in this scenario is 50/50 now.


Issued at 1250 PM CDT Thu Aug 17 2017

Convection this afternoon will be limited mainly to west Kentucky.
KPAH and KOWB could be impacted in the first few hours of the
forecast, otherwise, there is little support for development with
the cold front tonight, so the remainder of the forecasts are dry.

Southwest winds will continue to gust up to 20kts, especially at
KEVV and KOWB for much of the afternoon. There will not be much of
a push with frontal passage tonight, so some fog development is
possible, especially at KCGI. A light northwest wind will pick up
in the morning well under 10kts.




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