Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 140536
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
1236 AM CDT Fri Jul 14 2017

.UPDATE...
ISSUED AT 1231 AM CDT FRI JUL 14 2017

Update for 06z TAFs for aviation section.

&&

.SHORT TERM...(Tonight through Saturday night)
Issued at 433 PM CDT Thu Jul 13 2017

Most of the morning shower and thunderstorm activity was forming
within southwest to northeast axes of higher theta-e air, combined
with subtle low to middle level shortwaves, aiding in overturning
of parcels. Given the low to mid level shear (<20 knots),
efficient transport of parcels for updraft/downdraft interactions
has been marginal, leading to outflow dominated storms.

Anticipate the pre-frontal convective activity over the northern
1/3 of MO and across central IL will continue to persist into the
evening hours, moving once again into the WFO PAH forecast area.
Diurnal convection, associated with differential heating from
earlier outflows from earlier storms, will continue to develop
this afternoon, especially in areas where the thermal cap is the
weakest (mainly over Southern Illinois). In the absence of the
approaching cold front and transient shortwaves traveling along
the slowly descending baroclinic zones, it will be hard to
maintain any convection across the southern 1/3 of the WFO PAH
forecast area this afternoon and early this evening. SPC`s
transition back to a Marginal Risk area (similar to their original
Day 3 outlook) may have been a prudent move.

Convection allowing models, as well as intermediate and medium
range model guidance, have had trouble resolving the effectiveness
of the thermal ridging aloft. This may be also due to variations
in model parametrization of precipitation schemes.

For now, have utilized blended guidance with an emphasis toward
the RAP and NAM-WRF model family guidance. While it is not the
most elegant solution for rain chances tonight into Friday, the
goal was to highlight the transition from north to south the
movement of rain chances through the WFO PAH forecast area
overnight into Friday. Until ridging builds in behind the
baroclinic zone Friday into Saturday, still need to leave a small
chance of precipitation in the area, mainly over southeast
Missouri.

For the rest of the sensible weather elements (temperature,
dewpoint, wind, etc...) started with a regional model
initialization, then leaned the gridded forecast output closer the
the NAM-WRF model suite.

.LONG TERM...(Sunday through Thursday)
Issued at 433 PM CDT Thu Jul 13 2017

The medium range models were in reasonable agreement in their mid
level flow pattern solutions in the extended forecast period, though
there were still smaller scale differences such as pcpn patterns and
the strength of the expanding ridge. We will start out with a
western ridge/eastern trof pattern initially.

Shortwave energy in the initially amplified pattern is progged to
swing through the Midwest, including the Great Lakes, during the
weekend and Monday. As this occurs, a weak, "backdoor" type surface
front is more or less progged to enter the PAH forecast area from
the northeast and eventually dissipate. We will continue to have a
warm, rather moist and unstable airmass in place, and this boundary
may act as enough of a trigger for a few showers or tstms to develop
north of I-64 on Sunday night, and across parts of the Evansville
Tri-State region Monday.

Further out in time, the aforementioned western ridge is progged to
expand eastward, with greater influence on the PAH forecast area.
Due to primarily the GFS pcpn fields, the initialization blend
contained "bullseyes" of PoPs, which seemed unlikely compared to the
CMC/ECMWF solutions and all that warm air aloft. In that spirit, we
collaborated with surrounding offices and WPC to go somewhat above
guidance with lows and highs next week. Heat indices of 100-105
should be common on Wed/Thu (Day 6/7) as deep moisture increases.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z Friday NIGHT)
ISSUED AT 1231 AM CDT FRI JUL 14 2017

Current thinking is the cluster of storms over KPAH will continue
to push south and weaken along with cluster near KPOF. Storms
south of KSTL should also weaken as they move southeast into
worked-over air. Should see a dramatic decrease in coverage
between 08-10z. Storms will likely re-fire around 18-19Z mainly
over the southern half of the area. Scattered variety makes it
difficult to put timing into KCGI/KPAH TAFs.

&&

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

$$

UPDATE...PS
SHORT TERM...Smith
LONG TERM...DB
AVIATION...GM/PS



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