Area Forecast Discussion
Issued by NWS Paducah, KY

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115
FXUS63 KPAH 181124 CCA
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
623 AM CDT Fri Aug 18 2017

.SHORT TERM...(Today through Sunday night)
Issued at 119 AM CDT Fri Aug 18 2017

A weak surface ridge and dry airmass across the region today will
result in mostly sunny skies with slightly lower humidity. Upper
level energy will dive SE tonight from the Upper Midwest to the
Ohio Valley region, with a weak surface reflection moving into the
area as well. Marginal moisture and some instability warrants
slight chance PoPs moving in from the west /northwest late
tonight, shifting to southern sections of the area by Saturday
afternoon. Again only slight chances given marginal overall
moisture (the theme recently). Will have a dry forecast Saturday
night through Sunday night. However on Sunday, hints of weak
return flow and marginal moisture means isolated convection cannot
be ruled out completely. Confidence in coverage and occurrence
too low to include in the forecast. Temps will be a blend of
persistence and MOS. Wx elements a straight up model blend given
low confidence in any one solution.

.LONG TERM...(Monday through Thursday)
Issued at 335 am CDT Fri Aug 18 2017

The extended forecast period begins with the passage of the solar
eclipse through the WFO PAH forecast area between 16z-19z Monday.
Will address specifics with regard to the eclipse in a separate
section below.

The GFS and GFS Ensemble, followed by the Canadian and ECMWF are
all continuing the trend of eroding the western edge of the ridge
into the WFO PAH forecast area, as a series of waves continue to
be ejected from the coastal southern California low. To make
things more interesting, the GFS hinted at a low to mid level
trough extending from the western Gulf of Mexico and arching into
MS/AL in yesterday`s runs. The 00z deterministic GFS and the GFS
Ensemble continue to display this feature, now developing a low in
the northwest Gulf of Mexico. The 00z ECMWF and 00z Canadian
present more widespread and disorganized convection in the Gulf
of Mexico.

The aforementioned projected development of a low or general
disturbances in the Gulf of Mexico may impact the clean trajectory
of moisture from the Gulf into the lower and middle Mississippi
Valley to support deep layer convection early on in the forecast
period.

However, as the influence of the closed low off the California coast
wanes, a ridge builds into the western U.S. and the elongated
eastern Canada low begins to retrograde and dive progressively
southward into the area very late Monday night into Tuesday into
Wednesday. The general moisture trajectory from the Gulf of Mexico
may be sufficient in time to phase with the dynamic shortwave
driving down from the upper Midwest on Tuesday. Given the more
impressive thickness gradient, could see some robust thunderstorm
activity (severe?) late Tuesday and Wednesday.

Beyond Wednesday, the western U.S. ridge builds into the area,
inhibiting convection and lowering max/min temperatures some 3-7
degrees below normal for late August.

Have 50-60% forecast confidence on the timing and intensity of the
cold frontal passage late Tuesday into Wednesday and the reduction
of temperatures in the middle and latter part of next week.

...Friday morning discussion on Solar Eclipse Day for the WFO PAH
forecast area...

Although there is still quite a bit of uncertainty on the mesoscale
and storm scale level activity for Monday, the trend from the
GFS/European/Canadian/NAM-WRF family of guidance is that that the
influence of any Mesoscale Convective System (MCS) existing over
KS/OK/AR/MO area may be waning during the morning hours on Monday.
The trajectory of any mid-upper level cloud cover would likely have
to come from any potential MCS from the west of the WFO PAH forecast
area. The benefit of any morning clouds would help slow the reaching
of convective temperatures for the development of more widespread
lower level cumulus cloud fields.

All in all, the cloud forecast much better than projected by the GFS
last weekend and early this week. The ECMWF has been  consistently
drier than the GFS and Canadian Guidance.

Temperatures during the three hour period of the eclipse transition
will range from 85 to 91 degrees, with a relative minimum along the
central axis of the 100% totality. Of some concern will be heat
index values, which will spike into the middle and upper 90s around
3 pm CDT, after the eclipse.

For travel, any shower or thunderstorm activity should now remain
minimal, except over southeast Missouri. Depending on the origin
point and evolution of an MCS in the middle and upper Mississippi
Valley Monday night, most persons remaining in the area should be
ok. Travel may be tricky on Tuesday, as showers and thunderstorm are
forecast to work southward with coverage expected to peak Tuesday
afternoon and overnight.

Again, there is still a number of smaller scale weather phenomena
which may impact the forecast between now and Monday morning. Stay
tuned.

&&

.AVIATION...
Issued at 623 AM CDT Fri Aug 18 2017

VFR conditions today through tonight. West winds below 10 kts today
will gradually back to SSW and become light by tonight.
Will monitor the slight chance of convection into southeast MO and
southern IL toward daybreak Saturday.

&&

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

$$

LONG TERM...Smith



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