Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 201739

Area Forecast Discussion
National Weather Service Paducah KY
1239 PM CDT Sun Aug 20 2017

Issued at 1239 PM CDT Sun Aug 20 2017

The outflow boundary from early morning convection over Missouri
is beginning to light up over southeast Missouri just north of our
area. Winds ahead of the boundary over most of our area are veered
to some extent which should limit convergence about it. There is
also a convergent boundary oriented north to south just west of
Van Buren and Doniphan MO. The strongest convection currently is
along the intersection of this boundary and the outflow boundary
over Reynolds county. Look for scattered to numerous storms over
most of southeast Missouri as the outflow continues to push south
this afternoon. Will keep slight chances going into southern
Illinois just in case a stray storm can get going near the
boundary there. Certainly cannot rule out a strong to severe storm
this afternoon over southeast Missouri, but it should be pulsy in

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

UPDATE Issued at 727 AM CDT Sun Aug 20 2017

Minor update during the past hour to reflect ongoing thunderstorm
activity over southwest Indiana.


.SHORT TERM...(Today through Tuesday night)
Issued at 355 AM CDT Sun Aug 20 2017

Subtle zone of instability within a narrow surface to mid-level
moisture plume, added by a mid-level shortwave has generated
shower activity over southern Illinois within the past hour.

Ironically, the 4km NAM-WRF (NMM version) height and mass fields
hinted at this feature, and keeps this moisture/instability plume
in place in a southwest to northeast fashion, bisecting the WFO
PAH forecast area. Had originally considered this instability axis
to be overdone, but the NAM-WRF families were suggesting greater
moisture at low levels along the ridge than the other guidance
suite. With that in mind, beefed up measurable rain chances for
this morning and afternoon across the WFO PAH forecast area.

Mid-level ridging and weak southwesterly flow build into the area
overnight and should diminish any remaining convection overnight

Will address the Monday forecast in the eclipse forecast section

The 12km NAM-WRF provides a reasonable presentation of the
channeled vorticity in the west-southwest flow ahead of a
progressive shortwave across Iowa and Wisconsin Monday evening
with a slowly decaying ridge influence over the WFO PAH forecast
area. The dynamic nature and narrow zone of instability/deep layer
moisture will keep rain chances close and just behind the main
frontal zone as it moves southeast during the day on Tuesday.
Increasing shear and instability late Tuesday afternoon and
evening should support robust updrafts within organized linear
convection to support some strong to severe thunderstorms. Was not
particularly surprised when SPC expanded a slight risked back to
the Land Between the Lakes to near Benton Illinois, with a
marginal risk extending back through the remainder of the forecast
area. Given the progressive nature of the line, most of the
significant convection should exit the area after midnight into
Wednesday morning.

For persons traveling away from the WFO PAH forecast area after
the eclipse, earlier travel may be preferred to avoid strong
convection late in the day.

...Sunday Morning forecast discussion for the Solar
Eclipse on August 21st...

The 4km NAM-WRF (NMM Version) suggests some weak lift and
instability within a zone of channeled varieties along and south
of the St. Louis metro area, working southward into Southeast
Missouri during the 18z-21z (1 pm- 4pm CDT) time frame on Monday.
The GFS, which is usually overdone with rainfall (QPF) generation,
was more robust with the addition of showers and thunderstorms
over southeast Missouri into extreme southwest Illinois. Given the
track record and variability of the GFS, have low confidence in
its solution.

However, will utilize the NAM-WRF guidance and keep a small chance
of showers and thunderstorms over southeast Missouri and possibly
into extreme southwest Illinois, but mainly after the peak eclipse
passage time.

Cu(cumulus)rule estimates suggest that most of the area will see
clear to scattered cloud cover, generally between 4-6kft agl in
level during the eclipse passage. The notable exception may be
from Ava Illinois, southwest to Perryville and Poplar Bluff
Missouri, generally between 1 pm and 2 pm CDT. Development of
isolated thunderstorms will likely generate a cirrus (high cloud)
canopy, which will slowly expand east-northeast after 3pm CDT.
Given the weak steering flow, the expansion of the cloud cover
should not adversely inhibit viewing of the eclipse for most of
the WFO PAH forecast area, with the possible exception of the
aforementioned area in southeast Missouri. This has been a concern
for several days now.

Of greater concern will be heat index values at noon and again
after 2 pm (outside the main eclipse passage). Heat index values
are expected to peak during the middle of the afternoon, with heat
index values 100-103 degrees expected over southeast Missouri and
southwest Illinois. Heat index values in the middle to upper 90s
will be the rule across the rest of the WFO PAH forecast area.
Will once again highlight with statements and possibly a graphic
of heat index values around 1pm or 3 pm CDT.

Cloud cover will gradually increase from the west and northwest
during the late afternoon on Monday. Any rain chances will
diminish in the west with sunset and not return to the I-64
corridor until daybreak on Tuesday.

.LONG TERM...(Wednesday through Saturday)
Issued at 355 AM CDT Sun Aug 20 2017

Wednesday will be the first day of the newly arriving Canadian air
mass. After any lingering showers end in southern Kentucky, drier
air should bring clearing skies in the afternoon. A fairly strong
surface high over the upper Mississippi Valley will produce a north
breeze. Highs will be mostly in the lower 80s.

Thursday and Friday, the core of the coolest low-level air will pass
across the Ohio Valley and Great Lakes. 850 mb temps will fall to
around 12C. Highs will struggle to reach 80 at most locations, and
will likely stay in the upper 70s along the I-64 corridor. Lows
Wednesday and Thursday nights will fall into the chilly 50s. In
fact, some locations may even challenge record low temperatures for
the day. The surface high will weaken and move off to the east next
weekend, allowing winds to turn more southerly and a slow warming
trend to begin.


Issued at 1239 PM CDT Sun Aug 20 2017

TSRA will be most numerous over southeast Missouri this afternoon,
but not sure if it will impact KCGI with anything more than an
outflow. Will monitor radar and amend when confidence increases.
An isolated TSRA will be possible throughout the area today, but
expected coverage is too sparse to mention.

Winds should go calm tonight, so we could see a repeat of this
morning`s fog, with the worst conditions at KCGI and KPAH. Some
isolated to scattered convection will be possible over southeast
Missouri by late morning Monday, but it should be to the west and
northwest of KCGI.




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