Area Forecast Discussion
Issued by NWS Paducah, KY

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000
FXUS63 KPAH 131752
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
1252 PM CDT Sun Aug 13 2017

.UPDATE...
Issued at 1249 PM CDT Sun Aug 13 2017

For aviation section only.

&&

.SHORT TERM...(Today through Tuesday night)
Issued at 325 AM CDT Sun Aug 13 2017

Surface winds are forecast to veer more easterly today as an area
of high surface pressure, centered over the southern Great Lakes,
begins to move off to the east. Meanwhile, a minor ripple in the
slightly cyclonic mid level flow, indicated in some model
solutions, might result in measurable rainfall in the form of an
isolated shower or tstm near the AR state line in southeastern MO
during the day. The remainder of the PAH forecast area will
remain mild and dry, albeit with quite a bit of cloud cover in
the southern third of the region.

The models, including short and medium range, continue to show
some inconsistency in how far south shower and tstm activity will
be (north of the AR/TN state lines, or not), through Mon night.
Our region apparently will be just north of a more active flow
regime and deeper moisture. Basically, parts of southeastern MO
and western KY, mainly closer to the AR/TN state lines, could
receive a shower, or even a tstm, anytime through Mon night.

The large scale pattern will begin to change on Tue. In response
to a developing long wave trof in the western CONUS, mid level
heights are progged to start rising over us. Meanwhile, low level
moisture and instability will increase. This will be accompanied
by the passage of a warm front from southwest to northeast
sometime during the day, acting as a trigger for isolated to
scattered showers/tstms, mainly in the southern half of the
region. PoPs ramp up overnight for the entire region Tue night,
as 850 mb moist fetch increases just ahead of mid level shortwave
energy moving through the now southwesterly flow aloft.

There will be a noticeable increase in dewpoints by Mon afternoon,
to around 70, increasing a few degrees more by Tue, as temps
gradually ease upward through the short term period. Heat indices
are forecast to be in the middle 90s by then.

.LONG TERM...(Wednesday through Saturday)
Issued at 325 AM CDT Sun Aug 13 2017

The circulation that was ejected from the Alaskan/western Canadian
Province cyclonic vortex was over North Dakota this evening and is
forecast to rotate around the eastern Hudson Bay low in Canada. This
circulation/shortwave sharpens a trough axis along and east of the
WFO PAH forecast area early in the week, building a low amplitude
ridge in the intermountain/central and northern Plains.  Further to
the west another series of shortwaves from the Alaskan circulation
dive down and sharpen a trough in the Pacific northwest and northern
California.

The net effect is to move the WFO PAH forecast area from a zonal to
west-southwesterly flow aloft. This combined with enhanced southerly
moisture flow from the Gulf of Mexico late Tuesday into Wednesday.

The faster jet flow (60-80 knots at 250 millibar level) appears more
focused across northern MO through northern OH through Thursday,
impressing a surface front boundary in the ageostrophic flow near
the entrance region of the jet.  This should keep the highest
chances of rain further to the north, with chance probabilities of
showers and thunderstorms across the WFO PAH forecast area further
to the south in the gradient of lesser instability, shear, and
moisture convergence.

The baroclinic zone, albeit weaker (partly due to lesser deep layer
moisture convergence), gradually shifts southward Friday and
Saturday.

At the same time, a coastal southeast U.S. (west to east)
oriented ridge axis retrogrades westward, pushing the WFO PAH
forecast area into northwesterly flow, with building heights to
the west and southwest. The ridge builds slowly toward the WFO PAH
forecast area, with heights building into the intermountain ridge
of the U.S and south Central Canada. The shortwave energy is
diverted from the eastern Canada low and the Alaskan/western
Canadian Provinces and away from a trajectory that would move
shortwave energy into the WFO PAH forecast area.

With the aforementioned ridge building in mind, kept the WFO PAH
forecast area dry after Saturday night.

The influx of moisture Wednesday and Thursday combined with the heat
will likely generate a return of afternoon heat index values on
those days, generally between 2 pm and 6 pm CDT, especially outside
rain areas.

...2017 Solar Eclipse Weather Outlook #2 for WFO PAH Forecast
Area...

The following information is based on output from the 00z Sunday
August 13th deterministic runs of the GFS, ECMWF, and Canadian
(CMCnh) numerical guidance at 1 pm CDT on August 21st.

The GFS continues to be consistent with the forecast of
precipitation across the WFO PAH forecast area. This model is the
slowest of the aforementioned guidance in building a ridge and/or
diminishing moisture convergence into the area. In fact, it still
keeps the local area in zonal flow with the passage of minor
impulses with overall 70-90% relative humidity over the entire WFO
PAH forecast area. In this model scenario...opaque cloud cover
and rain would make viewing of the eclipse very poor.

On the other hand, the ECMWF and Canadian guidance have varying
degrees of amplification of a westward tilted ridge to the southwest
of the WFO PAH forecast area, with no significant shortwaves
impinging on the area along the eastern limb of the ridge. The mean
1000-500 mb moisture ranges between 40-50% for both models.

The Canadian guidance has been more variable than the GFS or ECMWF
guidance.  However, from an initialization perspective, the ECMWF
and Canadian have been better than the GFS.  This forecaster
preference is for the drier/less cloudy solution of the
ECMWF/Canadian guidance. However, we are slightly over a week away
from the event and mesoscale influences hours ahead of the event may
still cause havoc. Stay tuned.

&&

.AVIATION...
Issued at 645 AM CDT Sun Aug 13 2017

As high surface pressure retreats to the east, surface winds less
than 7 knots will veer to the east today, then go calm or near calm
overnight. VFR cloud cover will be most common.

&&

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

$$

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



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