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000
FXUS63 KPAH 141147
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
647 AM CDT Mon Aug 14 2017

.UPDATE...
Issued at 646 AM CDT Mon Aug 14 2017

Refinement of morning forecast just issued to reflect influence of
elevated warm frontal zone moving across southwest Kentucky and
extreme southeast Missouri at this time. Mid-level southwest to
northeast trough/weak deformation (roughly near 10-12kt MSL)
aiding in the light precipitation over parts of southeast Missouri
and southern Illinois at this time.

Blended 06z 12km NAM-WRF and ESRL HRRR guidance in depicting the
better chances for rain over the southern Purchase/Pennyrile
regions of west Kentucky through the morning hours. The trend
should be for most of the precipitation to remain along the west
Kentucky/Tennessee border.

Adjusted high temperatures and dewpoints to more closely reflect
the impacts of cloud cover and precipitation today.

12z TAF forecast provided below.

&&

.SHORT TERM...(Today through Wednesday night)
Issued at 409 AM CDT Mon Aug 14 2017

The main surface frontal boundary remains well south of the WFO PAH
forecast area this morning. However, the combination of lift from a
mid-level shortwave embedded in a broader zone of isentropic lift
will continue to support isolated showers further north into
forecast area early this morning.

As ridging builds in behind this shortwave during the day, the focus
of the lift will be sharpened over west Kentucky during the
afternoon and evening. As the westward tilted surface to upper level
ridge settles over the area tonight, boundary layer stabilization
combined with increased moisture (dewpoints) and variable cloud
cover will lead to the development of patchy fog and some stratus,
mainly over the southern Pennyrile of west Kentucky late tonight.

Most of the forecast area, east of the Mississippi River will
remain dry on Tuesday as the influence of the ridge and lack of deep
layer moisture/lift suppresses convection. On the western limb of
the ridge (eastern limb of a deeper trough in OK/TX) moves eastward
toward the area, scattered showers and isolated thunderstorms will
be possible over southeast Missouri and the Purchase area of west
Kentucky. during the day on Tuesday.

As this trough lifts and becomes more progressive into the middle
and upper Mississippi Valley Tuesday night and Wednesday, rain
chances will increase from the west and northwest over the WFO PAH
forecast area. There may be a sharper delineation on the leading
edge of the showers and thunderstorms from Tuesday night through
Wednesday night. The eastward limit of the precipitation may be
impacted by the focus of the upper level jet and accompanying
ageostrophic forcing accompanying the faster flow east of the trough
axis and surface frontal reflection.

There is medium confidence on the timing/coverage and precipitation
in the short term. Outside of the regionally blended guidance
initialization, utilized the PoPs and weather with a blend of the
Canadian, GFS ensemble, the 12km NAM-WRF and 4km NAM-WRF (ARW
version), with more higher resolution input on temperature, winds,
etc.

.LONG TERM...(Thursday through Sunday)
Issued at 409 AM CDT Mon Aug 14 2017

At the start of the extended period, we will be in the warm
sector of a somewhat vigorous storm system. The main energy will
be in the Great Lakes region, but there should be enough positive
vorticity advection moving through the PAH forecast area, along
with deep moisture and instability, to trigger scattered to even
numerous showers and tstms across the entire region early in the
day. The medium range models had slightly different placement of
the core of the mid level shortwave (north/south), but the timing
of the surface wind shift to the northwest Thu night was
synchronized fairly well (GFS the fastest). Bulk shear values and
mid level lapse rates were not impressive for this time frame, but
there could perhaps be a few strong storms around the region
midday Thu. Pcpn should begin to diminish from the west by
evening, and there may be lingering showers in the southeastern
half of the region after midnight as cooler, drier air gradually
replaces the very humid initial airmass, which is forecast to have
dewpoints in the middle 70s and heat indices over 100 in some
locations.

The next pcpn event is more iffy in the model data. The 00Z GFS
depicts the aforementioned shortwave in the northern stream as the
lead shortwave of two, close together, while the ECMWF/CMC showed
it as one. Further out in time, the 00Z ECMWF/CMC showed
additional, broader shortwave energy digging through the Midwest
instead of a sharp wave (GFS). This resulted in a different
surface/850 mb pressure pattern between the models, and
southwesterly low level moisture advection in the GFS case vs. a
northwesterly dry fetch in the ECMWF/CMC data. Model consensus
suggests a limited possibility of showers and eventually some
isolated tstms in the western half of the region Sat morning,
perhaps expanding across the entire region through the evening.
The drier scenario may be the one to go for. The rest of the
weekend looks dry, warm and mostly clear.

Now, for the Great EClipse forecast. While the Monday period is
not yet officially in our seven-day forecast on this shift, enough
data exists in the deterministic and ensemble data to suggest that
the PAH forecast area will be under a building mid level ridge.
This will be conducive to subsidence, meaning very little chance
of pcpn development, and little cloudiness. This is a preliminary
forecast, and is subject to change, but at this point we have
reasonable confidence that it will hold.

&&

.AVIATION...
Issued at 646 AM CDT Mon Aug 14 2017

Isentropic lift and lift ahead of mid-level shortwave this morning
should keep the potential for showers within the vicinity of each
TAF location in place through 18z. Radar and model trends will
keep a categorical mention of precipitation in the KPAH TAF site
this morning.

Kept VFR conditions in place for most locations today as trough
and boundary work southeast and ridging builds in briefly tonight.
Reduced visibilities as boundary layer saturates this evening and
overnight.

&&

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

$$

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