Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 222153
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
453 PM CDT Sat Apr 22 2017

.UPDATE...
Issued at 453 PM CDT Sat Apr 22 2017

Dynamics, moisture, and forcing associated with the convection
over the Tennessee Valley have decoupled somewhat, leaving the
mean trough axis over Southern Missouri, Southern Illinois and
Indiana separated from the mean flow. Until the jet
dynamics/vorticity rotate around the sheared out low and deepen
the it as the jet moves along the eastern side of the trough,
will need to reduce PoPs and cloud cover on the northern end of
the WFO PAH forecast area. Satellite trends still showing some
lift along the southeastern section of the WFO PAH forecast area,
so anticipate some isolated to scattered shower activity will
regenerate as the low attempts to reform toward Southeast
Missouri. Anticipate most of the lift will be confined to the
south and east of the low through the overnight hours, so kept the
mention of shower activty limited to that quadrant.

The ESRL HRRR and the Canadian hinted at this potential since this
morning, but had low forecast confidence in the timing and speed
of the transition to lesser cloud and precipitation coverage. Oh,
well.

&&

.SHORT TERM...(Tonight through Monday night)
Issued at 300 PM CDT Sat Apr 22 2017

With the 850 mb low east of the WFO PAH forecast area, the threat
of thunderstorms has diminished greatly for the rest of this
afternoon (first period). Latest GOES-R ("test" GOES 16) water
vapor imagery from the 6.19 to 7.34 micron channels are still
suggesting a slightly cyclonically curved jet axis (possibly two) oriented
across the southern third of the WFO PAH forecast area.

On the northern end of the entrance side of the jet, an
increasingly sheared out closed low is still evident over the St.
Louis Metropolitan area, with a deformation zone and noticeable
stretching occurring across southern Illinois into Southwest
Indiana and Northwest Kentucky. With sufficient moisture still in
place, felt it was prudent to keep measurable QPF/PoPs, and
Weather in place until the low and deformation zone are shifted
northeast of the WFO PAH forecast area through the early evening.

Likewise, there is some northward propagation of moisture and jet
dynamics across the southern Pennyrile and extreme southeast
corner of the Purchase area of West Kentucky during the last hour.
Will keep a gradient of higher chances of rain across this area
through the evening, gradually shrinking the chances of rain
toward daybreak on Sunday.

Have been pleased with the Canadian (CMCnh) and higher temporal
resolution GFS deterministic guidance last night and today (and
even back to Thursday). Both depicted the zone of instability and
the progression of the thunderstorms quite well this morning.

Along with the regionally blended guidance, utilized the Canadian,
GFS, and the high resolution NAM-WRF model family, as well as the
ESRL HRRR guidance for the PoP/Weather/QPF of the departing system
tonight.

The NAM-WRF, HRRR, and GFS MOS guidance also did well with the
initialization of temperatures into early next week, so weighted
the current forecast closer to these values.

After the last of the rain moves east of the area late tonight,
dry conditions should prevail through Monday night. Sunday will be
the transition day with respect to the rise in temperatures, as
most of the insolation used to provide thermal gradients for
mixing and sensible heat exchanges (evaporation of rain-soaked
ground) and remnant cloud covers in the east. In addition, there
will be the issue of gradually weakening cold air advection in the
boundary layer to contend with as well early on Sunday.

.LONG TERM...(Tuesday through Saturday)
Issued at 300 PM CDT Sat Apr 22 2017

Forecast confidence is rather high into Thursday, then it
decreases considerably heading into next weekend.

Troughing will develop aloft over the Rockies and high Plains by
Tuesday, which will take the primary storm track from there
northeast to the Great Lakes. An initial storm system will push a
cold front through our region Wednesday afternoon or evening, and
there should be a nice coverage of showers and thunderstorms near
the front. Heavy rainfall will be a concern with precipitable water
values around 1.5" which is near the 99th percentile compared to
climatology. A few strong to severe storms will be possible as well.

There is quite a bit of variability in the guidance in the timing
and track of the next storm system for Thursday night into Saturday.
The 12Z GFS seems to be the outlier here, with a much stronger upper
ridge off the Carolina coast, while the ECMWF and GEM are weaker and
farther east. The result of the GFS solution for us is that the next
storm system digs into the southern Plains, and pushes the front
back well north of our area by Friday. The other models trend
farther south with the storm track, more over our region heading
into next weekend.

We will keep the PoPs mainly at or below 50% for most of the area
Thursday night into Saturday, and wait for more of a consensus in
the details.

Temperatures will generally be warm in the absence of convection and
associated cloud cover. Tuesday will see many locations approach 80.
The GFS-based MEX guidance has a high of 87 in Paducah on Friday,
which gives an indication of just how warm we could be if the front
lifts back to our north.

&&

.AVIATION...
Issued at 1251 PM CDT Sat Apr 22 2017

Showers will continue to impact terminals over much of the region
this afternoon, and scattered activity may linger well into the
evening hours over KPAH and KCGI. Any thunder activity this
afternoon is expected to remain over west Kentucky to the south of
the TAF sites.

Most guidance seems to indicate lower ceilings than are currently
being observed, so tried to go on the optimistic side with the
ceiling forecast. KEVV is already flirting with VFR conditions and
should be there soon. All sites should climb into VFR territory
at some point by the overnight hours.

Northeast winds will continue to gust over 20kts at times through
the period.


&&

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

$$

UPDATE...Smith
SHORT TERM...Smith
LONG TERM...DRS
AVIATION...DRS



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