Area Forecast Discussion
Issued by NWS Paducah, KY

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433
FXUS63 KPAH 230453
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
1152 PM CDT Sat Apr 22 2017

.UPDATE...
Issued at 1152 PM CDT Sat Apr 22 2017

Updated aviation discussion.

&&

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 1152 PM CDT Sat Apr 22 2017

Low surface pressure over Dixie will roll away to the east
overnight, while a sheared upper low to our northwest deepens and
dives into TN. After sunrise, additional low surface pressure may
develop over central TN in response. This should maintain the
surface pressure gradient across most of our region during the day
with little change in wind direction. Winds may gust at or above 20
kts out of the north northeast by then. Cigs are expected to stay
VFR through the TAF period, except for in the Pennyrile region of
KY, where they may hang on for much longer, along with light pcpn.
However, the KOWB TAF site is only expected to have somewhat more
cloudiness than the others.

&&

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

$$

SHORT TERM...Smith
LONG TERM...DRS
AVIATION...DB



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