Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 192104

Area Forecast Discussion
National Weather Service Paducah KY
404 PM CDT Sat May 19 2018

.SHORT TERM...(Tonight through Monday night)
Issued at 403 PM CDT Sat May 19 2018

Tough mesoscale situation to determine eastern extent and limit of
convection for the remainder of the afternoon and this evening. In
short, leaned closer to the ESRL HRRR/13km RAP guidance to reflect
the eastern limit of thunderstorm activity through 7 pm CDT. The
local mesoscale environment along and ahead of the convective
lines over east central Missouri may persist further east near
Interstate 57 by 6-7 pm before collapsing with the loss of
insolation and weak/moderate shear. The 13km RAP Mesoscale
Convective System (MCS) maintenance probabilities still suggest
significant weakening between 23z-01z, so will hold with the
current forecast of thunderstorms limited to mainly along and west
of the Mississippi River in southeast Missouri and over the
Purchase area of west Kentucky through sunset.

To complicate things even more, the stable boundary layer from the
fog/low clouds over southeast Missouri into southern Illinois
has created differential heating zone over the area, helping to
maintain isolated convection into northern sections of southeast
Missouri and then again in the lower delta regions of southeast
Missouri and the Bootheel onward to the border counties of west
Kentucky and west Tennessee.

More intense LAPS surface theta-e gradient across northwest
Tennessee edging into the southern Purchase area of west
Kentucky. A broader and much weaker theta-e axis extends from the
Land Between the Lakes northward into southeast Illinois. Although
there is good surface CAPE (2000-2500 j2/kg2) and weak helicity
(100 m2/s2 or less) in this area, thermal profiles (better lapse
rates) are lacking and may not support much more than showers
through mid-evening.

As the boundary layer stabilizes, anticipate most of the moisture
and instability will become elevated and shifted to the northern
end of the WFO PAH forecast area, where low level moisture convergence,
stretching, and lift is forecast to be maximized. Therefore,
significantly reduced PoPs across the entire WFO PAH forecast area
during this evening, then increase PoPs near a weak developing
surface trough in extreme western sections of southeast Missouri,
then along the I-64 corridor after midnight and through daybreak
on Sunday near and impressed warm front.

Although not favorable, there may be an outside chance that the
convection overnight may generate enough of a balanced cold pool
for MCS development well south into the WFO PAH forecast area. It
will bear monitoring early Sunday morning.

Sunday will bring significant potential CAPE of 3000-5000 j2/kg2
even during the morning hours in the warm sector south of the
convective activity expected along the I-64 corridor. The low and
mid level shear ranges from 10-30 knots, with the highest values
obviously near the convection on Sunday. Between thunderstorm
outflows from the north and larger scale isentropic lift,
anticipate a slight chance of showers and thunderstorms area wide
by late morning and through midday. This activity should continue
through the afternoon and become enhanced as a weak shortwave
glances the area from the northwest with the primary wave going
into Wisconsin and the Great Lakes region.

With the WFO PAH forecast area moving into a faster southeasterly
flow aloft and weak moisture advection, small non-diurnal chances
for rain will exist the rest of Sunday through Monday.

.LONG TERM...(Tuesday through Saturday)
Issued at 403 PM CDT Sat May 19 2018

There remains a mention of mainly diurnal showers and
thunderstorms across the WFO PAH forecast area through the remainder
of next week. However, continue to have low forecast confidence
for any widespread precipitation Wednesday through Friday, as high
pressure dominates at middle and upper levels of the atmosphere.
At this time, looked for opportunities to lower rain chances
across the WFO PAH forecast area, especially Wednesday through


Issued at 1255 PM CDT Sat May 19 2018

Somewhat difficult to pin down the dividing line for KPAH and
KCGI for ceilings less than VFR category, given the expected
proximity of thunderstorms and attendant ceilings generated by
thunderstorm outflows. At this time, will leave KCGI and KPAH in
VFR category and will amend should mesoscale thunderstorm trends
suggest worsening flight conditions.

The rest of the WFO PAH TAF sites will see transfer toward MVFR
conditions with the new convection developing late tonight and
Sunday morning.




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