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637
FXUS63 KPAH 252310
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
610 PM CDT Thu May 25 2017

.UPDATE...
Issued at 610 PM CDT Thu May 25 2017

Aviation update.

&&

.SHORT TERM...(Tonight through Saturday night)
Issued at 338 pm CDT Thu May 25 2017

As is sometimes the case, the short term forecast can be rife
with smaller scale uncertainties. However, the consistent signal
the past several days has been for the main convective threat to
occur late Saturday into early Sunday. The uncertainty comes into
play where sub-synoptic thunderstorm clusters (Mesoscale
Convective Systems - MCS`s) generate their own environment and
change the scale and scope of the severe weather threat with time.

For tonight through the daytime hours on Friday, the middle and
upper level ridge behind the closed low and trough that moved is
east of the region this morning will dominate the WFO PAH forecast
area.

During this tranquil weather period, winds will turn southerly
overnight, funneling in higher dewpoints in the 60s completely
into the region by 10 am CDT Friday. Dewpoints will reach into the
70s by Saturday morning. Needless to say, CAPE will be sufficient
to robust convection.

In the Day 2 SPC Outlook (Friday), the WFO PAH forecast area is
included in a Marginal Risk area. Looking at the Canadian, GFS,
NAM-WRF (ARW version) deterministic runs, there is a suggestion
that the lead shortwave rotating around the closed low along the
Canadian/U.S. border this afternoon will serve to dampen the ridge
slightly. Some of the CAM/short range/medium range guidance hint
at an MCS developing over parts of IA/northern IL Friday afternoon
and evening. The outflow and attendant thunderstorms that develop
in its wake maybe the contributing factor for some renewed
thunderstorm development in our northern counties late Friday
night/early Saturday morning. Believe at this time that the best
threat for any severe weather in the WFO PAH CWA will be very late
Friday night (after midnight). Any boundaries left from outflows
will provide differential heating/moisture boundaries for new
convection on Saturday.

On Saturday, there may be two distinct episodes possible. The
first may be generated along an impressed surface boundary already
stretching from southwest to northeast across the area. An morning
insolation (sunshine) will be unknown, but shear will be
increasing as a faster zonal flow develops over the WFO PAH
forecast area. This may provide the focus for isolated to widely
scattered strong updrafts leading to isolated severe convection
during the afternoon hours. The second episode will be likely tied
to the approach of an existing MCS moving from
Nebraska/Kansas/Iowa/Missouri into the region Saturday night.
Depending on the speed and maturity of the MCS will determine the
degree of convective potential and severe weather coverage
overnight. As the potential wanes, the focus will shift toward
heavy rain potential for the pre-dawn hours on Sunday morning, as
training of storms will be most likely in the presence of
significant precipitable water. WPC already has a slight risk
potential for excessive rainfall over southern sections of the WFO
PAH forecast area.

As mentioned at the beginning, mesoscale and storm scale
influences can and will make short term adjustments in the overall
severe potential this weekend. Given these uncertainties, will
need to err on the side of caution, given the abundance of holiday
travelers this weekend.

.LONG TERM...(Sunday through Thursday)
Issued at 338 PM CDT Thu May 25 2017

Models show the cold front moving on through the PAH forecast area
Sunday, and this will result in showers and thunderstorms tapering
off from northwest to southeast by 00z Monday.  Weak high pressure
will build into the region, and dry weather will return for Sunday
night through Tuesday.  GFS shows a small chances of showers/storms
in our far west counties Tuesday evening, and ECMWF has gone back
and forth on that possibility, so included a slight chance for now.
Better chances return late Wednesday into Thursday with models
showing a developing upper level trof over the Central Plains moving
toward the middle Mississippi Valley.  Kept possibilities in the
chance to slight chance categories for now due to timing and track
uncertainties.

Temperatures will remain close to seasonal through the extended.
Sunday will be pretty humid, but dew points will be dropping off
into the 50s behind the cold front, giving our region more pleasant
conditions.

&&

.AVIATION...
Issued at 610 PM CDT Thu May 25 2017

VFR conditions tonight through Friday. Light south winds tonight.
SSW winds 10+ kts with gusts around 18-23 kts midday through late
afternoon.

&&

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

$$