Area Forecast Discussion
Issued by NWS Paducah, KY

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000
FXUS63 KPAH 191754
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
1254 PM CDT Sat May 19 2018

.UPDATE...
Issued at 1255 PM CDT Sat May 19 2018

Late morning forecast adjustment to add a formal mention of
severe storms potential into gridded, tabular, and text products
for this afternoon. Main area of concern this afternoon will be
the southern half of southeast Missouri and the southern sections
of the Purchase area of West Kentucky.

As mentioned earlier this morning, the numerical guidance was
underestimating the low level instability (sharper lapse rates
through 850 mb). The big question has been how much mid-level
warming will be in place downstream of the mid-day convective
activity to suppress any additional shower or thunderstorm
activity over southern Illinois, southwest Indiana, and the
Pennyrile region of west Kentucky.

The current convection over south central missouri (as of 1240 pm
CDT) appears to be congealing updrafts and developing a uniform
cold pool somewhat balanced by the upshear environment ahead of
it. A second area is also developing across Carter, Ripley, and
Wayne County Missouri. If it holds together, may see some
organized severe storms develop upscale and expand across the
southern half of southeast Missouri toward the bootheel and
southwest Kentucky, tapping into a southwest to northeast axis of
higher thet-a air nosing just south of KPAH toward the Land
between the Lakes region of West Kentucky. Mesoscale influences
with weak to moderate shear would maintain this convection through
the remainder of the afternoon.

With the loss of daytime heating and the development of an
inversion, thunderstorm activity should drop off quickly over
southeast Missouri and southwest Kentucky, becoming re-established
near an elevated zone of instability near the Interstate 64
corridor late tonight.

The 13km RAP has been a decent proxy for the forecast
thunderstorms this afternoon and was used as background in the
gridded forecast update.

UPDATE Issued at 1014 AM CDT Sat May 19 2018

Will be updating the text and tabular products within the hour to
remove mention of locally dense fog from today`s forecast. Also,
will be increasing rain chances over southeast Missouri and over
the tri-state area (southeast IL, southwest IN, and Pennyrile
region of west Kentucky) through midday.

The ridge that was expected to minimize coverage of shower and
thunderstorm activity today is a little more narrow and oriented
northwest to southeast (about 90 degrees counter-clockwise from
previous model run orientation).

The current and recent model guidance are initializing the 925 mb
temperatures (around 3kft agl) about 1 degree Celsius to cold,
plus or minus 1 degree Celsius at 850 mb, and about 1 degree
Celsius too cold at 700 mb. In addition, the short and medium
range guidance was underplaying the intensity (shown in the water
vapor and IR imagery on GOES-E) near KOWB around 12z this morning.

The upper air pattern suggests more unstable thermal and moisture
profiles through at least 5 kft agl over southeast Missouri than
forecast by the guidance. The 13z 13km RAP appears to be resolving
these differences and was utilized for the morning update.

There appears to be an impressed warm front/weak convergence zone
over southeast Missouri, extending northwest (where WFO SGF was
issuing severe thunderstorm warnings in southwest MO) and to the
southeast over parts of northeast Arkansas. Anticipate general
convection over to continue developing and moving east-northeast
over southeast Missouri this morning. If there is enough lift
along the boundary, a few updrafts maybe able to overcome the
mid-level cap to produce isolated strong to severe storms. The
upper flow would cast cirrus blow-off from the thunderstorms,
supporting better storm development to the north-northeast. Will
add a mention of some strong to severe storm potential in the HWO
for southeast Missouri.

Renewed convection may occur along the eastern side of the ridge
near the shear axis this afternoon, but for now anticipate any
updrafts for strong to severe storm development to be quite
limited.

&&

.SHORT TERM...(Today through Monday night)
Issued at 350 AM CDT Sat May 19 2018

Pesky upper level low was finally exiting our region early this
morning. Some showers still lingered across western KY, southwest
IN and southern IL on the back side of the low. These showers will
eventually lift northeast of the kevv area later this morning.
Areas of fog and low stratus will burn off early this morning, but
there is still the potential for dense fog patches through
sunrise.

A 500 mb shortwave ridge will build over the lower Ohio Valley
this afternoon. This should result in plenty of sun, which will
boost temps well into the 80s. Surface dew points will climb to
near 70, which will contribute to some strong buoyancy. RAP and
NAM mixed-layer capes climb above 2000 this afternoon in parts of
se Missouri, with little if any capping. It is difficult to
identify a trigger for convection, but any storms that manage to
develop would have plenty of juice to work with. Some hail and
gusty winds are possible in the presence of steepening mid-level
lapse rates and dcapes near 1000. The href mean qpf indicates
isolated convection forming from Missouri into west Tennessee late
today. The forecast will contain slight chance pops this
afternoon.

Surface cyclogenesis will take place over the central Plains
tonight, with a sharpening warm front extending eastward to the
Wabash Valley. Some of the model guidance indicates convection
will develop along the warm front late tonight, mainly in sw
Indiana and parts of southern Illinois.

On Sunday, the air mass will become quite unstable with daytime
heating as dew points hover near 70. Not much in the way of lift
would be needed to initiate convection, which would again contain
the potential for hail and wind. The proximity of the surface low
over the Missouri Valley and the warm front extending eastward
to the Wabash Valley justifies small chance pops in the forecast.
Highs will be near 90.

Sunday night through Monday night, a 500 mb shortwave will
traverse the upper Mississippi Valley and Great Lakes region. The
associated weak surface low will move east across central
Illinois. Although the strongest forcing and wind fields will
remain just north of our region, there will be enough shear and
instability to maintain the risk of some locally strong
convection. Again, this risk is conditional on the development of
storms this far south into the warm sector. Monday afternoon
appears to be the most probable time frame for convection as the
surface front makes some southward progress.

.LONG TERM...(Tuesday through Friday)
Issued at 350 AM CDT Sat May 19 2018

Our region will remain in an unsettled pattern through the extended.
Models show a frontal boundary sinking south into the middle
Mississippi/lower Ohio River valleys early in the work week.  This
front will meander in our region through the extended forecast
period.  Daytime heating will play the most significant role in our
shower/thunderstorm chances with models, the ECMWF even more than
the GFS, indicating measurable and more widespread QPF in the 12z to
00z time frames each day.  ECMWF pretty much keeps our area rain
free each night.  However, looking at the GFS solution and
superblend, and with the boundary in our vicinity and plenty of
moisture available, confidence in having completely dry overnights
is pretty low.  Went ahead with a dry forecast Tuesday night, but
included slight chances mainly in our western half of counties
Wednesday night and Thursday night.  As for daytime chances, went
with slight to low chance categories for now.

Southerly winds will continue to pump unseasonably warm air and
plenty of moisture into the region.  Both highs and lows will be 8 to
10 degrees above seasonal normals, with readings in the middle to
upper 80s and the middle to upper 60s respectively.

&&

.AVIATION...
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 willl see transfer toward MVFR
conditions with the new convection developing late tonight and
Sunday morning.

&&

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

$$

UPDATE...Smith
SHORT TERM...MY
LONG TERM...RST
AVIATION...Smith


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