000
FXUS63 KPAH 281118
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
618 AM CDT Wed Jun 28 2017

.SHORT TERM...(Today through Friday night)
Issued at 337 AM CDT Wed Jun 28 2017

Above average forecast confidence (60-80%) with the short term
forecast period rain chances. For at least six days, medium range
guidance has been showing a consistent signal for precipitation this
Friday. Even though there was a two day period where some models
introduced precipitation as early as Wednesday night, the focus for
initial widespread convective activity remains focused starting on
Friday.

The 00z Wednesday ECMWF and Canadian guidance are quite similar in
moving the upper level ridge axis through the WFO PAH forecast area
this Wednesday morning, placing the region in a general zonal flow
with a slight southwesterly orientation, influenced by the
previously mentioned (in earlier discussions) the low (centered near
the LA/TX coastline) and associated trough.  This southern U.S. low
does have a good surface reflection, but evidence of the trough is
not apparent until you get close to 700 mb and above. Despite the
absence of the a deep layer trough there is still deep layer
moisture advection through the layer across the Lower Mississippi
Valley through Thursday, mainly east of the Mississippi River. The
focus of weak/moderate mid-level lift increases rain potential
working up toward the West Kentucky Pennyrile late Thursday night.

Despite any lack of convection Wednesday into Thursday, moisture
advection will increase across the area with dewpoints increasing
some five degrees (into the upper 60s) in the next 24 hours
beginning at 7am CDT today, reaching near 70 degrees by mid-morning
Thursday and holding there into Friday morning.

For Friday, enhanced (raised) rain chances (PoPs) above the
regionally initialized model blend guidance, closer to the
ECMWF/Canadian numerical model solutions. Both models take the
previously mentioned TX/LA low and move into into southeast Missouri
Friday morning as an open wave (pre-frontal surface trough). The
aforementioned model guidance then slowly take the surface cold
front, which was stalled in the Upper and Middle Mississippi Valley
on Thursday and move it slowly through the WFO PAH forecast area
Friday afternoon and night. This frontal passage is in associated
with an west-east oriented shortwave over the western Canadian
Provinces this morning gradually sinking southward into the Great
Plains and becoming positively tilted over the Central Plains and
Upper Midwest Friday morning.

There are some differences between the ECMWF and Canadian guidance
on how this shortwave evolves through the WFO PAH forecast area
Friday night. The ECMWF maintains a positive tilt, but shears the
shortwave northeast of the WFO PAH forecast area, while the Canadian
becomes more negatively tilted and sharper over the WFO PAH forecast
area.

Regardless of the convective mechanism, layer precipitable water
(PW) will be 1.5" Friday morning, ranging up to 2.30" Friday night
(per Canadian guidance), which falls within the 75 to 99 percent
range of normal PW for this time of year.  Would not be surprised to
see some excessive rainfall, especially over parts of Souheast
Missouri and West Kentucky late Friday night and early Saturday
morning.

SPC already has part of the Day 3 (Friday/Friday night) in a
Marginal Risk for severe storms. Most of the area is mainly north of
the Ohio River, where slightly better shear will be in place. Of
course, this will be conditional, depending on the trajectory of the
shortwave as it moves through the area.

The increase in dewpoints will definitely have an impact on apparent
temperatures, with heat index values in the middle and upper 90s
likely in Southeast Missouri Thursday and Friday.

.LONG TERM...(Saturday through Tuesday)
Issued at 337 AM CDT Wed Jun 28 2017

A typically warm and moist summertime pattern is expected this
weekend into the July 4th holiday. A weak cold front will
dissipate over our region on Saturday, followed by a southwest
flow of very warm/humid air early next week. Timing of convective
precip is usually a challenge during weak summertime flow
regimes, and this is no exception.

On Saturday, a weak cold front will lose its identity as it enters
the usual summertime graveyard for cold fronts over our region.
The air mass will be very moist, and some showers or storms will
likely precede the front or convective outflow boundaries.
The coverage and intensity of storms will depend on the timing of
the front and any potential cold pools from earlier convection on
Friday night. Given the weak low-level flow, the air mass will be
slow to recover from the stabilizing effects of any overnight
convection. Precip chances will be highest south and east of the
Ohio River, where the remnant front will be located during peak
heating hours.

Saturday night and Sunday appear somewhat drier than previous
model runs indicated. Weak high pressure will temporarily form
over the Lower Ohio Valley in the wake of the Saturday system.
The brunt of the moisture and instability will be suppressed
southward into southeast Missouri and the Tennessee border region.
The forecast is now mainly dry, especially for southwest Indiana
and parts of southern Illinois. A slight chance of afternoon
storms still exists south and west of the Ohio River, where dew
points will still be in the lower 70s.

Early next week, the models are in pretty good agreement that a
compact, slow-moving 500 mb shortwave will take up residence over
the Missouri Valley. If this does occur, it would induce a
strengthening southwest flow of very warm and unstable air over
our region. Monday through Tuesday would be convectively active,
at least compared with the weekend. The forecast will feature
elevated pops Monday through Tuesday, though not quite into the
likely category.

There will be little day-to-day variation in temps, with highs
mainly in the mid to upper 80s and lows around 70.

&&

.AVIATION...
Issued at 618 AM CDT Wed Jun 28 2017

VFR forecast continues with the 12z Wednesday Routine WFO PAH TAF
issuance. Just a few to scattered cloud deck between 4-5kft agl
today and tonight, with forecast groups separated by wind speed
variations due to daytime mixing of air parcels and increasing
surface pressure gradient with time.

&&

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

$$

SHORT TERM...Smith
LONG TERM...MY
AVIATION...Smith