Area Forecast Discussion
Issued by NWS Paducah, KY

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000
FXUS63 KPAH 240009
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
709 PM CDT Sat Jun 23 2018

.UPDATE...
Issued at 709 PM CDT Sat Jun 23 2018

Amended aviation discussion for 00Z TAFs.

Current public forecast is generally representative of current
conditions. A broken line of thunderstorms has formed across
portions of southern Illinois and southeast Missouri early this
evening. The storms are tied to the eastward propagation of a weak
mid level disturbance and located near the intersection of
1500-2000 J/kg SBCAPES across the southern half of the area and 30
to 40 knots of 0-6 km bulk shear over the northern half of the
area.

Numerous storms have displayed weak mid level rotation with mini
supercell characteristics. This activity is largely expected to
continue its movement southeast through mid evening, with a
localized threat of large hail and damaging winds. Despite the mid
level rotation, the tornado threat appears to be smaller given
observed weak 0-1 km bulk shear. Some decrease in intensity is
expected by mid evening as instability begins to dampen due to
loss of daytime heating.

&&

.SHORT TERM...(Tonight through Monday night)
Issued at 149 PM CDT Sat Jun 23 2018

The short term modeling, esp the HRRR, has really downplayed near
term pcpn chances. We`ve bought into that a little, but will
collaborate for best final pic. That`s likely to be a slight
chance to low chance from north to south across the area, based on
consistency of previous model solutions/insistence. While
confidence tonight is down, still like tmrw morning`s chances
with all models showing wave opening up and high chance to low
likely pops as has been consistently advertised. More vort energy
continues these pops into Sunday night while we will warm sector.

We`ll still anticipate more off and on chances into the early work
week, with the newly forming Low moving into Iowa by the end of
the short term. Having warm sectored by then, temps will start to
heat up and juice up, with heat indices reaching toward triple
digits by Monday pm, which could be the beginning of consistent
such readings for several days in a row, if not the entirety of
the work week.

.LONG TERM...(Tuesday through Saturday)
Issued at 149 PM CDT Sat Jun 23 2018

The extended portion of this routine forecast issuance begins with
the transition of a nearly vertically stacked closed shortwave
length low moving from southern Minnesota/northern Iowa at 7 am CDT
Tuesday to just south of Hudson Bay Canada by 7 pm CDT Wednesday.

The sensible weather for the WFO PAH forecast area will include warm
frontal zone lifted and isentropically lifted convective activity by
Tuesday afternoon, followed by pre-frontal and weak cold frontal
convection late Tuesday night through Wednesday. With the middle and
upper low moving into southern Canada, the majority of the deep
layer shear will drift north of the WFO PAH forecast area.  The
medium range GFS carries a little more shortwave energy in the west-
northwest flow behind the aforementioned low versus the 00z Saturday
ECMWF and 12z Saturday Canadian numerical model guidance. Even the
12z NAM-WRF is not consistent with bringing any additional shortwave
energy behind the closed low versus the GFS. At this point, will
lean away from a greater influence of the GFS versus a combination
of the NAM-WRF/ECMWF/Canadian solutions.

Beyond the passage of the closed low on Wednesday and the weak
surface frontal zone reflection through the WFO PAH forecast area,
all of the guidance build in a ridge over the area through Friday
morning. The middle and upper level ridge eventually moves east of
the WFO PAH forecast area Friday night into the Ohio Valley and the
northeast U.S.   A progressive trough/low centered over Idaho will
gradually shift the upper level flow to the southwest on the eastern
side of the trough during the day Saturday into Sunday. Again the
GFS differs from the ECMWF and Canadian guidance by retrograding the
departing high pressure ridge westward into the Ohio and Mississippi
valley area.

Even though low level (below 850 mb) flow from the western Gulf of
Mexico allows for a trajectory of moist air through the entire
forecast period, deeper upper level flow/moisture/lift does not
become available until Friday and Saturday.  With that in mind,
diurnally-based PoPs/Weather (mainly afternoon and evening
time frame) will be introduced along the gradient zone of
moisture/shear/instability in the wake of the ridge.  This will most
likely place the greatest PoPs/Weather over eastern sections (mainly
west Kentucky, southwest Indiana, southeast Illinois) of the WFO PAH
forecast area.

With any of the convective activity during the extended forecast
period, general most unstable CAPE values will be 1500-2500 j2/kg2,
with 0-6 km cumulative shear between 15-30 knots. There may be the
potential for isolated to scattered strong to severe thunderstorms
Tuesday and Wednesday.

The more long term concern will be the heat and humidity for the
extended period. Afternoon and evening heat index values will exceed
100 each day in some part of the WFO PAH forecast area through the
extended forecast period. From Wednesday through Saturday, several
locations will see heat index values top out near 105 degrees,
although any occurrence on Wednesday will be conditional based on
precipitation and cloud cover. More widespread heat index values
near 105 will be expected Thursday through Saturday and may warrant
a future heat advisory issuance.

.AVIATION...
Issued at 149 PM CDT Sat Jun 23 2018

VFR conditions may temporarily restrict to MVFR, with vicinity
mention of pcpn possible late this pm, and during the late
night-early morning time frame. Should see best chance of
deteriorated conditions (cigs and vsbys) coming then, when wave
opens up/lifts across mid Ms valley and spreads best such chance
pcpn southwest to northeast across the area.

&&

.AVIATION...
Issued at 709 PM CDT Sat Jun 23 2018

A weak mid level disturbance will keep the potential for widely
scattered thunderstorms in the forecast through mid evening,
especially over the southern half of the area. Included a VCTS
mention at KCGI and KPAH. Mainly dry weather should prevail from
late evening through the overnight and into much of Sunday
morning. Thereafter, a better chance of scattered thunderstorms
returns to southern portions of the region Sunday afternoon in
association with increasing mid level energy along an approaching
baroclinic zone. This chance should slowly spread northeast into
the remainder of the region Sunday night. VFR conditions should
largely prevail through the forecast period, with the exception of
brief reductions due to thunderstorm activity. Winds will be light
and variable tonight, then southwest around 5 knots on Sunday.

&&

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

$$

UPDATE...RJP
AVIATION...RJP



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