Area Forecast Discussion
Issued by NWS Paducah, KY

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949
FXUS63 KPAH 222239
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
539 PM CDT Sat Jul 22 2017

.UPDATE...
Issued at 535 PM CDT Sat Jul 22 2017

Updated Aviation discussion for 00Z TAF Issuance.

&&

.SHORT TERM...(Tonight through Monday night)
Issued at 306 PM CDT Sat Jul 22 2017

Increasing moisture over the next 24 hours will help to take the
edge off the heat, but the specifics are yet to be determined.
There is a wide range of possibilities for Sunday, depending on
the timing and coverage of convection. Some of the biggest factors
the models disagree on include cap strength and timing/location of
a surface boundary.

Although there is a lot of variability, the best estimate is that
the cap will hold until at least midday Sunday. This should allow
enough sunshine for temps to reach at least the lower 90s. Dew
points will pool along the southward moving boundary, generating
high heat indices. Peak heat indices based on the cap holding
through 18z are in the 101 to 107 range. Again, peak heat indices
will depend on the timing of convection or convective cloud
debris.

Once convection develops, there are likely to be some intense
updrafts with all models indicating capes over 3000 j/kg. The 12z
gfs indicates mixed-layer capes over 4000 along the boundary in
the afternoon. Deep layer shear values will be only around 20 kt,
but this may be enough for pulse-type severe storms given the
strong instability. Coverage of convection may be limited by
weak frontal convergence and upper forcing, so pops will remain
in the chance category. In addition, some elevated convection is
possible late tonight and early Sunday, which further complicates
the temp forecast.

As for Sunday night and Monday, the models agree the boundary will
become stalled over our region. Dew points are forecast to remain
pooled in the lower to mid 70s in west Kentucky and se Missouri,
but dew points in the 60s should make it into southwest Indiana
and parts of southern Illinois. There will be a lingering chance
of showers and storms along and south of the front through Monday
night. However, rising 500 mb heights in the wake of the Great
Lakes shortwave will limit chances Sunday night into Monday night.

.LONG TERM...(Tuesday through Saturday)
Issued at 306 PM CDT Sat Jul 22 2017

On Tuesday, models show surface high pressure over the northeast
U.S., with an upper level low over the southeast U.S.  Models
indicate the upper level low will bring a chance of scattered
showers and thunderstorms to the southern half of the PAH forecast
area during the day Tuesday.  We should be dry Tuesday night, then
confidence drops a little bit for Wednesday into Wednesday evening
with models going back and forth with precip potential.  For now
will keep our area dry from 00z Wednesday to 06z Thursday.

Confidence increases after 06z Thursday with ECMWF and GFS in good
agreement with the approach of a cold front.   The latest GFS is
about 6 hours slower than the ECMWF, but both models have the front
well into the PAH forecast area Friday evening.  Chances of showers
and thunderstorms will spread across the region from the north late
Wednesday night into early Thursday.  Good chances are expected
Thursday afternoon into Friday area wide as the front gradually
makes its way across the middle Mississippi and lower Ohio river
valleys. A few showers and storms may linger across southern
portions of our area Friday night depending on the timing of the
front.

Models show surface high pressure building into the central U.S.
late Friday night into Saturday, bringing a return of dry
conditions.

Conditions will be heating and moistening up into mid week.  Highs
will reach around 90 degrees Tuesday, with dew points in the upper
60s to lower 70s.  Wednesday and Thursday will see highs in the
lower 90s, with dew points in the lower to middle 70s.  Afternoon
heat indices Wednesday and Thursday will climb back into the 100 to
105 degree range.  Temperatures will drop several degrees for
Friday, and dew points by Saturday will drop off into the middle to
upper 60s.

&&

.AVIATION...
Issued at 535 PM CDT Sat Jul 22 2017

While gusts will diminish with sunset, enough pressure gradient
is maintained to allow light southwesterlies to continue
overnight. A frontal boundary will drop south and introduce
scattered low VFR bases with potential for mid cloud broken
bases/cigs as well. Isolated or vicinity mention inserted at KEVV
(and KOWB) will be kept this package as all models produce
convection, albeit scattered, along boundary upon its drop
southward.

&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...Excessive Heat Warning until 7 PM CDT Sunday for ILZ075>078-
     080>094.

MO...Excessive Heat Warning until 7 PM CDT Sunday for MOZ076-086-087-
     100-107>112-114.

IN...Excessive Heat Warning until 7 PM CDT Sunday for INZ081-082-
     085>088.

KY...Excessive Heat Warning until 7 PM CDT Sunday for KYZ001>022.

&&

$$



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