Area Forecast Discussion
Issued by NWS Paducah, KY

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631
FXUS63 KPAH 180716
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
216 AM CDT Tue Jul 18 2017

.SHORT TERM...(Today through Thursday night)
Issued at 136 AM CDT Tue Jul 18 2017

06Z surface map shows dew points ranging from near 70 east to the
upper half 70s in our west. Highs Monday ranged from 89-93F with
00Z 850mb temps at 18-20C and H5 heights at about 590DM. 24 hours
later at 00Z Wed, models say H5 heights rise about 40DM as 850
temps warm to 19-21C. That should translate to surface high temps
a degree or two of yesterday`s...which puts the entirety of the FA
into the lower half of the 90s. Combined with dew points in the
lower 70s, that`ll translate to Heat Indices near 100.

Fast fwd 24 more hours to Wed and 48 to Thursday. We see another
10-20DM rise in H5 heights as its upper anticyclone center
anchors over the mid Mississippi/Tennessee river valley by 00Z
Friday. 850 mb temps ranging from 21-22C expand their grip during
this time frame. With overnight Lows now hovering in the low to
mid 70s, we should see a subsequent bump up in surface high temps
of at least another degree or two...translating to mid to upper
90s for the entirety of the FA. Dew points should still be pretty
much in the low end 70s, maybe mixing to upper end 60s at times
during the peak heating hours. That will equate to Wed-Thu Heat
Indices of 100 becoming commonplace with some locations starting
to nudge to 105. All these numbers are close to WPC guidance,
which most offices have been running with to overcome the blended
cool bias deficit. We`ll continue that strategy here as well, and
this all fits the narrative of the Heat Watch headline issued
Monday and beginning Wednesday. All things staying the same,
collaborative agreement is to anticipate an upgrade to near term
headlines beginning either this pm (for the later 2nd period
event) or tonight (for the later 1st period event).

The other near term forecast challenge is convection. We still
see some ongoing just outside the FA, to our northeast. HRRR and
RAP, while not perfect, did model today`s popcorn variety
development that affected mainly the northern half of the FA,
where the capping temps were a degree or two cooler. That inverts
today, with popcorn convection modeled with heat of day hours for
approximately the southern 2/3 of the FA, or where capping temps
run 2-3C cooler. We should see late day/early evening shut off
coming from the north as H7 temps warming to 13C are modeled into
the northern 1/3 of the FA, but some fireline development could
continue along the differential gradient until the loss of diurnal
fueling comes with nightfall. For Wed and Thu afternoons, we
think the 10-12C H7 temps that become more commonplace under the
building upper ridge/high pressure subsidence should preclude
anything more than a silent mainly diurnal pop mention.

.LONG TERM...(Friday through Monday)
Issued at 136 AM CDT Tue Jul 18 2017

Confidence remains medium through the extended at least until day 7.

The upcoming heat wave continues to be the main threat in the
extended. With overall guidance nos coming into better agreement
with high temperatures through the extended, the excessive watch
will stand. However with the one and ten hour fuels (grass) still
green with transpiration in full process, would not expect to
encounter ambient air temperatures in the triple
digits...especially with dew points in the 70s. However before the
heat wave ends, fuels will definitely start to cure. This may
allow the heat wave to persist longer than expected. Blended
warmer WPC numbers for highs into the extended. However models are
trying to bring a cold front through Sunday night bringing an
abrupt halt to the heat wave. I do believe a front will approach
from the north but not convinced it will plow through the region
like the models scenario. Leaned a little more toward WPC progs
with holding the front up across the northern section of the area.
Models also cool us down with north winds. I tempered this
scenario and kept WPC temps going for day 7 which are also still a
little warmer than the blended forecast. Either way heat index
values will still be in the 100 to 105 degree range through most
of the extended at least through the weekend. Next week we are
expected to fall into the low 90s for heat index values. We will
monitor next weeks projected cool down closely to add confidence
for that time period.

&&

.AVIATION...
Issued at 136 AM CDT Tue Jul 18 2017

Patchy early day fog may offer vsby restrictions, but will burn
off quickly with the rising sun. With dew points in the lower 70s,
maybe mixing upper end 60s at times, should see scattered diurnal
bases starting around 4-5K FT AGL, with temporary periods of
broken bases/cigs in range same thru the pm hours, perhaps
lifting another 1K FT AGL. Vicinity CB/TS mention is possible but
not included at this writing with site specific Pops still at 20
percent or less.

&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...Excessive Heat Watch from Wednesday afternoon through Sunday
     evening for ILZ075>078-080>094.

MO...Excessive Heat Watch from Wednesday afternoon through Sunday
     evening for MOZ076-086-087-100-107>112-114.

IN...Excessive Heat Watch from Wednesday afternoon through Sunday
     evening for INZ081-082-085>088.

KY...Excessive Heat Watch from Wednesday afternoon through Sunday
     evening for KYZ001>022.

&&

$$



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