Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 180004
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
704 PM CDT Mon Jul 17 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Tuesday)
Issued at 305 PM CDT Mon Jul 17 2017

19Z water vapor imagery shows an upper disturbance still over
northwestern KS and southwestern NEB with storms recently
developing in south central NEB. Otherwise a broad ridge in mid
and upper levels remained over the southwest and south central
U.S. At the surface, A low pressure system was seen over the
northern plains in an otherwise diffuse pattern.

For tonight and Tuesday, the main thing to watch is the upper
disturbance to the west of the forecast area. Models generally
bring some energy across northern central and northeast KS late
tonight and into Tuesday morning and tend to shear it out by
Tuesday. By this time there isn`t much in the way of instability
and the convective allowing model progs struggle to maintain
precip this far east with the loss of heating. So only have POPs
in the 20 to 40 percent range for now. The biggest question is
where the disturbance will end up by peak heating on Tuesday.
Based on latest guidance shearing the energy out and lifting it
northeast, have a dry forecast by the afternoon.

With a little more of a gradient in the surface pressure, winds
are forecast to be a slightly stronger overnight and into Tuesday.
This is expected to keep the boundary layer a little better mixed
and keeping lows in the lower and mid 70s. For highs, there is
some indication of a little warm air advection. With this in mind,
highs are expected to be a degree or two warmer than today. Heat
indices should be in the 100 to 105 range for most areas. Since
this is likely to be the beginning of a prolonged period of hot
weather, we decided to go ahead and issue an excessive heat
warning beginning Tuesday even though the indices may not actually
reach criteria. The prolonged nature is the deciding factor with
what looks like 4 or 5 consecutive days of hot weather.

.LONG TERM...(Tuesday night through Monday)
Issued at 305 PM CDT Mon Jul 17 2017

The Wednesday through Saturday timeframe will be characterized by
the return of summertime heat with a broad Southern Plains
blocking high slowly translating eastward during the course of
the week. It will not be until Saturday evening and Sunday that
the ridge breaks down and allows a trough to dig southward and
bring any decent chance of precipitation. On Tuesday night, a
decaying convective complex may propagate southward in response to
a 30-40 kt LLJ and graze NE Kansas in the morning hours on
Wednesday. The 17.12 NAM bufr soundings along the KS/NE border
from 06-12Z on Wednesday do show a 700-600 mb EML that would
result in 1500 J/kg of MUCAPE, but the lack of shear would limit
any organized storm threat.

The effective surface warm front will retreat northward during the
course of the day on Wednesday and looks to linger across the
Siouxland region for the mid to latter part of the work week.
H850 temps during this time will hover around +22 to +28 C with
surface temps in the upper 90s to low 100s. With dewpoints in the
upper 60s to mid 70s, afternoon heat indices will top 100 to 110
degrees. A lee cyclone anchored on the Front Range will eject
with the approaching aforementioned upper trough on Friday and
drag a cold front southward across NE Kansas during the day on
Saturday. Model solutions diverge on how far south this boundary
will sag for the weekend and the resultant precip timing, but it
appears like the boundary will hang up somewhere over Kansas and
result in multiple rounds of rain.

Challenges with this forecast revolved around high temperatures and
dewpoints during the extended heat wave from Wednesday to Saturday.
Given recent model trends of underdoing high temps and daytime
dewpoints by several degrees, leveraged the BCCONSMOS and BCCONSAll
blended grids for these variables. This resulted in raising dewpoints
by several degrees in the east, which makes sense given that the
guidance is likely not quite resolving the local
evapotranspiration effects from the increased vegetation in
eastern Kansas. Highs were also raised in the river valleys by a
couple of degrees, also consistent with trends over the last few
days. Finally, given the modest pressure gradient and increased
mixing with the heat, pushed daytime winds higher towards the
CONSMOS guidance through the end of the work week. When all of
these adjustments were said and done, the duration of daytime heat
indices in the 105-110 degree range would easily justify the
excessive heat warning through Saturday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 640 PM CDT Mon Jul 17 2017

For the 00Z TAFs, have held off any mention of showers or storms
for the overnight period at the terminals as a slow moving upper
wave remains west of the area. LLJ is active but also remains west
overnight. Winds should increase especially near KMHK into the
afternoon Tuesday.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning from noon Tuesday to 8 PM CDT Saturday
for KSZ008>012-020>024-026-034>040-054>056-058-059.

&&

$$

SHORT TERM...Wolters
LONG TERM...Skow
AVIATION...Drake



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