Area Forecast Discussion
Issued by NWS Topeka, KS

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000
FXUS63 KTOP 252339
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
639 PM CDT Tue Jul 25 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Wednesday)
Issued at 316 PM CDT Tue Jul 25 2017

This afternoon a broad upper ridge was centered across the southern
plains. Temperatures early this afternoon were in the lower to mid
90s with dewpoints in the upper 60s to lower 70s. The heat indices
were already above 100 degrees. I expect the Heat indices across the
eastern and southern counties of the CWA to top out in the 103 to
106 degree range late this afternoon.

Thunderstorms will develop across western NE and northeast CO late
this afternoon and evening. A complex of thunderstorms may develop
across west central NE into northwest KS. Most models show this
complex of storms weakening before reaching the CWA. However, any
MCV that develops from this complex may provide enough ascent
along with weak isentropic lift for elevated storms to develop
across north central KS during the early morning hours of
Wednesday. These elevated showers and storms should begin to
weaken during the late morning hours of Wednesday and should
remain north of I-70.

An upper trough across UT and northwest CO will round the upper
level ridge axis tonight across the northern high plains, then
will dig southeast cross eastern NE and northeast KS Wednesday
afternoon and phase with a more amplified upper trough across the
Great Lakes by 00Z THU. A surface cold front across central SD and
western NE this afternoon will slowly push southeast into the
northern counties of the CWA by 21Z WED. High temperatures ahead
of the front should reach the mid 90s to around 102 degrees. The
mixing may be a bit deeper across the western half of the CWA
ahead of the front, thus dewpoints may mix down into the mid 60s
but given highs around 102 degrees, heat indices should reach the
104 to 107 degree range. The mix boundary layer will not be as
deep across the eastern counties of the CWA, thus surface
dewpoints may remain in the lower to mid 70s during the afternoon
hours. The combination of temperatures in the mid and upper 90s
and dewpoints in the lower to mid 70s will cause heat indices in
the eastern counties of the CWA to reach the 105 to 110 degree
range. There will be some question on high temperatures across the
northern CWA, especially if the elevated storms in the morning
continue into the early afternoon and residual cloud cover
continues through the afternoon. This would cause highs to be 5 to
8 degrees cooler and prevent heat indices to reach above 103
degrees. But there is a better chance that there will be a break
in the cloud cover during the afternoon hours. I will include most
of the CWA in a heat advisory Wednesday afternoon, except for the
far northern counties where heat indices should remain below 103
degrees.

As the surface front moves southward across the CWA late tomorrow
afternoon and evening, the increasing surface convergence along the
front and the synoptic scale ascent ahead of the H5 trough will
cause thunderstorms to develop across the northern counties of the
CWA. Most numerical models forecast between 2,000 and 3,000 J/KG of
MLCAPE developing ahead of the front. 0-6 KM effective shear is
forecasted to be between 25 to 35 KTS. Storms that develop may
become severe given the environment of moderate CAPE and some
vertical windshear. The primary hazard will damaging wind gusts and
large hail, especially within rotating updrafts. The scattered
storms across the northern counties may merge into a line or line
segments which may produce damaging wind gusts across east central
KS during the early evening hours.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 316 PM CDT Tue Jul 25 2017

Additional convection is expected to develop southwest along the
frontal boundary aft 00Z Wednesday evening. Even though peak heating
has passed at this point, still believe there to be enough elevated
instability and effective shear near 30 kts for a few severe storms
capable of producing damaging winds. Highest threat for the severe
weather however still remains across east central and far northeast
areas where low to mid level shear parameters are stronger. By
Thursday morning, much of the activity will clear the CWA with
northerly winds around 10 mph during the afternoon and partly cloudy
skies. A cool, Canadian 1020 mb sfc ridge settles across much of the
midwest, providing much needed heat and humidity relief. Consensus
highs are generally in the lower to middle 80s through Saturday with
overnight lows in the lower 60s.

Eastern periphery of the ridge axis edges towards central KS on
Friday evening, with weak perturbations developing scattered showers
and thunderstorms towards the western high plains. Depending on
where the ridge sets up, it`s possible a few of these may impact
central KS. Otherwise, the area remains dry through the beginning of
next week as deep troughing occurs across the eastern CONUS,
retrograding the upper ridge further west and placing our area under
northwest flow. Models continue to differ on how long this pattern
holds through mid week with the GFS trying to advertise another
upper trough entering the plains around Wednesday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 634 PM CDT Tue Jul 25 2017

Predominantly VFR conditions are expected through the period,
but confidence is increasing in precip pushing across the area
towards sunrise tomorrow. Did not introduce restrictions with the
00Z TAFS, but if model/radar trends continue, expect some mention
of precip in the 06Z TAF package. Winds will remain from the
south and increase with daytime heating tomorrow, but a front
will begin sinking southward after noon tomorrow.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT this evening for KSZ011-012-021>024-
026-034>040-054>056-058-059.

Heat Advisory from noon to 8 PM CDT Wednesday for KSZ021>024-026-
034>040-054>056-058-059.

&&

$$

SHORT TERM...Gargan
LONG TERM...Prieto
AVIATION...Skow



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