Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 190442

Area Forecast Discussion
National Weather Service Topeka KS
1142 PM CDT Tue Apr 18 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Wednesday)
Issued at 256 PM CDT Tue Apr 18 2017

Currently, WV and Vis imagery show a shortwave and associated
cyclone over the Great Lakes region.  Subtropical moisture is
advecting north into eastern Texas.  Broad southwest flow is setup
over the western CONUS with mostly zonal flow over the central CONUS
and local area.  With WAA advection increasing this afternoon, the
current frontal system has stalled out over eastern NE into western

The rest of the afternoon will continue to see south winds in the
mid teens.  Mixing has allowed temps to increase into the lower to
mid 70s.  Highs today should top out in the upper 70s at many

Tonight, as isentropic lift increases and maximizes generally around
and north of the KS/NE state line, there is a chance for some
elevated thunderstorms to become severe for a period of time.  The
main threat with any storms that do develop into the northeastern
and north central KS will be that of a hail threat.

Wednesday, severe weather will be the main concern heading into the
afternoon as a vigorous shortwave pulls out of the Central Rockies.
The boundary layer should become well mixed along with WAA
increasing into the afternoon with temps in low 80s and dewpoints
generally in the 60s. While the atmosphere should remain well capped
into the afternoon hours, it will be possible that late afternoon
could see that cap degrade and allow some storms to begin forming
mainly over north central KS before becoming more widespread later
in the evening.  With MLCAPE values from 1500-2000 J/kg and deep
layer shear in the 30-40kt range, storms could find themselves in a
favorable environment for rotating updrafts to grow with the
stronger discrete cells.  While, not an ideal tornado threat, there
will be a better severe hail and wind threat in place with these
storms before they grow upscale and form more a line.  The NAM and
ARW solutions suggest more suggest a stronger push from the cold
front once it becomes activated than some other solutions.  The GFS
and EC don`t appear to be as aggressive of solutions suggesting a
more broken line of storms.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 256 PM CDT Tue Apr 18 2017

Thunderstorms should be ongoing along a cold front bisecting the
CWA from NE to SW at the onset of the period Wednesday evening.
With SBCAPE values of 1500 to 2000+ J/kg and 0-6 km shear values
of 30-45 kts, there remains a threat for strong to severe storms
into the overnight hours. With H500 winds at about a 45 degree
angle to the boundary orientation, discrete cells will be possible
initially before growing upscale into a line segment as the cold
pools merge. Large hail would be the main threat with these storms
along with pockets of damaging winds. Given the SSW winds out
ahead of the front and 0-3 km shear values of 10 to 20 kts, the
tornado threat will be minimal. However, this threat would need to
be reassessed should any outflow boundary from morning convection
propagate south into the area.

Modest CAA behind the boundary will drop H850 temps from +16 to +7 C
between 00Z Thu and 00Z Fri. High temps by Fri will likewise fall
into the 50s with lows in the 40s. The GFS has come more in line
with the EC with a further south and east track of a developing
surface cyclone late Friday and Saturday. This track will pull the
associated wrap-around precip also further south and have started to
trend down POPs slightly, especially in the northern CWA. High
pressure will shift east of the area by midday Sunday, allowing
warmer air to flow back north into the area.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1126 PM CDT Tue Apr 18 2017

VFR conditions expected through tomorrow late afternoon. A line a
storms will form west of the taf sites tomorrow and move eastward
during the evening. Reduced visibility and gusty winds will be
likely with the storms. The details on the flight conditions are
uncertain at this point, but brief periods of MVFR/IFR are
possible. The rain may be coming to an end at MHK towards the end
of the period.




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