Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 201746

Area Forecast Discussion
National Weather Service Topeka KS
1146 AM CST Wed Feb 20 2019

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 342 AM CST Wed Feb 20 2019

Upper trough axis on water vapor imagery this morning continues to
lift northeast over the western high plains. Bands of light snow
mixed with freezing drizzle continues to develop, lifting north and
east at the current hour with an embedded vorticity max noted within
the 500 mb flow from southern KS through central and eastern Kansas.
An additional half to inch of snow is possible through 12Z this
morning with precipitation exiting the CWA after 8 AM.

Much drier air surges northward today, eroding cloud cover by the
afternoon. Sided with consraw guidance for highs today given the
gusty westerly winds coming off the snow covered ground. Readings in
the low 30s are common with perhaps middle 30s southeast of
Interstate 35 where they did not receive as much snow.

Surface high pressure shifts into the Missouri valley tonight with
clear skies and light winds. Latest NAM and SREF are hinting at the
potential for shallow fog to develop, however given the potential
for mid and high clouds to increase northward by 12Z have not
mentioned it in the forecast for now.

The active upper flow pattern persists with another stout upper low
developing across the Intermountain West on Thursday. Inverted sfc
trough deepens in response to the southwest with qpf developing on
the GFS/Canadian/ECMWF in vicinity of a front. Uncertainty varies on
the available moisture at the time with forecast soundings showing
ample dry air below 700 mb. The GFS solution is also above freezing
in the afternoon so precip type could transition to a light rain and
snow mix. Little to accumulations are expected as the better forcing
quickly exits the area aft 00Z. Elsewhere, mid clouds increase with
a gradual warmup into the upper 30s and lower 40s.

.LONG TERM...(Thursday through Tuesday)
Issued at 342 AM CST Wed Feb 20 2019

The long range forecast continues to be dominated by a diverse
weather pattern.  A mix of winter and spring type weather looks
possible for the Saturday time frame with the Saturday afternoon and
evening period shaping up to be quite diverse in terms of weather
types from northwest to southeast.

A digging western CONUS high amplitude trough will advance east on
Friday with main trough axis from the northern Rockies through the
Desert Southwest vicinity.  A lead wave works out of the Rockies on
Friday afternoon enhancing a WAA regime over the region.  Thus,
expecting widespread showers and rain to develop over the forecast
area into Friday afternoon.  Temperatures and dewpoints should
generally be on the rise along with PWATs.  This sets up and
increasingly unstable environment that should support mostly rain
over much of the area with MUCAPE on the rise as well.

This should foster an environment into Saturday where thunderstorm
potential will be on the rise especially over east central KS as
dewpoints increase to the upper 40s and possibly low 50s.  Saturday
will see a potent shortwave lift into the area with strong PVA and
ascent ahead of the main axis.  Increasing lapse rates ahead of
strong height falls will foster quick deep cyclogenesis as the wave
takes on a negatively tilted aspect.  There is a degree of
variability on the actual position of the surface and upper low
feature as well as the overall strength.  But, regardless of the
solution, the timing looks to be consistent.  The differences come
in to play with regards as to how much instability can develop over
the region on Saturday.  Best chance for thunderstorms looks to be
over the east central areas of KS as 0-6km shear will be very
impressive.  Instability actually looks a bit more impressive than
previous model runs as well, so can`t rule out thunderstorms for a
period of time until the axis of instability and the warm sector
advance east of the region.  Perhaps the biggest question at this
point comes on the backside of the system and developing deformation
zone.  CAA is stronger in the low levels with the ECMWF which
suggests a quicker transition to a freezing rain setup before full
transition to all snow with cold enough temperatures through the
column to support snow.  The deepening nature of the system suggests
that strong kinematic responses will be in play as well.  This means
very strong winds, especially with CAA component.  This could mean
that Saturday night would see not only snow but blowing snow work
across north central areas and counties along the KS/NE border
region before the system finally exits the region overnight and
early Sunday morning.

Overall colder temperatures work into the area before the upper flow
begins to transition to a more progressive zonal flow pattern with
another quick shot of mixed precipitation and transition to snow
late Monday into Tuesday time frame.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1146 AM CST Wed Feb 20 2019

MVFR stratus deck has become more prominent as daytime mixing
increases and TOP and FOE could have some brief cigs resulting.
Drying of the low levels should bring VFR conditions to all sites
by 21Z. Will need to continue to monitor wind and cloud trends
with some potential for lower visibility in BR/FG to develop in
the 07Z-15Z window. Melting of the fresh snow will likely add to
the boundary layer moisture in an otherwise dry lower troposphere,
though some increase in both winds off the surface and cirrus
takes place after 09Z. Will include a 6SM mention at this point.




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