Area Forecast Discussion
Issued by NWS Kansas City/Pleasant Hill, MO

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FXUS63 KEAX 212309

National Weather Service Kansas City/Pleasant Hill MO
609 PM CDT Sat Oct 21 2017

Issued at 303 PM CDT SAT OCT 21 2017

Early afternoon satellite imagery, surface observations and radar
returns all point at a large trough, who`s axis is currently
swinging through the Rocky Mountains into the Northern Plains, is
helping push a strong cold front quickly east this afternoon. This
has been well advertised by the many models for many days now, with
details in the latest batch of model runs simply reinforcing the
concerns for strong to severe thunderstorms this evening, though
only for a small window in time.

Winds ahead of the front this afternoon have continued to be very
strong with sustained southerly speeds in the 20 MPH range, gusting
to near 40 MPH, thanks to the mixing ahead of the front and a strong
low level jet, as 850mb winds are noted around 45 to 50 knots. A
little scattering in the skies ahead of the front has allowed
surface temperatures to climb into the low to mid 70s just ahead of
the front this afternoon, resulting in surface CAPE values in
between 1500-2000 J/KG. This has helped kick off a line storms,
currently stretching form Wichita to Topeka; with a fine line of
the actually front still about 20 miles farther back to the west.
Shear is rather strong ahead of the front thanks to the jet
structure, with 45 to 55 knots of Effective bulk shear, oriented
to the northeast, noted with the line of developing storms.
Convection is struggling a little at this time as shear is likely
over powering the instability, but think as the afternoon wears on
there will be sufficient lift, especially as the front catches up
to the line of storms, for the storms to quickly grow upscale
into strong to severe storms. The main mode of severe weather will
be the wind given the orientation of the shear, but strong low
level helicity mean that an isolated tornado can not be ruled out
with storms that might lead the front and/or occur within the
inflection points along the progressive squall line. That said,
the window of opportunity will be a bit small for severe weather
this evening as the progressive front will likely undercut
convection. But, that might then lead to hydrologic concerns as
precipitable water values around 1.4 inches are noted, which will
allow over running moisture to produce several hours of efficient
rain behind the leading line of strong storms. Current forecast of
rainfall totals would be for a widespread inch to to inch and a
half with locally higher amounts around 2 to 3 inches in areas
where discreet embedded circulations enhance rainfall rates.

As for timing...currently, given radar, satellite and model trends,
thoughts are that the main line of strong to severe storms will
start affecting our section of extreme northeast Kansas and
northwest Missouri between now and 5 PM, with the front and
associated storms nosing into the west side of Kansas City between
5 and 6PM, and areas to the south of Kansas City between 6-8 PM,
with areas farther south waiting closer to 8 PM. As the front
undercuts the strong storms, expect the severe threat to taper off
quickly past 8 PM this evening as the line moves east.

Rest of the forecast...Sunday will still be rather nice as wind
speeds will be more in the 5 to 10 MPH range under clearing skies,
allowing afternoon highs to reach through the 60s. However, there
will be some cooler temperatures later this week as a prevailing
northwest flow dominates our section of the Plains States,
allowing temperatures to roller coaster up and down a couple times
this week.


.Aviation...(For the 00Z TAFS through 00Z Sunday Evening)
Issued at 603 PM CDT SAT OCT 21 2017

Strong thunderstorms will affect the terminal sites through the
early evening ahead of and along a cold front. Aside from KSTJ,
where ceilings will likely remain MVFR, VFR cloud bases are
expected during peak activity. MVFR to possible IFR visibilities,
however, are likely as the main line of storm pushes through the
early period. Gusty surface winds are also likely out of the SSW,
with gusts approaching 30 to 35 kts. Rain showers will then
continue overnight behind the frontal boundary before conditions
quickly improve early Sunday morning.




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