Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 201125

Area Forecast Discussion
National Weather Service Topeka KS
625 AM CDT Fri Oct 20 2017

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 355 AM CDT Fri Oct 20 2017

Surface low pressure trough over eastern Colorado this morning will
deepen through the day increasing the pressure gradient across
central and eastern Kansas. Gradient winds are expected to increase
to advisory levels generally along U.S. 81 Highway by afternoon with
gusts to near 42 mph and sustained winds around 30 mph. Therefore
will issue a wind advisory for Ottawa, Cloud and Republic Counties.
Water vapor imagery this morning at 08Z shows a shortwave over
southeast New Mexico as well as an increase in high level moisture
across the Plains with increasing cirrus over Kansas. Today the wave
over New Mexico this morning is forecast to lift northward across
Western Kansas this morning then into the Dakotas by evening. Model
sounding s show increasing moisture centered around 850 mb by this
afternoon with cumulus developing across northeast Kansas. High
temperatures today are expected to be a cooler than yesterday due to
the presence of clouds, but will still be above normal with readings
in the upper 70s.

Tonight good moisture advection occurs as the low level jet
increases. Models are in agreement with increasing cloud cover as
stratus deck forms across central and eastern Kansas. There may be
enough ascent amid an increasing EML tonight for a few isolated
thunderstorms, but looks to be better focused across Nebraska and
Iowa. Will leave a small chance in after midnight and expect most
areas to remain dry. Lower boundary layer will remain mixed tonight
and along with the cloud cover will keep temperatures mild with lows
in the mid 60s.

.LONG TERM...(Saturday through Thursday)
Issued at 355 AM CDT Fri Oct 20 2017

Forecast remains largely on track for a few elevated showers and
storms on Saturday morning before the main neutrally elongated
progressive upper trough emerges from the Rockies into the
afternoon. This trough will cause lee side cyclogenesis to take
place ahead of it over southern KS increasing moisture transport
into the area.  Mid 60 degree dewpoints should pool along and ahead
of the cold front.  This will set the stage for destabilization to
take place into the early and mid afternoon time frame.  Concerns
with convection firing too early are due to a fairly strong EML in
place over northeastern and east central KS.  But most model
sounding data has this EML more quickly eroding into the late
afternoon as lapse rates aloft steepen with the advancing trough.
Mid level lapse rates should be around the 7C/km range which yields
MLCAPE values in 1500-2000 J/kg range.  Can`t rule out any hazards
with the a quickly developing line of storms expected as the cap
breaks with strong cold frontal lift and forcing for ascent moving
overhead with effective shear values easily between 40-50 kts
overspreading the area.  Also, 0-3 km helicity values could easily
be above the 200 m2/s2 range, which presents some concern for low
level rotation as well.  Initial storm modes would likely be
discrete supercellular (with a hail and non-zero tornado threat)
then quickly forming a line of storms as shear vectors run mostly
parallel to the front with strong winds becoming the main hazard.
PWAT values will be high with plenty of moisture in the atmosphere,
so heavy rain will be of concern with all storms, but the
progressive nature should minimize the flash flooding threat

Main message for Saturday continues to be the awareness of severe
weather safety with many outdoor activities typical of a fall
football Saturday.

The rest of the weekend after Saturday night and into the work week
should remain dry and pleasant temperature wise for fall with more
seasonal temperatures in place.  High temps will see some low 70s
but mainly remain in the 60s while lows should bottom in the 40s and
perhaps some 30s.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 625 AM CDT Fri Oct 20 2017

Winds will be the main concern for this forecast. LLWS will end
around 14Z this morning. Winds will increase to around
18 to 20 kts with gusts to 28 kts by 18Z then continue through the
end of the period. MVFR cigs forecast to develop after 08Z


Wind Advisory from 11 AM this morning to 7 PM CDT this evening
for KSZ008-020-034.



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