Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 210456
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1156 PM CDT Fri Oct 20 2017

.UPDATE...
Issued at 1151 PM CDT Fri Oct 20 2017

Elevated convection has developed ahead of an approaching H300
shortwave trough that is lifting through south central KS. These
showers will continue to race northward over the next few hours
and mostly push out of the CWA by 06-07Z with the wave. A few
isolated showers may linger near the NE border through the
remainder of the night.

&&

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Saturday)
Issued at 401 PM CDT Fri Oct 20 2017

The short term period is expected to consist of continued gusty
winds along with the threat for severe thunderstorms Saturday
afternoon and evening.

As of 19Z Friday afternoon, a broad upper level trough was
positioned across the western CONUS. Water vapor imagery reveals
an embedded shortwave trough within the cyclonic flow extending
from eastern WY, southeast to southeastern TX. Two vort maxima
were also noted, one near Elkhart, KS and the other near Lawton,
OK. Only resultant impact from the 2 features has been increased
mid and high level clouds. Expect this trend to continue as they
lift north and east across the central Plains. At the surface, lee
cyclogenesis continues across eastern CO coupled with a broad
surface anticyclone across the southern CONUS has yielded a tight
pressure gradient across the CWA. Much of the area has seen
sustained winds near 20 MPH with gusts of 30-35 MPH. Richer
boundary-layer moisture continues its march northward, with low
60s dewpoints approaching the KS/OK border. Transitioning into the
overnight hours -- WAA and gusty winds will continue in response
to the persisting surface pressure gradient and a 60 knot H85 jet
overhead. That being said, isentropic ascent will remain limited
across the area through tonight. Maintain chance PoPs overnight,
mainly north of I-70 where the strongest ascent appears to take
shape. Temperatures will only fall into the mid 60s with
increasing clouds and a coupled BL in place.

Saturday -- widespread cloud cover is expected across the area
Saturday morning as Gulf of Mexico moisture continues advancing
northward. By midday Saturday, surface dewpoints are forecast to
reach the low to middle 60s ahead of the cold front. Good
agreement amongst synoptic and hi-res guidance with the position
and movement of the cold front through the CWA. By midday
Saturday, solutions suggest the front will be positioned from
north-central to central KS. Expect thunderstorm development along
the front during the early afternoon hours in association with
upper jet divergence and surface heating immediately preceding the
front. Capping associated with an impressive EML above H85 should
limit thunderstorm coverage initially. Steep midlevel lapse rates
near 7.5 C/km associated with the EML surmounting boundary-layer
dewpoints in the low to mid 60s will yield MLCAPE values near 2000
J/kg. Effective shear values of 35-40 kts will yield organized
convection, mostly likely as supercell structures initially.
Ultimately, the primary hazards will include large hail and
damaging winds. Although, hi-res solutions suggest impressive low
level shear profiles, with effective helicity values approaching
300 m2/s2. Thus, some tornado potential will exist with semi-
discrete storms in the 19-22z timeframe. As the front continues
its east-southeastward progression, trough-preceding ascent and a
strengthening LLJ should encourage an increase in thunderstorm
coverage into late afternoon and evening, with convective modes
transitioning to quasi-linear segments. The risk for severe wind
and hail will continue, although any pre-existing vorticity from
parent supercells could yield a QLCS tornado threat through the
early evening hours. Any lingering thunderstorm activity should
exit the region early Sunday morning.

.LONG TERM...(Saturday night through Friday)
Issued at 401 PM CDT Fri Oct 20 2017

After severe weather exits the area Saturday evening main focus is
on the deep 500 mb trough forming over the Central Plains on
Thursday night and Friday.  12Z ECMWF and GFS are in good agreement
in developing a 500 mb trough over Alberta Canada on Thursday and
then deepening it and dropping it into the Central Plains into
Friday.  Expect to see an area of precipitation late Thursday night
into Friday as the trough ejects into the plains. Forecast soundings
support the mention of a rain/snow mix as the low levels cool with
the strong cold air advection behind the front Thursday night.
Guidance temperatures on Friday look too warm based on forecast
soundings but this will be dependent on how fast skies clear on
Friday.  Good chance the area will see a hard freeze Friday night
and Saturday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1151 PM CDT Fri Oct 20 2017

MVFR stratus is expected to develop across much of the area
between 08-10 Z this morning and persist into the early afternoon.
Gusty southerly winds will likewise persist throughout the night
and into tomorrow, switching to the northwest as a cold front
pushes in from the NW. This front will also spur the development
of thunderstorms this afternoon, some of which may be severe.
These storms should push east and be clear of the area by 00-02Z.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...Skow
SHORT TERM...Baerg/Cohen
LONG TERM...Walawender
AVIATION...Skow



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