Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 140447
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1147 PM CDT Fri Oct 13 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Saturday)
Issued at 313 PM CDT Fri Oct 13 2017

A shallow, northeast-southwest baroclinic zone is stalling over
the forecast area, as a frontal wave is drifting northeastward
from central KS. Visible satellite imagery depicts a well-defined
inflection along the eastern edge of low stratus, marking the wave
that is migrating along the corresponding surface front. As this
wave continues to progress toward the lower/middle MO Valley
region tonight, a few showers or storms could develop on the cool
side of the surface boundary in response to nocturnally
strengthening isentropic ascent (the mass response to boundary-
layer decoupling within the warm sector). Present indications are
that antecedent capping aloft, and the limited magnitude of the
ascent, will yield substantial conditionality regarding any
marginal severe hail potential for tonight -- and related mention
has been removed from the HWO.

For late tonight into Saturday, the boundary will advance
northward across the forecast area beneath a long fetch of
southwesterly flow aloft, while an amplified shortwave trough
approaches from the central Rockies. Isentropic ascent ahead of
this wave will episodically be enhanced by a series of minor
midlevel perturbations advancing northeastward ahead of the
trough. The influx of partially modified Gulf moisture --
characterized by middle 60s dewpoints -- and the ascent could
facilitate a substantial increase in convective coverage by early
Saturday morning, with activity spreading northeastward through
the day. This activity and related cloud debris may tend to mute
boundary-layer destabilization on the warm side of a sharpening
cold front that will spread eastward/southeastward from the
afternoon into the evening. While ample flow throughout the
troposphere, associated with limited hodograph curvature, will
support a conditional damaging- gust potential -- for both along-
front and pre-frontal convection -- substantial uncertainty exists
regarding prospects for pre-frontal diurnal destabilization. As
the front consolidates and overtakes richer boundary-layer
moisture, the strongest-storm potential may tend to focus
southeast of a Holton to Council Grove line. Limited mid- level
lapse rates and the anticipated quasi-linear convective mode tend
to suggest that isolated damaging wind gusts should be the primary
risk with this activity.

.LONG TERM...(Saturday night through Friday)
Issued at 313 PM CDT Fri Oct 13 2017

The vast majority of the sensible weather impacts for the period
will be on Saturday night as the region undergoes an airmass regime
change. The main issue will be a strong wind threat on the backside
of the departing storms.

Shower and thunderstorm activity attendant with the passage of cold
frontal boundary should be ongoing at the start of the period--
primarily south and east of a line from Abilene to Hiawatha. This
precip will quickly exit the region as colder and drier air
infiltrate from the north. This push of CAA, combined with sharp
pressure rises of 4-7 mb/3hr and a tight pressure gradient, will
lead to a short period of very strong winds on the heels of this
departing complex. CAA in the BL will steepen low-level lapse rates
and permit the mixing of stronger winds to the surface. NAM bufr
soundings show this layer mixed to 900 mb between 00-12Z Sunday with
winds at the top of the layer pushing 40+ kts. This period of strong
wind looks to only last about 3 to 6 hours in any one location
before decoupling and a relaxation of the pressure gradient take
place. Have raised the forecast winds to the high end of the model
guidance spectrum for tomorrow night with sustained values in the 20
to 30 kts range with gusts nearing 40 kts.

After tomorrow night, the pattern settles down for much of the
upcoming week. Surface high pressure will settle in across the
Southern Plains on Sunday and Monday. Widespread lows in the upper
30s are expected for Sunday night, with patchy frost possible in low-
lying areas that decouple. Split upper level flow will dominate the
weather pattern over the region for the remainder of the week with
light southerly winds/WAA gradually increasing as the week
progresses, allowing surface temps to rebound back into the 70s for
Monday and beyond.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1139 PM CDT Fri Oct 13 2017

VFR prevails at KTOP and KFOE with the potential for MVFR
stratus to spread into KMHK in the 08Z-11Z time frame from
northern Kansas. Increasing ascent tonight will develop scattered
showers with isolated thunder entering from the southwest in the
09Z-11Z time frame. Other concern is the strong winds this
afternoon veering from the south to the northwest with the fropa.
Current guidance times the shift in the 22Z to 02Z time frame.
Accompanying the front will be scattered TSRA and likely
temporary,restricted ceilings. Northerly winds behind the front
are actually stronger above 15 kts sustained through 06Z.

&&

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

$$

SHORT TERM...Cohen
LONG TERM...Skow
AVIATION...Prieto



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