Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 211730

Area Forecast Discussion
National Weather Service Topeka KS
1230 PM CDT Sat Oct 21 2017

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 314 AM CDT Sat Oct 21 2017

Water vapor satellite imagery at 0730Z shows shortwave that
developed isolated convection has moved off to the northeast and was
located over southwest Iowa. Further west the upper trough was
progressing across the Rockies with 50 to 55 kt mid level jet
rounding the base of the trough. Clouds have started to form across
eastern and central Kansas with an increase in stratus noted since
06Z. EML will continue to increase near and above 850 mb through the
morning hours today. Tight pressure gradient continues across the
area this morning east of the surface low in southwest Nebraska
which has kept gusty winds going through the night.

For today, the main concern will be severe weather potential with
the approach of the upper trough and attendant cold front. Models
bring the front into north central Kansas by mid day with the front
then progressing southeast across the CWA through the afternoon and
early evening hours. Still looking like the onset of convection
should be by early afternoon with increasing ascent and steep mid
level lapse rates around 8 C/km overspread northeast Kansas as well
as convergence along the front. Dew points in the lower 60s are
expected ahead of the front along with good moisture transport
across northeast Kansas in the afternoon and evening hours. MLCAPE
values around 2000 J/kg are forecast across the area this afternoon
and 35 to 45 kts of effective shear. High resolution models show
250-300 m2/s2 of helicity primarily across south central to
northeast Kansas along the frontal boundary this afternoon.
Initial semi discrete cells are expected with the onset of
convection with all modes of severe weather possible before
transitioning into a wind and hail threat. There is the potential
for an isolated QLCS tornado into the early evening hours. Most of
the precipitation should come to an end as the front progresses
through the cwa this afternoon and tonight. Additional concern
will be locally heavy rainfall, especially across east central
Kansas where rainfall will be efficiency will be best as strong
moisture transport continues through the evening hours. Isolated
rainfall amounts could exceed 3 inches mainly south of a Lawrence
to Burlington line. Highs today will only warm into 70s before
cooling off into the upper 30s and 40s overnight.

.LONG TERM...(Sunday through Friday)
Issued at 314 AM CDT Sat Oct 21 2017

Forecast remains on track for northwest flow set up to take shape
into early week through next weekend.  A couple deeper troughs dig
into the northern Rockies/Plains and make their way into the Central
Plains as well.  The first cold front arrives late day Monday and
should be a dry passage but manifest itself as the first shot of
cool Canadian air which will quickly slide off to the east of the
area.  Overnight lows should bottom around 40.  Highs remain around
normal, generally around 60-70 degrees.  A deeper upper trough
arrives with an Alberta type low pressure system Thursday night into
Friday morning. Although right now, there is too much overall model
variability to tell for sure, the general sense based on profile
temps, overall profile saturation, and ensemble data is there will
be a chance that some showers on Friday could transition into a
rain/snow mix. But, right now that may be complicated by overall low
level temps not cool enough for the daytime hours.  Regardless, fall
type weather is in store for the week ahead.  Temps overnight on
Friday could fall to near freezing.  Probably a good chance for
frost and some areas could see freezing temps.  A little far out to
be too confident in how low temps can go, but something to focus on
at this point.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1230 PM CDT Sat Oct 21 2017

A cold front will move across the region later this afternoon
into the evening. Thunderstorms with gusty winds, hail, and
restricted visibilities/ceilings will be possible with this
activity. TOP and FOE will be the most likely TAF sites impacted
by this activity, in the 21Z-23Z timeframe. Ahead of the front,
gusty southerly to southwesterly winds are expected, with winds
becoming westerly to northwesterly following the frontal passage.
Ceilings should rise behind the front with decreasing cloud




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