Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 182321

Area Forecast Discussion
National Weather Service Topeka KS
621 PM CDT Fri Aug 18 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Saturday)
Issued at 323 PM CDT Fri Aug 18 2017

Early this afternoon the mid- level trough axis was tracking just
east of the CWA, with water vapor imagery highlighting two
embedded waves: one tracking over far eastern Nebraska and the
second one over far southeast KS tracking into southwest MO. At
the surface, low pressure continued to advance eastward across the
area with surface observations showing the cold front hovering
near the KS/NE border and a pre- frontal boundary extending
roughly from Council Grove to Topeka as of 19z. Regional radar
showed scattered showers and thunderstorms developing along the
far western edge of the IA/MO border near the cold front and over
southwest MO from the embedded shortwave. As this cold front
gradually tracks southeastward across the CWA late this afternoon
through this evening, there is still model uncertainty in the
exact timing and coverage of thunderstorm development. Southerly
flow ahead of this boundary allowed for some moisture advection
into far eastern KS, with dewpoints rising into the upper 60s to
low 70s. These dewpoints combined with high temperatures in the
upper 80s to near 90 degrees resulted in decent destablilzation
across eastern KS with CAPE values of at least 3000 J/kg expected.
Additionally, 0-6km bulk shear values still look decent at
35-45kts. These parameters combined with little to no cap in place
are certainly supportive of the development of some strong to
possibly severe thunderstorms. The main question is whether or not
the front can move into this favorable area in time to provide
adequate forcing to allow these storms to initiate in the first
place. Latest short-range guidance is starting to show a few more
storms developing along this boundary, however the timing of this
initiation has steadily been trending later (around 23Z and
after). The main focus for this scattered thunderstorm development
will be generally along and southeast of I-70, with scattered
storms diminishing across far east central KS by late this
evening. The main hazards with any strong storms that develop will
be large hail, damaging winds, and some locally heavy rainfall.

Cloud cover will quickly diminish overnight behind the front, with
light and variable winds.  This combination of clear skies, light
winds, and some surface moisture from the evening storms may result
in some areas of patchy fog developing by Saturday morning,
primarily across portions of east central KS.  Dry conditions are
expected for Saturday, with the return of southerly flow allowing
for decent warming into the afternoon hours. High temperatures are
expected to rise into the low/mid 90s from east to west across the
CWA, with afternoon heat indices into the mid/upper 90s.

.LONG TERM...(Saturday night through Friday)
Issued at 323 PM CDT Fri Aug 18 2017

There continues to be some chance for precip Saturday night
through Tuesday evening. Aside from a couple features noted in the
model solutions, precip chances are more conditional based on
mesoscale details. The first feature models show some agreement on
is a convectively induced vort max that they propagate east along
the NEB state line Saturday night. The NAM and GFS along with
their respective ensembles show some decent precip across northern
parts of the forecast area during the overnight hours associated
with the forcing. Because of this have increased POPs Saturday
night to around 50 percent. If models show some consistency with
the vort max, we`ll need to increase POPs for nocturnal
thunderstorms. The second feature noted in the model solutions is
a synoptic shortwave that moves through the northern plains and
upper midwest, bringing a frontal system through the forecast area
on Tuesday. There seems to be some agreement among the long range
solutions for this and so the forecast has some likely POPs in
for the early morning hours Tuesday as the upper level wave begins
passing through. The frontal system eventually pushes south of
the forecast area Tuesday evening with a surface ridge building
south effectively ending precip chances for the rest of the week.
In between the two waves, models depict a west or southwesterly
flow aloft with the potential for perturbations within the flow.
So with a conditionally unstable airmass some low chance POPs
remain in the forecast for Sunday through Monday evening.

For Monday and the eclipse specifically, models continue to show
moisture over eastern KS with signals for mid level clouds through
the day. So there is no change from the previous forecast for
partly cloudy skies during the eclipse.

Temps for Sunday and Monday look to remain warm with highs in the
lower and mid 90s while lows bottom out in the lower 70s. The
frontal system on Tuesday is expected to keep highs a little
cooler with readings forecast to be in the lower 80s for Wednesday
through Friday due to the surface ridging progged to be over
northeast KS.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 616 PM CDT Fri Aug 18 2017

Isolated TSRA is possible near terminals between 00Z and 02Z.
Latest short term guidance keeps most of the area dry so will
remove the VCTS mention. As the weak frontal boundary passes
through by 01Z, light winds veer to the northwest before shifting
to the south by 05Z. There is a consistent signal for MVFR fog to
develop in low lying areas near KTOP. Mentioned 4 SM at this
issuance however timing and visibility may need to be adjusted.




SHORT TERM...Hennecke
LONG TERM...Wolters
AVIATION...Prieto is the U.S. government's official web portal to all federal, state and local government web resources and services.