Area Forecast Discussion
Issued by NWS Topeka, KS

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149
FXUS63 KTOP 211746
AFDTOP

AREA FORECAST DISCUSSION
National Weather Service Topeka KS
1246 PM CDT WED SEP 21 2016

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 259 AM CDT Wed Sep 21 2016

General west to southwest flow aloft has caused a frontal boundary
to stall out over portions of NE. Southerly winds will increase
later today once daytime mixing occurs. Gusts could reach the 25 to
30 mph range especially in central KS. High temperatures are
expected to be in the upper 80s to lower 90s, while dew points
remain in the upper 60s to lower 70s. This will result in slightly
lower heat indices this afternoon than yesterday. This afternoon and
evening a shortwave trough will lift out over the stalled boundary.
With a weakening cap and added lift storms should develop along this
front across northwestern KS and central NE. A majority of the
models keep the main forcing confined to that area therefore chances
are fairly low that storms reach the forecast area. Later tonight
chances for showers and storms increase as isentropic lift develops
in the mid levels out ahead of the boundary. Soundings indicate that
the instability will be around 1500 to 2000 j/kg in the evening, but
decrease to below 1000 j/kg tonight. Therefore the threat for any
strong to severe storms appears almost non existent. Perhaps if a
complex were to form along the boundary and become progressive it
could reach portions of northern KS in the evening bringing a threat
for strong wind and hail. None of the models really depict this
scenario therefore general thunder is more likely.

.LONG TERM...(Thursday through Tuesday)
Issued at 259 AM CDT Wed Sep 21 2016

An upper level trough will amplify across the western US Wednesday
through Friday then move east across the plains Saturday and Sunday.

Thursday through Friday night, As the upper trough amplifies across
the western US a downstream upper ridge will amplify across the
eastern plains and lower to mid MS river valley. Expect warm
conditions with highs in the upper 80s to lower 90s. Southerly low-
level flow will keep dewpoints in the mid 60s to around 70 degrees,
thus afternoon heat indices will reach the mid 90s to around 100
degrees across the eastern counties of the CWA.

Saturday through Sunday night, The upper trough will move east into
the high plains on Saturday. The environment may be favorable for
severe thunderstorms to develop along a surface dryline Saturday
afternoon across west central KS given good vertical windshear and
2,000 to 3,000 J/KG of MLCAPE. Some of these strong to severe
thunderstorms may move into the western counties of the CWA during
the afternoon and evening hours.

Eventually the mid and upper level flow will become more meridional
which should end the threat for severe thunderstorms. However, deep
gulf moisture in combination with strong ascent ahead of the deep
upper trough will cause periods of showers and thunderstorms to
continue through Saturday night into Sunday. The GFS model forecast
PWATS to increase to 1.6 to 2.0 inches Saturday night into Sunday.
The ECMWF continues to forecast 2 to 4 inches of rainfall across the
CWA from Saturday night through Sunday. If we see training of
thunderstorms across sections of the CWA Saturday night into Sunday
then there may be some flooding possible. If the models continue
to show this heavy rainfall potential a flash flood watch may need
to be posted for portions of the area for Saturday night into
Sunday.

The upper trough axis and surface front will move southeast across
the CWA Sunday afternoon and evening. This should cause the heavier
showers and thunderstorms to push southeast of the CWA during the
afternoon and evening hours of Sunday.

Highs on Saturday should reach the mid to upper 80s before the
showers and thunderstorms develop during the afternoon hours. Highs
on Sunday will be cooler as a surface front will move southeast
across the CWA during the day plus there will be cloud cover with
periods of showers and thunderstorms. Highs Sunday will range from
the lower 70s across north central KS to the upper 70s across east
central KS.

Monday through Wednesday, the ECMWF and GFS solutions diverge quite
a bit. The ECMWF shears the upper trough moving across the plains
and amplifies the southern section of the H5 trough into a closed
upper low over western TX, then rotates and amplifies this upper low
northeast across central KS into eastern NE by Wednesday. If the
ECMWF solution verifies then we will keep a chance for showers and
thunderstorms across the CWA from Monday afternoon through
Wednesday. The more progressive GFS pattern will keep dry conditions
with northwesterly flow aloft and a large upper low amplifying
across the upper Midwest. Highs temperatures will cool back into the
70s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1246 PM CDT Wed Sep 21 2016

VFR conditions should persist. Somewhat drier surface RH values
anticipated tonight for even less chance for any BR impacts. Will
need to keep an eye on wind shear potential around 11Z as well,
but too unlikely to mention at this range.

&&

.TOP Watches/Warnings/Advisories...
NONE.
&&

$$

SHORT TERM...Sanders
LONG TERM...Gargan
AVIATION...65



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