Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 220456
AFDTOP

AREA FORECAST DISCUSSION
National Weather Service Topeka KS
1156 PM CDT WED SEP 21 2016

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Thursday)
Issued at 308 PM CDT Wed Sep 21 2016

West-southwest flow continues aloft with frontal boundary from north
central to southwest Nebraska becoming oriented in a similar
fashion. High cloud continues to stream into the area from tropical
plume with weak returns showing on upstream radars. Cloud heights
remain at or above 12000 feet in Kansas however. Dewpoints slightly
lower than yesterday`s in most areas with values into the lower 60s
in eastern Oklahoma.

Not much change to previous forecast. The most notable alteration
was to trim precip chances a bit more tonight. The front should
remain well north of the area, and models are in good agreement in a
weaker-forced and less wet mid/upper level atmosphere. Weak
isentropic lift still ramps up a bit overnight in the north and west
around 600mb to support small precip chances into early Thursday
morning. Have kept lows a bit cooler than recent days with a
slightly drier boundary layer but increasing cloud cover should help
keep them in the lower 70s for the most part. Should see more
sunshine develop through the day Thursday for another day of highs
around 90.

.LONG TERM...(Thursday night through Wednesday)
Issued at 308 PM CDT Wed Sep 21 2016

By Thursday night a mid-level ridge will be centered over the
Central and Southern Plains with a deep mid-level trough stretched
across the western U.S.  At the surface, low pressure will be
located over the central Rockies, resulting in southerly flow into
the area through Friday.  This southerly flow will support decent
moisture and warm-air advection through the day, which will push
afternoon high temperatures roughly 10F degrees above the seasonal
normals into the upper 80s to around 90 degrees. This deep mid-level
trough will begin advancing eastward across the Rockies Friday night
into Saturday, which will shift the surface low pressure and
associated cold front into western KS early Saturday morning. Models
show that this boundary may track into north central KS by Saturday
afternoon with showers and thunderstorms expected to develop along
and ahead of the front. Model soundings show the cap weakening
during the early/mid afternoon hours to support the development of
surface-based storms. With the southerly flow ahead of the front
helping to advect warm, moist air into the area and with steep mid-
level lapse rates present, expect modest instability to be present
with 1000-2000 J/kg of CAPE. There may be a gradient of 0-6km bulk
shear values near north central KS (across the far western edge of
the forecast area) where models show values ranging from 30-40 kts,
so cannot rule out the potential for some stronger storms that
develop along the boundary to sustain themselves as they advance
into north central KS Saturday afternoon. With this in mind, have
increased to likely PoPs for portions of the CWA Saturday afternoon.
The increasing cloud cover through the day will limit temperatures a
bit with highs in the low/mid 80s.

For Sunday through early next week, no significant changes to the
current forecast.  Any storms ongoing will likely organize into a
line of storms along the slowly advancing cold front and thus slowly
move through the area overnight into Sunday as deep upper level
troughing is slow to move into the Central Plains region.  With the
more parallel flow setup along with deep moisture and lift along
the cold front it is possible that showers and storms may train
over the same areas for a prolonged period of time with continued
ascent along the boundary. PWAT values of over 1.50 to 2.00
continue to hint at very moist atmosphere in place for late
September. Should this type of situation continue to appear to be
likely then a flash flood watch will be possible and conditions
favorable for this setup look to last through at into early
afternoon Sunday.

Have kept small chance POPs in the forecast Monday through mid week
favoring higher chances lifting north with time as model guidance
seems to be different timing wise but in agreement in the fact that
the overall upper system becomes more cut-off in nature and
vertically stacked.  Therefore, some wrap around precip could be
likely mainly in the form of light showers.

Temperatures align into a more seasonal pattern with the upper
trough moving into the region and the cold front ushering in cooler
dewpoints.  High temps for the period top out in the mid 70s while
overnight lows settle into the lower 50s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1154 PM CDT Wed Sep 21 2016

VFR conditions are likely through the TAF period with increased
wind speeds during the late morning through afternoon hours. 2000
foot AGL winds will be in the 30-35 kt range through 12Z, which
should be slightly less than LLWS criteria.

&&

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

$$

SHORT TERM...65
LONG TERM...Drake/Hennecke
AVIATION...Barjenbruch



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