Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 311141

641 AM CDT Sun Aug 31 2014

...Update to aviation forecast discussion...

.SHORT TERM...(Today and Tonight)
Issued at 418 AM CDT SUN AUG 31 2014

--Large Scale Overview---------------
At 3 AM, surface low pressure was centered in north central SD with
a trailing cold front just entering extreme northwest Nebraska. A
modest low level jet was bringing a bit of low level moisture
advection into the central plains. A rather sharp short wave trough
axis was entering western Kansas while stronger short wave energy
and a jet of stronger mid-level westerlies was quickly moving into
Wyoming. Scattered thunderstorms had developed in western and north
central KS in advance of the incoming short wave trough and
isentropic ascent on the LLJ ahead of the trough axis.

--Through Early Afternoon------------
The area of scattered showers and embedded thunderstorms will track
east through the morning hours, likely decreasing in intensity and
coverage after 10 AM or so as the LLJ wanes. Instability is meager
and elevated within this airmass and no severe storms are expected
through early afternoon. As this initial trough axis moves over the
area, expect a period of a few hours of mid-level subsidence in its
wake. Some models suggest that this will aid in a decrease in
boundary layer dewpoints due to the combination of mixing and
subsidence. If this occurs, expect less in the way of afternoon
instability, although low-level moisture would still likely increase
late in the day in advance of the next trough. During this period,
the cold front will advance into central Nebraska and northwest
Kansas while new surface low pressure intensifies over western
Kansas. Temperatures will warm quickly as early clouds dissipate and
move east, with highs likely surging into the middle 90s.

--Mid Afternoon Through Tonight------
Expect thunderstorms to develop near the cold front in central
Nebraska initially, with activity probably increasing into the
locally backed flow ahead of the surface low in western Kansas by
late afternoon. This early activity should move to the northeast and
east, and should for the most part avoid the forecast area this
afternoon. However, this will depend on the cold front location by
mid afternoon, and could see a few of these storms develop into
north central KS as early 4 PM. The effective front will progress
southeast during the evening as the surface low moves into central
KS and the low level jet rapidly intensifies across the entire area.
Enhanced convergence in the frontal zone will result in widespread
thunderstorm development, eventually making its way into the local
forecast area. The southward progression of the front should slow
markedly in the local area during the early morning hours, which
will result in the northern half of the area having a very good
chance for thunderstorms while east central KS will be on the
edge...but will be likely to have storms as well. Even with the
front still in the area by sunrise, the upper wave and associated
thunderstorms should push off to the east by 12Z.

--Specific Hazards--------------------
The main uncertainty in the extent of various hazards lies in some
question of the dominant storm mode at any given time. Early on,
expect supercells and QLCS structure with embedded supercells. As
forcing along the front increases during the evening, expect storm
mode to transition into dominant QLCS or bowing segments with
embedded rotation. LCL heights early in the event are expected to be
rather high locally, so if storms are in the area during the
afternoon and early evening would expect the primary hazards to be
hail and wind...perhaps in combination. As the evening progresses,
the evolution of storm mode will likely begin to favor damaging
winds with embedded areas of hail. The sharply increasing low level
wind shear is likely to maintain a non-zero tornado threat through
the evening and perhaps into the early morning as 0-1 km helicity in
the 300-500 m2/s2 range will be common and LCLs will decrease with
time. Also expect the storms to remain surface based or near-surface
based well into the evening. So, this non-zero tornado threat will
need to be monitored throughout the event, although some recent
model guidance indicates that an increased potential for widespread
damaging winds may be prevalent. In addition, as the front slows its
progression, could see some back-building along the front or perhaps
a series of more than one progressive bow echoes in northeast KS.
Atmospheric moisture content is very high and some parts of northern
KS are a bit saturated so a mainly localized flash flood threat may
materialize later tonight.

.LONG TERM...(Monday through Saturday)
Issued at 418 AM CDT SUN AUG 31 2014

By Monday morning, a broad mid-level trough will be in place over
the Northern Plains with an area of low pressure slowly tracking
eastward over the region. The cold front that will be supporting
thunderstorm development tonight is expected to be very slow moving,
with most models showing this boundary stretching from central to
northeast Kansas by Monday morning and barely exiting east central
Kansas by Monday evening. Some of the thunderstorm activity from an
MCS that may develop overnight tonight near the Kansas/Nebraska
border may linger into the morning hours across far eastern Kansas.
While this scattered thunderstorm activity may diminish during the
morning and early afternoon hours, several models suggest a
redevelopment of storms along and ahead of the boundary across east
central Kansas as a passing embedded shortwave looks to provide
additional support for this storm development. Also, with this
expected brief break in precipitation through the day, it should be
enough to boost temperatures into the upper 80s/near 90 degrees
across east central Kansas, helping to increase the instability
across the region. Some of these storms may be strong to severe
during the afternoon and evening hours as 2000-3000J/kg of CAPE are
possible along with 40-60kts of 0-6km bulk shear. The primary
threats with the strongest storms will be large hail and damaging
winds. As the mid-level trough associated with this system lifts
toward the Great Lakes region, the mid-level flow will transition to
a more zonal pattern, and it should help to finally push the front
just south of the forecast area Monday night. Scattered showers and
thunderstorms should diminish from north to south Monday night into
Tuesday morning with this slight southward shift in the front.
However, models continue to trend toward having this front lift back
northward over the region as a warm front Tuesday night into Wednesday.
This boundary, with the support of a few weak embedded shortwaves,
should support additional isolated to scattered thunderstorm
development, however there is still model uncertainty with regards
to the coverage of this thunderstorm activity across the forecast
area. Temperatures should warm up quickly on Wednesday as the
precipitation clears out, with highs in the upper 80s to mid 90s.

By Wednesday morning, models show the next mid-level trough moving
into the Pacific Northwest. However, models become more inconsistent
with the development of this trough as it progresses into the
Northern Plains, with the GFS lifting much of the trough into Canada
while the ECMWF deepens the trough over the Rockies and Northern
Plains by the latter part of the week. This translates to model
discrepancies with how far north the mid-level ridge over the
southern U.S. will be able to extend into the Central Plains.
Additionally, while both models agree that this trough will help to
push an area of low pressure through the Central Plains, there are
notable differences in the timing of this next frontal passage. The
ECMWF is more progressive (due to its solution of a deepening
trough) and has the front moving through Thursday night into Friday,
while the GFS is slower and brings the front through Friday
afternoon into Saturday morning. As a result, there is plenty of
uncertainty in the forecast for the end of the week for
precipitation chances, temperatures, and winds. There is the
potential for more widespread showers and thunderstorms with this
late-week frontal passage, but it`s difficult to pin down the timing
of these precipitation chances at this time.


.AVIATION...(For the 12Z TAFs through 12Z Monday Morning)
Issued at 635 AM CDT SUN AUG 31 2014

A round of scattered thunderstorms will likely impact MHK between
12Z and 16Z, with primarily VFR conditions likely even during TS.
These TS may make it as far east as TOP/FOE, but expect weakening
and they should dissipate as they move east. Will have gusty
south winds for much of the day/evening, but otherwise VFR until
strong thunderstorms impact TAF sites this evening into the early
morning. These TS may produce damaging winds as well as large
hail. Expect IFR conditions or worse during strongest storms but
with uncertainty in exact timing, have highlighted the general
time frame with MVFR vis.




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