Area Forecast Discussion
Issued by NWS Topeka, KS

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000
FXUS63 KTOP 302034
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
334 PM CDT Tue Sep 30 2014

.SHORT TERM...(This Evening through Wednesday)
Issued at 330 PM CDT TUE SEP 30 2014

Broad long wave trough continues a slow progression east today as a
very strong short wave trough has surged into South Dakota and then
slowed today. Several weaker short wave troughs are moving through
the flow of the main system with influence as far east as north
central Kansas. One upstream wave evident in central CO will
influence the area tonight while a much stronger short wave and
attendant jet streak was evident in water vapor in central Utah back
into northern CA...and will impact the area on Wednesday. At the
surface, scattered showers and embedded thunderstorms continued much
of the day over north central KS. Here, a zone of weak low level
convergence has been in place, extending from a surface low on the
OK panhandle northeast and then arcing back to the central SD
surface low. Modest instability of 1000-1500 J/kg has developed just
west and southwest of this precip in central KS where there has been
more ample sunshine today.

Through Tonight:
Current shower and t-storm activity in northern KS will gradually
shift northeast and remain un-impressive. However, additional storms
are expected to continue to develop from north central KS back to
the SW amidst the modest instability. Effective shear of 40+ kts
will be sufficient for storm organization in areas with sufficient
instability. Mid level lapse rates are weak though, and it seems
that it will be rather difficult to get a strong updraft...the
exception being if supercell structure can develop. Also, as this
activity moves east through the evening and overnight, it will
encounter weakening instability even of the elevated variety. While
severe weather is thus unlikely overnight, the LLJ will focus over
the area with substantial 850 hPa moisture transport locally. After
4 AM, the LLJ will veer a bit and may become favorable for a slow
backbuilding situation in northeast KS and perhaps a training
situation prior to the backbuilding. This in combination with PW
values in the 80-90th percentile for this time of year suggests
heavy rain likely. Flash flood guidance is quite high (3-4" in 6
hours) so will not issue a flash flood watch at this time, but
certainly can not rule out a bit of flash flooding especially around
sunrise.

Wednesday - Wednesday Evening:
This period remains complicated. There is potential for severe
weather across the area but a bit of uncertainty in how severe it
will be and where the preferred locations will be. The current
thinking, subject to change, is this:

Morning thunderstorms, still potentially exhibiting some
backbuilding and heavy rain characteristics will continue but shift
east of the area by around noon as weak large-scale subsidence
overspreads the area in the wake of a short wave trough. Clouds may
or may not break up a bit, but the interesting thing is that it
seems that moderate instability of 1500+ J/kg of CAPE will develop
even if clouds don`t break up as the driving factor for increased
instability will be an influx of low level moisture (upper 60s
dewpoints). IF breaks in the clouds allow for more heating, the
severe threat seems likely to increase from an already elevated
state. A surface low will intensify over central KS by afternoon
with a synoptic warm front/inverted trough is likely to extend
northeast across north central KS. A punch of dry air will wrap
south of the surface low into south central KS by early evening
while an intense upper jet streak moves into the area for the
evening as well. Low level winds up to 800 hPa will back in advance
of the intensifying sfc low by late afternoon, greatly intensifying
the low level helicity over eastern KS (and to a lesser extent
northern KS) by evening. Some model guidance indicates that outflow
boundaries from early day convection may play a role in the
afternoon convective evolution but this remains uncertain. If
outflow boundaries exist, they would certainly become a focus for
initiation and enhanced vorticity. Wherever storms develop during the
late afternoon and evening hours, the likely storm mode is discrete
to semi-discrete supercells with all hazards possible. The tornado
threat appears to be maximized in east central KS between 6 PM and
10 PM while still possible on either side of that window and to a
lesser extent in northern KS. Given the magnitude of low level
shear/SRH in conjunction with the strong deep shear, a few strong
tornadoes may be possible. Inhibition will increase by late evening
with a decreasing potential for surface based storms...and thus
decreasing tornado potential...although still non-zero through
midnight. Wind and hail potential will continue through the period
with any organized storms, and localized heavy rain/flash flooding
will also be possible but more dependent upon training as storms
should be progressive.

.LONG TERM...(Wednesday Night through Tuesday)
Issued at 330 PM CDT TUE SEP 30 2014

By 06Z Friday widespread showers and thunderstorms should be
ongoing. A strong low level jet will persist mainly across the
southeastern half of the forecast area. Although the better
convergence on the the nose of the jet could push into MO during the
pre-dawn hours. By sunrise the sfc front should be located along or
just west of the KS turnpike. The ECMWF is still the fastest
solution with the progression and timing of the front. During this
timeframe the main trough and shortwave energy eject into the
central and southern plains supplying plenty of large scale lift. A
majority of the models still depict a decent amount of elevated
instability, and the deep layer shear increases to 50-60 kts
therefore storms in the morning could still be capable of producing
large hail.

By late morning instability ahead of the front increases
significantly, and soundings show a brief window for surface
convection by noon mainly across east central KS. As the main trough
passes over central KS thunderstorm coverage increases along the
front, and expect that given cape around 3000 j/kg and 50 kts of
deep layer shear there is a risk for severe. The severe threat will
quickly diminishes once the front leaves the area, which should exit
sometime in the early afternoon. If the ECMWF solution is correct
the front should clear our area by noon limiting the severe
convection. Behind the front weak instability across north central
KS will limited thunder, but expect an expansive area of rain
associated with the trough axis moves through far north central KS.
Most of the activity exits by 00Z Friday as strong northwest flow
advects cooler air into the region. By 12Z Saturday morning dew
points may be in the upper 20s. With a pressure gradient still
present mixing could prevent the temps from completely bottoming
out. Therefore have morning lows in the upper 30s to low 40s, and
the possibility of frost. Forecast still remains dry with northwest flow
aloft and temperatures rebounding early next week.

&&

.AVIATION...(For the 18Z TAFs through 18Z Wednesday Afternoon)
Issued at 1233 PM CDT TUE SEP 30 2014

Main issue through the period will be timing and intensity of
TS. Area of storms west of MHK should stay west in the short
term. May see additional development near MHK through the rest of
the afternoon. Any TS would have VFR cigs with short periods of
lower VIS. While there is a chance of TS through the evening, most
likely timing of TS with more persistent IFR VIS and occasional
lower CIGS is expected after 05Z (MHK) and 09Z (TOP/FOE) with
+TSRA possible. These storms should exit the area between 15Z-18Z.


&&

.TOP WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Barjenbruch
LONG TERM...Sanders
AVIATION...Barjenbruch





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