Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 010541
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1241 AM CDT Mon Sep 1 2014

.MESOSCALE DISCUSSION...
Issued at 1240 AM CDT MON SEP 1 2014

The midnight through 3 AM period will continue to see at least some
low end potential for marginally severe hail, and perhaps localized
damaging wind gusts in the strongest storms able to downburst and
transport momentum to the surface through the inversion (refer to
recent SPC mesoscale discussion for more details).

Aside from severe potential, the potential for flash flooding is
increasing and could see additional warnings needed during the
overnight hours. The outflow from the complex of storms has pushed
into southern Kansas and stretched west-to-east while the upper
trough and overall storm motion remains east southeast nearly
parallel to the effective boundary. Moisture transport is very
impressive into and over the frontal zone into central and east
central Kansas while precipitable water values are running 2 to 2.2
inches according to RAP analysis. Dual-pol signatures have at times
indicated very heavy rainfall rates to around 2" per hour with the
heaviest activity and spotters have reported 15-20 minute periods
with 1 to 1.5 inches of rain. The orientation of the front and storm
motion suggests training of individual convective updrafts over the
same areas from southern Dickinson through Morris, Lyon, Coffey,
Anderson, southern Osage and southern Franklin counties. Dual-pol
rain estimates as of midnight range from 4" in southern Dickinson
county to 2.5" in Lyon county to 2" in Osage/Franklin counties.
Given current trends and expectations, could end up seeing a broad
swatch of 2 to 5" with locally 6+ inches possible.

There are some indications that the rear edge of the convection is
beginning to forward propagate toward the southeast which may help
put a slightly faster end to the very heavy rain, but should
continue to see development in advance of the upper trough axis and
north of the outflow which could persist even behind any forward
propagating segments. This heavy precip is also falling across the
parts of the area that have been driest as of late, and can handle
more water. However, the torrential rates alone will produce
substantial runoff and may be sufficient for flash flooding even in
the dry conditions.

&&

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Monday)
Issued at 311 PM CDT SUN AUG 31 2014

Elevated activity has weakened and exited with good insolation in
recent hours in weak subsidence allowing for a quick increase in
winds and temps. 19Z surface observations suggest cold front from
south central Nebraska to the SW corner of Kansas with some CU
development on the north end. Recent water vapor imagery shows
main upper wave pushing through southern Wyoming with convection
in NC Colorado well behind the front.

Overall setup continues to support a rather wet next 12-18 hours
with cold front coming into northern and western areas overnight and
upper trough following, with low level jet ramping up to around
60kts. Nearby 12Z observed precipitable water values were at to just
above normal and continued advection of deeper moisture to the south
will pump these values up.

Still having some question on the early hours of the event. With
surface low pressure in far SE Colorado, convergence along the front
is not great, and the main upper trough well behind will not boost
convergence until perhaps the daytime hours of Labor Day. Various
higher resolution models are delayed with the convective initiation
until around 23-0Z and could occur well into the CWA. Dewpoint
depressions should remain in the 25-40F range for somewhat high
cloud bases, but increasing low level wind fields will support
strong low level helicity. Again, with the trough lagging, mid level
winds are somewhat low to support a long-lived supercell, and wind
and hail continue to the be the main severe weather concerns with ML
CAPE of 2000-2500 J/kg and downdraft CAPE around 1200 J/kg currently
analyzed. The high-res models are also similar with a secondary
impulse of convection moving through in the late evening to
overnight hours, perhaps via a sustained MCS from the current NC
Colorado storms. This could track over northern areas again and
bring an increased flooding concern, but with recent rains spotty at
best, am not confident enough for a Watch. Convection should overall
slowly clear from northwest to central areas overnight.

Cold front still appears to sag south into southern areas for the
afternoon hours of Labor Day. This again is not a clear-cut forecast
with convective activity possibly affecting the location of the
effective front. If at least some insolation can occur, cooling mid
levels behind the upper trough should allow for moderate instability
to again develop along and ahead of the effective front with plenty
of shear to support severe storms. This time, upper winds will be
stronger but weaker near the surface.

.LONG TERM...(Monday Night through Sunday)
Issued at 311 PM CDT SUN AUG 31 2014

For Monday night and Tuesday, have continued chances for precip as
the expected outflow boundary from tonight`s convection gets hung
up across southeast and south central KS. In general would anticipate
POPs diminishing and becoming increasingly confined to far
southern portions of the forecast area with time through the day
Tuesday as the upper forcing moves east of the area and the low
level jet veers to the southwest decreasing convergence along the
front.

Tuesday night may be more interesting in terms of strong to
possibly severe thunderstorms. Most model guidance is pointing to
strong theta-e advection overnight with good isentropic upglide.
Additionally model forecast soundings have fairly steep mid level
lapse rates with potentially 1000 to 1500 J/km of elevated
instability. The NAM also shows around 30 KTS of effective bulk
shear, so it is not out of the question that there could be some
elevated storms capable of producing large hail. This activity may
linger into Wednesday morning. After that the forecast is
expecting a dry period through Thursday before models bring a
frontal system into the area Thursday night through Friday night.
Because of this boundary, have continued with some chance POPs for
Thursday night through Friday. By Saturday, both the GFS and ECMWF
build a surface ridge into the central plains with dryer air
advecting in. Because of this, have trended POPs down for Saturday
and kept Sunday dry.

Temps are expected to remain warm through Thursday until the
frontal system enters the region with chances for rain. The
warmest days look to be Wednesday and Thursday as 850 temps warm
to around 25C. Therefore have highs in the mid 90s with lows
around 70 to the lower 70s. Temps should trend cooler by the
weekend due to cold air advection behind the front. May see lows
in the 50s with highs in the upper 70s to around 80 by Sunday.

&&

.AVIATION...(For the 06Z TAFs through 06Z Monday Night)
Issued at 1240 AM CDT MON SEP 1 2014

For the 06z TAFs, widespread showers and embedded thunderstorms will
persist through the overnight hours into Monday morning. Some models
are suggesting that areas of fog may develop before sunrise,
especially near KTOP/KFOE. There is still uncertainty with regards
to whether or not this will occur as winds may stay just high enough
to diminish this threat, but went ahead and added a TEMPO group at
this time and will continue to monitor the trends through the
overnight hours. Expect this precipitation to shift south of the TAF
sites during the morning hours with winds veering to the northwest
and eventually north-northeast with the frontal passage. There is a
chance for additional scattered thunderstorm develop late afternoon
into early evening. However, the TAF sites may be near the northern
edge of this precipitation development, so have only mentioned VCTS
at this time due to the uncertainty in the exact location of these
storms. Any storms that do develop near the TAF sites should shift
southward by mid to late evening.

&&

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

$$

MESOSCALE...Barjenbruch
SHORT TERM...65
LONG TERM...Wolters
AVIATION...Hennecke





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