Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 221745
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
1245 PM CDT Tue Jul 22 2014

...Update to near-term convective and heat discussion, and
aviation forecast discussion...

.MESOSCALE DISCUSSION...
Issued at 1218 PM CDT TUE JUL 22 2014

Heat: Temperatures have increased much more rapidly today than on
Monday, as would have been expected after the very warm start to
the day. Dewpoints have been quite high in the east, with several
observations in the 74 to 76 range through mid day. Winds also
have a more westerly component today with the moisture a bit
shallower as evidenced by the morning TOP RAOB. So, while the
temperatures will almost certainly be hotter today, the dewpoints
are expected to mix into the middle to upper 60s from west to
east, with only far northern and eastern KS holding in the lower
70s by this afternoon. The forecast continues to have max heat
index values in the 108 to 112 range...flirting with the criteria
for an excessive heat warning. However, with the distinct
possibility (and expectation) for a drop off in dewpoints this
afternoon will maintain the "upper end" heat advisory. Should note
though that if dewpoints do not drop as much as expected, could
easily have a few sites in the 115 range late today.

Thunderstorms: Model guidance is very diverse regarding potential
thunderstorm development this afternoon evening. It seems that the
diversity is related to the very fine atmospheric balance that
will be in place by this afternoon. The deeply mixed but still
relatively moist boundary layer ahead of the front will lead to a
very unstable atmosphere, but parcels will need to be lifted
through quite a depth to reach the LFC. This will be highly
dependent upon the strength of the front, and also any additional
ascent that can be lended by a weak short wave trough moving out
of central Nebraska and northwest Kansas. If the trough axis is
too quick in moving through northern KS, it could add subsidence
to the frontal zone and reduce precip chances. Also, while the
frontal zone left over from this morning near the Nebraska border
has little to no convergence, observations in east central
Nebraska indicate that a reinforcing front is now surging south
with north winds gusting to around 20 kts. This could provide the
necessary deeper convergence to initiate storms. All of this said,
expect a few thunderstorms to develop along the frontal zone in
far northern KS between 4 and 6 PM, and these should move/develop
to the east southeast through 8 PM. How long they persist is in
question, but given the thermodynamic setup, expect ANY storm with
even a moderate updraft to produce strong wind gusts. Stronger
storms could produce rather intense but localized wind gusts with
downbursts. So despite strong enough shear/instability parameters
for supercell development, expect it to be quite difficult to have
persistent supercell structures due to the strong downbursts and
cold pools. This should reduce the severe hail threat to only
those storms that can persist with mid level rotation...and if
hail develops would see a good chance for it to be wind driven.

&&

.SHORT TERM...(Today and Tonight)
Issued at 411 AM CDT TUE JUL 22 2014

Water vapor imagery this morning showed a broad mid-level ridge
stretched across the Southern Rockies and the Southern and Central
Plains, with a modest embedded shortwave skimming eastward along the
Canadian/U.S. border toward the Great Lakes region. An area of
surface low pressure was associated with this shortwave, with a cold
front across Minnesota extending southwest into far northern
Nebraska this morning. With the region wedged between this
approaching surface trough and surface high pressure over the
southeastern U.S., southerly winds prevailed overnight with high
clouds streaming over the southern two-thirds of the forecast area.
As a result, overnight temperatures remained mild in the mid/upper
70s. A very weak embedded shortwave was noted across southeast
Nebraska/southwest Iowa, which helped to develop a few spotty
showers and isolated thunderstorms early this morning just north of
the forecast area.

Another very warm day is expected today as we will be starting the
day already with mild conditions and model soundings show deep
mixing occurring through this afternoon. The strong shortwave to the
north will strengthen into a more well-defined mid-level trough over
the Great Lakes region today. This deepening trough combined with
advancing surface high pressure behind the shortwave over the
Northern Plains will help to push the cold front southward toward
the area. Models are in pretty good agreement in having the front
situated near the Nebraska/Kansas border between 16z-18z. The front
should slowly sink southward, bisecting the forecast area from west
to east by early this evening and advancing south of the area early
Wednesday morning. Winds look to be a bit slow in veering to the
north behind the front this afternoon across far northern Kansas, so
expect high temperatures to reach into the mid 90s to low 100s
across the entire forecast area. While much of the region should see
dewpoints in the low/mid 70s, the models are in good agreement in
low-level moisture mixing out strongly today across portions of
north central and central Kansas by this afternoon. However, feel
that the models may be a bit too aggressive in dropping dewpoints
into the low/mid 60s, so only trended with dewpoints dropping into
the mid/upper 60s across the far southwest corner of the forecast
area. Also expect increased low-level moisture across far northern
Kansas behind the frontal passage this afternoon, so should see
rising dewpoints over this area. Will need to monitor these
conditions closely today as they will have an impact on heat
indices. At this time, expect heat indices to rise into the 105 to
110 degree range this afternoon, so the existing Heat Advisory will
remain unchanged for today.

The next area of focus is the potential for scattered showers and
thunderstorms to develop along and behind the cold front late this
afternoon through tonight. Models continue to vary greatly in
whether or not they think storms will even be able to develop. Model
soundings show that any storms that develop will likely be
high-based. The environment will need to be able to overcome the
modest CIN in place across the region and be able to lift parcels
through the deep mixing layer to tap into the 3000-4000+ J/kg of
CAPE aloft. In general, 0-6km bulk shear values are fairly weak but
may reach upwards of 25-35kts. With the notable inverted-V
soundings, damaging winds will be the primary threat with any storms
that develop and cannot rule out some marginally-large hail. The
models that are picking up on this potential for post-frontal storms
have scattered activity developing across far northern Kansas by
early evening and advancing southeastward. Additional scattered
showers and storms will be possible overnight as an embedded
shortwave approaches the area from the northwest.

.LONG TERM...(Wednesday through Monday)
Issued at 411 AM CDT TUE JUL 22 2014

Wednesday morning the cold front should be across southern Kansas
with some post frontal precipitation as far north as east central
Kansas. Have left small chances of precipitation in the morning then
dry for the afternoon hours. Cooler temperatures are also expected
on Wednesday, generally near our normal around 90 for late July.

Temperatures expected to be a couple of degrees cooler on Thursday
with warmest temperatures toward central Kansas in the nose of the
thermal axis.

Next chance of showers and thunderstorms comes Thursday night along
and east of a warm front moving across central and eastern portions
of Nebraska and Kansas. Elevated thunderstorms will be possible in
the waa regime/isentropic lift along with some upper support from a
weak shortwave. The precipitation may linger in the far east Friday
morning.

For the weekend a cold front is expected to move through the area
slowly keeping a small chance of showers and thunderstorms in the
forecast through Sunday evening. Models show the upper ridge moving
southwest as an upper trough deepens over the Great Lakes and into
the eastern states with northwest flow aloft redeveloping across the
Central Plains. There is some uncertainty with the timing of the
upper level forcing and have kept precipitation chances low. Cooler
temperatures are expected for Monday as high pressure builds into
eastern Kansas from the north as the upper trough deepens in the
eastern CONUS and upper ridge builds over the Rockies and west.

&&

.AVIATION...(For the 18Z TAFs through 18Z Wednesday Afternoon)
Issued at 1218 PM CDT TUE JUL 22 2014

A front will approach TAF sites during the 3 hour window
surrounding 00Z with the most likely timing around 01Z. Before the
front, expect VFR with west southwest winds. Isolated TS are
expected to develop near the front between 22Z and 02Z and may
approach TAF sites. If storms do approach TAF sites, expect
localized downbursts with very strong winds. Winds then turn out
of the north to northeast, mainly less than 10 kts. With drier air
moving in, feel that vis restrictions are not likely on Wed
morning.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
HEAT ADVISORY until 9 PM CDT this evening FOR KSZ008>012-020>024-
026-034>040-054>056-058-059.

&&

$$

MESOSCALE...Barjenbruch
SHORT TERM...Hennecke
LONG TERM...53
AVIATION...Barjenbruch





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