Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 161716

Area Forecast Discussion
National Weather Service Topeka KS
1216 PM CDT Tue May 16 2017

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 321 AM CDT Tue May 16 2017

08Z water vapor imagery showed a closed low over Las Vegas with
another upper low moving southeast along the British Columbian
coast. Southwest flow aloft was noted over the plains. At the
surface, the remnants of an old outflow boundary was noted across
north central KS. On a larger scale, low pressure was seen over the
central high plains with ridging along the MS river valley. A few
shows and thunderstorms from yesterday have persisted long enough to
move into western portions of the forecast area. Overall the trend
has been for these showers to diminish and this should continue with
isentropic surfaces showing some drying in mid levels occurring.

For this afternoon and tonight, the main question is whether the
capping inversion will hold or if there will be some cooling in the
850 to 700MB layer to weaken the cap. The forecast soundings from
the NAM and GFS hang onto a stout cap suggesting thunderstorm
chances may not be that great late in the afternoon and early
evening. Conversely the RAP cools temps in the 850 to 700MB layer and
allows convection to develop and spread into north central KS by the
late afternoon. For what ever reason, the convective allowing models
have also developed thunderstorms by the late afternoon within the
instability axis while the larger scale models maintain the CAP and
suppress thunderstorm development. Because there has been some
consistency with the HRRR over the last several hours, have POPs
increasing to likely by 00Z across far western sections of the
forecast area. However I don`t have a great deal of confidence in
the HRRR solution since most guidance, including the RAP, redevelop
a surface low over eastern CO which causes the dryline to remain
over western KS suggesting the better low level convergence to help
break the cap would remain to the west of the forecast area. So it
comes down to whether the RAP cooling and weakening of the cap is
correct or if the NAM and GFS maintaining a strong cap is the right
solution. If storms develop, the airmass should be very unstable
with CAP values around 4000 J/kg. Deep layer shear of 35KT to 40KT
would suggest discrete storms could become supercells. However 0-1KM
shear parameters are not that impressive until after 00Z when the
low level jet increases. So while conditions for tornadoes could be
better, there may be a window before storms evolve into an MCS for a
greater tornado risk. Again this is all conditional on whether the
cap holds or not. The main severe weather threats will be large hail
and damaging winds.

Think that convection from the south could move into eastern KS
during the pre dawn hours Wednesday as a negatively tilted shortwave
lifts across the area. Models show very good dynamics moving over a
warm and moist airmass so I have higher POPs for that area than what
the QPF progs may suggest based on the expected forcing. Think highs
today will be back in the middle 80s even though models want to cool
things from yesterday. There shouldn`t be much change in the airmass
and think there will be good mixing of the boundary layer with an
increasing pressure gradient. So have highs a couple degrees warmer
than MOS guidance. Gusty south winds through the night should keep
lows Wednesday morning in the mid 60s.

.LONG TERM...(Wednesday through Monday)
Issued at 321 AM CDT Tue May 16 2017

Wednesday through Tuesday...

The extended period consists of several thunderstorm chances.
Depending upon mesoscale feature, severe storms may come into play.
Thunderstorm chances continue through the onset of the period
Wednesday morning. Supercell thunderstorms are expected to form
along a dryline across portions of the panhandles and western KS
Tuesday afternoon. As the night progress, storms are expected to
congeal into a linear mode in response to the main H5 shortwave
ejecting across the plains. Guidance has continued to trend slower
with the progression of the dryline through eastern KS Wednesday
morning. If this slower solution comes to fruition, strong to
severe thunderstorm become possible early Wednesday afternoon.
Good agreement amongst guidance of a 6 to 10 MB pressure gradient
taking shape across the outlook area Wednesday afternoon,
resulting in sustained winds near 30 MPH with gusts 40 to 50 MPH.
As the surface low pushes northeast of the area winds will
decrease Wednesday evening.

The next complicated severe weather setup takes shape for Thursday.
00z operational runs continue to suggest an intersection of a warm
front and inverted stationary trough across portions of central KS.
Dew points are forecast to surge back into the mid to upper 60s
Thursday afternoon within the warm sector resulting in MLcape values
approaching 3000 J/KG with little inhibition. This conditionally
unstable environment coupled with effective shear values between 40
and 50 knots, supercell structures can be anticipated. Primary
hazards at the moment include large hail and damaging. Depending
upon thunderstorm evolution after 00Z Friday, the tornado threat
could increase in response to a strengthening LLJ.

Next, we turn our attention to Friday, where 00z guidance has also
slowed the progression of the dryline. Widespread convection is
anticipated Thursday night/Friday morning along the inverted surface
trough. Its evolution and remnant cloud cover will play a big role
in the potential for severe thunderstorms. Stay Tuned.

The multiple rounds of heavy rainfall may result in localized
flooding throughout the week. Otherwise, surface high pressure
then builds into the central plains over the weekend as the closed
upper level low pushes into the northern plains. Shower and
thunderstorm chances return early next week in response to an
upper level trough traversing the central and northern plains.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1213 PM CDT Tue May 16 2017

VFR to start the forecast period as gusty south winds prevail
above 15 kts sustained through tomorrow. TSRA is expected to form
off the dryline and quickly race north and east into the area.
Initial, severe round is more likely to impact KMHK aft 01Z with
lesser confidence in TOP/FOE for this issuance so just have VCTS
mention. There may be a secondary round of TSRA that comes through
aft 06Z, clearing the terminals aft 14Z. Forecast soundings are
hinting at the potential for high end MVFR cigs aft 00Z this
evening, otherwise would not expect limiting categories with the
exception of being under TSRA.




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