Area Forecast Discussion
Issued by NWS Topeka, KS

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000
FXUS63 KTOP 171743
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1243 PM CDT Wed May 17 2017

...Update to near-term and aviation forecast discussion...

.UPDATE...
Issued at 1236 PM CDT Wed May 17 2017

Thunderstorms developing at mid day will continue to expand in
coverage and intensity as the move rather quickly to the northeast
at 40-50 mph. Deep layer shear is not particularly impressive in
close vicinity to the surface low in north central KS as the upper
jet is displaced into east central and northeast KS. However, deep
layer vertical vorticity is impressive even where the effective
shear is not...and a tornado watch was warranted for much of
north central and northeast KS. The greatest risk will be in far
northeast KS as it will take a bit of time for these storms to
organize with any rotation. This is a bit of a quasi-low-topped
supercell scenario but not a classic one as the 0-1 km shear is
minimal but more impressive in the 0-2 km layer. Bottom line is
that any strong updrafts will need to be closely monitored for
isolated hail/wind/tornado threat through the afternoon with any
severe threat likely to exit the area by 4 PM.

&&

.SHORT TERM...(Today and tonight)
Issued at 323 AM CDT Wed May 17 2017

08Z water vapor imagery shows the upper trough beginning to lift
into southern KS with a negative tilt. Another closed low was noted
over OR and ID moving southeast. At the surface, an area of low
pressure was located near the OK/CO/KS boarder. An inverted surface
trough/front stretched north from the low across north central KS
and into eastern NEB.

For today the upper trough is expected to rotate over the forecast
area with steep lapse rates and little inhibition progged by the
models. The best forcing appears to be impacting the area now with
some new convection moving into east central KS. However with the
thermodynamic profiles favorable for precip through the day as the
upper wave moves through, think there could be scattered showers and
thunderstorms redevelop as the boundary layer begins to heat up. So
will keep some chance POPs through the day. Perhaps the greater
impact from the weather today will be the winds. All of the models
are in pretty good agreement with a strengthening pressure gradient
and winds becoming isallobaric across east central KS. With a
dynamic system and strong low level jet, there is some concern that
wind gusts could be over 40 MPH today with sustained speeds around
30 MPH. So will issue a wind advisory for parts of northeast and all
of east central KS. Think there may be some sunshine break out by
the afternoon across at least southern parts of the forecast area.
Therefore have maintained the prev forecast highs of around 80 along
and south of I-70. A little more cloud cover and some weak cold air
advection late in the day may hold highs across northern counties in
the mid and upper 70s. Increasing subsidence and a stabilizing
boundary layer should bring an end to precip chances early this
evening. There may be some chance for fog in the pre dawn hours
Thursday with winds becoming light. Confidence in fog formation is
not high enough at this point to include a mention in the forecast,
but later shifts can reevaluate this potential. Lows should fall
into the lower and middle 50s with some cooler and dryer air moving
into from the northwest.

.LONG TERM...(Thursday through Tuesday)
Issued at 323 AM CDT Wed May 17 2017

The extended period consists several thunderstorm chances. Some
of which may be severe, specifically Thursday and Friday.

Thursday [2017 May 18th]: Thursday morning the entire outlook
area will reside in the cool and stable sector. A warm front
draped across central and northeast Oklahoma will begin
progressing northward. The exact placement of the aforementioned
warm front will play a huge role in the severe potential Thursday
afternoon and evening. Model guidance is beginning to converge on
a solution in which the warm front stalls across the southern
counties of the CWA. Dew points south of the front will surge into
the mid to upper 60s. This plentiful moisture coupled with steep
mid level lapse rates approaching 8 C/KM will result in MLcape
values approaching 3500 J/KG. However, a stout EML will reside
above 800 MB, which should preclude any thunderstorm development
through the daytime hours.
**If an isolated thunderstorm would develop along the warm front,
all modes of severe weather would be possible, given the moderate
instability, effective shear near 50 knots and sufficient low level
vertical wind shear.***

Thursday night: thunderstorms along the dryline across western OK/KS
are expected to grow upscale as a negatively tilted shortwave ejects
across the central plains. Once again, guidance is converging on the
solution of an MCS traversing the CWA after 06Z Friday morning.
Given 0-3KM shear values approaching 50 knots after 06Z the threat
for bowing segments and damaging winds would become the primary
hazard. Good model agreement with QPF amounts as well, the vast
majority produce 1 to 2 inch amounts across most of the CWA.
Flooding may become a concern given the multiple rounds of rainfall
expected.

Friday [2017 May 19]: The closed upper level low will remain
within the lee of the Rockies. The evolution of convection
Thursday night/Friday morning will play a big factor in the severe
potential Friday afternoon/evening. Model solutions begin to
diverge Friday afternoon regarding the placement of the warm
sector. 00z GFS/ECMWF suggests the warm front will lift through
the entire area, placing the CWA within the warm sector. The 00z
NAM is the complete opposite, leaving the warm sector south of the
CWA. Hedging my bet towards the ECMWF/GFS given the trajectory of
the upper low. A sharpening dryline and surface low will develop
across portions central and western KS during the afternoon hours.
Dew points will once again surge into the mid 60s resulting in
MLcape values approaching 3000 J/KG. Effective shear values near
50 knots would suggest supercellular structures would be possible
with any sustained updraft. The closed low will eject across the
central and northern plains late Friday and Saturday, pushing any
lingering shower and thunderstorms east of the area by Saturday
evening.

Surface high pressure will begin building in over the weekend. Rain
chances return early next week as an upper level trough traverses
the central and northern plains. Not only does this particular
trough bring precipitation chances, it appears to be bringing much
cooler temperatures as well. Low temperatures are expected to dip
into the 40s by Wednesday morning.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1236 PM CDT Wed May 17 2017

Aviation forecast focused on short term thunderstorm potential
through 21Z and tried to focus on the most likely hour of impact
although it could vary a bit. Winds will shift through the night
turning out of the east by Thursday morning with a good chance for
MVFR cigs to build over TAF sites after sunrise.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 7 PM CDT this evening for KSZ024-026-035>040-
054>056-058-059.

&&

$$

UPDATE...Barjenbruch
SHORT TERM...Wolters
LONG TERM...Baerg
AVIATION...Barjenbruch



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