497
FXUS63 KTOP 212331
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
631 PM CDT Wed Jun 21 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Thursday)
Issued at 307 PM CDT Wed Jun 21 2017

A subtle upper level wave moving through central Nebraska helped
spark scattered showers and thunderstorms over far NE Kansas and
locations further north and east late this morning. This wave and
the associated shower activity will push east of the area by the mid
afternoon hours, with a second wave nipping at the departing
system`s heels over SE Wyoming. A dryline across western Kansas will
once again be the focus for thunderstorm development later this
afternoon and evening. Given the lack of steering flow aloft,
convection will not move terribly quickly off the dryline, and have
hence slowed the arrival of POPs in the forecast after 21Z. This
idea matches most of the CAM solutions such as the HRRR and high-res
ARW/NMM cores.

The main threats from this event would be large hail and damaging
winds in the far western CWA given the SB/MLCAPE values over 4000
J/kg and near adiabatic low-level lapse rates from the surface to
above 800 mb. Deep shear values will be lacking, thus the convective
clusters will likely be non-steady state and new convection will be
driven by previous convective cold pools. Steep mid-level lapse
rates above a 30-40 kt LLJ may drive elevated convection into the
night over central portions of the CWA, but the lack of deep shear
should once again keep the severe threats to a minimum.

The upper level pattern will amplify over the northern Rockies
during the day on Thursday as a longwave trough digs southward from
western Canada. As this wave approaches, the cold front over
Nebraska will begin to shift ESE and reach the NW corner of the CWA
at the end of the period. High temperatures on Thursday will be near
what they reached yesterday and today--reaching the low 90s east to
upper 90s west. Heat indices will push 100 degrees in most locations.

.LONG TERM...(Thursday night through Wednesday)
Issued at 307 PM CDT Wed Jun 21 2017

Thursday night into Friday morning, broad cyclonic flow resides over
the northern tier of the CONUS.  A deep upper level closed low will
amplify short wave energy over the Northern Rockies and eventually
send this impulse into the Northern and Central Plains.  The closed
low will begin to fill into the overnight and thus weaken as it
shifts east into the Northern Great Lakes.  Best forcing for large
scale ascent stays north of the forecast area and leaves
northeastern KS under a capped environment much of the day with a
strong EML in place.  NAM and GFS solutions are more consistent with
higher moisture set up and a well mixed boundary layer in place but
also strong CINH in place.  Both solutions focus gradient of
instability along the front as it moves through southern NE late
afternoon and into the northeastern KS by early evening.  MLCAPE and
SBCAPE on the order of 1500-2000 J/kg as it makes its way into
northeast KS within marginal deep layer shear with best deep layer
shear lagging well behind the front should lead to isolated to
widely scattered storms probably at best.  Most CAM solutions that
go out far enough to include this forecast period are suggesting
that any discrete multicells that do develop quickly become a
disorganized cluster or quasi linear set up into the evening.  The
ECMWF solution is similar to the overall concept that the NAM and
the GFS arrive at, but slow the overall progression of the cold
front slightly with overall less moisture pooling to help
initiate activity in the first place. All this being the case, it
leads to a low probability set up that severe storms will be long
lasting or maintained. The threats that do exist would be possibly
some large hail and mainly a wind threat lasting for a short time
frame (before boundary layer stabilzation takes place) where
storms can break the cap and get going. Therefore, have gone with
slight to some chance POPs in the early evening on Thursday.

On Friday day time frame high pressure ridge noses into the area and
should leave a nice day in place with subsidence aloft.  Temps will
be in the 80s with dew points in the 50s.

Saturday, there may be some showers and storms that make their way
into portions of the area coming off the high plains associated with
another impulse rotating around the broad cyclonic flow aloft that
just continues to skirt the area once again.  It`s a low chance set
up and one that won`t have a lot of support but too much model
variance at this point to determine one way or the other.

Latter part of the weekend into the first half of the upcoming week,
the pattern becomes more progressive with a low amplitude ridge
working into the area and activating the lee trough.  This will
bring some return flow of moisture back into the area and possibly
lead to a couple on and off chance of precip with isentropic lift
developing and advecting over the area.  All in all, temperatures
will be more near normal and comfortable compared to recent heat.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 625 PM CDT Wed Jun 21 2017

VFR prevails at terminals with main concern being the low level
wind shear and dissipating convection across western Kansas. Sfc
winds are expected to become light around 5 kts aft 01Z with the
low level jet increasing around 1000 feet up to 40 kts. This is
expected to persist through sunrise before southerly sfc speeds
increase above 10 kts near 14Z. Current convection in western
Kansas is expected to weaken as it shifts east and southeast.
There is a slight chance for a few thunderstorms to reach KMHK by
11Z, hence the VCTS mention.

&&

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

$$

SHORT TERM...Skow
LONG TERM...Drake
AVIATION...Prieto