Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 170444

Area Forecast Discussion
National Weather Service Topeka KS
1144 PM CDT Wed Aug 16 2017

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Thursday)
Issued at 128 PM CDT Wed Aug 16 2017

An active afternoon and evening is in store for areas southeast of
the Kansas Turnpike with showers and thunderstorms developing around
2-3pm and moving southeastward until clearing the area by 10pm.
Large hail, damaging winds, and heavy rain will be the main threats.

The 18Z water vapor imagery showed a seasonably strong, negatively-
tilted upper level wave lifting northeastward across the High Plains
with a 70 kt jet max wrapping around the base of the trough. Subtle
perturbations lifting northeastward along the eastern flank of the
shortwave have been observed on the 1-min GOES-16 visible data over
the course of the morning with a stout EML evident in the low to mid-
level water vapor imagery channels. The cold pool/outflow boundary
from this morning`s MCS has laid out over SE Kansas and into SW MO
with the main synoptic cold front now bisecting the CWA near
Manhattan. Stable stratocu developed ahead of the approaching upper
level impulse, with the latest scans showing some agitation along
the leading edge of the field.

There is some uncertainty as to how much the thermodynamic
environment can recover from this morning`s MCS. The latest
SB/MLCAPE mesoanalysis fields still show 150-200 J/kg of inhibition
in place across the area, though the agitation in the CU field
would lend credence to the cap eroding quickly. Given the upper
level support and satellite trends, convective initiation is
expected by 3pm with initially discrete cells fueled by 2000+ J/kg
of MLCAPE and 30-50 kts of effective bulk bulk shear. The cells are
expected grow upscale into a linear complex given the nearly
unidirectional mid-level flow parallel to the boundary. Large hail
and damaging winds will be the main threat early in the event,
transitioning to wind and heavy rain as the complex evolves.
Localized flash flooding with be possible with any prolonged or
training cells with PWATS of 1.5 to 2 inches and ample moisture
transport northward from the Gulf. The tornado threat looks to be
limited given the lack of low-level wind shear/helicity and winds
already starting to veer to the SSW ahead of the boundary.

Recent CAM runs have been trending southward with the placement of
storms this afternoon and moving them out by 03-04Z, which was
reflected in this forecast update. A few post-frontal showers are
possible this evening and overnight, but the bulk of the precip will
be done before 06Z. Surface high pressure builds southward by
tomorrow morning and will bring clearing skies. This high will pass
south of the CWA by this afternoon and will result in return flow
and WAA with highs in the 80s in most places.

.LONG TERM...(Thursday night through Wednesday)
Issued at 128 PM CDT Wed Aug 16 2017

Overnight Thursday into Friday morning could bring some convection
into the western counties initially as storms move off the high
terrain and move eastward in the overnight LLJ regime. EC and GFS
vary in strength with next upper shortwave and associated front
to move in for Friday afternoon, with the GFS more pronounced with
features overall and produces more qpf/convection as a result.
With afternoon onset of storms, some could be strong to severe.
Either solution clears out clouds and precip for Saturday through
Sunday morning. Highs forecast in the upper 80s to around 90 with
overnight lows in the middle to upper 60s.

For the solar eclipse on Monday...The upper flow flattens over
the northern into the central plains late in the weekend. Upper
trof just offshore of the southern California coast helps
reestablish moisture return into our area by late in the day
Sunday, with additional features noted over the Gulf on the
westward side of the SE US high, also aiding in an increase in
moisture from the south. Our area lacks strong triggers overall,
but with storm track just to our north, and small scale features
moving through the overall flow, cant rule out cloud cover or rain
chances on Monday afternoon. GFS is stronger with an overnight
MCS Sunday night over central Nebraska and into our Western half
by Monday morning and moving eastward through the day, which would
bring cloud cover along the eclipse track as well. Perhaps a
stronger convective signal could generate subsidence behind the
wave and improve chances for clearing through the mid day Monday.
EC is farther north with storms and earlier on Monday, but still
indicating some cloud cover and a weak signal for some afternoon
QPF. In other words, uncertainty remains as to how much will be
visible for our area.

Tuesday/Wednesday...Upper trof in the Great Lakes in the EC
pushes a front southward into early Tuesday morning with another
chance of rain across our area as that front moves through. If
cooler drier high pressure moves far enough southeast on Wednesday
then storm track should also shift westward, which could cool
highs back down a few degrees toward normal, in the middle 80s.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1143 PM CDT Wed Aug 16 2017

VFR conditions are expected through the period.




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