Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 092331
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
631 PM CDT Wed Aug 9 2017

...AVIATION UPDATE...

.SHORT TERM...(This evening through Thursday)
Issued at 350 PM CDT Wed Aug 9 2017

As of 20z, just as several days in the past, upper level ridging
continues to dominate the central and southern Rockies. Slow
moving cold front remains draped across northwestern NE. Weak
isentropic lift within the 300-310K layer continues to maintain
scattered showers across central and eastern KS. Pwat values
across the area remain in the 1.5 to 2.0 inches, resulting in a
brief heavy rainfall potential. Instability remains rather meager
with mesoanalysis suggesting upwards of 500 J/KG. Therefore, only
expect a few rumbles of thunder through the afternoon and evening
hours. Initial batch of showers and thunderstorms will push
eastward of the area during the evening hours. Thunderstorms have
began to develop along the aforementioned boundary in northwestern
NE in response to an embedded shortwave within the northwest
flow. Current short range models tend to steer the bulk of
thunderstorm activity towards the better instability axis across
central and western KS. That being said, I still expect scattered
showers and isolated thunderstorms to traverse the CWA overnight
as the subtle shortwave passes. Behind any shower activity,
forecast soundings suggest a drying column with clearing skies.
With Td depressions approaching 0 and nearly calm winds, I expect
patchy fog to develop overnight. Especially for areas along and
north of Interstate 70.

As we transition into Thursday, lingering showers will exit the
southeastern counties by mid morning. Additional development is
expected during the afternoon and evening hours. The
aforementioned surface boundary in NE will be positioned along the
KS/NE by 21Z Wednesday afternoon. Another subtle shortwave is
noted by most short and mid range models. I expect thunderstorms
to develop in response to sfc heating, convergence along the front
and additional lift from the wave. Great disparity exists amongst
guidance regarding instability. Forecast soundings from the GFS
near MYZ mix the BL to near H750, resulting in MLcape near 600
J/KG while the NAM mixes to H850 with ~2000 J/KG of MLcape. Have
hedged my bet towards the NAM solution, given agreement with Hi-
Res solutions. Effective shear is forecast to approach 30 to 40
knots. Given these 2 parameters, I expect organized convection to
develop with large hail and damaging winds. Warm cloud depths
approaching 15,000ft and pwats remains in the 1.5 to 2 inch range
could result in heavy rainfall.

.LONG TERM...(Thursday night through Wednesday)
Issued at 350 PM CDT Wed Aug 9 2017

The end of this week into the weekend will be characterized by
continued unsettled weather, with the pattern gradually
transitioning towards a drier setup by early to mid next week.
Temperatures look to remain seasonably cool through the period.

A surface boundary currently extending from eastern South Dakota
to western Kansas will bisect the CWA by the start of the period
Thursday evening. An approaching shortwave/H500 vort max embedded
in a broad central CONUS longwave trough will aid in thunderstorm
development across western Kansas and eastern Colorado Thursday
afternoon. Models have for days been in agreement with the
progression of this thunderstorm complex southeastward along the
(convectively enhanced) baroclinic zone, though the track
placement for this complex has varied. Recent runs of the
NAM/EC/GFS have taken the bulk of the precip south and west of the
CWA, though 305 K isentropic lift aloft and mid-level kinematic
forcing may tap mid-level monsoonal moisture being advected
eastward with the subtropical jet and spread precip northward into
the CWA.

With SBCAPE values ahead of the boundary in NE Kansas around
1000-2000 J/kg with little inhibition per domestic model bufr
soundings and roughly 30 kts of effective mid-level bulk shear,
there a chance for thunderstorm development along the boundary in
the early evening hours ahead of this main MCC. This possibility
is reflected in both the GFS and NAM, which both have a linear
precip band developing along the KS/NE border between 00-03Z,
decaying after 06Z as it encounters subsidence attendant to the
main convective complex to the south and its low-level moisture is
cut off. Have continued with chance POPs for most of the CWA
given the deeper moisture in place and uncertainty in the track of
the complex/initial line of storms. There is some chance of
stronger storms with the convection along the boundary, mostly in
the form of hail/wind given the lack of low-level wind shear.
Heavy rainfall will be a concern given the amble deep moisture and
weak steering flow.

This complex as a whole should shift southward Friday morning
with surface high pressure building south from the Northern
Plains. The aforementioned baroclinic zone will stall out over
central Oklahoma late Friday and then advance northward Saturday
morning. Upslope convection from the High Plains will once again
track ESE along this baroclinic zone Friday night, lifting
northeastward with the boundary and attendant isentropic upglide
ahead of this effective warm front on Saturday morning. The
boundary will oscillate over the area through Saturday and Sunday
before high pressure becomes more established over the Northern
Plains on Monday.

While confidence is high that the region will remain in an active
pattern through at least the weekend, confidence in the timing of
the precip (especially beyond Friday morning) remains low given
the lack of any prominent synoptic boundary and the likelihood of
convection from the previous day influencing the timing and
location of subsequent storms. Confidence in how soon the region
will break from this active weather pattern is also in some doubt
given how the planetary scale models have been delaying the
arrival of the West Coast meridional ridge--not surprising given
this amplified blocking pattern. However, one benefit of this
pattern will be the cooler weather (somewhat unusual for early to
mid August). H850 temps struggle to vary much between +15 and +18
C, which will translate into highs in the low 80s and lows in the
60s through the period.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday)
Issued at 631 PM CDT Wed Aug 9 2017

Surface obs and latest visible satellite show a relative area of
stable air across north central KS from the early day clouds and
rain. So am not confident storms across south central NEB will
hold together in order to impact the terminals. The better chance
for precip may come from isentropic upglide this evening and
overnight. There are some mixed signals from the convective
allowing models and it appears the storms across central KS may be
slightly further south than the 22Z HRRR depicts. So confidence
in precip is marginal to include in the forecast for the terminals
at this time. If there were to be some elevated showers, it may
be between 01Z and 05Z so will keep an eye out for this. Models
keep some cloud cover across east central KS through the night
mitigating chances for fog. But if skies were to clear out, there
could be some IFR or even LIFR VSBY through the morning. Will also
monitor trends for fog as the evening progresses.

&&

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

$$

SHORT TERM...Baerg
LONG TERM...Skow
AVIATION...Wolters



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