Area Forecast Discussion
Issued by NWS Topeka, KS

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000
FXUS63 KTOP 160802
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
302 AM CDT Fri Jun 16 2017

.SHORT TERM...(Today and tonight)
Issued at 301 AM CDT Fri Jun 16 2017

07Z water vapor imagery shows a closed upper low south of the Hudson
Bay and north of MN with another upper low over the Gulf of Alaska.
This has left the mean westerlies across the northern tier of the
county with a quasi-zonal pattern from the Pacific northwest into the
northern plains. Surface obs indicate a meso high over the Ozarks
associated with last nights convection. Otherwise a broad surface
trough of low pressure was noted from the southern high plains into
the eastern Dakotas. Profiler data across OK shows generally a south
and southeast low level jet with dewpoints along the Red River in the
lower and middle 70s.

For today and tonight, models show little in the way of large scale
forcing impacting the forecast area keeping organized energy to the
north with the mean westerlies. Although with southerly low level
winds, the boundary layer moisture is expected to recover with
dewpoints warming into the lower and middle 70s across eastern KS
and around 70 across north central KS. This is expected to lead to a
very unstable airmass with little CIN by peak heating. Models prog
MUCAPE values to be 4000 J/kg or greater. And 0-6 KM shear is
expected to be reasonable in 40 to 50 KT range. So while the
dynamics do not look all that great for thunderstorms, the
thermodynamics are very favorable for convective development. Given
weak inhibition for storms from diurnal heating, it appears that the
trigger may be low level convergence within the surface trough. So
the expectation is for storms to potentially develop to the
northwest and possibly move across the forecast area as a MCS.
Confidence is only so-so because the models seem to focus storms
over different parts of the central plains, the GFS over central KS
and the ECMWF across western IA. So only have chance POPs in the
forecast for late this afternoon and through the evening.

Highs today are expected to be similar or slightly warmer than
yesterday as models advect some warmer air at 850 over the forecast
area and bring the thermal ridge more into central KS. So the
forecast has lower 90s across the eastern counties and upper 90s for
the western counties. Lows tonight could be tricky if another MCS
does move through. The pattern with a synoptic trough remaining
mainly west of the area with a southerly surface wind argues for
mild lows. Since I only have a chance for storms overnight, have
gone with the more mild low temp forecast with readings expected to
be between 69 and 73.

.LONG TERM...(Saturday through Thursday)
Issued at 301 AM CDT Fri Jun 16 2017

Impactful weather is expected across the entire outlook area for
Saturday as two hazards are being monitored: high heat indices and
severe thunderstorms.  By Saturday morning, a mid-level trough will
be advancing into the Northern Plains, with the trough digging
southward into the Central Plains Saturday afternoon and evening. At
the surface, low pressure and the associated cold front will stretch
from southwest to north central KS.  By early afternoon, models show
this cold front making a slight southeastward progression across the
CWA, stretching from central to far northeast KS.  Decent
southwesterly flow ahead of this frontal passage will allow for
continued warm, moist air advection with a nose of 24C-28C 850mb
temperatures extending from central KS into east central and far
northeast KS.  As a result, expect hot and humid conditions with
afternoon highs in the mid/upper 90s and upper 60s to low/mid 70
dewpoints pushing afternoon heat indices into the 100-105+ degree
range.  As a result, a Heat Advisory has been issued for portions of
east central KS for Saturday afternoon, and will need to monitor
conditions to see whether or not this advisory needs to be expanded.
These warm, moist conditions combined with steep mid-level lapse
rates will allow conditions to quickly destabilize through the
afternoon, with models suggesting MUCAPE values of at least 5000
J/kg.  By mid to late afternoon, the advancing mid-level trough axis
will help to provide a better southeastward push to this cold front,
with the front looking to exit the CWA by mid to late evening. Model
soundings show the loss of a cap by late afternoon as the cold front
begins to progress southeastward.  The combination of a highly
unstable atmosphere, modest convergence present near the front, and
0-6km bulk shear values of 40-50kts will likely support the
development of severe thunderstorms late afternoon into the evening
hours.  The primary hazards will be large hail (and possibly some
very large hail) and damaging straight-line winds.  The severe
thunderstorm threat should diminish by mid to late evening as the
cold front exits the CWA.  However, additional scattered showers and
thunderstorms are expected behind this frontal passage across the
CWA as an embedded shortwave within the mid-level trough tracks over
the area overnight into Sunday morning.  Instability will have
decreased significantly by this point, so do not expect any severe
storms from this overnight activity.  Once this overnight
precipitation exits east central KS Sunday morning, we can finally
expect a brief stretch of dry weather and more seasonal conditions
from Sunday through Tuesday morning with highs dropping into the
80s.

With the mid-level trough shifting east of the area and becoming
centered over the Great Lakes region, expect northwesterly flow over
the region with a string of embedded shortwaves bringing off-and-on
scattered thunderstorm chances Tuesday through Thursday.  High
temperatures during this period will also be moderating back into
the 90s

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1115 PM CDT Thu Jun 15 2017

The wind speeds on the backside of the rain may continue to gust
up to 40 mph in the next few hours. There is a chance for storms
again this afternoon and evening, but confidence in the timing and
coverage is fairly low at this point.


&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon to 7 PM CDT Saturday for KSZ026-039-040-
055-056-058-059.

&&

$$

SHORT TERM...Wolters
LONG TERM...Hennecke
AVIATION...Sanders



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