Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 161747

Area Forecast Discussion
National Weather Service Topeka KS
1247 PM CDT Wed Aug 16 2017

Issued at 1120 AM CDT Wed Aug 16 2017

Morning showers and thunderstorms associated with a decaying MCS
are in the process of departing the area with clearing skies noted
in visible satellite between the departing complex and the
synoptic front pushing across Washington to Ottawa counties.
Timing extrapolation puts this front along a line from Council
Grove to Holton near the expected convective initiation time of
20-21Z. The overall progressive eastward motion of the line should
limit prolonged heavy rainfall over a given area; however,
training storms/heavy rain along the boundary are still possible
given the mean 850-300mb wind parallel to the boundary and PWATs
near 2 inches.

All modes of severe weather will be possible this afternoon along
and ahead of the front with 40 kts of deep shear and 2000-3000
J/kg of SBCAPE, with the initial threat of large hail with
discrete convection, transitioning to wind and heavy rain as the
cells congeal. Also fine tuned the departure of the precip this
evening per recent CAM runs which push the bulk of the rain out by
03Z with lingering post-frontal showers possible through 06Z.


...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 347 AM CDT Wed Aug 16 2017

A mid level trough axis currently stretches from CO to ND and is
responsible for a cold front advancing southward through NE and
western KS this morning. Widespread convection developed along the
front last evening, which has evolved into a large scale MCS. The
mucape ahead of the MCS is around 2000 j/kg and a strong low level
jet has set up over central KS. A well established cold pool is also
supporting the eastward progression. An additional area of
isentropic lift and moisture convergence is aiding storm development
across southeast KS. This area of lift is expected to move
northeastward over eastern KS this morning, and storm coverage
should expand over the next few hours. The HRRR is resolving the
current MCS and shows it somewhat maintaining strength as it moves
across the forecast area through the remainder of the morning. Also,
the low level jet is expected to veer during the late morning hours,
which could continue supporting new convection above the cold pool.
Other 00z CAMs did not resolve the convection as well therefore are
struggling with the current trends. Brief periods of moderate rain
and gusty winds will be possible as the storms pass through
especially in north central KS.

The main question for today will be how much precipitation and cloud
cover lingers across the area before the front pushes through. As of
now a majority of the models are hinting at some clearing in the
early afternoon allowing for the environment to destabilize ahead of
the front. If an MCV from the current MCS were to move over eastern
KS the timing could be such that subsidence behind the wave could
aid in clearing skies. With the lower to mid 70s dew points in place
and modest mid level lapse rates mucape is forecasted to reach 2,500-
3,000 j/kg. With the mid level trough progressing over the plains
expect deep layer shear around 30-35 kt. This mid level trough could
also provide the lift along with the front needed to break any cap
in place. Model consensus for storm development would be around 3-5
pm mainly along a line from Abilene to Marysville. Initially the
storms may be discrete and supercellular, but given the deep layer
shear vector is parallel to the front storms will likely evolve into
a linear complex. Large hail will be the a threat early in storm
evolution, but damaging wind gusts will be the primary threat
through the late afternoon and evening. The threat for severe storms
seems to be supported by the WRF ensembles and HRRR time lagged
probabilities for decent updraft helicity, and 10 m gusts greater
that 50 kt. The low level directional shear is marginal at best so
the tornado threat is rather low as compared to the backed flow
across IA closer to the low pressure. Sufficient moisture will lead
to pwats approaching 2 inches so heavy rainfall will be possible
with these storms, with the better chances across the southeastern
half of the forecast area. Storms should depart the area between 10
pm and 1 am as the front advances eastward.

.LONG TERM...(Thursday through Tuesday)
Issued at 347 AM CDT Wed Aug 16 2017

Thursday surface high pressure will move into northeast Kansas
behind the cold front through the day. Expect mostly sunny skies as
residual clouds move out during the morning hours. Temperatures will
be below normal in the low to mid 80s. A ridge will be in place over
the northern Rockies with northwest flow across the Plains. Another
upper level wave will move southeast Thursday night and Friday
across the northern Plains and into the Great Lakes to the middle
Mississippi Valley by 12Z Saturday. This system will bring the next
round of showers and thunderstorms to the area as a warm front lifts
northeast by early Friday morning. Best forcing will be from
northeast Kansas northeast into Iowa Friday night. An upper level
ridge builds into the central Plains Saturday allowing the area to
dry out through Sunday. Sunday night into Monday return flow sets up
across western and central Kansas into Nebraska. Mid level
shortwaves move eastward through the Plains with models differing on
the placement with some solutions north across the northern Plains
with others further south across Nebraska. Main takeaway is that
there will be a small chance for showers and thunderstorms Sunday
night and early Monday morning. A frontal boundary is forecast to
move into southern Nebraska or northern Kansas Monday night into
Tuesday morning. Good moisture transport is forecast Monday night
into Tuesday ahead of the front and attendant waves moving through
the Central Plains.

.Solar Eclipse for Monday, August 21st...

Latest model guidance indicates that there is uncertainty with the
timing and placement of precipitation across northern Kansas and
southern Nebraska Sunday night into Monday. That said, there remains
a slight chance of precipitation for the area. Some drying is
forecast into the early afternoon hours behind departing wave with
decreasing cloud cover. Model fluctuations and changes to the
forecast will continue through the week.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1245 PM CDT Wed Aug 16 2017

High MVFR CIGS across north central KS will gradually lift into a
low VFR deck with numerous showers and thunderstorms developing
along and east of a line from KMPR to KFNB around 20-21Z. These
storms may degrade conditions to MVFR or lower at times. Some of
these storms may produce large hail and winds in excess of 50 kts.
This line of storms will push SE and out of the area before 04Z
with winds backing to the NW overnight and becoming gusty




SHORT TERM...Sanders
AVIATION...Skow is the U.S. government's official web portal to all federal, state and local government web resources and services.