Area Forecast Discussion
Issued by NWS Kansas City/Pleasant Hill, MO

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000
FXUS63 KEAX 150526
AFDEAX

AREA FORECAST DISCUSSION
National Weather Service Kansas City/Pleasant Hill MO
1226 AM CDT Tue Aug 15 2017

.Discussion...
Issued at 313 PM CDT MON AUG 14 2017

Thunderstorms have formed along a subtle boundary across southeast
Nebraska and northwest Missouri this afternoon. This boundary is
colocated with the interaction between a weak surface low over
the High Plains and a weak ridge of high pressure over the
Northern Plains. Flow aloft is rather weak, so deep layer shear
associated with northwest Missouri is minimal for organized storm
development, with only 20 to 30 kts of present effective shear.
SPC mesoanalysis indicates around 2000 J/kg of ML CAPE, so some of
these storms will have strong enough updrafts to produce some
small hail and perhaps some gusty winds. Not anticipating severe
weather with any of this activity, but gusty winds up to 40 mph
will be possible with the downdraft activity. With the boundary
solidly in place over northern Missouri the best chance for rain
through the night will reside across the far northern Missouri
corridor through the overnight hours. Subtle isentropic upglide
apparent on the 305K ThetaE surface across that region is the
likely source of ascent, as low level winds are rather light and
no apparent source of midlevel vorticity will be present.

For Tuesday the trough at the surface will deepen a bit and push
the boundary well to the north of the MO/IA border, leaving all
of the region within the warm sector. Temperatures will become
more seasonal, with highs in the middle to upper 80s, with heat
indices pushing the middle 90s. As the boundary shifts northward
through the day more showers will continue to form along the
boundary through Tuesday morning. Coverage of this activity still
looks a bit spotty at the moment, but considering the orientation
of the boundary across northern Missouri and low level winds
running generally parallel to the boundary there will be a
localized heavy rain concern through the day on Tuesday, mainly
north of HWY 36. PWAT around 1.75 to 2.00 inches will make
rainfall rather efficient in this zone, so again, localized high
rain accums could be possible where these showers form.

Strong warm air advection processes pick up on Tuesday night into
Wednesday morning, which should create periods of showers and
thunderstorms across most of the area for through Wednesday. In
the midlevels the shortwave ridging and overall zonal patterns
will give way to southwest flow aloft ahead of an encroaching
shortwave trough. The best ascent with this trough will likely
arrive Wednesday afternoon and evening, which will likely coincide
with the warmest part of the day and perhaps ample instability for
strong thunderstorm formation. Clouds through the day present a
wild card to the situation, with more morning and afternoon
cloudiness associated with the warm air advection perhaps leading
to reduced instability. Should the sun be able to make its way
through the clouds temperatures could warm into the upper 80s with
heat indices approaching the middle to upper 90s. These
temperatures along with the quality low level moisture could yield
instability approaching 3000 J/kg. With the trough pushing through
the area deep layer shear will approach 30 to 40 kts, which should
yield ample conditions for strong to perhaps a few severe
thunderstorms on Wednesday afternoon. Mitigating the severe
potential will be veering low level winds, which will shorten
hodograph lengths, thus reducing the potential for rotating storms
and/or tornadoes. Again, with the cloud cover uncertainty the
strong/severe potential for Wednesday remains a bit conditional.
The best chances for rain during the afternoon/evening will likely
be along the advancing cold front where low level convergence and lift
are maximized across northwest Missouri. As this cold front
progresses south and east chances for widespread showers and
thunderstorms will increase through the overnight hours. High PWAT
values likely approaching or exceeding 2 inches will contribute to
efficient rain rates, however the progressive nature of the cold
front may mitigate the prolonged heavy rain potential with this
system.

Thursday will likely see cooler and drier air move into the area.
Highs will still be in the middle 80s, but with dry air moving in
conditions will feel a bit cooler with heat indices likely
hovering near air temperatures in the middle 80s. Return flow into
the area on Saturday will be the focus for the next chance for
some showers and thunderstorms. Amplifying the chances for
thunderstorms will a shortwave trough swinging through the
northwest flow. Early next week will see the active pattern
continue across the region as a series of shortwave troughs and
low level jet interactions could keep chances for rain alive
through the end of the forecast period. Specifics for the Monday
8/21 time frame can be found below in the "Solar Eclipse" section
of the AFD.

&&

.Specifics for the Total Solar Eclipse on August 21st...
Issued at 313 PM CDT MON AUG 14 2017

Model consistency still leaves a bit to be desired with respect to
the sky conditions for Mon 8/21 and the solar eclipse. Models
seemed to have been converging toward a solution indicating some
nocturnal thunderstorms forming along the leading edge of the
overnight low level jet across central and eastern Nebraska. This
solution would likely lead to thicker clouds over central Missouri
as the decaying complex pushes through the area. And while that
particular solution may have the best momentum at the moment the
most recent ECMWF solution continues to show a different solution,
hinting at a weaker LLJ and focused a little further north into
Nebraska. Still much too early to settle on one deterministic
forecast for Monday, so will continue to see how things progress
as we approach that time period.

&&

.Aviation...(For the 06Z TAFS through 06Z Tuesday Night)
Issued at 1208 AM CDT TUE AUG 15 2017

Low visbys are already starting to pop-up throughout portions of
MO, so look for this trend of decreasing visbys at the terminals
through sunrise this morning. Fog looks to be the dominant impact
to aviation interests, but can`t rule out lowered ceilings as the
overnight hours progresses. Once the fog dissipates in the
morning, an afternoon CU field will likely develop, although the
trend looks to be drier with respect to thunderstorm chances
within the terminal areas.

&&

.EAX WATCHES/WARNINGS/ADVISORIES...
KS...NONE.
MO...NONE.
&&

$$

Discussion...Leighton
Aviation...lg
Eclipse...Leighton



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