Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 161155

Area Forecast Discussion
National Weather Service Des Moines IA
655 AM CDT Wed Aug 16 2017

...Updated Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 427 AM CDT Wed Aug 16 2017

The main forecast concern was focused on timing of convection and
the severe weather potential today into the evening hours. The
hires models have a fairly good grasp on the current convection in
central and eastern Nebraska into western Iowa. Thus leaned
toward a blend of the HRRR/ESRL HRRR/NAM12 through 00z Thursday
for timing of convection and then more towards the ECMWF/NAM12
through 12z Thursday.

Per latest GOES-16 IR imagery, cloud tops are warming rapidly
suggesting the MCS is losing its strength and possibly diminishing.
This is likely the case wrt convection over eastern Nebraska and the
chances are decent that this convection diminishes before reaching
the forecast area. Delayed pops and left chances going across the
west in the event that the lingering convection trickles into the
western edges. However, the main line of convection in north-central
Kansas and south-central Nebraska seems to be holding together due
to an organized and strong cold pool. TOA suggest this line to move
into the southwest corner of the forecast area b/t 15-17z today but
a better chance of dissipating or remaining south in Missouri.
Again, the environment is not suitable for sustainable convection
and have higher confidence this will fall apart prior to the
aforementioned time frame.

Boundary remains draped across northern Missouri into southeast
Nebraska but is anticipated to push back north-northeast as a warm
front throughout the day today. Thus, redevelopment of convection
seems likely within the vicinity of the boundary pushing back
northward as the upper level trough and surface low pressure advance
further east throughout the day. Atmosphere looks to have some time
to destabilize this morning into the early afternoon with MUCAPE
values ranging from 1500-2000 J/kg by 16-20z today. LCL heights
range from 500-750m per NCAR WRF, updraft helicity seems sufficient
enough to support rotating updrafts this afternoon. Plus, there is
decent directional and speed shear this afternoon over central to
southern Iowa. Potential is there for an isolated tornadic threat
and also large hail. Severe weather timing remains the issue if the
atmosphere has the chance to rebound enough and doesn`t get muddled
with the current convection moving east out Nebraska this morning.
The current slight risk seems reasonable, but certainly can`t rule
out an upgrade to an enhanced risk across the south if the
aforementioned scenario pans out.

.LONG TERM.../Thursday through Tuesday/
Issued at 427 AM CDT Wed Aug 16 2017

Current system will pull east of the state on Thursday with a few
showers early across the north. However, strong subsidence and
drier air will push into the state with partial clearing in the
west and south. The quiet weather will be brief in duration as
another shortwave will drop into the Plain by Thursday night
across the Dakotas. Warm advection will start early Friday morning
with theta-e advection increasing into the Missouri River valley.
Forcing will spread into the state during the day although
moisture advection will remain modest. Scattered showers and a few
thunderstorms are expected to develop by afternoon and persist
into the night, although with limited instability, widespread
organized severe weather is not expected at this time. This system
will pass by Saturday morning with much of the weekend remaining
dry with seasonable temperatures.

Focus then shift towards Monday and expected weather during the
upcoming eclipse. Both GFS and Euro 00Z runs have Iowa on
southern edge of stronger westerlies with a surface boundary just
north of the state. This will be the main focus in the Midwest for
precipitation holding the bulk of precipitation north of the state
during the day. Given the changeable solutions over the past few
runs, this is in no way certain and will continue to look for
consistency. However, with both models coming closer with their
respective solutions, confidence will grow that the threat of
precipitation will be low for much of the day and sky conditions
decent for eclipse viewing. It appears the best threat for
precipitation will be Monday night into Tuesday as the front drops
through the state. Will continue to monitor closely.


.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 655 AM CDT Wed Aug 16 2017

Lingering IFR stratus over FOD should quickly dissipate this
morning with the sunrise. A few brief stints of MVFR visibility
due to fog is possible elsewhere, but left out mention due to the
expected short duration. Expecting convection to develop later
this afternoon into the evening hours and only have VCTS mentioned
as still some uncertainties in timing. With the cold front late
tonight into Thursday morning, have mention of MVFR stratus and
some gusty winds.





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