Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 231643

Area Forecast Discussion
National Weather Service Des Moines IA
1143 AM CDT Tue Aug 23 2016

Issued at 1143 AM CDT Tue Aug 23 2016

Have updated the grids to reflect the latest pop/qpf trends.
Complex of thunderstorms across NE Nebraska is expected to
continue to track NE into NW IA by mid-afternoon. Intensification
of the storms is possible with additional development across
western Iowa toward late afternoon/early evening. Heavy rainfall
threat still possible, with a shift toward the northern half of
Iowa expected with the latest trend in the CAMS. Shear parameters
still not great for widespread severe storms, but enough for an
isolated to scattered strong to severe storm with damaging winds
still the main threat with a secondary severe threat of large hail
or an isolated tornado if more discrete storms develop and sfc
winds end up backing a bit more to increase directional shear in
the low levels.


.SHORT TERM.../Today/
Issued at 418 AM CDT Tue Aug 23 2016

Primary concern today will of course be convective trends. Immediate
attention upstream relates to a compact MCS over south central NE
which appears to be driven by immediate kinematic forcing with mid
level short wave and theta-e advection surge mainly attributed to
advancing tight gradient of teen H85 dewpoints rather than warm
advection. Instability and shear currently decreases downstream into
the immediate Missouri River Valley so expect this cluster to
diminish somewhat. However the sustained degree of forcing and the
fact that the better moisture and instability should advect
northeast with the convection should sustained it to some degree and
maintain its strength. Extrapolation would bring it into northwest
sections of the forecast area 19-20z.

The attention will then turn further upstream to the cold front
developing across the high Plains. The front and primary convergence
should be positioned over eastern Nebraska during peak heating with
models suggesting 2500-3000 j/kg MLCapes will be realized with
convection spreading into western Iowa 21-00z. Our afternoon airmass
will be appreciably different than the past few days with humid
dewpoints pushing 70F driving the MLCapes. Models suggest the deep
shear will be around 30kts at best, but sufficient for organization.
Some convection allowing models /CAMs/ depict elevated updraft
helicities supporting supercells and this thinking. The predominate
threats would be hail and wind, but would not be surprised to see a
tornado west. A few models surge 0-1km shear and SRH near a subtle
warm front.

.LONG TERM.../Tonight through Monday/
Issued at 418 AM CDT Tue Aug 23 2016

There will be an extended period of unsettled weather in the long
term.  Beginning tonight...a strong shortwave will be pushing across
the Northern Plains with a surface low and cold front approaching
Western Iowa at 00z.  Strong theta-e advection will be in place
across Iowa and storms should be ongoing across the West at the
beginning of the period gradually spreading East overnight. There is
strong low level shear in place but somewhat weaker deeper shear.
Still the presence of a boundary and the strong low level shear will
keep the potential for a tornado going across the West into the mid-
evening hours.  Overnight the concern shifts to more of a heavy rain
event with PWATS approaching an inch and three quarters across the
North and West.

Wednesday the front will move through the forecast area keeping the
threat for storms continuing through the day.  The focus for heavy
rain and severe storms will be generally South of the rain that will
come tonight so flash flooding threats are somewhat diminished
though there is still a local threat from storms that produce heavy
rainfall.  Soundings showing PWATS across the North Wednesday into
Wednesday night are around three quarters of an inch while further
South they are closer to an inch and three quarters.

The front finally pushes South and East on Thursday with rain
chances virtually nil except for the far South/Southeast and only
slight chances in that area.

Friday we return to a warm advection regime with decent theta-e
advection across central Iowa Friday into Friday night and another
short wave passing through the region. Strong storms are again
possible Saturday along with a heavy rain threat. Sunday should be
drier and with a return to zonal flow we should see temps in the
80s. However with weaker shortwaves around and passing through
from Sunday into next week lower chance pops were left in the
forecast at this point.


.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 700 AM CDT Tue Aug 23 2016

The approach and passage of a frontal boundary will bring uncertainty
across several elements through the period. Initially mainly VFR
convection should cross the MO River later today possibly
affecting TAF sites late this afternoon and into the night. An
extended period of precip is possible, but have gone with windows
of VCTS and VCSH during most the likely times to avoid too long a
mention of uncertain thunder. Extrapolation expects the current
NE complex to stay just north and west of sites. Later tonight
stratus and potentially fog is expected to develop during and
after the convection just ahead of the front. Most models are in
agreement with this and with high low level moisture in place have
dropped ceilings to IFR overnight.





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