Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 231747

Area Forecast Discussion
National Weather Service Des Moines IA
1247 PM CDT Wed May 23 2018

Issued at 1247 PM CDT Wed May 23 2018

Will be updating zones shortly to include some thunder chances
over portions of central and western forecast area...north toward
the MN border. Pronounced outflow boundary has reinforced sfc
boundary over western into central Iowa early this afternoon.
Airmass south of the boundary quite unstable with a few isolated
showers/convective elements already showing up at 1740z. Question
regarding how much northward translation of boundary vs.
convergence along it this afternoon will determine coverage and
perhaps - intensity of convection. For now will include slight to
chance PoPs along and near the frontal zone. Confidence remains
low to medium.  /rev


.SHORT TERM.../Today through Tonight/
Issued at 322 AM CDT Wed May 23 2018

Early morning storms across northern Iowa will gradually clear out
to the northeast by midday, with an early risk for some small hail
and locally heavy rainfall. Once those storms clear out the forecast
is dry through tonight. A deep-layer thermal ridge will begin to
build across the region, making this the first of many unseasonably
warm days. It will truly begin to feel like summer as temperatures
climb into the mid 80s with dewpoints in the mid 60s this afternoon.
This will be the first time we have felt heat like this since last
year, so if engaging in outdoor activities please be sure to stay
hydrated and take breaks. Temperatures will not fall far tonight
either, with modest southerly flow keeping lows in the mid to upper

.LONG TERM.../Thursday through Tuesday/
Issued at 322 AM CDT Wed May 23 2018

The state will be well entrenched within the warm sector on Thursday
with very warm temperatures as upper ridging slides east of the
state.  A shortwave will be riding along the northern edge of this
ridge on Thursday into Friday.  This will push a boundary toward the
state on Thursday night into Friday although the front will be
weakening as the upper system outruns the boundary.  Convection is
forecast to fire across the Plains later Thursday and gradually work
toward the forecast area on Thursday night as the low level inflow
veers toward the state.  The storms will likely be outflow dominated
given the relatively weak shear and anticipate a weakening trend as
they enter the forecast area overnight into Friday morning.
Redevelopment is expected on Friday afternoon across central Iowa
near the weakening boundary and once again the weak shear will limit
overall organization.  Most of the convection should become cold
pool dominate with maturation limiting severe potential.  However,
the decent CAPE values would lend to a brief window of large hail or
even a downburst wind gust.

With the passage of the upper shortwave, ridging will build back
into the the central United States through the weekend into early
next week.  This points to very warm or even hot conditions through
the weekend into next week.  GFS seems to have convective issues
with all kinds of convection on a daily basis throughout the Midwest
through Day 7.  Given the proximity of the upper ridge a relative
lack of forcing, feel it is well overdone and has contributed to
POPs being to aggressive during this time.  Have tried to limit the
influence of this in the forecast as much as possible/reasonable and
lean much more toward the drier Euro solution.


.AVIATION.../For the 18Z TAFS through 18Z Thursday/
Issued at 12457PM CDT Wed May 23 2018

Main challenge will be possible convection again this afternoon
and evening near/along the outflow/synoptic boundary now draped
over western to central Iowa. Instability has quickly increased
over the southwest. For now have included mention of -tsra at
KFOD. Some chance that convection could fire southward toward KDSM
but more limited toward KMCW/KOTM/KALO at this time. Winds will
remain modest through 00z...and probably strong enough from 00z to
13z west to mitigate fog formation. Farther east in the Cedar
River Valley at KALO and south toward KOTM...some fog may occur
toward 12z; but not a strong enough signal at this time to include
any mention. Will leave that for the next package to address in
more detail. /rev




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