Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 232049

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

.SHORT TERM.../Tonight through Thursday/
Issued at 348 PM CDT Wed May 23 2018

Confidence: Medium

Relatively unstable atmosphere with little forcing for now... though
cluster of storms over northeast Nebraska and southeast South Dakota
may outflow and track east along the weak warm front lifting north
this evening across Iowa. Hires models also suggest some scattered
convection possible over the northwest this evening with later
development from convection west.  Winds overnight will remain south
southeast as gradient relaxes this evening.  Temperatures likely to
fall off to the mid to upper 60s by sunrise. Despite the high low
level moisture expected...winds will remain strong enough overnight
to limit/remove threat for any widespread fog development.  Some
patchy valley fog is also possible by morning. Highs tomorrow will
be warm again with upper 80s common for the entire area.

.LONG TERM.../Thursday night through Wednesday/
Issued at 348 PM CDT Wed May 23 2018

.../Thursday Night through Saturday/ Confidence: Medium to High

Main concern will be increasing threat for thunderstorms Thursday
night into Friday night.  Period of greater threat for convection
likely to be during this time. With weakening boundary and weak to
modest forcing moving across the region Thursday night...
instability will favor some hail/wind and another round of locally
heavy rainfall due to high water content of moisture expected
through the period. Confidence on details and coverage of stronger
convection somewhat limited...though with instability waning toward
06z...expect the better chances for any impactful weather to be
prior to that time and mostly over the northwest third to half of
the forecast area. After the storms diminish on Friday a quick
return to warm H850 temperatures will occur over the area. Heat will
be increasing over the region through Saturday as a ridge builds
aloft into the region beginning Saturday afternoon into Sunday and
Monday.  Highs Saturday are likely to move into the upper 80s to
lower 90s and heat indices in the lower 90s as well in areas that
hit 90 For higher. For now...will continue messaging heat in social
media and outlooks rather than product headlines. Will evaluate
again tomorrow for various options as event nears.  Obviously given
the holiday weekend with a first real taste of summer...heightened
awareness for heat related problems will be needed.

.../Saturday Night through Monday/ Confidence: Medium to High

Warm to hot conditions will continue for Sunday into Monday.
Depending on cloud cover and mixing Sunday...highs may reach the
lower 90s or higher and this may be the hottest day of the 3 day
period. Still some time to dig further into the details. By Monday
temperatures will remain very warm though hint that more clouds may
keep highs slightly cooler...though the additive effects of the heat
will need to be considered. Some low chances for thunder will remain
over the region.

.../Monday Night through Wednesday/ Confidence: Low to Medium

Models diverge toward the end of the period with the GFS faster than
the Euro with regard to the midweek arrival though both models are
trending toward weakening the approaching front as it nears the
region Tuesday night into Wednesday. Cloud cover and some increase
in rain/thunder chances should lower highs and lessen heat during
the period.


.AVIATION.../For the 18Z TAFS through 18Z Thursday/
Issued at 1245 PM 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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