Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212041

Area Forecast Discussion
National Weather Service Des Moines IA
341 PM CDT Sun May 21 2017

.SHORT TERM.../Tonight through Monday/
Issued at 339 PM CDT Sun May 21 2017

Cyclonic flow will be subsiding this evening with stratus deck
eroding away this evening.  Weak surface ridging will slide through
central Iowa during the later half of the evening as warm advection
begins overnight.  This is in response to a shortwave dropping
southeast into the region on the backside of the large upper closed
circulation.  Therefore, while daytime highs today were very cool,
overnight temperatures are not expected to drop considerably as the
warm advection and southwest winds increase later tonight.  In
addition, some mid and high clouds are expected to arrive toward
daybreak in advance of the system.

Theta-e advection increases into southern Iowa by Monday morning
with the threat of scattered showers in southern Iowa as the
moisture returns.  This moisture will continue to spread northward
during the day in advance of the surface low with instability
increasing through the afternoon.  Scattered showers and
thunderstorms are expected to develop ahead of the frontal boundary
by afternoon in northwest Iowa and move quickly eastward. Given
relatively low freezing levels and inverted-V soundings, both large
hail and some strong winds will be possible with any of the stronger
cells.  Limiting factor will be amount of instability available to
the storms as moisture as a whole is not overly robust. Temperatures
will be warmer on Monday with a thermal ridge quickly moving back
into the state as southwest winds increase late tonight into

.LONG TERM.../Monday night through Sunday/
Issued at 339 PM CDT Sun May 21 2017

The cold front will continue to cross the forecast area Monday
evening and current trends keep it in the southeast through much of
the overnight as well. Instability albeit weak wanes fairly
quickly through the evening so the chance for thunderstorms will
quickly diminish by mid evening. Showers will persist through much
of the night however.

On Tuesday the upper a second upper low finally drops south across
Iowa keeping the precip going and instability returns across the
east in the afternoon.  I have slight chance/isolated pops going
but colder air is in place by then and don`t see much in the way of
convection.  Isolated covers it about right.

Not much change in the extended with Wednesday and Thursday mainly
dry.  I did keep slight chances going Wednesday far south because
models hint that a lobe will pivot around the upper low now in IL
and this may produce enough forcing to pop some isolated showers.

On Friday the next large upper low drops in to the northern Plains
and breaks down the ridge up the region as well as pushing it East
now.  This may bring some showers/storms to the area as earl y as
Thursday night through Friday.  As that low takes it`s time pivoting
across the region, it will keep chances for showers and storms going
over the forecast area through the holiday weekend but temperatures
will remain seasonal.

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1236 PM CDT Sun May 21 2017

Broad cyclonic flow across the state today is helping stratus to
linger into the afternoon. However, this influence will wane this
evening with low cloudiness dissipating across the area.
Therefore, the widespread MVFR conditions of this afternoon will
give way to VFR conditions overnight into Monday morning.
Visibilities will be unrestricted for the duration of the forecast
with west to northwest winds becoming south to southwest with




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