Area Forecast Discussion
Issued by NWS Des Moines, IA

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017
FXUS63 KDMX 192015
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
315 PM CDT Wed Apr 19 2017

.SHORT TERM.../Tonight through Thursday/
Issued at 315 PM CDT Wed Apr 19 2017

Early afternoon surface analysis places the warm front over far
southern Iowa along or south of Highway 34 with low pressure nearing
Concordia, KS. South of this boundary, dewpoints are in the low and
middle 60s with temperatures in the 70s. 19z operational and non-
operational visible satellite imagery shows cumulus clouds along
this boundary with stable wave clouds just north of the front up
towards Des Moines eroding. Over northern Iowa, the atmosphere is
relatively stable as clouds, showers, and thunderstorms are in the
process of exiting central Iowa as the warm front slowly trudges
northward. 19z SPC mesoanalysis shows SBCAPE values peaking near
2000 J/kg and bulk wind shear of 50 to 60 knots in the vicinty of
the warm front continue to favor supercell development. Steep mid-
level lapse rates east of the surface low into far southwest Iowa
will continue farther into the warm sector as the low approaches and
some mid-level cooling takes place. 0-1km SRH is around 200 m2/s2
east of the surface low along the warm front keeping the main
tornado threat in this area, especially closer to the surfac low.
Large hail will also be possible near and just north of the warm
front as storms cross the front and become elevated.

Current thinking with the slow progress of the warm front is that it
may approach the Interstate 80 corridor by late this afternoon and
early this evening. Expect convective initiation around 4 or 5 this
afternoon in eastern Nebraska and western Iowa as the low nears the
state. A broken line of thunderstorms will develop along the cold
front that will move into southwest Iowa this evening with the
primary threat with these storms being large hail and damaging wind
gusts. Heavy rainfall will be possible, but the progressive nature
of the thunderstorms today should largely minimize the flash flood
threat. These storms should push through central Iowa (e.g. Des
Moines metro) mid-evening and be largely out our CWA by 9 to 10z.

For Thursday, cold air advection will occur as the low moves into
the Great Lakes with gusty winds from the northwest prevailing. Did
bump up winds a bit for tomorrow from initial guidance. Clouds will
gradually clear from southeast to northeast throughout the day.

.LONG TERM.../Thursday night through Wednesday/
Issued at 315 PM CDT Wed Apr 19 2017

Few concerns after system moves east tonight.  Temperatures will
cool off following the event with H850 temperatures from 2 to 4C
through Saturday during the daytime hours as high pressure builds
into the area and the storm track is displaced south during the
early portion of the extended. This will keep afternoon highs in the
50s to lower 60s over most of the region.  By Sunday low pressure
will be developing over the high plains with return southerly flow
across the region.  This will allow for H850 temperatures to climb
back to 6C to 7C for Sunday into Monday prior to a cold front
approaching from the west. This will result in afternoon highs back
in the 60s to lower 70s for at least a two day period. The front
should spark some showers and thunderstorms Monday night into
Tuesday.  A quick passage of the boundary will be followed by a
quick return to southerly flow with another southern stream short
wave and strong warm air advection back into the region with some
potential for showers and thunderstorms.  The majority of the area
will be far enough north of the better instability to see mainly
rain/showers and a fairly tight surface gradient resulting in a
breezy rainy period from late Tuesday night into Wednesday.
Depending on the timing the system may be just about out of the
region by Wednesday afternoon. There is a little more uncertainty
regarding afternoon highs Tuesday into Wednesday...depending on the
track of the system at that time...highs may be a bit cooler
especially if the low is just south enough to promote east northeast
surface flow over the area.  Much less diurnal range would be
anticipated. Some time to iron out the details in the next couple of
days.

&&

.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1242 PM CDT Wed Apr 19 2017

Plenty of clouds along with showers and thunderstorms are lifting
northward into northern Iowa with a mixture of VFR or MVFR
ceilings. Over central and southern Iowa, breaks in the clouds
are allowing for primarily VFR ceilings with a few localized areas
of MVFR ceilings. Scattered thunderstorms are expected to develop
later this afternoon and evening and have tried to time the
period of when they will pass the sites. IFR ceilings/visibilities
are possible as these storms pass, but not enough confidence in
timing or severity of conditions so will handle with amendments
as needed. As a cold front sweeps through the state overnight,
winds will become from the northwest with MVFR ceilings gradually
giving way to VFR over the south tomorrow morning and likely late
in the day or evening over northern Iowa.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Ansorge
LONG TERM...REV
AVIATION...Ansorge



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