Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 291715

Area Forecast Discussion
National Weather Service Des Moines IA
1213 PM CDT Mon May 29 2017

...Updated for 29/18Z Aviation Discussion...

Issued at 624 AM CDT Mon May 29 2017

Showers have develop over southern Iowa on a weak PV anomaly that
has moved in from Nebraska ahead of the shortwave trough/PV
anomaly mentioned in the short term discussion below. Have updated
PoP/Sky/Wx grids for the next few hours based on current trends
and HRRR/RAP guidance.


.SHORT TERM.../Today through Tonight/
Issued at 314 AM CDT Mon May 29 2017

A shortwave trough and associated PV anomaly will drop through Iowa
in the broad northwest flow today. With cooler air aloft behind this
trough/PV anomaly, diurnally driven clouds and scattered showers
with perhaps a few peals of thunder will develop late this morning
and afternoon. Used hi-res models over global models to bring PoPs
farther south toward the Highway 30 corridor. Winds from the
northwest will also increase this morning and become gusty similar
to yesterday. Forecast soundings across central Iowa show deep
mixing once again, though perhaps not as deep as Sunday.
Nevertheless, with dry adiabatic lapse rates from the surface to the
top of the mixed layer, will be able to mix down wind gusts of 25 to
35 knots. It is also worth noting that soundings show an inverted V
profile. This may allow showers to entrain drier air into their
downdrafts yielding higher wind gusts compared to the synoptic gusts
mentioned above. While highs were adjusted a degree or so upward,
they will still be cooler than yesterday and below normal for late

Showers will end shortly after sunset and winds will decrease
as well as the boundary layer decouples with the loss of daytime
heating. Skies will become partly to mostly clear. Lows will be in
the upper 40s to low 50s. The next shortwave trough/PV anomaly will
be dropping out of the Dakotas toward Iowa in the northwest flow
by 12z Tuesday.

.LONG TERM.../Tuesday through Sunday/
Issued at 314 AM CDT Mon May 29 2017

Expectations have changed little from this time yesterday. Our
weather will continue to be dominated by high amplitude broad
cyclonic flow at the start of the period before transitioning to
weaker nondescript flow to end the week. At onset, Tuesday looks
to play out much like today with the potential for unorganized
weak peak heating convection north and east. Winds should again be
brisk from the NW possibly gusting to 30 mph in deep mixing
considering low humidities in this dry airmass.

This pattern will finally break into Wednesday as surface high
pressure slides into the MO Valley with a cool start to the day.
Warm advection will finally return again later into the night
however as a weak short wave crosses the Plains with lowering
heights pushing the baroclinic zone back into Iowa. This will
likely trigger elevated convection and a weak MCS which will
conclude with much of Iowa being back into the warm sector
Thursday pushing highs and humidity levels closer to what is
typical for this time of year. The ECMWF and GFS are both in good
agreement with a weak warm front near the IA/MN border during the
evening possibly triggering surface based convection which will
sustain itself into the night due to persistent inflow and
moisture transport into deep convergence. Precipitable water and
specific humidity values are nothing anomalous for early June, but
even seasonal levels may still produce appreciable amounts,
especially considering the weak flow and shear. MLCAPEs may reach
2-3K j/kg during peak heating so there still could be a brief
thermodynamically dominated severe weather window through early

Confidence in details diminishes somewhat into Friday due to
uncertainties of where outflow may re-focus the weak warm front,
but the general scenario will likely repeat with late peak heating
surface convection following by an MCS into the night with best
chances north and east. Both the ECMWF and GFS suggest some form
of a weak short wave in the maturing NW flow Saturday keeping at
least low PoPs for convection in the forecast. Although the
deterministic GFS suggests some PoPs are necessary Sunday, have
gone dry for now as the GEFS and especially the ECMWF and its
ensemble are less aggressive with the NW flow that would drive it
with stronger ridging into the Dakotas.


.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1213 PM CDT Mon May 29 2017

Mainly VFR conditions are expected during the valid TAF period.
Mixing of the boundary layer will result in surface wind gusts of 25
to 35 kt this afternoon. Scattered to isolated showers are psbl this
afternoon mainly along and north of US Hwy 20, which may contain
enhanced gusty winds and perhaps a brief period of MVFR cigs. Any
lingering showers will dissipate and winds will abate this evening.





SHORT TERM...Ansorge
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