Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 210349

Area Forecast Discussion
National Weather Service Des Moines IA
1049 PM CDT Sun May 20 2018

.SHORT TERM.../Tonight through Monday/
Issued at 341 PM CDT Sun May 20 2018

Upper ridging has passed to the east placing Iowa on the backside of
the ridge.  A shortwave is lifting up out of the Southern Plains and
will cross southeast Iowa tonight then head into Illinois and
Wisconsin.  To the west, low pressure is developing over the Plains.
This low is expected to weaken into an open wave and lift northeast
into MN and WI on Monday.  Deeper moisture will lift into southeast
Iowa later this afternoon and tonight while a second area of
somewhat better moisture, located ahead of the Plains low, lifts
across eastern Nebraska and the Dakotas and far western into
northwest Iowa.  Forcing will also increase across these respective
areas as the shortwave lifts up across southeast Iowa and the low
shifts east across far west and northwest Iowa.  This will bring
more showers and thunderstorms into southern and western Iowa this
evening lifting northeast overnight into Monday.  With unusually
weak flow for May, with little more than 20kts through almost 500mb,
not really considering any type of severe threat but stronger storms
could certainly produce some locally heavy rainfall...especially

By Monday afternoon we will only be dealing with the remnant of the
wave across northern/northeast Iowa for some lingering precip in
that area. Soundings indicate quite a bit of low level moisture so
low clouds will be problematic and will likely hang around through
Monday...especially across the north.  Southern Iowa may see sun in
the afternoon.  Temps will be pretty cool across the state Monday
due to the cloud cover and a northwest wind at the surface.  We will
generally see mid/upper 50s north to the upper 60s/low 70s south.

.LONG TERM.../Monday night through Sunday/
Issued at 341 PM CDT Sun May 20 2018

A ridge builds back over the region for Monday night/Tuesday for
somewhat of a break from precip.  However on Tuesday, most models do
suggest a shortwave breaking through the ridge, traveling across
southern/central Iowa for some potential for precip.  Being that it
is shifting east under general ridging and no real boundary around,
any precip should be very hit or miss and light.

Tuesday night and Wednesday a stronger wave rides up the ridge
bringing a better chance for showers and storms into western then
northern Iowa.  Through this whole period any support for severe
storms is marginal at best so any potential will be tied to areas
where better forcing and instability exist.  Since this will be
better determined in the coming days I will keep to the current
wording in products. One other item of note is with the ridge
building into the region, we will be seeing much warmer temps
and dewpoints into the mid 60s so much more humid than we have
been seeing as well.

Wednesday night through Saturday morning is the most interesting
time in the long term.  A deep trough over the west will lift into
the northern Rockies and across the Northern Plains into the Upper
Midwest. This trough will flatten the ridge over the region with
time.  Ahead of this, another decent shortwave will cross Iowa
mainly across the north Thursday night/Friday.  Showers and storms
will again be possible and given the strength of this wave, a few
strong storms will be at least possible.  Friday night and Saturday
the upper trough will deepen, sink south and pass to the east by
late Saturday. Additional chances for showers/storms will occur
through early Saturday over much of the state but by afternoon, the
trough axis should be east of the state and the best chances will be
across the east to southeast Iowa.  Drier and cooler air will work
into the state then and we will enter a quieter period.


.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1049 PM CDT Sun May 20 2018

Couple of concerns overnight into Monday. Cigs will remain
MVFR/IFR at times continuing to lower through 09z and remain
generally IFR until 18-20z with some improvement possible
KDSM/KOTM prior to other sites. Patchy fog likely as well toward
10z lingering through 15z. Upper level system tracking into Iowa
tonight will tap into abundant moisture and weak frontal zone
aloft...resulting in showers with brief heavy rainfall. May see
some impacts at KDSM/KOTM and KALO especially due to strength of
showers and rainfall thorugh 12z. /rev




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