Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 200908

Area Forecast Discussion
National Weather Service Des Moines IA
408 AM CDT Tue Mar 20 2018

.SHORT TERM.../Today through Tonight/
Issued at 335 AM CDT Tue Mar 20 2018

Upper wave currently moving through SD/NE and into IA/MN this
morning on water vapor and will continue to push ESE through the
day. It is expected to diminish in intensity as it moves southeast,
but will still bring enough lift to spread some light snow across
much of the west central to northwestern forecast area into this
morning. Fairly decent area of precipitation currently associated
with the trough across eastern SD/NE and it is beginning to push
into west central IA. Radar returns showing up across the northwest,
but precip is fighting a lot of dry air in the north to
northeasterly flow of the large area of high pressure in place
across Ontario into the Great Lakes region. However starting to see
some moistening of the column taking place near Estherville and Storm
Lake as ceilings have dropped to around 3-4kft versus the 7-10kft
that were in place previously over the past few hours. CAMS showing
decent area of precip expected to spread across the west/northwest
this morning, with lingering light precip into the afternoon hours
across the western half to two-thirds of the forecast area.
Additionally soundings showing cooling aloft in the mid levels with
some weak instability into the afternoon. Therefore expect precip to
take on more of a convective look into the afternoon hours and
become more scattered in nature. Even with temps expected to hover
near or just above freezing across the north, expect precipitation
in the form of snow. Given the warmer temps, did lower snow ratios
some but could still see around 1-2" of snow in the far northwest
near Estherville. This may end up with more accumulations mainly in
grassy areas as road temps are currently in the mid 30s and not
expected to drop much through the day. However if more of a
convective snow shower moves through, snowfall intensity may briefly
allow for some isolated slick spots to develop. As surface
temperatures warm across central Iowa expect any lingering
precipitation to likely start to mix with rain or change over to all
rain mainly along and south of Highway 20 this afternoon.

A weaker secondary wave is moving through ND currently. This wave is
expected to drop south toward Iowa this evening. This could result
in some lingering flurries/sprinkles in the far west, otherwise
should see dry conditions for the remainder of the night.
Temperatures expected to drop into the 20s with clouds remaining for
much of the night.

.LONG TERM.../Wednesday through Monday/
Issued at 335 AM CDT Tue Mar 20 2018

Pretty active pattern as several shortwaves are expected to ride
down the upper ridge over the Rockies and impact the weather over
the Upper Midwest.  On Wednesday the models are  similar in locating
a shortwave over the Dakotas.  Over the southeast U.S. a low will be
shifting off into the Atlantic.  The result of which will be to de-
amplify the ridge over the Rockies.  That will send the Dakotas
shortwave more east than southeast thus missing Iowa.  We may see a
fair amount of clouds mainly across northern Iowa but any precip
should remain north and east of the state.  Wednesday night and
Thursday a strong low will push into the Pacific Northwest shoving
the ridge into the Upper Midwest with a weak shortwave dropping down
the flow.  Meanwhile a surface low will develop over Colorado with a
frontal boundary extending east into KS/NE and eventually into
Northern Missouri.  Some weak forcing along this boundary will
interact with the shortwave dropping down and may produce some light
precip across south central into southeast Iowa during the day. I
have low confidence that we will see anything Thursday morning but
if we do, temps in the morning would support a rain/snow mix going
over to rain by late morning.  The better chance will be in the
afternoon and precip would be rain by then.

That wave moves off then Friday through Saturday the Pacific NW low
pushes into the west and develops a deep trough over the western
U.S. while the ridge sharpens over the Upper Midwest.  Models differ
here with the Euro deepening the surface low while the GFS is far
weaker and broad with the low though both models put a warm front in
the vicinity of southwest Iowa.  Friday night into Saturday a strong
shortwave ejects out of the western trough across the Plains. Both
models strengthen the surface low but the Euro is much stronger with
the low and a stronger warm front as well.  In addition, some
instability works into southern or southwest Iowa so not only will
rain move into the area but thunderstorms will be possible across
southern/southwest Iowa.  Further north, the precip will transition
to snow and both models print out some impressive qpf.  Snow will
occur across northern Iowa.  Strong forcing is forecast to occur
over the state on Saturday but just where this sets up is a little
too far out to tell.  There is definitely a potential for a heavier
band to set up, likely somewhere along the highway 20 corridor or
north on Saturday.  For this reason will start mentioning a
potential for snow north on Saturday.  By late Saturday that western
trough begins to move east.  This will push our system to our east
by late Saturday night though there still may be some snow Saturday

The western trough will then deepen as it moves towards the Rockies
but models really diverge on how it will evolve.  It will bring
periodic chances for precip to the Upper Midwest into next week but
with the model uncertainty, I went with a model blended solution for
Sunday and beyond.


.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1153 PM CDT Mon Mar 19 2018

Minor changes to forecast with main emphasis on extending MVFR
cigs into farther out in the period. Inverted trough still
expected to produce light rain/snow over most of the sites except
less if any at KALO/KOTM. Shield of lower cigs will spread east
and southeast...lingering into the post 00z timeframe over the
region. Hires models may keep MVFR clouds in through 06z...but
will need to evaluate again at 12z. /rev




SHORT TERM...Beerends
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