Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 171143
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
643 AM CDT Mon Apr 17 2017

.SHORT TERM.../Today through Tonight/
Issued at 358 AM CDT Mon Apr 17 2017

A weak shortwave is brushing the North/Northeast and will be East of
the area by 15Z.  Some weak vorticity is sliding across the far
North and radar is picking up sprinkles but these are not making it
to the ground per surface observations. Soundings verify that very
dry air is in place.  Other than some mid level clouds that will be
the only sensible weather from this feature.

By mid morning a ridge will build in aloft while a surface high over
the Northern Plains moves to the East.  Temps will be interesting to
forecast today as a surface low is taking shape over Wyoming in
response to the next shortwave to impact Iowa in the long term. This
surface low will have a trailing frontal boundary that will develop
and lie across Iowa...somewhere near the Highway 20 corridor.  So
with the upper ridge moving in, temps will generally be cooler than
yesterday by just a couple degrees.  However, by mid to late
afternoon or so the low over Wyoming will become better organized as
the upper wave deepens over the Rockies.  This will better establish
a warm front across Northern Iowa and as a result, temps across the
North will likely be the same or possibly a degree or two warmer
than Sunday`s highs.

Dry conditions will prevail across central Iowa through 09Z but
towards daybreak that low over the Plains will lift into West
central or Northwest MN pulling the warm front into Eastern MN and
Wisconsin and dragging a cold front into far NW Iowa.  Better
forcing and some theta-e advection will increase across much of
Western/NW Iowa ahead of the front and this will increase precip
chances across this area between 09Z and 12Z.  Soundings do not show
a great increase in moisture...and certainly not deep moisture so I
keep the area dry except for introducing some slight chance to low
chance pops across the far NW.  All the hi-res models do print out
some kind of precip in this area so although confidence is low, it
was worth a mention.

.LONG TERM.../Tuesday through Sunday/
Issued at 358 AM CDT Mon Apr 17 2017

An active period this week and it will be difficult to find a day
without precipitation in the forecast with the exception of
possibly Friday and Sunday. A strong PV anomaly has entered the
Pacific Northwest overnight and is currently centered over Oregon.
This system will move quickly east. Surface low pressure will form
over Wyoming and western South Dakota by late today as the system
crosses the Rockies, then will move east and be over west central
Minnesota by early Tuesday morning. Theta-e advection will develop
ahead of the system and bring a ribbon of moisture to the state.
This will help bring a chance for showers and a few thunderstorms
during the day along and ahead of an advancing cold front.
CAPE values will be mainly less than 1000 J/kg and will limit the
threat for severe weather.

The next system will quickly arrive as low pressure lifts through
Kansas and into Nebraska bringing a warm front into southern Iowa.
The low level jet will increase in advance of the system and will
bring increasing thunderstorm chances to late Tuesday
night/Wednesday morning over the south. Additional showers and
storms will be possible throughout the day with the best chances
for severe weather over southern Iowa during the afternoon and
evening. Conditions will be favorable for supercell development
and the potential for large hail and damaging winds. The tornado
threat still appears conditional with lack of backing of surface
winds to enhance helicity and low level shear. Areas of heavy
rainfall will also be possible with pwat values in the 1 to 1.5
inch range.

Temperatures will be cooler Thursday and into the weekend. The end
of the week still features an intense upper level low pressure
that will impact the state around Saturday. Still uncertainty on
the northward extent of precipitation chances. The GFS has been
the most consistent solution with precipitation reaching into
central Iowa while the GEM is too far north. The ECMWF is much
drier and keeps much of the state dry. This will be a cold system
therefore temperatures where rainfall does occur will likely be in
the 40s to even some 30s. The precipitation will exit by Sunday.

&&

.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 643 AM CDT Mon Apr 17 2017

VFR conditions will prevail for the next 24 hours, with only high
clouds at times. Light and variable winds will become Southerly and
increase after 00Z ahead of the next system to impact TAF locations
after this valid period.

&&

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

$$

SHORT TERM...FAB
LONG TERM...Donavon
AVIATION...FAB



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