Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 252339
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
639 PM CDT Sun Jun 25 2017

.SHORT TERM.../Tonight through Monday/
Issued at 327 PM CDT Sun Jun 25 2017

Confidence: Medium to High

Northwest flow continues over the area through the period though we
will be in early stages of a transition to a warmer regime by the
end of the forecast period. Water vapor imagery this morning
confirms an upper level system over the eastern Dakotas with yet
another mid to upper level short wave in southern Canada which will
drop south and catch the Dakotas wave late tonight into Monday. The
more northerly system also inducing a surface cool front which will
drop south over the area on Monday as well. The combination of the
upper level support and developing boundary through the column will
be enough to generate showers and a few thunderstorms through the
forecast area late in the overnight and on Monday. The synoptic
models are handling the Dakotas shortwave slightly better than the
mesoscale models at this time...though there remain some
uncertainties regarding coverage of precipitation. Have taken a
similar tact to previous package and increased PoP through the
evening hours and overnight bringing the areal coverage to about I80
by 12z.  After 12z the area will drift south through the remainder
of the day generally tied to the surface front/wind shift. Models
differ on the amount of instability accompanying the system
overnight into Monday as well. Have trended toward less instability
overnight given lack of real unstable layers within Bufkit soundings
until mid to late morning Monday. Euro model may be overdone on
mucape through the overnight hours in our area. Showers and a few
thunderstorms should continue into the afternoon hours and may
increase in intensity as they get closer to the border in the mid to
late afternoon. Lows tonight will again fall into the 40s north to
50s in the south...though cloud cover is likely to prevent the same
widespread cooling we experienced this morning. Highs Monday will be
similar to today...a degree or so cooler in most locations given the
cloud and shower potential.

.LONG TERM.../Monday night through Sunday/
Issued at 327 PM CDT Sun Jun 25 2017

Monday Night through Tuesday...Surface high pressure will have
already begun to slide into the state from the northwest at the
beginning, bringing about quiet conditions. With the cold airmass in
place, clear skies, and light to calm winds at times, lows will dip
into the upper 40s and even mid 40s in a few locations. Brrrr for
this time of the year.  While dry, Tue will see high slide off and
WAA return throughout the day as surface high departs and weak upper
level ridging and zonal flow moves in. Highs will see modest boost
into mid-upper 70s/around 80 along with good moisture return
beginning.

Tuesday night through Friday...High temps generally 80s, and looks
to be quite an active period with multiple opportunities for strong
to severe storms. First up, wave moving off the Rockies Tue will
initiate storms/MCS back across Nebraska and South Dakota which will
work their way into Iowa after midnight Tue night. Given timing,
severe potential on the lower end with elevated overnight storms,
but potential for strong to severe winds should storms orient
linearly and organize in a borderline shear environment. Main
concern more in line with heavy rainfall with Pwats in excess of 1.5
in, but antecedent dry conditions and progressive storms should curb
flooding concerns this round.

Second up, previous wave will drag a weak boundary into the state
through Wed with the surface low reflection north of the area.
Should be the focus for afternoon/evening convection across the
state. Certainly question as to how morning convection plays out,
boundary location, and if the atmosphere can recover enough to
realize the areas of 3000+ J/kg SBCAPE currently depicted. Should
that be the case, tie in 0-6km shear in excess of 40 kts and severe
potential certainly along with more heavy rainfall. However again,
questions as to how first round plays out.

Third up, upper low/trough to pivot through and affect the area
later Thur and into Fri. Model disagreement comes into play here.
GFS reflection of upper low essentially develops it over the NW
Great Lakes area as Canadian energy/wave ingested, while Euro sweeps
the Canadian low across Northern Plains. As it pertains to Iowa, GFS
then shunts wave coming off the Rockies south of the state while
Euro fires storms across the state. Euro solution more
reasonable/consistent, but confidence in altering forecast heavily
not present just yet. By this point, with heavy rainfall possibly in
play once again flooding, river and over land, may be of increasing
concern.

Saturday through Sunday...As upper trough departs, quiet Sat in
store, but with return of WAA by Sun, storms once again possible.
Highs to remain in the upper 70s to mid 80s as well.

&&

.AVIATION.../For the 00Z TAFS through 00Z Monday evening/
Issued at 639 PM CDT Sun Jun 25 2017

VFR conditions will prevail through the TAF period with northwest
winds diminishing this evening, becoming light and variable at
times overnight, and re-establishing themselves by Monday
afternoon. Scattered light showers are expected across parts of
the area overnight, mainly between midnight and sunrise around
FOD/DSM/OTM. However, ceilings are expected to remain VFR and any
reduced visibilities will be spotty and brief so have not been
advertised in the 00Z TAFs beyond a VCSH group.

&&

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

&&

$$

SHORT TERM...REV
LONG TERM...Curtis
AVIATION...Lee



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