Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 110455

Area Forecast Discussion
National Weather Service Des Moines IA
1155 PM CDT Thu Aug 10 2017

.SHORT TERM.../Tonight through Friday/
Issued at 353 PM CDT Thu Aug 10 2017

At mid-afternoon, a strong, compact shortwave trough was located
over southern Minnesota and identifiable by the counter-clockwise
rotation of clouds in GOES-16. At the surface, low pressure was over
central Wisconsin with a cold front stretching southwestward. At
3pm, the front was east of a line from Prairie Du Chien to
Marshalltown to Lamoni continuing to move east. The strongest
thunderstorms were over eastern Iowa pushing into northwest
Illinois, which is in an area characterized by MLCAPE of 2000 J/kg
and deep layer bulk shear of around 40 knots. Farther southwest over
south central Iowa, isolated thunderstorms are trying to form, but
bulk shear is lower near 30 knots. Main concern if these storms
strengthen will be strong wind gusts with DCAPE values over 1000
J/kg and dry low levels per forecast soundings, which would help to
accelerate wind gusts to the surface. Also, hail may be possible
with hail growth CAPE around 800 J/kg, though mid-level lapse rates
are not too steep. The front will slide southeastward the remainder
of this afternoon and evening with storms out of southern Iowa by

In the wake of the front this afternoon, have seen winds switch and
blow from the northwest with drier air coming in behind the front.
Cooler air will arrive overnight as high pressure over Saskatchewan
this afternoon drops southeastward into the upper Midwest. Lows were
adjusted from initial Superblend guidance by a degree or two and on
the cooler side of available guidance. Forecast concern overnight is
possible fog and low clouds. Model cross sections show abundant low
level moisture over northern Iowa and to a much lesser extent over
southern Iowa. Forecast soundings over the north also show
saturation near the ground with an inversion, indicating a possible
fog setup. However, short term guidance (SREF, WRF cores, CONSShort)
show low clouds arriving/developing over north central and northeast
Iowa after midnight. At this time, think there will be a mixture of
the two over the north with low clouds more prevalent roughly east
of I-35 and patchy fog more likely roughly west of I-35. Part of the
thinking for low clouds east of I-35 is that winds are just a touch
higher and that may be enough along with the clouds to mitigate the
fog potential there.

Fog that does form will dissipate within a few hours after sunrise
Friday with low clouds gradually lifting and scattering. By the
afternoon hours, except plenty of sunshine with highs around 5
degrees below normal as breezes blow from the north.

.LONG TERM.../Friday night through Thursday/
Issued at 353 PM CDT Thu Aug 10 2017

For the next several days Iowa will remain beneath broadly
cyclonic west northwesterly flow aloft. Meanwhile at the surface a
high pressure ridge will build in from the north on Friday night,
then shift eastward and remain anchored over the Great Lakes
region through the weekend, with Iowa on the western periphery of
the ridge. The combination of these factors will lead to a
continuation of temperatures a bit below normal, with generally
dry conditions prevailing. However, a fairly weak mid-level
shortwave trough will move slowly through the cyclonic flow over
Minnesota and Wisconsin over the weekend, producing some rain
chances for our forecast area between Saturday night and Sunday
night. Instability and atmospheric moisture will be limited and a
relative lack of strong flow/shear will preclude severe weather,
but scattered locations will receive light to moderate rainfall at
times. Next week we will enter a roughly zonal pattern aloft,
leading to alternating, low amplitude troughs and ridges. This
will allow temperatures to moderate to near normal and also bring
period thunderstorm chances, maximized around midweek. However, no
organized severe weather is anticipated and impacts should be low


.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1155 PM CDT Thu Aug 10 2017

Cold front will pass out of far southern Iowa overnight with
cooler Canadian air moving into the state. Some light fog may
develop toward daybreak and may have to contend with some stratus
in the northeast during the same time. However, widespread VFR
conditions are expected for much of the forecast period with
increasing north winds on Friday becoming light on Friday evening.





SHORT TERM...Ansorge
AVIATION...Cogil is the U.S. government's official web portal to all federal, state and local government web resources and services.