Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 302026

Area Forecast Discussion
National Weather Service Des Moines IA
326 PM CDT Tue Aug 30 2016

.SHORT TERM.../Tonight/
Issued at 319 PM CDT Tue Aug 30 2016

The primary question for the period will be convective trends.
The first glance at the water vapor imagery wouldn`t suggest much
of a frontal passage occurring with steady state moisture
continuing to stream SW-NE into the upper Mississippi Valley ahead
of the CO short wave. However other waves along the ND/Canada
border and the very tail end of the Hudson Bay long wave trough
will just be sufficient to push an initial surface trough and then
trailing cold front through IA this evening. The trough appears to
be across southern IA per the 19z analysis while the true cold
front may be lagging across ND/MN/WI. The airmass across IA is
currently uncapped with around 1000 j/kg MLCapes, low LCLs, weak
shear and a relatively high percentage of the MLCape in the 0-3km
layer. Thus could see a few funnel cloud reports for several
hours. Precipitable water values are pushing two inches south with
warm cloud depths to 4km, but low level moisture convergence is
weak with 925/850mb winds already going north of 270 degrees so do
not feel widespread or sustained convection is much of a threat.

Any precip should be over by later tonight with a gradual north
to south decrease in cloud cover. The rate of drying south may be
slow enough and winds light enough to allow for some patch fog
development south toward daybreak.

.LONG TERM.../Wednesday through Tuesday/
Issued at 319 PM CDT Tue Aug 30 2016

Main forecast concerns include precip chances from Saturday
onward into the middle of next week.

Cold front will bring Canadian air mass into Iowa tonight. This
air mass will linger into Friday night. Made minimal changes to
temps during this time. Seasonably cool temperatures are expected
into Saturday.

Upper-level flow switches from northwest to southwest on
Saturday. Models depict area of forcing due to shortwave lifting
northeast across the state on Saturday. Another more potent
shortwave will eject northeast through the flow on Sunday.
Moisture and instability will be available for SHRA/TSRA far west
on Saturday, and overspreading most of the rest of the CWA
Saturday night into Sunday.

Cold front dropping southeast from the Dakotas will draw closer
to the CWA on Monday and will remain quasi-stationary into
Wednesday. During this time, ample moisture and instability will
remain for prolonged SHRA/TSRA chances. Heavy rainfall will be a
possibility. Best chances for svr wx will come Tuesday into
Wednesday as instability and shear line up best. In addition,
surface cold front will drop southeast through the CWA on
Wednesday as the western U.S. longwave trof slides eastward.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 104 PM CDT Tue Aug 30 2016

The initial concerns will be convection and cloud trends. Light
showers continue to drift across central and southern sections
with patchy sub 2Kft MVFR ceilings. The likelihood of thunder and
non-VFR conditions is expected to decrease through sunset as a
weak pressure trough along I-80 drifts southward through IA. Thus
only have VCSH and short term MVFR wording pending current trends
until confidence increases at any particular point. Some MVFR fog
may develop south immediately behind the frontal passage toward
daybreak, but otherwise VFR conditions are anticipated into Wed




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