Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 211734
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1234 PM CDT Sun Aug 21 2016

.SHORT TERM.../Today/
Issued at 402 AM CDT Sun Aug 21 2016

Forecaster Confidence = High

A large upper trough continues to pivot through the upper
Mississippi River valley with Iowa now residing in an area of upper
level height rises and broad subsidence. Primary surface low is now
well northeast over Lk Superior as a weak sfc high builds into the
region.

Keeping an eye on possible fog development this morning as many
areas have seen winds decouple with T/Td spreads small.  Limiting
factor appears to be dry air and turbulence right off the surface as
KDMX VWP shows winds at 2k feet around 30 kts.  Otherwise...today
should be an almost ideal late summer day with partly to mostly
sunny skies /fair weather cumulus/ and low humidity as sfc dewpoints
remain in the mid-50s. Weak warm air advection aloft combined with
plenty of sunshine should allow readings to warm 3 to 4 degrees from
yesterday with highs in the low 70s north and the middle to upper
70s central and south

.LONG TERM.../Tonight through Saturday/
Issued at 402 AM CDT Sun Aug 21 2016

The primary sensible weather concerns remain wind during peak mixing
Monday afternoon and then two precip events, one around midweek and
another into the coming weekend. Tranquil weather is expected at
onset with surface high pressure departing tonight. This will
initiate a gradual increase in heat and humidity with noted
warm/theta-e advection into Monday, especially through the Siouxland
area. The forecast remains dry during this period however due to
the lack of moisture and instability but would not be surprised to
see a few weak elevated showers northwest late tonight. Iowa will
be back into more seasonal temp and dewpoints Monday with fairly
strong return flow. This will bring non-convective wind we haven`t
seen for awhile. Fairmont MN bufr soundings suggest mixing to 1km
may bring gusts pushing 40 mph to northwest sections by afternoon.

Winds should subside into the evening with the attention turning to
precip into Tuesday. Our brief dry zonal flow will switch to
southwest as the upper low tracks along the US/Canadian border.
Although the surface front remains to our west, most models depict
a weak wave passage with increased moisture reflected in a
secondary theta-e surge and the return of appreciable instability.
This precip could be followed by remnants of surface based
convection to our west into the night. Deep shear will be weak so
our severe weather window will likely be more tied to peak heating
with heavy rain potentially a greater threat into the night.
Precipitable water values and warm cloud depths are high but not
extreme, but the potential may be more tied to duration with
persistent moisture transport into the night, the mean wind
parallel to the front and Corfidi vectors quite weak. This may
continue into Wednesday morning as well due to larger scale
forcing contributions ahead of the approaching upper system. None
of the deterministic models are too excessive with their QPF
amounts, but it is interesting to note that GEFS 24hr Mean QPF for
several periods ending Tue Night and Wednesday is over an inch
over a decent area and is in the 99th plus M-climate percentile.
There could also be another window of severe weather potential
central and southeast if recovery can occur during peak heating
Wednesday with shear increasing a bit.

Drier weather with cooler less humid weather is anticipated into
Thursday, but the next system will bring a fairly coupled increase
in thermodynamic and kinematic forcing per both the 00z GFS and
ECMWF. With both in decent agreement, confidence is fairly high for
this lead time.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1233 PM CDT Sun Aug 21 2016

Widespread VFR conditions will occur for the duration of the
forecast with surface ridge passing through the state this
evening. Northwest surface winds will become light from the south
overnight and increase quickly on Monday morning. Cu field will
dissipate with sunset with unrestricted visibilities and ceilings
tonight into Monday morning.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

SHORT TERM...Fowle
LONG TERM...Small
AVIATION...Cogil


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