Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 210537
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
1133 PM CST Fri Jan 20 2017

.SHORT TERM.../Tonight through Saturday/
Issued at 338 PM CST Fri Jan 20 2017

Similar concerns again tonight. Fog likely to fill in and thicken
quickly after sunset. Have segmented 2 areas with north/northwest/
and east/southeast river valleys quickly dropping or continuing down
for the remainder of tonight. Then over the south/southwest will see
lowering vsby as the upper level low moves north with light rain and
advection fog. Some improvement in vsby possible far southeast by 10-
13z...though will not try to figure exact ending of fog there and
leave it to the overnight shift for any adjustments. Fog may be more
transient in nature tonight...with visibility jumping around a bit.
However...expectation is that it will be down long enough to fog
criteria that fog advisory warranted. Mins will not move far from
present with only a few degrees drop over most of the region. With
warmer air moving in Saturday...highs will eventually warm to upper
30s north to lower 50s south. Rain should peak late evening and
early morning over the area. Clouds will likely hold on over most of
the area...with some breaks in the south possible in the afternoon
hours.

.LONG TERM.../Saturday night through Friday/
Issued at 338 PM CST Fri Jan 20 2017

Fog remains the forecast challenge though Saturday night and into
Sunday morning especially over northern Iowa. Low level profiles
remain quite saturated though that region of the state while a
weak boundary stalls across that area. The SREF visibility probs
during this time continue to support the persistence of fog. The
low level moisture will deepen on Sunday. A switch to northwest
surface winds may lead to the increased moisture depth resulting
into stratus as opposed to lingering fog. Much like Saturday, high
temperatures on Sunday may be a bit optimistic should the
status/fog linger though the day.

The most significant system during the extended remains the system
to impact the Midwest Tuesday and Wednesday. Relatively good
agreement remains on the upper level features though the typical
spatial and temporal differences remain. Have continued with a
consensus approach with a lean towards the EC/Gem solutions as
the GFS is slightly faster/farther north and drier on the southern
edge. General timing of this system should remain consistent with
no blocking high pressure in advance of the system and unrestricting
flow from the southwest. This would place highest chances for
precipitation by Tuesday afternoon and into Wednesday.
Precipitation type remains rain or snow with snow to the
northwest of the sfc low pressure and rain and/or snow to the
south. As mentioned in yesterdays discussion, near surface
temperatures/dew points and wet bulb may be warm initially even
across the north and may result in rain before a transition to
snow later. Therefore, snowfall amounts still are unclear. Colder
temperatures will follow for the end of the week with 850 mb
temperatures by Thursday dropping to -8C to -12C.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1133 PM CST Fri Jan 20 2017

LIFR cigs and vsbys will persist through 15Z before gradually
improving and Northern TAF locations may remain in LIFR conditions
through much of the day.  After 00Z a low pressure system will slide
East across Southern sections of the plains.  This will not impact
Iowa TAF`s directly but it will cause a wind shift to the Northwest
which may be responsible for more dense fog...especially over
Northern TAF locations.

&&

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

$$

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


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