Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202139
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
339 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 18Z TAFS through 18Z Saturday afternoon/
Issued at 1205 PM CST Fri Jan 20 2017

While visibilities have improved at some locations, low ceilings
will persist this afternoon at most of the terminals keeping IFR
or LIFR conditions prevailing. Fog should become thick again
tonight with LIFR conditions. Showers will also move across TAF
sites after sunset tonight through tomorrow morning.


&&

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

$$

SHORT TERM...REV
LONG TERM...Donavon
AVIATION...Ansorge


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