Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 242127
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
327 PM CST Fri Feb 24 2017

.SHORT TERM.../Tonight through Saturday/
Issued at 326 PM CST Fri Feb 24 2017

Deformation area passing through forecast area at this time as upper
low swings across the state.  Strong mid level QC forcing is ongoing
with impressive Q-vector convergence over central Iowa.  This will
slide east this evening with forcing and lift gradually waning from
west to east through 06Z.  Unfortunately, peak snowfall rates in I35
corridor near DSM metro and AMW will occur over the next few hours
during peak rush hour traffic and will remain problematic.
Therefore, see no reason to end the advisory there.  Farther north,
still a few locations with 1/4 mile or less visibility in light snow
and blowing snow across north central Iowa.  While blizzard has been
somewhat marginal, given the poor conditions continuing for the next
few hours, will continue headline but trim as snow ends and
conditions improve from west to east this evening.  Clouds will also
clear overnight with strong drying on backside of the system in area
of deep subsidence.  Temperatures should drop nicely with the clear
skies and fresh snow cover in northern and portions of central Iowa.

Quiet weather is expected into Saturday with ridging sliding toward
the state.  A weak shortwave will approach later in the day and with
the onset of warm advection late in the period, some cloudiness will
once again spread across the state.  However, dry low levels will
prevent any sort of precipitation from occurring.  Temperatures over
the snow field will remain hard pressed to climb much during the day
although late warm advection should help a little.

.LONG TERM.../Saturday night through Friday/
Issued at 326 PM CST Fri Feb 24 2017

The new snow field will dictate temperatures through much of the
extended period. High pressure ridge will begin sliding east
Saturday night with return flow developing over central Iowa into
Sunday. Clouds will be on the increase Saturday night in response
to weak deep layer theta-e advection in addition to the approach
of a short wave trough. This system appears to have a
moisture/forcing timing discontinuity with the deeper moisture
arriving late Saturday night and the primary forcing lagging into
the day Sunday. That said, not completely confident in no
precipitation during this period and any precipitation. Model sounding
profiles seem to be much to mixed too much given the much drier
wet bulb temperatures and potential cloud cover. This certainly
may hold temperatures down but also any precipitation would trend
towards light snow, though little to no accumulations would be
expected. Regardless, high temperatures will linger in the low
30s over the north, readings should rise into the mid to upper 40s
central and south given better potential for some sun and mixing.
Weak high pressure will pass through the region Sunday night. The
potential is there for a good radiational cooling night with
clear skies and light winds and this would have the greatest
impact over the snow area north. Should this occur, overnight lows
very well may drop into the single digits.

Warm advection will return again on Monday as lee side
cyclogenesis commences over eastern Colorado. This low pressure
will will lift east northeast through Kansas/northern Missouri and
into southeast Iowa from Monday night into Tuesday. Warm dew
points will lift into Iowa ahead of the system and should have wet
bulb temperatures at 34 or high as far north as Highway 20. With
this current track, any precipitation along and south of this line
will be primarily rain and light snow or a rain/snow mix to the
north. Temperatures will be reliant on clouds breaking allowing
better mixing in the afternoon. The higher dew points overriding
the snow over the north may lead to fog development Monday night
into Tuesday then eroding as drier air behind a passing boundary
late in the day become established.

Another short wave fragment will move through around Wednesday and
may bring light precipitation to the state. General northwest flow
will persist behind the system on Thursday then a transition to a
more broad zonal flow toward Friday. Mainly quiet weather to end
the period with temperatures near to above seasonal values central
and south and lagging a bit yet north with the snow.

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1125 AM CST Fri Feb 24 2017

Snow showers and stratus are producing widespread MVFR conditions,
with areas of embedded IFR or lower visibilities and ceilings.
This will continue through the afternoon and into the evening
before steadily improving overnight as the snow moves out. Have
taken best estimate at timing improvement of flight categories,
but amendments are anticipated.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight CST tonight for IAZ027-
028-037>039-047>050-057>060-070.

Blizzard Warning until midnight CST tonight for IAZ005>007-016-
017-023>026-033>036-044>046.

&&

$$

SHORT TERM...Cogil
LONG TERM...Donavon
AVIATION...Lee


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