Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 171813

Area Forecast Discussion
National Weather Service Des Moines IA
1213 PM CST Sat Feb 17 2018

.SHORT TERM.../Today through Tonight/
Issued at 337 AM CST Sat Feb 17 2018

We currently reside within a weak warm air advection regime ahead of
an approaching shortwave trough. Mid-level forcing associated with
the trough is generating light snow upstream over South Dakota, and
this will progress across northern Iowa later this morning. Have
increased POPs to categorical up north of Highway 20, but amounts
will be light, generally under an inch with only a dusting at best
down toward Highway 30. There should be little impact from this
snowfall, especially on a Saturday morning with no typical morning
commute traffic. The surface trough will sweep across Iowa this
afternoon, quickly scouring out any lingering snow, but there will
be no noticeable cold air advection behind the trough and
temperatures will still climb into the 30s north and 40s south.
Tonight southerly flow will return late, ushering in yet another
warm air advection regime that will strengthen toward Sunday as
discussed in the long term section below.

.LONG TERM.../Sunday through Friday/
Issued at 337 AM CST Sat Feb 17 2018

Focus is on the approaching system from early Monday into Tuesday
as large western trof develops with broad southwest flow across
the central United States. A frontal boundary moving through Iowa
will do so slowly as the upper flow will be nearly parallel with
the front allowing precipitation to linger.

Initially, strong warm advection will be underway across the state
on Sunday with strong southwest flow and the thermal ridge
approaching the state. Temperatures are forecast to be quite warm,
generally in the 40s with 50s across the southern half of Iowa
where snowpack is much less. Surface front will enter northwest
Iowa by late in the day and gradually settle southeast overnight.
Placement of the front will remain a top concern over the next
few forecast cycles as GFS is slightly more progressive than the
Euro. This will have important implications on location of
precipitation type placement for portions of central Iowa on
Monday. Have continued with a somewhat compromised position
between the two and this will continue to be closely watched.
Overall, will see the lower levels of the atmosphere saturate on
Sunday night with increasing forcing as the first in a series of
shortwaves approaches. Anticipate precipitation to break out
overnight with the increased lift with rain along and ahead of the
front, possibly even some thunder in the far south. In the
northwest third of so of the forecast area. Soundings all
indicate saturation in the lowest 5-10kft of the atmosphere with
the mid levels remaining somewhat dry with little ice
introduction. Appears freezing drizzle/light freezing rain will
be produced in these areas, mainly along and north of an Audubon
to Iowa Falls to Waverly line. This precipitation will persist
into Monday morning ahead of an approaching dry slot. The threat
of shower/few thunderstorms will remain in the southeast with
freezing drizzle/rain in the northwest. At this point, advisory
criteria icing is likely and should see good accretion given the
winds and will have to maintain vigilance to make sure warning
criteria is not met. Will certainly be hitting messaging for the
upcoming potential of some decent icing in the northwest third or
so of the forecast area.

Dry slot will cross much of the forecast area by later Monday into
Monday night save the southeast. Given the boundary will be
sinking well south of the forecast area at this time, there
appears to be an increasing threat of freezing rain across the
southeast third to half of the forecast area Monday night into
Tuesday morning. This potential is now in the forecast and will
mention the increasing threat in the southeast during this time.
The system should sink fully southeast of the forecast area by
late Tuesday with quiet weather into the middle of the week. The
upper pattern remains somewhat active however, as the western trof
remains in place with numerous shortwaves passing through the
broader trof into next weekend. While not all of these will impact
the state, it does appear another system is set toward the end of
the period.


.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1213 PM CST Sat Feb 17 2018

Area of snow and lower visby/cigs quickly moving out this
afternoon. Some mixing has followed the clearing and will be
updating winds at KDSM/KOTM through 21z. Overnight some increase
in clouds again...and some uncertainty whether lower stratus/br
deck will reform due to todays snowfall/melt and tonight clearing
skies. For now have not included...but hires models suggesting
some potential. Winds increase aloft aft 12-14z Sunday with
potential llws across the area. Will pass along to evening avn
forecaster to evaluate these two items further. /rev




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