Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 161723

Area Forecast Discussion
National Weather Service Des Moines IA
1120 AM CST Fri Feb 16 2018

.SHORT TERM.../Today through Tonight/
Issued at 324 AM CST Fri Feb 16 2018

A cool area of surface high pressure is building in from the west
early this morning and will cross Iowa later today into this
evening. Blustery northwest winds ahead of the high pressure center
will affect the area during the morning hours, especially before
sunrise, but remain well below advisory criteria. Meanwhile
lingering clouds and a few flurries will blow out to the southeast
within the next few hours, yielding to a sunny but relatively cold
day across the forecast area. As the high pressure area moves
rapidly away to our east tonight, it will be replaced by modest low-
level warm air advection and redevelopment of clouds. Meanwhile, a
weak shortwave impulse rounding the base of broad cyclonic flow
aloft will move along the Iowa/Minnesota border late tonight,
producing some light snow well after midnight. Only a dusting is
expected near the border through 6 am Saturday, with more impacts
later Saturday morning as discussed in the LONG TERM section

.LONG TERM.../Saturday through Thursday/
Issued at 324 AM CST Fri Feb 16 2018

Theta-e advection will continue to lift across Iowa Saturday morning
and will bring a light snow chance to the area. Best chance for
minor snow accumulations will be over the northeast portion of the
state. Profiles will mix out during the afternoon as the deep layer
flow becomes westerly and this will help push temperatures into the
30s and 40s during the afternoon. Saturday night into Sunday will
feature a sub-tropical high pushing westward towards the southeast
CONUS while a large trough carves out across the intermountain west.
This will lead southwest flow amplifying across Iowa and will drive
a warm front through Iowa. Strong southerly flow will develop by the
afternoon as high pressure moves well east and low pressure
approaches from the west. Low level gulf moisture will stream
northward through the flow and reach southern Iowa by late afternoon
and spread northward overnight. This moisture will be accompanied by
stratus and possibly fog as dew points in the 40s to even 50s reach
the southern portion of the state. Drizzle or light rain will likely
develop overnight over the south.

The most impactful weather during the forecast period is expected
Monday. A cold front will bisect the state from northeast to
southwest. The primary issue with this boundary is the cold air will
undercut warmer air aloft creating precipitation type issues and the
potential for icing. Both the GFS and ECMWF are bringing convective
instability into the southeast half of Iowa, and some of this
convection may stray onto the cold side of the boundary. The other
issue where convection does not occur, is the lack of ice
introduction. With the forcing expected and deep saturation from
near surface to around 700 mb, all point to the potential for
widespread freezing rain/freezing drizzle on the cold side.
Therefore, have introduced freezing rain into the forecast with the
potential for one to two tenths of an inch of ice, generally north
of a Waterloo to Ames to Atlantic line. The icing efficiency will be
aided by breezy northerly winds that will accelerate the heat
transfer and cooling/freezing process. This area has remained
relatively consistent the past few nights. Obviously the surface
boundary will be very critical.

Much warmer temperatures over the southeast with dew points rising
into the mid 50s to near 60s. This moisture could lead to surface
instability being realized as the boundary settles southeast during
the day, in addition to any elevated thunderstorms farther to the
northwest into central Iowa. Have introduced thunderstorms to the
forecast but expect the area will need to be expanded further. The
boundary will be nearly parallel to the upper flow through Tuesday
as the subtropical high holds over the southeast CONUS coast. This
setup would suggest that model guidance may be pushing the surface
boundary southeast to fast during this period, especially the faster
GFS. Little to no significant weather the remainder of the forecast
period with near to below normal temperatures forecast with a modest
warm up late in the week as a more broad westerly flow develops.


.AVIATION.../For the 18Z TAFS through 18Z Friday night/
Issued at 1120 AM CST Fri Feb 16 2018

There will be a chance for light snow from 09Z through 18Z...mainly
over KMCW and KALO TAF locations and perhaps as far south as KFOD
closer to the 09Z-12Z timeframe.  MVFR cigs/vsbys would occur with
the snow otherwise expect VFR conditions.  Very strong SW winds just
off the surface is creating a LLWS setup for most locations with the
exception of KOTM.




LONG TERM...Donavon
AVIATION...FAB is the U.S. government's official web portal to all federal, state and local government web resources and services.