Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 191028

Area Forecast Discussion
National Weather Service Des Moines IA
428 AM CST Fri Jan 19 2018

.SHORT TERM.../Today through Tonight/
Issued at 420 AM CST Fri Jan 19 2018

Much like yesterday, temps will be the only concern in this
period. A short wave over Saskatchewan early this morning will
track across Ontario into tonight but moisture is negligible and
any lift will remain to our north. The only real sensible weather
reflection will be its associated weak frontal boundary dipping
into the state tonight. We have continued our warming trend with
09z temps generally 10-15 degrees warmer than this time yesterday.
Confidence is not great in the degree of warming today with
questions on how much of the inversion can be eroded, but with
snowpack similar to yesterday would expect maybe a third of it to
be mixed out resulting in highs in the 40s, possibly pushing 50 in
spots south and west.

.LONG TERM.../Saturday through Thursday/
Issued at 420 AM CST Fri Jan 19 2018

The primary concern in this period remains the large system
crossing the MO and MS Valleys late in the weekend through the
start of the work week. For the most part, little has changed
since this time yesterday but with somewhat better agreement in
timing. Confidence in details remains low, especially with regard
to precip types and amounts in the transition zone across the
northwest half to third of Iowa but there is higher confidence in
some larger themes regardless of model solution which supports
two likely periods of hazardous travel. Among the higher
confidence items will be areas of drizzle and freezing drizzle
expanding Saturday Night into Sunday with model soundings
depicting deepening low level moisture from the bottom up
reaching 1-2km in time. Surface temp and dewpoint freezing lines
should be slowly moving northward through this period with the
entire forecast area warm enough to support just drizzle by Sunday
afternoon. This may still support mainly light ice accums along
and north of I-80 over that period, although some tenth plus
accums may occur farther north which will stay coolest longest.

There should be a lull in travel impacts Sunday evening with a
transition to all liquid but all models show a maturing
deformation zone later in the night into Monday with deep thermal
and kinematic support, including noted frontogenetical Fn
convergence response. Unfortunately this deformation band will be
right on the NW edge of the forecast area lowering confidence and
all models showing varied degrees of north and west dry slot
progressions. This dry slot ushers in precipitation type questions
by Monday morning with some northern soundings showing no
moisture above -10C but plenty of lift below per H85/H7 QG
forcing and 290K isentropic lift. This raises considerable
questions on whether this precip will be realized in liquid or
frozen form across NW and northern Iowa Monday. The CMC and ECMWF
have a pronounced dry slot through the entire state while the
domestic models /NAM/GFS/ are farther east with the deformation
zone and mid level moisture, and the former never really get it
too far west with diminished moisture and lift. This would also be
the time winds increase sharply with sustained speeds NW to near
30 mph and wind gusts pushing 40 mph. Thus this low confidence
results in sensible weather impacts ranging from near blizzard
conditions to more freezing rain/drizzle than snow, and possibly
even just liquid for a while toward Mason City Monday morning if
low level wetbulb temps stay warm enough. Have played a middle of
the road approach for now with 1-4 inch accums for far NW, Algona
to Estherville, and an inch or less farther south and west. A wide
variability in outcomes is still possible for northern Iowa.

The system should exit into Monday Night with normal to above
normal temps into the next work week and only minimal precip
chances. Still have some low end PoPs included giving credence to
the 12z ECMWF, but 00z models have trended dry so if this stays
steady run to run subsequent forecast could end up dry with fairly
benign and mild NW flow aloft.


.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1134 PM CST Thu Jan 18 2018

VFR conditions are expected through the TAF period with modest
southwest breezes and thick cirrus clouds. The threat of patchy
fog overnight has diminished and it now appears unlikely. However,
low-level wind shear has been included at OTM/DSM in the next
several hours and may become more widespread Friday afternoon and





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