Scientific Forecaster Discussion
fxus63 kdmx 170001
Area forecast discussion
National Weather Service Des Moines Iowa
601 PM CST Sat Dec 16 2017
Short term.../tonight through Sunday/
issued at 331 PM CST Sat Dec 16 2017
Relatively quiet as low pressure lifts out of the Southern Plains
into Missouri and finally into far southern/southeast Iowa before
pushing off to the east Sunday afternoon. Short term models confine
the precip a little further south than previous runs and slow the
saturation as the low lifts north. Probably on track given the
large dewpoint depressions currently in place over Iowa. At any
rate between 09z and 13z saturation occurs and for the most part,
brings low VFR or MVFR to the state from south to north. Between
12z and 15z there is a pretty good push of forcing and lift and some
weaker Theta-E advection across south central into southeast Iowa.
This is the period where precip and perhaps some fog will occur.
Thermal profiles indicate the precip will mainly be rain though the
more you go north of Highway 34, the more a wintery mix is possible.
Then from 15 to 18z lift and forcing begin to push off to the north
and east so while precip may linger far east or southeast, most
locations across the far south will see an end to precip but cloud
cover will remain all day.
There will be a pretty big contrast in temps tomorrow with locations
north of the warm front that is associated with the low in the mid
30s while south of the front (i-80 and south) temps will be in the
low to mid 40s.
Long term.../Sunday night through Saturday/
issued at 331 PM CST Sat Dec 16 2017
System will be departing to the east on Sunday evening with
quiet weather into the middle of next week. Upper flow across
much of the central United States will be relatively flat/zonal
allowing a mild airmass to remain in place through Wednesday.
Daytime temperatures will be above to much above normal during
this time with little if any chance for precipitation as the
westerly flow will contain limited moisture and there is little
This will begin to change by later Wednesday as a system drops
into the intermountain west. The main upper low will drop south
with an elongated trof extending northward into central Canada.
This will be in conjunction with an advancing cold front across
the plains entering into Iowa by Thursday morning. Forcing greatly
increases by Thursday across the state ahead of the northern
stream shortwave with precipitation developing during the day with
isentropic lift also increasing. Strong cold advection on the
backside of the advancing front will quickly transition any light
rain into snowfall which will persist into the evening before
diminishing overnight as the system quickly pulls east. The GFS
remains more progressive than the Euro but in either case, it
appears that much of central Iowa will see some measurable
snowfall for the first time this year.
After the passage of this system, the upper flow become northerly
with the Arctic opened up to the Midwest. Much colder air will
spread into the state next weekend with a progressively colder
airmass pushing into the region.
Aviation.../for the 00z tafs through 00z Sunday evening/
issued at 545 PM CST Sat Dec 16 2017
Weak surface front will stall over southern Iowa. Moisture
advection will increase overnight with upper trough in the
Southern Plains lifting northeast into the Midwest. Expecting a
gradual lowering of ceilings overnight with moisture plume over
running weak surface front. Potential for fog development near and
north of boundary after 06z. Sref prob ceilings signaling IFR
possible north of the boundary near 10z. Will monitor ceiling
trends and further lower ceilings in 06z tafs as needed. Light
rain showers possible near kdsm and kotm after 12z.