Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 092105

Area Forecast Discussion
National Weather Service Des Moines IA
305 PM CST Sat Dec 9 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 303 PM CST Sat Dec 9 2017

Northwest flow remains over the region today. Stratus and some
flurries mixed with sunshine at times as a series of troughs/cold
front pass southeast through the region. The second front is nearing
the IA MO border now...with the stronger cold air advection east of
I35 where clouds will hold on longer this afternoon/early evening.
West of I35 there is some moderation in temperature aloft along with
a gradual shift toward slightly more westerly component in the wind
fields this afternoon. Despite some clouds lingering over the west
this afternoon...there has been a tendency for some breaks in the
clouds. By 23z and shortly thereafter...we should begin to see more
clearing for a time prior to the upstream warm air advection mid
level deck propagating southeast overnight. Thickness/H850
temperatures all on the mend tonight into midday tomorrow.  The warm
air will be pinched to the south during the afternoon as a cold
front quickly overtakes the area at that time. Despite that highs
Sunday will warm to the lower to mid 30s Northwest to the mid to
upper 40s over the southwest. Winds aloft behind the boundary are
slated to increase again and mixing on Sunday will result in
afternoon gusts in the 15 to 25 mph range until around sunset.  Some
clouds are likely to pass over the area during the day...with a sun
cloud mix on the way. The airmass looks too dry to produce any

.LONG TERM.../Sunday night through Saturday/
Issued at 303 PM CST Sat Dec 9 2017

Little change in the overall pattern with a large ridge anchored
over the western United States and a deep trough planted over the
Great Lakes and east.  For the most part this will mean we will stay
in a deep northwest flow and remain susceptible to passing
shortwaves that will bring shots of cold and low confidence for the
slightest chances for light precip. At this point none of the models
are pointing at any big system hitting Iowa as most of the systems
in the flow either remain north of Iowa and just graze our north and
northeast, or they dive way south of the state, and in any event,
moisture is really lacking.

Having said that the forecast challenges at this point remain a
decent shortwave and cold front coming through on Monday and a much
weaker shortwave diving south out of Canada for Tuesday
night/Wednesday. There`s also some potential for another wave
Wednesday night/Thursday but models really diverge by this point.

For the Monday system...a strong shortwave dives across the state
but the bulk of the forcing moves from northern Iowa to southwest
Iowa.  A vort max slides from north central Iowa to southeast Iowa.
Models are showing a pretty dry sounding so not really seeing much
of a chance for precip but the far north central to northeast could
see something being closer to the low/vort max and seeing a little
better moisture to work with than further south.  The bigger story,
and one which the models consistently underdo, is that we will be in
strong cold advection all day and winds, at least based on bufkit
soundings, are grossly underdone.  I have bumped wind speeds up
based on winds in the mixed layer in the soundings.  Looks like
sustained winds on Monday in the 25 to 35 mph with gust in the 40 to
45 mph range.  It is too early for any kind of wind headline yet but
if trends continue, one would expect a headline to be forthcoming.

The mid week system has resolution problems within the models with
the Euro coming in faster and further northeast and the GFS later
and further southwest.  The Canadian just washed this system out all
together.  Confidence is too low at this point to veer from
persistence.  These differences extend into the next system on the
heels of the Tuesday night/Wednesday system.  Thus for Wednesday
night/Thursday, the Euro drops the next system in faster and
southwest while the GFS is just a little later but further
northeast.  Given the time of day, ptype is also in question, giving
more of a mix than snow during the day then switching over to all
snow by early evening for any remaining precip.  Keep in mind though
that with models continuing to show a distinct lack of moisture with
these systems, there is low confidence in the possibility of precip
with any of these systems.

Finally towards the end of the long term period, a rather large low
pressure system dives across Canada which acts to flatten the west
coast ridge put the Upper Midwest in more of a zonal flow which
means warmer temps for Friday onward.  After highs in the mid 20s to
mid 30s for the first half of the week, and pretty big swings with
these passing systems, the latter part of the week will see highs in
the mid 30s to mid 40s.


.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1127 AM CST Sat Dec 9 2017

Main challenges will be clouds and winds this period. Shortwave
energy continues to rotate southeast with periods of MVFR cigs
along with gusty winds and flurries. Lowered confidence on timing
of any breaks in clouds...but have used guidance to best estimate
any rise in cigs. More mid level clouds to arrive by 02z northeast
at KMCW/KALO and farther west sites KFOD/KDSM toward 12z as very
brief warm air advection arrives from northwest. Winds also
increase aft 13z Sunday in response to passage of cold front./rev




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