Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 051745

Area Forecast Discussion
National Weather Service Des Moines IA
1145 AM CST Tue Dec 5 2017

Issued at 845 AM CST Tue Dec 5 2017

Updated cloud cover central and south areas...and added isolated
flurries with passing cloud deck. A few flurries reported with
deck to our west. Clouds should be on the decrease in the
afternoon. Temperatures may be a few degrees cooler with the
cloudy areas as well. /rev


.SHORT TERM.../Today through Tonight/
Issued at 339 AM CST Tue Dec 5 2017

The cold front is through us as evidenced by the nearly 40 degree
drop in temps from this time yesterday and the strong northwest wind
in place.  For today the upper trough will swing through the state
and with a tight pressure gradient still in place...mainly over the
eastern half of the state...winds will once again be very strong.
There may in fact be a bit of a relative lull in winds through mid
morning but some increased forcing come through late morning through
mid afternoon and looking at BUFKIT soundings, we still have 38 to
40kts in the mixed layer that will likely get mixed down so we have
another wind advisory going into effect for late morning through mid
afternoon to account for the strong winds.  Highs today will only be
in the mid 20s north to the upper 30s far south and with the wind
expected across the north, wind chills today will be around 10 so it
will really feel like winter outside today.

Tonight a shortwave drops through the state and this will actually
switch the winds briefly to the west then southwest before bringing
them back to the northwest.  Some weak vorticity and forcing
associated with this wave drops through the state.  Hi-res models
all hint at the possibility of some very light snow or flurries with
this feature.  Upstream obs show light snow every now and then so I
did but a brief period of light snow in for the evening but not
expecting any real accumulation or hazard from this.  Once this
passes winds will diminish and cloud cover will keep temps from
tanking but we will still see mid teens to mid 20s and with the wind
at 10-15kts, we`ll see wind chills in the single digits north to the
teens to around 20 south.  Not out of line for December for vastly
colder than what we`ve seen in recent past.

.LONG TERM.../Wednesday through Monday/
Issued at 339 AM CST Tue Dec 5 2017

An amplified ridge across the western CONUS and into western
Canada combined with low pressure around the Hudson Bay region
will keep Iowa in cold north/northwest flow through much of the
period. Numerous short waves will be dropping through the flow.
The first short wave will arrive during the day Wednesday and will
arrive with an attendant cold front. Good mixing will yield
another windy day with gusts 30 mph possible. Cold advection will
arrive by the afternoon and may arrive with a few flurries.
Otherwise, little to no precipitation is expected with relatively
dry air in place. The boundary layer is expected to remain mixed
Wednesday night and along with passing clouds, will keep
temperatures from a big drop Wednesday night.

Less certainty in the forecast for Friday and Saturday as the GFS
continues to suggest a closed upper low dropping well south into
northern Illinois and Indiana by Friday night. The ECMWF and the
GFS Ensembles suggest a significant portion of this energy will
break off and shift east with a closed low north into southern
Ontario while a weaker fragments drops south. The past several
runs of the GFS have shown a positive linear regression slope at
500 mb over the area and the GFS confidence plots are below normal
for that time period. Considering this to go along with the ECMWF
and GFS ensembles in addition to no snow cover and good mixing,
temperatures should warm higher than what guidance suggests. A few
flurries or even light snow may occur Friday into Friday night
with this system.

The western ridge will recycle over Saturday night into Sunday and
will allow warm advection to move across Iowa and bring slightly
above normal temperatures for Sunday with highs in the 30s to 40s.
The next chance for precipitation will arrive Monday night into
Tuesday. Still low confidence in exact details and moisture
availability but current set up would support light snow potential
during this period.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1145 AM CST Tue Dec 5 2017

Main challenge will be wind this period and scattered flurries.
Winds remain brisk with mixing to about 900 mb...gusts approaching
38 kts at times north sites KMCW/KALO with lesser gusts expected
over the south. Most areas will see sustained winds 25 mph
approaching 30 mph through 21z...then winds and gusts begin to
relax. Another weak upper level wave will drop southeast aft 00z
with periods of --sw over the region once again. Northwest flow
and stronger gusts again aft 15z Wed.  /rev


Wind Advisory until 4 PM CST this afternoon for IAZ004>007-



LONG TERM...Donavon
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