Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211727

Area Forecast Discussion
National Weather Service Des Moines IA
1125 AM CST Wed Feb 21 2018

.SHORT TERM.../Today through Tonight/
Issued at 341 AM CST Wed Feb 21 2018

A brief break from the active weather is expected today with
large Canadian high pressure moving across the state. Widespread
subsidence and dry airmass will lead to sunshine for much of the
day although some lingering cirrus is expected east of I35 this
morning from departing storm system. With thermal trof nudging
into the state today, temperatures are forecast to remain well
below normal although the relatively light winds will take some of
the bite out of the cold.

However, with the broad southwest upper flow in place, a return to
warm advection will quickly follow this evening as the surface ridge
departs to the east.  The gulf will have remained open as well
during this time so moisture transport into the state will also
increase into the overnight.  Isentropic lift will intensify through
the evening with clouds spreading quickly northward.  Deeper
saturation occurs after midnight with increasing threat for snowfall
as pressure deficits diminish, especially in the west and south. GFS
is generally the most diffuse with forcing and QPF with Euro/NAM/GEM
a bit more consistent with pcpn placement and amounts.  Snowfall
amounts will generally be an inch or two with highest amounts in the
west/southwest area and lighter amounts east of I35. Non-diurnal
temperature curve will be needed overnight with lows in the evening
and steady to slowly rising temperatures overnight as clouds and
winds increase.

.LONG TERM.../Thursday through Tuesday/
Issued at 341 AM CST Wed Feb 21 2018

Light snow will be ongoing early Thursday morning over southern Iowa
with it spreading into northern Iowa as a shortwave trough ejects
out of the base of the longwave trough over the western US. Forecast
soundings show saturation within the ice introduction layer allowing
for snow; however, mid-level dry air will begin to arrive in the
morning removing ice from the column. In addition, a warm, above
freezing, wedge of air will arrive in the mid-levels. The NAM and
CMC brings in the mid-level warm wedge faster by a few hours and
farther north compared to the GFS. Collaborative discussions with
neighboring offices favor a slower solution so blended in GFS with
initial guidance. Overall, there will be a progression of snow
changing to freezing rain and then freezing drizzle from south to
north. The period of freezing rain may be rather brief as the loss
of ice introduction occurs and moisture becomes shallow changing to
drizzle. As was seen last night, high rates of rain in subfreezing
air can mean lower ice accretion compared to a light rain or
drizzle event that can be more efficient in accretion. For parts
of our southeast forecast area (roughly east of I-35/south of
I-80), there will be enough low-level warm air for the freezing
drizzle/rain to change to primarily drizzle with the shallow
moisture. Ice accumulations will generally be under a tenth of an
inch, though a few places may have around 0.15 inches. Snow
amounts will generally be one to two inches north of Highway 3. As
the shortwave departs the area, subsidence will arrive bringing
an end to the precipitation.

A zone of high pressure will pass north of Iowa on Friday providing
a brief dry spell. However, the longwave trough over the western US
will begin to lift and move toward the area. This will bring
precipitation back to the state later Friday night into late
Saturday or early Sunday with a model consensus used for now as
there are quite a few discrepancies in strength and location of
the low pressure. For this time period, the models are in two
general camps with the GFS/GEFS/ECMWF in one and the NAM/CMC in
the other. The GFS is considerably stronger than last night and
farther north moving from southwest to northeast through the state
Saturday into early Sunday. Its ensemble, the GEFS, is similar in
timing, but is farther south by about 150 miles and not
surprisingly not as strong. The ECMWF is somewhat similar to the
GEFS. These models would favor a wintry mix as warmer air is drawn
northward ahead of the low pressure moving through the area. As
the low moves away from the area and colder air wraps into the
state, a changeover to snow would occur. Meanwhile, the CMC and
the last few frames of the NAM are much weaker and farther south.
These models would favor more snow and less liquid precipitation
with low pressure tracking near or south of the state.

Sunday into early next week will have high pressure moving from the
Plains into the Ohio Valley as quasi-zonal flow sets up aloft.
However, this flow will become more southwesterly as a new longwave
trough begins to develop over the western US. A weak system may
brush the area later Tuesday into Wednesday, though confidence is
low with model differences prevailing.


.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1125 AM CST Wed Feb 21 2018

Warmer air aloft will move in and produce cigs which will lower
between 09Z and 14Z to the MVFR category.  Initially TAF locations
will be cold enough for some light snow and northern locations will
remain cold enough for the precip to remain as snow through the
valid time of the TAF forecast.  Occasional IFR conditions are
possible aft 15Z for all TAF locations but this would be very
localized. Southern TAF locations will see precip change over to
freezing drizzle as some drier air near surface noses in.





LONG TERM...Ansorge
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