Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 092353
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
553 PM CST Fri Dec 9 2016

.SHORT TERM.../Tonight through Saturday/
Issued at 350 PM CST Fri Dec 9 2016

Warm advection will work it`s way into central Iowa this evening
increasing mid level clouds from West to East. A very weak
shortwave will pass through the North this evening and produce
enough lift that despite little moisture we could see some
flurries or a period of light snow. Better snow (still light
though) may fall to the North and West where models hint at a warm
front trying to develop. In general though I have increased
clouds with the lowest ceilings across the North by 12Z and nudged
lows up a degree or two from the previous forecast and a category
or so from last night.

Saturday begins a prolonged period of snowfall for central Iowa. A
surface low develops over the lee side of the Rockies with a warm
front extending along the highway 20 corridor. Moisture will
increase through the morning as series of weak shortwaves pushes
across Northern Iowa and locations North through the day. There
will be an initial shot of forcing earlier in the day which will
produce snowfall mainly across the North. From late tonight
through 18Z Saturday we could see 1 to 2 inches across locations
North of Highway 30 and West of Highway 71. There may be a brief
respite late Saturday morning into early afternoon as the first
shot of forcing passes but mid to late afternoon a second stronger
shot comes in which will increase snowfall again across Northern
locations...mainly North of Highway 20 and East of I-35. 2 to 3
inches of snow is possible during the afternoon hours in these
locations. There will be a fairly stout cut off to the snow but it
looks like the Northeast third or so will see accumulating snow.
Temperatures will be warm enough that the snow will be on the
wetter/stickier side which may cause travel issues as we get later
into the day. Winter weather advisories have been posted
beginning mid morning West and mid afternoon East for these
reasons. This will be a longer duration event with periods of
light to moderate snow with the bulk of the forcing coming
Saturday night but the North will have a 2 to 3 inch head start
from the snow on Saturday.

.LONG TERM.../Saturday night through Friday/
Issued at 350 PM CST Fri Dec 9 2016

The main forecast concerns were focused on snow over the weekend
and then the Arctic cold temperatures by mid-week. Leaned closer
to the GFS as it has been the most consistent model over the past
several runs wrt to the snow location and timing over the weekend.

Saturday night into Sunday...Surface low pressure is expected to
churn over the central to southern Plains before finally getting
pulled eastward by a strong upper level shortwave Sunday
afternoon. A wide plume of deep moisture persists over central to
northern Iowa Saturday evening into Sunday morning before dry air
begins to advect into western Iowa by around 18z Sunday. Snow
will be ongoing Saturday evening and looks to increase in
intensity b/t 06-12z Sunday as the aforementioned shortwave
enters northern Iowa/southern Minnesota. Weak omega anywhere
within the column persists for several hours and much of this
forcing is in the dendritic layer. The slightly stronger omega
within the dendritic layer moves across northern Iowa b/t 06-12z
Sunday and thus confident to increase QPF and snow amounts during
this time. Some locations over northern Iowa, mainly from
Estherville to Waterloo line northward to the Minnesota border are
likely to see over 7 inches and even cannot rule out some locally
higher amount close to 10 inches by Sunday morning. Much of this
is expected to fall over a 24-30 hour period which is near the
warning criteria. Certainly a high-end advisory but the likelihood
of the locations mentioned above needing to be upgraded continues
to increase. Winds look to remain southeast and somewhat gusty
Saturday evening into the overnight hours towards sunrise Sunday
morning. Visibility restrictions are a concern. A sharp gradient
is likely to set up roughly west-northwest to east-southeast
orientation from Sac City to just north of Oskaloosa. This
includes Polk and Dallas County where the southern portions may
only see trace amounts, but the northern edges look to see 1-4
inches by Sunday. Advisory seemed viable due to the potential for
long duration snow and first widespread snowfall of the season in
this location.

Monday through Thursday...Surface high pressure builds into the
region Monday before a quick moving shortwave cuts across northern
Iowa Monday night into early Tuesday. The latest GFS and ECMWF
have come in slightly further south and may result in another
round of light snow for northern Iowa. Quick burst of weak forcing
and much of the column resides within the -10C to -20C range
Monday night. Enough to mention flurries/snow showers if models
continue to trend toward this solution. Next, a band of snow
looks to develop over southern Iowa as the strong Arctic front
sags southward across the state Tuesday night into Wedensday
morning. Very large surface high pressure builds into the region
Wednesday into THursday providing a very cold airmass during this
time. 850mb temperatures are as cold as -22C Wedensday night.
Potential for a good radiational cooling event by Thursday
morning, especially with the fresh snow pack across much of
northern Iowa. Thus, lowered temperatures Wednesday night and
Thursday.

&&

.AVIATION.../For the 00Z TAFS through 00Z Saturday evening/
Issued at 551 PM CST Fri Dec 9 2016

Confidence in snow developing across northern TAF sites through
the period is high but intensity and timing details are only
medium. Initially there may be some VFR flurries this evening, but
confidence is too low to mention at any particular location. From
midday Sat into the afternoon, most likely scenario has at least
IFR conditions (visibilities and ceilings) in snow developing
north, possibly LIFR at times, and MVFR south a bit later in the
afternoon.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 2 PM Saturday to 6 PM CST Sunday
for IAZ006-007-016-017-025>028-036>039-047>050-059>062.

Winter Weather Advisory from 9 AM Saturday to 3 PM CST Sunday
for IAZ004-005-015-023-024-033>035-045-046.

&&

$$

SHORT TERM...FAB
LONG TERM...Podrazik
AVIATION...Small


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