Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 101837
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1237 PM CST Sat Dec 10 2016

.UPDATE...
Issued at 1235 PM CST Sat Dec 10 2016

A narrow band of snow was moving eastward across eastern IA and
has been producing up to a quick inch of snow. This band of snow
should weaken as it pushes northeast. In the meantime, as the low
levels saturate the snow will become widespread across much of the
cwa as the afternoon and evening progresses.

Headlines: have issued a winter storm warning in our northern
counties roughly north of Highway 30. This is where the higher
snow/liquid ratios will be and a longer duration of heavier snow.
Total snow amounts will be in the 6 to 10 inch range in that area.
Farther south to the I-80 corridor amounts will be in the 3 to 6
inch range and less than an inch in our far south.

&&

.SYNOPSIS...
Issued at 330 AM CST Sat Dec 10 2016

The latest sfc analysis was indicating lingering elongated ridge
axis extending from ND...down to the western OH RVR Valley. Some lee
side to central Rockies llvl cyclogenesis taking place in response
to lead impulse that will be later today`s weather maker. Associated
elevated warm air advection and isentropic lift induced wing of snow
already pushing acrs western into central NE and southwestern SD
ATTM.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 330 AM CST Sat Dec 10 2016

Today...assessing several lift and frontogenesis surfaces on the
latest model runs and comparing them to what is taking place
upstream, will utilize the latest 00z run ECMWF and to some extent
the 06z NAM run for today. These solutions, as well as others are
now less bullish on extent of lift and upward omegas thru the
dendritic layer, as well as less saturation this afternoon into the
evening. Thermal profiles support not as high of LSR`s as well, more
in the range of 13:1 to 15:1...several factors making for more of a
lighter snow event if the model trends are correct. Initial precip
to skirt in from the west mainly aloft in virga form by late
morning, but ongoing saturation of column seen on lowering Cond P-
Defs of 290-295K sfc`s will allow for snow arrival at the surface as
the midday into afternoon progresses. Mainly light snow with a few
bursts of moderate after 20z this afternoon, will make for snow
accums by 6 PM CST/00z mainly 1 to 1.6 inches along and north of the
HWY 30 corridor with the higher amounts generally west of a Dubuque,
to Anamosa IA. With a light north adjustment to forcing/saturation
swath, areas south of I80 of at least a tier of counties may now see
only a dusting to a few tenths by 00z. High temps today low to mid
20s north of I80, to the mid to upper 20s south of I80 in increasing
southeasterly return flow.

Tonight...Will keep sharp cut-off idea generally along and south of
I80 taking into account the latest accepted model solution forcing
and saturation trends. The first round of lift may move off quickly
after 7-8 PM for a temporary lull or decrease in snow intensity in
the early to mid evening window. But the next wave aloft and ongoing
saturation profiles support an increase again from the northwest from
late evening and into the overnight. New snow amounts for the 00-12z
Sunday period generally range from 1.5 inches at Cedar Rapids, to
over 4 inches at Freeport in NW IL. This will make for a 18 hour
totals by Sunday morning of 2-5+ inches again generally from just
south of HWY 30 to the northern reaches of the DVN CWA. Areas south
of a Fairfield IA, to Monmouth IL line may just get flurries or very
little snow at all through this period if the northern track of the
system verifies. Ongoing southeasterly sfc winds of 10-20 MPH and BL
WAA should allow for little drop off in temps overnight from the Sat
afternoon highs. The following discussion below will take into
account these updated snow amounts in the short term and timing for
overnight headline decision details.

.LONG TERM...(Sunday through next Friday)
ISSUED AT 330 AM CST Sat Dec 10 2016

Sunday will see a continuation of warm advection driven light to
moderate snow in the north 1/2 to 1/3 during the morning hours. This
is the last period of all snow for the CWA, before the warm nose
advects into the southern CWA offering a mix of rain and snow.

This prolonged light to moderate snow event, combined with the PM
hours event today will offer up a headline challenge. Extensive
collaboration has gone into the decision to issue a Winter Weather
Advisory to cover what is an event that is generally 3 to 8 inches,
over a 24 to 30 hour period. Factors in this decision are: Weekend
travel impacts being lower than work week. Generally a light snow
with rates under a 1/4 inch per hour. Questionable moisture feed
Sunday as the moist conveyer veers east, per roughly half the
guidance, and most importantly, little wind impacts. Thus this is a
on again, off again, light to moderate snow with gradually
increasing totals near 8 inches in the far northeastern counties,
and lesser amounts of 2 to 6 inches to the southwest, generally down
to Interstate 80. A very sharp gradient will exist even along and
south I-80, where little snow accumulates (no impacts), to plowable
snow in a distance likely under 40 miles. This too makes this
forecast winter headline a challenge for the perceived bust
potential.  Southeast Iowa, west central Illinois, and northeast
Missouri may see some light snow early Sunday, before changing to a
non accumulating or slushy snow/rain mix.

Unlike earlier runs, it appears the dendritic lift Sunday will not
be deep and intense, but rather narrow and weak. The smaller flakes,
combined with mild low levels should lead to snow ratios under 10:1,
thus Sunday afternoon`s QPF of 0.10 to 0.20 only should serve to add
1 to 2 inches in the northeast and less to the southwest.

Should higher rates than expect occur today and tonight, a warning
upgrade for a county or two would still be possible, as this is such
a long duration event that there shouldn`t be a rapid worsening of
any conditions.

The remainder of forecast is cold following the Arctic front early
Tuesday. Wind chills of -15 to -25 appear likely for the Wednesday
to Saturday period during the early morning hours. Some light snow
chances will also spread over the region in any isentropic lift, but
those are left to the model blend for now. The next significant trof
interaction may arrive by Friday with snow chances increasing.

ERVIN

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday Afternoon)
ISSUED AT 1235 PM CST Sat Dec 10 2016

Snow will spread across most of the taf sites through Sunday with
MVFR/ocnl IFR cigs/vis. Southeast winds 10 to 15 knots through the
period.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Winter Weather Advisory until 6 PM CST Sunday for Benton-Cedar-
     Clinton-Johnson-Jones-Linn-Muscatine-Scott.

     Winter Storm Warning until 6 PM CST Sunday for Buchanan-Delaware-
     Dubuque-Jackson.

IL...Winter Weather Advisory until 6 PM CST Sunday for Bureau-Henry
     IL-Putnam-Rock Island-Whiteside.

     Winter Storm Warning until 6 PM CST Sunday for Carroll-Jo
     Daviess-Stephenson.

MO...NONE.
&&

$$

UPDATE...Haase
SYNOPSIS...12
SHORT TERM...12
LONG TERM...Ervin
AVIATION...Haase



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