Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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000
FXUS63 KDVN 100344
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
944 PM CST Fri Dec 9 2016

.UPDATE...
Issued at 931 PM CST Fri Dec 9 2016

Will be adding flurries to forecast overnight for areas near
Hwy 20, with 1000-850 mb condensation pressure deficits lowering
to around 20 mb or slightly lower in developing weak isentropic
ascent. Temperatures look to rise a bit as the thickening clouds
continue to overspread area next few hours, then will likely see
fairly steady readings thereafter beneath the blanket of mid
clouds.

Only been able to peruse new 00z NAM and RGEM model data for next
storm system, and they remain in agreement on a very tight
southern gradient and sharp cutoff to snow Sat PM/night making for
a very tricky snowfall forecast for areas near I-80 southward to
Hwy 34. Forcing progs and fairly high low level condensation
pressure deficits suggest intensity of snow initially may be
mostly light to some pockets of moderate, with the intensity then
increasing a bit more Saturday night especially Hwy 30 northward.
Sent WSW products with no changes at this time.

&&

.SYNOPSIS...
Issued at 148 PM CST Fri Dec 9 2016

18Z surface data has high pressure from the Dakotas into the lower
Mississippi Valley while a storm system was developing in the
Rockies. Dew points were in the teens across most of the Midwest
with single digits above and below zero in the northern Plains.

&&

.SHORT TERM...(This Evening through Saturday)
ISSUED AT 148 PM CST Fri Dec 9 2016

A prolonged snow event will begin Saturday afternoon and continue
for the remainder of the weekend. Temperatures will continue to
average well below normal.

Quiet and dry conditions will be seen late this afternoon and
tonight as high pressure moves through the area.

Dry conditions will continue Saturday morning as high pressure moves
into the Great Lakes and the storm system approaches from the
Plains. Attention then turns to Saturday afternoon.

The question Saturday afternoon is not whether it will snow. The
question is how quickly will the atmosphere saturate. If saturation
takes longer than expected, then snow accumulations for the
afternoon hours will be lower.

As the atmosphere saturates, snow will develop and move from the
west northwest to the east southeast across the area. By sunset
Saturday, most areas north of an Ottumwa, IA to Galesburg, IL line
should see light snow. Areas along/north of I-80 may see 1-2 inches
of accumulation by sunset Saturday.

.LONG TERM...(Saturday Night through Friday)
ISSUED AT 148 PM CST Fri Dec 9 2016

Forecast forecast on winter storm to impact the region over the
weekend, followed by well below normal temperatures next week.
Weekend storm will be a prolonged 30-36 hour event, with two
rounds of precipitation. An initial warm advection wing of
precip will be seen Saturday night, with snow totals between 4 and
7 inches expected generally in a line from MCW to DBQ to RFD by
12z Sun. Thus after collaboration with surrounding offices have
issued a winter wx advisory for counties along and north of I-80
in Iowa and a winter storm watch for counties north of I-80 in
Illinois.

Snow will be ongoing across parts of eastern Iowa 00z Sun and
will continue in an east southeast direction throughout the
evening and overnight. Strong 850-700 mb fgen progs and omegas
suggest a heavier band may be possible around 06z Sun along or
north of highway 20, bringing a quick burst of snow and reduced
visibilities. With cold air in place and a rather deep dendritic
growth zone, am expecting SLRs averaging between 13 to 15:1 for
the duration of both events. Higher SLRs will be seen with the 1st
wave. A sharp gradient is expected along the southern periphery of
the snow totals in southeast Iowa and west central Illinois. Any
slight change in track of system prior to Saturday, will impact
snow totals.

Sunday through early Monday, an area of low pressure tracks over
the area bringing numerous forecast challenges. Temperatures both
at the surface and aloft suggest a changeover to all rain or a
mix between 15-21z Sun across the southern CWA, while the northern
areas remain all snow. The GFS is the warmest aloft and would keep
rain going for much of the morning in the south. However,
NAM/ECM/GEM all are colder and have sided more with these
solutions. Still expect the southern 1/5 of the CWA to see all
rain or a mix for a few hours Sunday morning, with no ice accums.
This second wave will bring more widespread snow, albeit lighter
snow rates than the first wave at this time. Snow should end from
west to east between 00z-06 Mon.

Looking ahead of this system, much colder air will filter into the
Midwest by mid week, with highs in the teens and single digits.
Depending on the amount of snow and cloud cover from this
weekend`s storm, these readings may even be too high. There is a
weak storm that the GFS is suggesting on Wednesday, but all other
solutions does not have it. Time will tell and will wait to add
any more detail until models come into better agreement.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday Evening)
ISSUED AT 543 PM CST Fri Dec 9 2016

Stratus with pockets of MVFR cigs and flurries will be in proximity
to MLI and DBQ early this evening before diminishing with the
passage of a weak disturbance. Otherwise, generally VFR conditions
expected tonight and Saturday AM, with light and variable winds and
increasing mid level clouds with cigs 7-12kft agl. Saturday afternoon
expect deteriorating conditions to MVFR and IFR in snow with
warm advection/isentropic lift increasing ahead of a storm system
emerging from the Rockies.


&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Winter Weather Advisory from 3 PM Saturday to 6 PM CST Sunday
     for Benton-Buchanan-Cedar-Clinton-Delaware-Dubuque-Iowa-
     Jackson-Johnson-Jones-Linn-Muscatine-Scott.

IL...Winter Weather Advisory from 3 PM Saturday to 6 PM CST Sunday
     for Rock Island.

     Winter Storm Watch from Saturday afternoon through Sunday
     evening for Carroll-Jo Daviess-Stephenson-Whiteside.

MO...NONE.
&&

$$

UPDATE...McClure
SYNOPSIS...08
SHORT TERM...08
LONG TERM...Gross
AVIATION...McClure



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