Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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470
FXUS63 KDVN 242342
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
542 PM CST Tue Jan 24 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 327 PM CST Tue Jan 24 2017

18Z surface data has a low in central Kansas. A warm front extended
east from the low into southern Missouri. Dew points were in the 20s
across the northern Plains and parts of the upper Midwest with 30s
from the Great Lakes into the central Plains.

&&

.SHORT TERM...(This Evening through Wednesday)
ISSUED AT 327 PM CST Tue Jan 24 2017

Complicated forecast. Confidence on snow amounts is low.
Changes...a winter weather advisory will be issued for the
highway 20 corridor west of the Mississippi. The area at risk are
the northern parts of these counties.

The dry atmosphere has saturated in a narrow band of strong forcing
that is lifting north toward highway 20. Ground and pavement
temperatures are above freezing so no accumulation will occur prior
to sunset.

This evening, light snow or a rain/snow mix will be seen north of
highway 30 through 9 PM as forcing slowly increases.

After 9 PM, forcing increases considerably as the low moves
northeast allowing rain or a rain/snow mix to expand in areal
coverage. Aloft, a layer of air becomes neutral to slightly
unstable. The strength of the forcing suggests the potential for
isolated but elevated thunderstorms. North of highway 30 and west of
the Mississippi, the increased forcing from the convection will
allow localized cooling of the atmosphere that should be enough to
force the precipitation over to all snow or possibly sleet. This
cooling may be enough to allow the snow or sleet to accumulate on
grassy and elevated surfaces.

Forcing will reach a maximum between 11 PM and and 5 AM Wednesday as
the surface low moves through the area. Pressure falls over the past
several hours suggest the low passing between the Quad Cities and
Cedar Rapids and then moving into southeast Wisconsin.

The arrival of the low overnight forces temperatures aloft to warm
which will change the precipitation over to rain. The concern is
that temperature increase aloft in the highway 20 corridor will be
enough to force the precipitation back over to all rain or a
rain/snow mix. If that occurs, then overall snowfall amounts would
end up being much less.

On Wednesday, the low moves into the Great Lakes. Cold air being
pulled down behind the low will allow the rain to slowly change over
to a rain/snow during the afternoon. Areas north and west of a
Dubuque to a Sigourney line may see a change over to all snow prior
to sunset.

Areas south of a Tama, IA to Lanark, IL line will not see any snow
accumulation until Wednesday afternoon at the earliest. North of
this line but south of highway 20 a dusting to possibly an inch
of accumulation is possible.

The heaviest snow amounts will be across the northern parts of
Buchanan and Delaware counties, and the northwest corner of Dubuque
county. There snow accumulations of 2 to 4 inches are possible by
sunset Wednesday.

.LONG TERM...(Wednesday Night through Tuesday)
ISSUED AT 327 PM CST Tue Jan 24 2017

Beginning of the long term will be focus on the current system
exiting the area with a chance for snow tomorrow night and then snow
showers and possibly snow squalls Thursday and Friday of this week.
Past this, quiet weather with cooler, more seasonable temperatures,
is forecast.  With the pattern shift, could see more of a clipper
path through the long term.

00z Thursday, sfc low is prog to be elongated from central MI west
to NE IL and SE WI.  Wrap around moisture with omega in the
dendritic growth zone suggests that the best chance for snow will be
between 00z and 06z Thursday.  With cooler air in place, snow ratios
will be high enough to warrant about an inch of snow.  The best
omegas for snow production should be east of the area by 06z and
there could be some flurry activity through 12z.  During the day
Thursday, under the upper low, lapse are large enough to introduce
some low end CAPE.  The snow squall parameter across the eastern CWA
exceeds 1, suggesting that there may be squalls Thursday afternoon.

Friday, continued NW flow aloft will usher in a vort max that could
lead to snow showers across the area.  Impacts from these snow
showers are not expected at this time.  Past Friday, ripples in the
NW flow will lead to chances of snow.  At this time, confidence is
low in any one solution.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday Evening)
ISSUED AT 536 PM CST Tue Jan 24 2017

Low pressure will move over eastern Iowa toward morning Wednesday,
with extensive low clouds through the period, especially north of
Highway 34. Heavy snow will fall over northern Iowa, but the
Highway 20 corridor in Iowa will not see continuous snow, sleet,
or rain. Intermittent rain/snow will be possible at all locations
north of Interstate 80 overnight, and in all locations through the
day Wednesday after winds turn toward the northwest. In sharp
contrast to the low conditions, will be far southeast Iowa this
evening, which is clear. In any case, those flying should expect a
likelihood of cigs under 800 ft, and visibility 1-4 miles in fog
and light rain/light snow through the period. ERVIN

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Winter Weather Advisory until midnight CST Wednesday night for
     Buchanan-Delaware-Dubuque.

IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...08
SHORT TERM...08
LONG TERM...Gibbs
AVIATION...Ervin



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