Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 091725

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1125 AM CST Fri Dec 9 2016


Issued at 314 AM CST Fri Dec 9 2016

The latest sfc analysis was indicating lingering cyclonic flow with
some embedded trofs acrs the western GRT LKS and to just west of the
MS RVR Valley, while elongated ridging occupied the northern into
southeastern plains as well as the MO RVR Valley. Aloft, a vort max
was noted in the current water vapor loop embedded in broad west-
northwesterlies, rotating down acrs eastern IA and continuing to
drive areas of flurries and light snow as well as ongoing stratocu


.SHORT TERM...(Today and Tonight)
ISSUED AT 314 AM CST Fri Dec 9 2016

Today...Although there could be several breaks in the cloud deck at
times today, will keep the theme as mainly cloudy. Feel that after
the main vort lobe aloft propagates east of the area this morning,
the flurries and areas of light snow will wane. Will not mention any
precip this afternoon, but any additional vort may wring out a few
flakes out of lingering stratocu deck. Most short range models
suggest the next wave aloft will not approach from the west until
toward sunset. Cloud cover, ridge sliding in from the wets for
rather weak wind mixing regime, and ongoing cold air advection to
make for high temps not much warmer then current values. A lot of
low to mid 20s, with a few northern areas having trouble getting out
of the upper teens.

Tonight...The first main impulse of this weekend`s complex snow
event/s starts to take shape this period just to the lee of the
central Rockies in flattening progressive steering flow. Cloud cover
to maintain under increasing elevated warm air advection/ return
flow tonight. Will keep non-mentionable POPs tonight, but would not
be surprised to see some bands of WAA-type flurries making it into
portions of northwest into the northern CWA later tonight into early
Sat morning. Low temps tricky depending on cloud cover and if light
south to southeasterly sfc winds increase, for now will advertise
around 10 degrees in the far north, to the mid to upper teens in the
south.   ..12..

.LONG TERM...(Saturday through next Thursday)
ISSUED AT 314 AM CST Fri Dec 9 2016

A complicated winter storm system will impact the region this
weekend. This has the potential for produce a significant snow
amount, over a prolonged period of time. Much like a pattern of
multiple MCSs in successive nights in the summer, there could some
impressive snow totals, but not everyone will see the overlapping of
the best forcing in this 36 hour long event, made up of two thrusts
of warm advection along a exceptionally tight baroclinic zone.

At this time, the main question for Saturday and Saturday night,
will be at what latitude saturation occurs. The best lift, including
a deep dendritic layer is found in the southern 1/2 of the CWA
Saturday and Saturday PM, but the models only saturated snow through
the surface over the northern 1/2 of Iowa and northern Illinois, and
locations north. That said, we will need to watch to see if with the
strong forcing, we can saturate farther south than model qpf
currently suggests. I have collaborated with the DSM office to bring
pops south a few more counties than the models suggest in 00Z qpf
forecasts. The fact that the saturation question is still in play
supports the idea to hold off on headlines on this issuance.
However, it should be said, that if we saturate early and south
through Highway 34 Saturday afternoon, we probably need a headline
for snow amounts at that time. It is just not something that carries
confidence yet.   Snow ratios, due to a deep dendritic layer and
cold boundary layer temperatures in Arctic origin air Saturday and
Saturday night should be on the order or 15 to 20:1, thus the model
blended qpf of 0.10 to 0.25 may be several inches.

Getting in the weeds of this forecast, the next question is whether
there really is a break in snow from Saturday evening into Sunday
morning. Models had indicated such, but with a saturated frontal
surface from earlier snow, I think there may be light snow
continuing the whole time, with a general lull in accumulations
later Saturday night through early Sunday. Pops have been increased
in the northern 1/2 for this thought.

Sunday through early Monday, the next wave of forcing arrives and
with it, a challenge of precipitation type and a gradient in snow
ratios. This period offers up potentially significant qpf, on the
order of 0.25 to 0.50, with the north 1/2 remaining all snow, and
the south seeing a change over to rain. For now, will use the model
blended solution, but there is a wide variance in forcing and
position of frontal zones for Sunday. The EC is weak, and father
north compared to the NAM/GFS/GEM. I will side towards a colder more
southerly solution for now, given our start in Arctic air, and
potentially a long continuous duration of snow keeping our
temperature profile from seeing a warm up above freezing. That was
the case last weekend if you recall, despite models indicating it
switching to rain over southeast IA/western IL, it was all snow in
the ongoing event that was actively snowing.

As far as headline potential, this period could be fully included
for lower end winter storm impacts. Amounts may be the primary
hazard to prepare for, but over a very long duration. In other
words, it won`t be all coming at once like last weekend.

In the wake of this system, bitterly cold air will arrive for mid
week. Depending on snow cover and cloud cover, our highs and lows
will possibly be confined to the single digits above and below zero.
By week`s end, other deep trof should exit the southwestern CONUS,
with a possibly broad over running snow event unfolding.



.AVIATION...(For the 18Z TAFS through 18Z Saturday Afternoon)
ISSUED AT 1122 AM CST Fri Dec 9 2016

MVFR conditions across northern Illinois and southeast Iowa will
improve to VFR through 00z/10 as high pressure moves east from the
Plains. After 00z/10...VFR conditions will be seen through 18z/10
as high pressure moves into the Great Lakes. After 18z/10...
conditions across eastern Iowa and northern Illinois will
deteriorate rapidly to IFR as a winter storm moves into the area.


IA...Winter Weather Advisory from 3 PM Saturday to 6 PM CST Sunday
     for Benton-Buchanan-Cedar-Clinton-Delaware-Dubuque-Iowa-

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.



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