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December 10-12 Potential Snow Event


Hoosier

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GRR worried, in the good way. They think the system may under up OVER performing

.UPDATE...
Issued at 933 PM EST Sat Dec 10 2016

Will be staying the course with this update in terms of headlines,
but I am a bit concerned this system may overperform. The snowfall
occurs in a 24-30 hour period with fairly high liquid equivalent
totals. The lowest QPF is via the ECMWF, on the order of 0.40 to
0.60 across the southern half of the CWA. The GFS is a bit more
0.60 to 0.70. The NAM is the high end, at 0.80 to 1.00. Warm air
advection snow events typically are some of our heaviest and we
see a warm air advection mode almost the entire duration. On
Sunday especially, we see strong isentropic upglide on the 290K
surface with 40-50 knots straight up the surfaces, so strong
pressure advection with condensation pressure deficits of only
5-10mb's.

We want to be able to peruse the entire 00z suite of guidance
before making any changes/upgrades to the current forecast. The
current forecast has 6-9 inches over 24-30 hours. If the models
nudge up just a bit overnight in terms of QPF we may need to
upgrade some counties even with the 24-30 hour time frame. My
area of concern would be about the southern half of the forecast
area. The snow will be working from a 20:1 snow tonight to closer
to 10:1 by Sunday evening. Bottom line is it looks to be an
impactful snow in both amount and the fact that it will become
increasing dense as we move through the event.

Another item to keep an eye on will be the lake enhanced snow
that is going on right now. This snow will work up the Lake
Michigan shoreline tonight. The flow is swinging from SW to S
which will eventually take the laminar batch of snow off shore,
but between now and 09z or so heavier snows will be working
northward through the lakeshore counties.
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CHICAGO NWS UPDATE                                               
FXUS63 KLOT 110259
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
859 PM CST Sat Dec 10 2016

.UPDATE...
855 PM CST

Evening Update...

No changes planned to forecast or headlines at this time, though
will be making some minimal changes to near term pops based on
radar/obs trends.

Initial region of warm-advection forced light snow overspread the
region late this afternoon as anticipated, with heavier (mainly
moderate) snowfall within a region of frontogenetically-forced
northwest-southeast bands. Recent radar trends show this banding
has weakened and lifted northeast across southeast WI, Lake
Michigan and northern IN at this time. However, 00Z DVN sounding
depicts a fairly deeply saturated column with nice veering wind
profile signature associated with strong warm-advection. Within
this region of persistent upglide, latest high-res guidance
continues to support regeneration of transient f-gen banding which
should continue to result in periods of moderate or greater
intensity snowfall. While we`re currently in a lull in radar
returns especially across the western parts of the cwa, stronger
returns are blossoming across portions of eastern IA, and would
expect an uptick in coverage and intensity later this
evening/overnight as forcing and moisture advection persist. With
the first band working to moisten initially dry low-levels, will
likely see additional bands more efficient in lowering vis and
producing heavier snow, and latest RAP soundings suggest a decent
period of deeper dendritic growth after 05-06Z.

Initial bands of snow have produced 2+" of new snow in many spots
across the warning area, which is in line with going forecast
amounts through 06Z. Thus not inclined to make any big changes at
this time, with expected redevelopment and increase in snow
coverage and intensity later tonight.
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5 minutes ago, Chinook said:

Areas in Indiana, Lima and Defiance, and Findlay, all getting -SN reports, and 1" storm report from central Indiana. Perhaps some decent snow accumulation rates west of Fort Wayne.

5jMK15x.jpg

That -SN is sticking to surfaces here north of Lima. Still just trace amounts though. Looking forward to those heavier bands as the night goes on.

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After the column was nice and saturated for hours, we've now gone back to a virga storm here, as dry air has intruded from the southwest.  Gonna take awhile to resaturate the column, and start the whole process over again.  I think our best chance for appreciable precip comes with the final wave arriving mid-morning tomorrow.  May get a quick inch of slop out of that before it jets east. 

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