December 4-6 Potential Winter Storm

A disturbance is forecasted to track eastwards from the Pacific Northwest & Southwestern Canada across the northern Plains, bringing accumulating snow to the region before intensifying as an additional piece of energy interacts with this original system around and east of the Great Lakes.

This event begins during the day and evening on December 4th, with an upper-level disturbance riding east-southeast along a band of tight upper-level confluence and spawning a surface low-pressure system in the Dakotas. The above image shows forecasted precipitation types and sea-level pressure contours as of 12pm Central Time, December 1st. While the bulk of the precipitation is seen in the Rockies, the eastward progression of this system begins at this time as an arm of light snow pushes eastwards into the Dakotas.

By 6am Central Time on December 5th, the surface low has pushed east in accordance with the eastward progression of this upper-level system. Some moisture fetch from the South U.S., evidenced by weak rainfall in MO/AR/TX, adds to the system and helps gently boost snowfall totals in Minnesota and Wisconsin. Accumulations are expected to be in the 2-4" range across the Dakotas, northern Minnesota and northern Wisconsin over this December 4-5 period.

Things become much more interesting overnight December 5th into the morning of December 6th, as our surface low pushes eastwards and the upper-level piece of energy finally gets an opportunity to strengthen after being squeezed by a progressive and prohibitive jet stream back in the West and Central. As shown above, the latest European model run sees moderate to heavy snow in southeast Canada and into extreme northern parts of New England, turning into a mixed precipitation event as a surge of warm Southern air brings rain to most of the region.

The GFS model is a little better with snowfall chances in New England (1-3" in western and central New York), but in general we should see the best shot for over 4" of snow in southern Canada. The latest ECMWF snow accumulation chart for this event is shown below, and there is some question as to how strong this system could actually get. I don't think the ultra-aggressive totals shown by the ECMWF will come to fruition, but it seems plausible that some areas along or just north of the U.S. / Canada border in New England could hit 6".

To Summarize:

• A winter weather event is expected to impact the northern Plains, Great Lakes and New England in the December 4-6 timeframe
• Snow accumulations of 1-3" appear possible in the Dakotas, northern MN and northern WI during the December 4-5 window
• Some heavier accumulations nearing 6" may be possible in extreme southeast Canada, but questions remain as to how notably this storm system will intensify as it nears the Atlantic

Andrew