Winter storm watch: complex Thursday snow system

If weather forecasting is the circus, snowfall forecasting is a dicey high-wire act.

We're setting the table for what looks like the most significant snow system so far this season for parts of Minnesota on Thursday. As usual there are a wide variety of model forecast solutions. And this storm has one interesting feature that makes for an even higher degree of forecast difficulty than usual.

In the blog, I talk about what makes this system tougher than most, and why snowfall totals may vary dramatically across just a county or two.

So you want to be a meteorologist when you grow up, kids? Place your bets.

Finicky system

Pay no attention to that map on the right hand side above. I jest some, but I still think it's too early to start nailing down spatial coverage and quantitative snowfall totals with this system.

Here's why. This stretched out, taffy-like system is elongated. That means it will likely produce an unusually narrow band of the heaviest snow. That heavy snow band could be only one county wide. A 60 mile shift north or south will produce dramatically different snowfall totals.

Here's the National Oceanic and Atmospheric Administration's Global Forecast System model, which places a narrow heavy snow band just south and east of the Twin Cities by Thursday morning.

High model variance

There is still a wide variance in model forecast snowfall totals across the Upper Midwest. Most models agree on a narrow band of heavy snow that could produce 4 to 8 inches-plus in the sweet spot. But the placement of that narrow band varies from northwest of the Twin Cities (Canadian model) to right on top of the Twin Cities (NOAA North American Mesoscale Forecast System model) the southeast of the Twin Cities (NOAA GFS model).

Here's a look at snowfall output for the Twin Cities from the various American models.

Narrow heavy snow zone

NOAA's 3 km NAM model illustrates the forecast challenge with this one. Look how narrow the heaviest snow zone is. A thin zone of 4 to 8 inches embedded within a wide 2- to 3-inch snow zone?

A snowfall range of 2 to 7 inches across the greater Twin Cities metro? It could happen.

Sweet spot

We're still not quite into the 12 to 24-hour sweet spot where the forecaster Venn Diagram of accuracy meets actionable preparation time. Buyer beware with this one. Things could look very different tomorrow before the snow flies.

Stay tuned.

Thaw continues for now

Meanwhile we enjoy two more mild days ahead of this week's snow chance. Then the bottom drops out late this week as arctic air pours south once again.

Stay tuned.

Winter Misery Index

So how's your misery gauge reading this winter? Believe it or not this winter is actually running behind last winter so far on the Winter Misery Index. The lack of snowfall has kept points down in spite of the persistent cold of late.

Here's an update from the Minnesota State Climatology Office:

As of January 8, 2018 the WMI for the 2017-18 winter is at 23 points: 21 points for cold, 2 points for snow. The winter of 2016-17 was ahead of this winter with 32 points at this time, mostly due to much more snowfall. As of January 8 the winter of 17-18 has only 7.2 inches of snow so far, 17.7 inches below normal.

The WMI for the winter of 2016-17 finished with 49 points, enough for 2016-17 to be categorized as a "mild" winter. The total WMI points for the 2016-2017 winter were 23 for cold and 26 for snow: 47 points.

The WMI for the winter of 2013-14 in Twin Cities was 207 points, or in the high end of the "severe winter" category. This was the 9th most severe winter on record based on WMI points. The lowest WMI score was the winter of 2011-2012 with 16 points. The most severe winter is 1916-1917 with 305 WMI points.

Note, this could also be called the "Winter Fun Index" depending on your perspective!