Snow cover effect; Still watching snow chances Thursday & Sunday

Clipper Thursday to bring snow to southern Minnesota

Modles differ on system track Thursday (Rochester or Metro?)

Bigger storm still likely Sunday for Minnesota

GFS shifts track slightly north in latest model runs?

+2 hours of additional daylight since December 21st!

Sunshine returns to the weather lab today!

Sunglasses Alert!

You'll need to find the shades today! The combination of some sunshine and bright "highly reflective" snow cover means it will be brighter out there than it has been for much of this winter.

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It's called "albedo." That's the reflectivity of various ground covers on incoming sunlight.

Fresh snow cover is the most efficient solar reflector, returning about 80% to 90% of the sun's incoming energy back into space before it can heat up the air near the ground.

Bare ground can absorb and "reradiate" about 80% of the sun's rays. That energy is then used to heat the air near the surface.

The result? Temps can run a good 10 degrees cooler on days like today with fresh snow cover in Minnesota. If we had bare ground today like most of the winter, we'd likely make a run well into the 40s again; with fresh snow we'll stall somewhere in the upper 30s in most areas.

Thursday Clipper: Which track is best?

The models differ a bit on the track of Thursday's Alberta Clipper sailing in from the northwest.

The clipper looks to feature a relatively narrow band of snowfall about 60 miles wide.

The GFS lays that band out along the Minnesota River towns of Redwood Falls and Mankato southeast toward Rochester, and suggests a 2" to 4" snowfall.

The overnight NAM run shifts the system north...from Willmar into the Twin Cities. It should be noted that the overnight NAM is sometimes less reliable, but it bears watching to see what the next couple of model runs do today!

Sunday storm: Still on track, but which track?

Looking ahead to Sunday's potential snow storm I am reminded that there are 3 main factors that determine snowfall amounts in winter storm systems.

1) Storm Track:

The surface low pressure track is a key to where heavy snow may fall in winter storms. Generally speaking, the heaviest sow band favors an area about 90 to 120 miles north & west of the surface low pressure track.

This can vary of course from storm to storm, but it's a pretty good rule.

2) Moisture Profile:

Obviously, the amount of moisture a storm can draw in effects snowfall totals. The biggest factor in overall snowfall totals for Minnesota is usually the availability of moisture from the Gulf of Mexico.

If southerly winds ahead of the storm have time to pull up enough Gulf moisture, we get dumped on.

3) Temperature Profile:

Temps within the overall storm are critical to determine precip type and snow intensity. Generally, warm air is drawn up ahead of the system, and cold air wraps in behind mid latitude cyclones in Minnesota and the Upper Midwest.

Often, the freezing, or "rain-snow" line sits just south of these low pressure centers. That's why the heaviest snow lies north of the low track, with mixed precip reducing snowfall totals as you go south.

Sunday's potential system shows some interesting trends.

The overnight GFS runs have shifted the track slightly north. If that pans out, the heaviest snow band could run from near Fargo to Brainerd and Duluth, instead of Redwood Falls to the Twin Cities.

The modes will likely show more shifts in the days ahead.

As usual, I'll wait until about 24 hours ahead of the onset of snowfall to issue my "final" snowfall predictions. This is the window where model accuracy and preparation time for people affected by the storm usually meet. At this point the best information is to say there will likely be a snowfall event Sunday, with specific totals to be determined as we approach the weekend.

Stay tuned!

PH