It’s November. Do you know here your driveway salt and shovels are?
Call it our winter dress rehearsal. The first real snowflakes of the season and winter weather advisories are flying for many Minnesotans.
As we work through the nighttime hours into Wednesday morning, one of the interesting things to watch is precisely where the moderate to heavy snow bands set up on radar.
The forecast models are history now. We’re in ‘nowcast’ mode. Radar, satellite trends and surface reports are king now. It’s an hour by hour, play by play to see how the system evolves.
Still on track
Overall, our weather system is still on track for a snowy night in central and southern Minnesota, with rain favoring southeast Minnesota. One band of snow has set up west of the metro in Hutchinson and Buffalo late Tuesday afternoon.
The way this band is setting up I’m starting to think we may see some heavier totals favor the northwest metro now, which is a change from earlier thinking.
Let’s see how that evolves tonight.
Infamous Snow Doughnut
Many MPR listeners have asked me why we often see a “snow doughnut” right over the Twin Cities as snow approaches in winter storms.
The Doppler Tuesday afternoon was a perfect example.
What causes these precip free rings on radar?
As moisture with weather systems moves in aloft, the radar beam shoots out just above the horizontal and rises in elevation above ground as it pulses away from the radar site. Eventually it gets high enough above ground to encounter the saturated layer — the rain or snow several thousand feet above the ground.
The result is that it looks like rain or snow say, 40 miles form the radar site but not overhead since the Doppler beam is looking out, and not up.
That’s why you see a snow doughnut on radar, but the saturation layer is actually overhead too. It just hasn’t worked it’s way down to the ground yet where the Doppler beam can see it.
This graphic gives you an idea how the Doppler beam increases with height away from the radar, and why it cannot see moisture directly overhead.