Snowy PM rush hour dusting; why do trees like it cold?

There’s one group of Minnesota residents who absolutely love this extreme cold.

The trees.

Trees in winter.  Paul Huttner | MPR News

Temperatures hit the magic number of -40 across northern Minnesota in the past week.

Magic number

Forestry experts like Lee Frelich at the University of Minnesota say -40 is a magic number for Minnesota forests. Temperatures of -40 produce high insect mortality rates. That means those little pine bark beetles that can decimate our northern forests took a big hit last week.

Pine Bark Beetle deforestation in the Black Hills, via U.S. Geological Survey

So even though you can’t feel your face, there are benefits to this extreme cold. Silver linings people.

Snowy dusting for PM rush hour

A weak front sails south across Minnesota Tuesday. Moisture is scant in this arctic air mass, but there may be just enough to squeeze out a light snowy dusting just in time for afternoon rush hours.

The National Oceanic and Atmospheric Administration’s High-Resolution Rapid Refresh model shows a narrow band of light snow pushing through the Twin Cities between about 4 p.m. and 6 p.m. Tuesday.

Cue the Minnesota Department of Transportation traffic cams.

NOAA HRRR model, via tropical tidbits.

Less cold

The worst of the bitter cold appears to be behind us for now. It’s still going to be respectably cold, but temperatures run about 5 to 10 degrees “less cold” this week. Teens on Saturday and 20s Sunday will feel remarkably good.

NOAA, via Weather Bell

Super-refraction 

Extreme cold like this can create temperature inversions aloft that can bend radar beams. When that happens the bean tends to follow the earth’s curvature, and bounce off tall objects.

NOAA

These days the Doppler sees wind farms in Iowa and Minnesota.

Polar vortex

The atmospheric culprit for our extreme cold is the dreaded polar vortex.

What is the polar vortex?

The polar vortex is a large area of low pressure and cold air surrounding both of the Earth’s poles.  It ALWAYS exists near the poles, but weakens in summer and strengthens in winter. The term “vortex” refers to the counter-clockwise flow of air that helps keep the colder air near the Poles.

Many times during winter in the northern hemisphere, the polar vortex will expand, sending cold air southward with the jet stream (see accompanying graphic).  This occurs fairly regularly during wintertime and is often associated with large outbreaks of arctic air in the United States.

The one that occurred during January 2014 is similar to many other cold outbreaks that have occurred in the past, including several notable colder outbreaks in 1977, 1982, 1985 and 1989.

Stay warm Minnesota.