First Frost and Flakes

Bemidji, Buena Vista Ski Area

Call it Sept-ober.  The chill in the air has been more reminiscent of October as temperatures have been a good 10 degrees below normal.  The cool and cloudy conditions will stay through the weekend.  You have to admit, though, that this is great riding weather.  The bugs are (almost) gone, the humidity has dropped, and the trails are fast.  If only the migrating ducks were this lucky.  This weekend is the earliest duck opener since World War II.

Saturday at exactly 9:49 AM, the Autumnal Equinox occurs…the first day of fall (astronomically speaking).  On this day, the length of day and night are approximately equal around the entire globe, the sun rises due east and sets due west and the sun’s most direct rays are perfectly parallel to the Earth’s equator.

It might be the first day of Fall but the weather may be telling you otherwise.  Another potent upper level low swings through eastern Minnesota Saturday morning kicking off light rain in the metro but a mixture of rain and snow in northern Minnesota & Wisconsin.

Weather map showing 500mb vorticity. The black circle over Duluth depicts a potent bundle of energy in the mid atmosphere. This will be enough to produce light rain and, if cold enough, rain & snow showers for the northland.

There are many, many factors that go into predicting snow.  Typically, the 850mb temperature has to be at or below 0°C.  The 850mb level is at the top of the troposphere and at a high enough elevation where friction from the ground doesn’t affect the wind flow.  Saturday morning, the 0°C 850mb temperature is south of the Twin Cities as indicated by the red line.

The thickness of the air is also crucial when forecasting winter precipitation.  The thickness of air is directly related to temperature and when the thickness is low enough, snow can reach the surface.  Meteorologists tend to look at the 540 line which is the 5400 meter thickness of air between the 1000mb-500mb levels in the atmosphere.  The white line below shows the 540 line which, again, is south of the Twin Cities.

So both the 850 mb temperature and the 540 thickness line show that the atmosphere is cold enough aloft for snow…but…it won’t snow in the metro.  Why?  The ground is too warm.  Temperatures right at the surface will stay above freezing (32°) through the night.  Some of the favored cold spots in northern MN may see a few flakes around midnight Saturday morning. Northern Wisconsin, however, has the best chance of seeing some snow as the moisture hangs on a little longer into the early morning hours when it will be cold enough to support snow.  Check out the accumulated snowfall forecast below.

Now that I’ve bored you with all the nerdy weather talk…let’s get to the forecast.  The sky will clear Saturday night, the temperature will drop below freezing and the Twin Cities could have the first frost/freeze of the season come Sunday morning.

Some models are showing low temperatures dropping into the upper 20s!  A freeze like this could definitely end the growing season for central Minnesota.  For those that have a green thumb, time to protect the plants!

After a cool weekend, temperatures will slowly start to rebound closer to normal next week.  The chance of rain also looks rather slim the next few days.  Expect the trails to be dusty and fast…a sign that Minnesota’s drought is getting worse.

CycleCast

Saturday: First day of Fall. Mostly cloudy, clearing late with the potential for frost and freeze early Sunday. H: mid 50s

Sunday: Sunny. Still cool. H: upper 50s

Monday: Sunny and much warmer. H: near 70°

Tuesday: Partly sunny. H: upper 60s

Wednesday: Sunny H: Low 60s

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