Forecast Models: All over the map

Posted at 8:30 AM on December 17, 2009 by Paul Huttner

Last night's model run: GFS 500 millibar forecast for noon next Wednesday. Upper low in North Dakota suggests big snow for Minnesota.

Overnight model run: Upper low displaced south suggesting no snow for Minnesota.

This is getting a little old.

The medium range forecast models are all over the map these days. Literally.

There have been big differences and little run to run consistency in the forecast models beyond 48-72 hours lately. Even different models such as the European Model are coming up with differing solutions.

What's a weather forecaster to do?

An example is how the forecast models are handling next Wednesday, the big travel day before Christmas Eve. If you look at the 500mb charts above, you can see two upper air forecast maps for the same time period, noon next Wednesday. The top image features a low in North Dakota. This would bring snow to Minnesota.

The lower image places low pressure to the south and the northwest in Canada. This would suggest little or no snow. One thing the models do agree on is a big "blocking high" near Greenland.

Even the local Twin Cities NWS is doing some head scratching over the model differences these days in their forecast discussions.

Why the differences?

One reasons for the differences is the type of atmospheric flow pattern. A blocking pattern is hard for the models to resolve.

Another reason may be that the atmosphere is trying to evolve into a more classic El Nino pattern with time. This pattern features split jet streams. The northern branch is called the Polar Front Jet Stream. It often migrates north into Canada during El Nino years.

The southern branch is called the Sub-Tropical Jet Stream. It usually brings increased storms to southern states in El Nino years.

It's as if the models are trying to adjust to and find a workable solution to chaotic flow patterns.

The result is forecasters have little confidence these days past about 72 hours. Will there be snow next Wednesday? Bitter cold? Milder with just a few flurries? Or sunny?

Stay tuned.

PH

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