Updraft: December 17, 2009 Archive

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

Updated 90 Day Outlook: Milder than average

Posted at 4:11 PM on December 17, 2009 by Paul Huttner (2 Comments)

NOAA's Climate Prediction Center (CPC) 90 Day Outlook shows a higher possibility of warmer than average temperatures over the northern U.S.

That's their story and they're sticking to it.

NOAA's CPC released updated 90 day outlooks today for temperatures and precipitation. Though much of the country is experiencing cooler than average weather so far in December, the NOAA outlooks favor above average temperatures in the northern tier of states.

My observations and discussions with experts in ENSO, like Dr. Kevin Trenberth who heads up NCAR's Climate Analysis Division, suggest that the effects of El Nino often increase in the second half of winter. Translation; we have a better chance of shifting into above average temperature trends from January into March.

Today's discussion from CPC emphasizes this point.

"CURRENTLY MODERATE EL NINO CONDITIONS EXIST IN THE TROPICAL PACIFIC AND ARE EXPECTED TO PERSIST INTO AT LEAST THE FIRST FEW MONTHS OF 2010. THE EXPECTED CONTINUATION OF THE MODERATE EL NINO EVENT SIGNIFICANTLY INFLUENCES THE TEMPERATURE AND PRECIPITATION OUTLOOKS FOR JFM THROUGH MAM 2010. THE IMPACT OF EL NINO ON THE CLIMATE OVER NORTH AMERICA IS USUALLY GREATEST DURING THE LATE WINTER SEASON."

It should be noted that the chances of a milder than average winter season in Minnesota is about 71% in moderate El Nino years. There have been cooler than average winters around the region. There have also been winters where one month is below average, and two months are above average. The trends observed also are seasonal averages, and we can still certainly have bitter arctic outbreaks during the winter season. 20-below can happen in El Nino winters.

It will be interesting to see if the polar front jet stream shifts northward into southern Canada in January and February.

While temperature trends favor milder conditions in El Nino winters, snowfall trends do not show any statistical correlation in the Upper Midwest. Average winter season snowfall for the Twin Cities is 55.9".

The southern states do tend to show above average rainfall.

PH

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