Wednesday, March 20, 2013

Spring 2013: DELAYED

It’s spring!

The vernal equinox on March 20, 2013.  It is that time of year again (it happens twice) when the relative angle of Earth is perpendicular to the Sun, causing equal incoming solar energy to the Northern and Southern hemispheres - as well as equal day and nighttime. At 7:02 ET on March 20, 2013, Earth was at its equinox. At 7:45 ET, the GOES-13 satellite captured this full disk image of Earth.  Click on the image for a larger view.  Image Credit: NOAA.

Both meteorological and astronomical springs have arrived.  The vernal equinox marks the beginning of spring in the Northern Hemisphere.  This is when the vertical rays of the sun are directly over the earth’s equator and crosses the equator on its northward journey.

Funny, it doesn’t feel like spring.  Let me check the forecast:

The 5-day forecast for March 20, 2013 for Columbia, SC.  Image Credit: WLTX-TV.

Remember those groundhog forecasts from six weeks ago.  A couple of them got it right, but most got is wrong.  Not only did spring not come early, it does not appear to be in sight.

The 6-10 day outlook prepared on March 19, 2013 for the period of March 25-29.  Notice the unseasonably cold weather in the East.  Click on the image for a larger view.  Image Credit: NOAA/CPC.

March is running almost 2 degrees F below normal in Columbia, South Carolina.  This is in stark contrast to last year when March was almost 10 degrees F above normal.  It was the warmest March on record.

A comparison of March 2012 with March 2013.  Click on the image for a larger view.  Image Credit: Climate Central.

So what is going on?

It relates to a phenomenon in the Arctic known as the Arctic Oscillation.  I have mentioned this before in writing about the winter outlook.  The oscillation has two phases; a warm phase (positive) and a cold phase (negative).  An index has been developed to distinguish between the two phases.

When the Arctic Oscillation is positive the jet stream is stronger and tends to keep the cold Arctic air bottled up over the Polar Regions.  This is what happened last year resulting in the record warm March.  However, in the negative phase the jet stream is weaker and meanders more often.  This allows for outbreaks of very cold air into the mid-latitudes.

Observations of the Arctic Oscillation Index through the winter of 2012-13 and into early spring.  Note the extreme negative numbers recently.  Click on the image for a larger view.  Image Credit: NOAA/CPC.

This month the index has been near record levels on the negative side.  There has been a steady push of cold Arctic air into the Upper-Midwest and into the eastern U.S.  The forecast is for the index to retreat toward neutral over the next two weeks, but the effects may linger through the end of March.  Andrew Freedman of Climate Central has an interesting piece comparing last March and this month.

It is best not to plant anything sensitive to cold until April (after Easter).  There will be another shot at freezing temperatures before the end of March.  The month is likely to be below normal.  You can reassess you planting times at the beginning of April.