The Northeast may still be waiting for spring, but it has already arrived — early, even — in some parts of the United States.

In California and much of the West, the first half of this winter was remarkably warm. Temperatures in many Southern California cities broke records in January, with readings on some days more than 20 degrees above average highs.

The warm western weather was caused in part by changes in the jet stream that also brought Arctic air to the Northeast, across the Midwest and deep into the South. Gulf Coast states saw temperatures plummet in early January and again in February, contributing to the delayed greening in the region.

Some studies have suggested that climate change, specifically loss of Arctic sea ice as that region warms relatively rapidly, may be contributing to these jet stream changes.

Temperatures across southern portions of the Ohio Valley were also extremely warm since the beginning of the year, with Nashville about 8 degrees warmer than normal for the month of February, said Ken Clark, a meteorologist with AccuWeather. A strong ridge of high pressure air contributed to the warming.