From Penn State's Michael Mann writing in Real Climate, on the widespread warm temperatures all throughout the country and the early appearance of spring:
The widespread pattern of this warmth is what was so unusual. Usually when one part of the U.S., say the east coast, is experiencing unusually warm weather, other regions, say the Rocky Mountain states, are experiencing unusually cold weather. This has to do with the natural wiggles of the jet stream from one month to the next. However, the pattern we're seeing so far this year, where essentially the entire U.S. is anomalously warm, only occurs when the jet stream has retreated far north from its usual position. As we have noted before (see here and here), there is no way to ascribe any single anomalous weather event, or even an anomalous season, to global warming and climate change. But what we can say is that the temperature pattern we've seen this January is similar to the kind of pattern that models predict as being normal in just a few decades time given some anthropogenic forcing scenarios. Global warming is likely to "load the dice", making the kind of January temperatures that might seem remarkable by past experience increasingly probable, and hence increasingly more frequent.