All La Nina's are not the same but this one was really different and unusually weak. Normally, waters in the southeast North Pacific are cooler than normal during a La Nina episode characteristic of cool phase Pacific decadal oscillation. But not in 2017. Surface water southwest of California remains unusually warm. This latest "warm blob" prevented, in-part La Nina from realizing more intensity.
A stratospheric warming event occurring Dec. 21-25 across northern Canada caused an arctic air mass to emerge in southern Canada touching the northern U.S. Dec. 24-28. The southern periphery of the arctic cold gained snow cover in the following days with resurgent following arctic air growing more intense as the polar vortex intensified and snow cover widened/deepened. The result for early January in the Midwest/East U.S. was a record to near record cold for the first 6-7 days of the New Year.
Historic cold in the U.S. with 80-year record heat in Australia while conventional storms bring 100 mph wind to New Zealand and Switzerland...why? An unusually strong negative phase of the global atmospheric angular momentum index.