So far, despite brief periods of cold the U.S. is super warm for December 2017. The second half of the month according to the midday operational GFS (model) as depicted by Storm Vista Weather Models indicates eastern Canada turns frigid but most of the U.S. remains warmer than normal. What climate factor is the leading cause of the warm start to winter?
The historic 2010-13 cool phase of the PDO was followed by an even more impressive warm phase (2014 into 2017). The extreme PDO signature has ended. Analogs indicate neutral phase will continue for 2018. The analogs indicate that nationally a drier than normal climate will evolve which the 90-day precipitation trend is indicating that potential scenario has already developed.
Earlier in today's forecasts we mentioned stratospheric warming over northern Canada causing some extreme cold into the northern U.S. Christmas week. Now let’s evaluate the role of the MJO. The MJO forecasts vary but Climate Impact sides with the persistence forecast (GFS) for days 4-8/6-10 followed by a progressive MJO toward the Atlantic (as indicated by the ECMWF). The implications are an eastward shift of the anomalous warmth anchored West (recently toward the East Coast while the cold pattern in the East retreats to snow cover. In the 11-15 day period the MJO shift s to the Atlantic and coupled with stratospheric warming leads to an arctic outbreak into the Central U.S. Briefly very warm East ahead of that cold air.
In 2013 State Climatologist Nicholas Bond of the University of Washington noted an unusual warming of the northeast North Pacific which during 2014-15 strengthened and did not weaken until 2016. The large area of warm water correlated to an upper level ridge pattern associated with the downstream polar vortex winter of 2013-14 and 2014-15 and was a major contributor to California drought. A weaker version of this phenomenon formed in 2017 southwest of California (pictured) and has contributed to another drought and record 2017 fire damage.