Summary: So far, despite brief periods of cold the U.S. is super warm for December 2017 (Fig. 1). 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 (Fig. 2). What climate factor is the leading cause of the warm start to winter?
Fig. 1: U.S. temperature anomalies so far in December 2017 across the U.S.
Fig. 2: Storm Vista Weather Models depiction of the 1200 GMT GFS 15-day temperature anomaly forecast across the U.S.
Discussion: Yesterday NOAA/CPC issued a La Nina Advisory stating 80% chance that La Nina would carry through the northern hemisphere winter. The Bureau of Meteorology/Australia issued a La Nina Alert earlier this month. A borderline La Nina climate has been in place for 2-3 months (according to multivariate ENSO index, a measure of the atmospheric reaction to ENSO) therefore not surprising is a general La Nina climate affecting the U.S. during early meteorological winter.
To identify the historical reference (or analog) to judge La Nina influence on U.S. climate we identify La Nina episodes similar to late 2017 using years during the current climate cycle. Climate scientists identify the current climate cycle in 2 ways. Climate Impact Company uses the long-term cycle of the leading modes of climate variability which are El Nino southern oscillation (ENSO), Pacific decadal oscillation (PDO) and Atlantic multi-decadal oscillation (AMO). The long-term cycles of each flipped during the mid-to-late 1990’s and remain in-place today. There is also the climate cycle in which CO2 emissions into the atmosphere began influencing climate which stretches back to the 1980’s.
The current La Nina is weak. Forecasts indicate there is potential for strengthening to moderate intensity but an intense La Nina is very unlikely. Northern hemisphere winter seasons occurring with weak-to-moderate La Nina episodes in the current climate cycle include 2011-12, 2008-09, 2005-06 and 2000-01. What do these analog years say about January and February in the U.S.?
In the current climate cycle mid-winter tends to be very warm when weak-to-moderate La Nina is present (Fig. 3). The cause of a warm signature of this type is low zonal index of Pacific westerly flow aloft and lack of northern U.S. snow cover. The February analog is also warm East and South while the West finishes meteorological winter colder than normal (Fig. 4).
Fig. 3: Weak to moderate La Nina analog years used to predict the January 2018 U.S. temperature anomaly scheme.
Fig. 4: Weak to moderate La Nina analog years used to predict the February 2018 U.S. temperature anomaly scheme.
Are these analog depictions the be-all/end-all for mid-to-late winter 2017-18? Absolutely not! Canada had their snowiest early cold season on record and the climate pattern has shown eagerness to pool arctic air in Canada. The question becomes will there be a mechanism to cause the Canadian chill to drop into the U.S.?
Catalysts to produce a cold outbreak include sudden stratospheric warming episodes or the right phase of the Madden Julian oscillation. These intra-seasonal climate oscillations cause the jet stream patterns to amplify (i.e. strong ridge/strong trough) triggering big storms and following cold. (To follow these risks more closely please contact Climate Impact Company).
As 2017 closes the global oceans remain near record warm (Fig. 5) certainly contributing to warm risk across the U.S. La Nina is here but the warmer than normal ocean surface in the southeast North Pacific is very unusual (given La Nina) and helping the prevailing upper flow in the atmosphere to carry anomalous warm air masses into the U.S.
Fig. 5: A weak to moderate La Nina is developing in the equatorial East Pacific surrounded by unusually warm surface water in both hemispheres.