We can break the ENSO (a cycle of sea surface temperatures in the central pacific), up into 3 distinct phases. El Nino, which is characterized by 5 or more periods (1 period is defined as a spell of 3 months), during which sea surface temperatures extending westwards from Peru into the central and western Pacific, are warmer than average. ENSO neutral is, near normal temps and La Nina is a colder than average temperatures in the mentioned region.
Winter 2017 Forecast
Whether a La Nina, El Nino or ENSO neutral is in place, all 3 have very distinct, different affects on the weather, especially for equatorial regions, an EL Nino dries out Australia and increases precipitation rates in Peru and the Pacific Ocean, whereas La Nina causes increased rainfall in Australia and decreases precipitation rates near Peru and the Pacific Ocean. These patterns, no doubt, also have massive affects on the northern hemisphere jet stream, so is a major determining factor for winter weather in the United States, Canada and the UK and Ireland.
We can not just rely on ENSO to gauge a winter forecast. Lots of other factors like solar activity, northern hemisphere snow cover extent, etc, need to be taken into consideration. Using ENSO alone is not enough to “judge” the atmosphere. Lets get into it then. For Winter 2017/18, long range forecasting models are now trending towards the La Nina side of ENSO.
For the United States & Canada
Like Winter 2016/17 when a weak La Nina was in place during the start of the winter, which fizzled out later on, the north west was favored for snowfall, with some upland regions experiencing a heck of a lot more snowfall than you’d usually expect. Partly due to blame is of course the La Nina, which in theory increased the amount of high pressure further east, meaning the east coast of the U.S didn’t quite see as much snowfall as we were expecting. The strength of the “North East Pacific High” largely determines winter weather in the US.
La Nina, weakens the strength of this high pressure system, allowing Arctic air to feed into western Canada and north western states of the US, like Montana, Idaho etc. El Nino, strengthens the polar jet stream and allows parts of the “polar vortex” (not the whole thing, the polar vortex core always remains in situ, just “parts” of it sink south), to head into north eastern US & Canada, like we saw in 2014-15 during which time a weak to moderate El Nino was in place. When things go to the extreme, things often change regarding the resultant weather patterns. The El Nino that developed during winter 2015-16 was exceptional and, one of the two strongest on record. This often goes “above and beyond” in terms of strengthening the nor’easters, meaning they are so “big” that they start on the east coast and finish over the British Isles.
The resultant weather pattern meant that 2015-16 actually wasn’t a beast of a winter, like many thought (although there were a few potent nor’easters), as the USA El Nino pattern grew so large due to the record breaking El Nino that it covered almost the entire northern hemisphere.
La Nina increasing in strength over Summer 2017:
But what about this winter 2017-18, La Nina?
As you can see from the graph below, temperatures are forecast to slide away in the ENSO (El Nina/La Nina), region and a La Nina could well be designated later in the year. La Nina is known for bringing a potent winter to the north west so, like last year, I expect this region could well be affected by above average snowfall and below average temperatures, in fact we have already seen some significant falls above 4,000ft. It’s a little more complicated for central, lakes & eastern regions. Although it is well too early to issue any sort of forecast, I suspect this La Nina could promote some dry weather, for the North East, but I expect the strongest week to week variation to occur here with the potential for some notable blizzards. It may sound vague, however long range forecasting is a tough & uncertain field, never are long range forecasts 100% accurate.
What about the UK & Ireland?
A La Nina is certainly better news for snow and cold weather in this region, ENSO implications are much weaker here, in comparison to the United States, however combined with the QBO (set of winds in the upper atmosphere), trending towards it’s negative (easterly) phase, then I suspect the word “snow” could (potentially), be used a little more often this winter than the previous 4. We shall see what happens for the Winter 2017-2018 forecast.