Winter 2013/14 Weather Discussion

[Ban my account ffs!]

Various sources have been releasing their forecasts for the coming winter in the past month or so, and there is plenty of disagreement.

Unlike in the past 15-20 years, where a dominant El Niño or La Niña was present in the tropical Pacific ocean, both last winter, and this summer/fall have seen the tropical Pacific in a relatively benign, "neutral" phase.

So here are the factors going into winter 2013/14:

Low solar activity:  We recently passed the maximum of solar cycle 24 and it was the weakest solar maximum in a century.  All indications are that solar activity will gradually weaken.

This is important because there is a link between low solar activity and blocking of the jet stream, which favors cold air moving into the U.S., especially in the central and eastern U.S.  It would also favor cold weather in Europe with a southerly storm track there affecting Spain, France, and Italy rather than points north.

During the very high solar activity of the 1980s, 90s, and early 2000s, the polar vortex (the large naturally occurring low pressure center in the Arctic during winter) was strengthened along with the subtropical high pressure systems that are semi-permanent.  This causes the jet stream to move northward and flatten out, driving strong storms into the west coast and into northern Europe.

This is measured in the Arctic and North atlantic Oscillations (AO and NAO), which were in a strong positive phase for much of the 80s, 90s, and early 2000s.  Now the opposite is occurring, with a much weaker polar vortex which can even be replaced by high pressure in Greenland, and the weakening and push southward of the subtropical high pressure zones.  Storms are weaker, but with more cold air being funneled in from the north... it might not seem that way since large areas can see heavy snow and bitter cold.  The cold, snowy winters in the UK, for example, tend to be drier than average overall.  The same is somewhat true for the eastern seaboard of the U.S.

Winter 2013/14 is likely to see lots of variability, but with a bias towards cold and snow.  Accuweather is forecasting a very stormy/snowy winter in the western U.S. with plenty of cold while the eastern 1/3 of the nation might see a slow start to winter with mild conditions in the southeast.

But it will depend on where the blocking occurs.  If it occurs over Greenland and NE Canada, it will likely mean very cold conditions in the eastern U.S.  But if it occurs over the Gulf of Alaska and across SW Alaska, it could mean the core of the cold is in the Rockies and central U.S. with somewhat milder conditions along the coasts.

Some expect weak El Niño conditions to develop, which might favor a stronger southern branch of the jet stream... something that has been ambiguously absent in recent years.  This would favor cool weather in the southern U.S. with wet conditions that might bring beneficial rains to southern California and northern Mexico.  But since other factors will be in play, you shouldn't expect such conditions to prevail all winter.

This year, the madden-julian oscillation (MJO), an index that measures broad areas of enhanced or suppressed tropical convection (read: thunderstorms) that circles the globe, has been quite active.  This means that we will likely see enhanced storminess across the U.S. with plenty of precipitation.  Whether that's rain or snow will depend greatly on the factors at the time the storm passes.  In other words, this just isn't going to be a very predictable winter.  But odds seem to favor a cold one over a very warm one.

In the next couple weeks, a major shot of cold will move into the central and eastern U.S. and will bring snow, including lake effect, to parts of the north-central U.S.  We'll see if this lasts very long... but it will definitely be a reminder of winter for many people.