NOAA NCEP’s temperature anomaly forecast for North America and Eurasia for February 2013 – higher than average chances of cold spells for Northeastern US and Northern Europe, comparable to current mid-January weather charts. Skill mask shows forecast reliability is relatively high (close to 100% for February 2013).

ENSO has shown a mixed picture for months. In November 2012 an El Niño remnant (as negative sea surface temperatures (SSTs) was still visible around Australia, setting in place the current drought. It has since disappeared – and now La Niña-like negative SSTs dominate the northeastern Pacific, influencing winter scenarios for North America and possibly other parts of the northern hemisphere winter.

Cold air for northern US – Atlantic depressions?

With high pressure systems dominating over the Northeast Pacific chances for cold air snaps across southern Canada en the northern US increase. Combined with still relatively high SSTs along much of the North American Atlantic this scenario favours the formation of Atlantic depressions – sea current NOAA observed SSTs below:

Weakest solar maximum now qualifies as solar minimum

Meanwhile the current solar maximum seems to have almost collapsed, with the observed (NASA) sunspot number dropping below 50 – no longer qualifying as elevated solar activity – therefore high pressure build-up at boreal (sub-Arctic) latitudes is not hindered*.

[*) Such hindrance of high solar activity is weak anyway, but statistical studies do show high sunspot numbers can be correlated with both positive NAO and positive AO – and solar minima with the negative counterparts.]

Negative Arctic Oscillation versus Atlantic depressions: ambivalent NAO

A winter scenario with depressions over the Arctic Ocean and over the Northern Atlantic can lead to a near-neutral North Atlantic Oscillation (NAO). This scenario combines well with a dominant high pressure system over Siberia and northern Russia – and a cold winter for Scandinavia.

For West Europe this scenario is different. Cold easterlies are interrupted by the influence of Atlantic depressions. When these pass across Southern Europe such depressions can bring significant amounts of snow over West and Central Europe. But when they linger on the European coast [as shown in below GFS forecast for Tuesday 22 January 2013 – core moving from Ireland to France] southern winds bring in mild air and much of West Europe can experience thaw attacks.

Thaw attack for West Europe as Atlantic depression moves from Ireland to France and low countries and Germany experience (temporary) influx of milder air from the south – carrying snow and rain. GFS forecast for January 22.

© Rolf Schuttenhelm | www.bitsofscience.org