Sunday, December 2, 2012

ECMWF Corals Polar Vortex into North America

ECMWF 8-10 500mb height anomaly forecast

ECMWF Ensembles 10 day 500mb height anomaly forecast
Image from WeatherBell

The latest 8-10 day forecast from the 0z ECMWF and Day 10 forecast from the 0z ECMWF Ensembles (EPS) shows that the Arctic will be changing its game plan in coming days, with forecasts calling for a strong ridging system of high pressure anomalies to be setting up from the Bering Sea to North Asia, North Europe and Greenland in what could be a dazzling display of flexibility as far as ability to change the entire field of the North Hemisphere goes.

If these forecasts of a horseshoe-style high pressure arrangement actually happen, there is only one option for the polar vortex (a semi-permanent low pressure system over the Arctic, the base of the Arctic Oscillation): It must be forced south into North America. If the polar vortex is forced south into North America, severe cold could flood the region.

Arctic Oscillation (AO) forecasts for the long range from the Climate Prediction Center suggest that there remains a very wide spread (range between forecasts) among the ensemble members forecasting this oscillation. However, the consensus does appear to be for the AO to stay mainly negative, although a trend towards a weaker negative phase appears to be in order.

Image from Weatherbell.
The 12z GFS ensembles show their variance as well in the long range, suggesting that the AO will remain negative, but at a moderate negative stance and not as strongly negative as we have seen in recent weeks.

If the polar vortex does indeed come into North America, the US and Canada will be visited by many more instances of cold and snow, especially if that horseshoe blocking pattern does not dissipate for an extended period of time.

Andrew