Cold shot increasingly likely in the Northeast this weekend

A few weeks ago, while stagnant warmth continued to sit stubbornly East of the Mississippi River, the hemispheric weather pattern underwent a significant change. A pattern in the Pacific Ocean that had (not coincidentally) also been stagnant, finally budged. A large-scale retrogression of the wave pattern resulted in a dramatically changed pattern both in the Pacific and throughout the United States.

Two weeks later, an anomalous ridge has formed in the North/Central Pacific Ocean (known more affectionately as a -EPO) and has effectively dislodged arctic air from the higher latitudes into British Columbia, Western Canada, and the Northwestern United States. This has resulted in a large, persistently colder than normal temperature anomaly across these regions.

As we have discussed, this cold air has struggled to move eastward with any intensity. Airmasses have modified during their eastward movement, a result of a lack of high latitude blocking on the Atlantic side. Over the next few days, however, the alignment of the ridge in the North Pacific Ocean will change, and some brief help from the high latitude pattern in Central and Eastern Canada will allow cold air to slide eastward.

This will be a  huge change compared to what has been observed over the last several weeks. Forecast model and ensemble guidance is in good agreement on the idea that the cold airmass will be impressive (temperatures 4 to 8 degrees below average) but will also be expansive, with colder than normal air stretching from Montana into New England. The southward extent of the cold will be limited by a Southeast Ridge and — you guessed it — a lack of high latitude blocking.

Still, that doesn’t take away from the importance of the cold shot itself. ECMWF EPS suggest that 7-day temperature anomalies may reach 3 to 6 degrees below normal over quite a large area, and while we will have to closely monitor the intensity of the cold, such an anomaly would be quite an impressive feat in a sea of warm temperatures that has existed since August.

The longevity and duration of this cold shot is a whole other story. Forecast model and ensemble guidance, as well as an assist from tropical forcing and another reshuffle of the Pacific pattern, suggests the cold will be quite short lived. In fact, ridging with warmer temperatures could return to the East as quickly as Day 10. Still – there are signals that other pattern drivers could kick into action.

The ECMWF EPS for example, continue to suggest high latitude ridging will develop from Greenland towards the Davis Straight region. Such an occurrence would significantly disrupt circulations to the point where colder air could be forced southward into the Great Lakes and New England. Tropical forcing orientation suggests that this development isn’t simply something to be ignored.

We will be watching carefully for the development of high latitude blocking late in November, as it will serve to have a significant impact on the forecast moving forward.