Friday, January 5, 2018

Tropospheric Polar Vortex Dislodged, But Warmth To Prevail

Medium-range forecasts have the tropospheric polar vortex dislodged from the Arctic Circle, typically an opportunity for a colder than normal period in the United States, but model guidance foresees abundant ridging in the Rockies to disperse any such opportunities.

Both the GFS and ECMWF models, in their 8-10 day 500-millibar geopotential height anomaly graphics, show strong ridging pushing into the Arctic Circle, forcing pockets of cold air to lower latitudes. However, with a strong ridge over the Atlantic in a less-than-optimal position and a broad ridge over the Rockies just screaming to bleed east, any opportunities for cold weather look to be transient.

The ridge in the Atlantic looks to try and push northward, but an area of well below-normal geopotential height anomalies just east of Greenland will not only stop the ridge from moving north, but due to its positioning near Greenland will also promote conditions similar to the positive phase of the North Atlantic Oscillation (NAO). Such a pattern is conducive for warmer than normal conditions over the Central and Eastern U.S.

Also helping prospects for warmer than normal weather in the eastern two-thirds of the country is the ridge over the Rockies, something that may be recognized as the positive phase of the Pacific North American (PNA) index, but this is a 'false positive'. The 'passive' nature of this ridge, as seen in its relative weakness compared to the ridge over the Atlantic and the trough over the Aleutian Islands of Alaska, will let the Pacific jet stream just ride over the ridge and *not* proceed to plummet south into the Plains, as would typically happen in a +PNA set-up. Instead, it looks like the jet stream will be coerced into staying north, possibly even meandering in Canada in this timeframe, storing the colder air up there and in the three lobes of the tropospheric polar vortex, expected to be over the Aleutian Islands, northwest Europe and northern Eurasia.

In the longer-term, prospects for sustained colder than normal weather look similarly-cloudy, with model guidance foreseeing the Madden-Julian Oscillation (MJO) transitioning to Phase 4 by the middle of the month. Phases 4-6 are notorious for encouraging warmer than normal conditions in the eastern two-thirds of the United States during the winter month, and as such, further shots of cold air may prove to be transient through the end of January.

To Summarize:
- Following the recent period of colder than normal weather, warmer than normal (if not seasonal) temperatures for much of the country are expected for much of the remainder of January.