AM Update: Warmth returns to east, but for how long?

Good morning and Happy Wednesday to you all. There is something oddly special about the first day of November in the meteorological community. It’s not a holiday, it’s not the start of any official season. But it has significance for many who forecast or follow the weather – it’s the beginning of “winter season” per se, where the forecasts for the weeks and months ahead start to have more significant implications on what we can expect during winter.

As we move into November this year, the weather pattern will be dominated by a large -EPO ridge, developing in the Pacific Ocean as we speak. We discussed yesterday in detail how these EPO ridges can impact the weather pattern throughout the hemisphere. In this case, the large ridge in the Northern Pacific Ocean will act to dislodge colder than normal air from the arctic regions into British Columbia, and eventually the Northwestern United States as well.

These ridges are quite simplistic in their character, at least when it comes to understanding how they work. Large perturbations from the Western/Central Pacific cause these ridges to develop, and as tropical forcing aids a stagnant weather pattern, the ridge amplifies into the Northern Pacific Ocean. As it does so, it disrupts the hemispheric circulations around it (similar to high latitude blocking in a sense) and pushes cold, arctic air away from its usual resting place.

As this occurs, the cold air is dislodged into other regions, in this case British Columbia and Western/NorthWestern USA. What happens from this point forward is the most critical piece of any forecast when an -EPO ridge forms. Where does this air go, exactly? Does it sit stagnant in the Western USA, or are there other hemispheric features present to move the air further?

In our case, for the first 10 to 15 days of November, the airmass will sit stagnant over the Western and Northwestern USA, owing to the absence of any high latitude blocking over Central or Eastern Canada. Without this blocking in place, the airmass will modify (or adapt to its surroundings) as it moves further east into the Central and Eastern USA. In fact, the atmosphere will respond to a trough in the Western/Northwestern USA by developing a ridge in the South/Eastern USA. The result will be above normal temperature anomalies east of the Mississippi River, essentially from Texas to Michigan.

But what happens when high latitude blocking does develop? The changed pattern in Central and Eastern Canada can aid in different airmass movements – and allow cold air to effectively shift from British Columbia into the Northern 1/3 of the USA. This is a critical piece to understanding how -EPO ridges work, because they can act to bring cold air to the Northern 1/3 of the USA if the conditions are right throughout the hemisphere.

Forecast models are hinting at this development towards the second half of November, with cooler risks returning to parts of the Great Lakes and New England as the Southeast USA ridge is battled on its northern fringes. This aspect of the forecast will depend heavily, though, on the development of high latitude blocking to change the flow of air throughout the hemisphere. Without it, the stagnant pattern will continue through Thanksgiving with warmer air lingering in the East.

Forecast model and ensemble guidance should continue to offer hints over the next few days, when utilized in tandem with tropical forcing and other hemispheric pattern drivers to analyze the forecast. Stay tuned!