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EPO. No, it's not contagious, but rather it's an acronym relating to weather and climate. Here's a primer.

EPO: What you need to know

Tyler Hamilton

Friday, January 16, 2015, 2:59 PM - EPO. No, it's not contagious, but rather it's an acronym relating to weather and climate.

Eastern Pacific Oscillation (EPO) - A dipole pattern similar to the NAO in the Atlantic, but located in the eastern Pacific. There is a tendency for heights/pressures/temperatures to be higher to the north and lower to the south in the negative phase and lower to the north and higher to the south in the positive phase. The negative phase corresponds to widespread cooling over central and eastern North America and the positive phase to warming. - Source: Weather Bell Analytics.

I was asked a question on Twitter the other day, and wanted to speak to this specific teleconnection (climate and large scale anomalies and relationships measured over very large distances).

Just a note, it can often be dangerous to rely on a single teleconnection for forecasting — in part because there's a multitude of exchanges, processes, and drivers that move large scale atmospheric processes around the globe.

What's the root cause or driver of that specific teleconnection, or the old chicken and egg conundrum?

Often a myriad other factors such as sea surface temperature anomalies (abnormal ocean temperatures) and other factors can drive ridges and troughs like snow pack, so to talk about what's exactly driving the Eastern Pacific Oscillation (EPO) is a much more complex problem — one I'm not prepared to answer today.

Many different teleconnections exist and are used by forecasters and climatologists alike, but the EPO can be an excellent signal, as it's one of the main teleconnections just west of North America and British Columbia.

The jet stream typically flows west to east and weather downstream of this teleconnection in particular can be influenced by these quasi-stationary pressure ridges (high pressure/higher temperatures), and troughs (low pressure/lower temperatures).

Image: shows an anamalous high pressure system in the Gulf of Alaska; not ideal for building a healthy snow pack and often interrupts the storm parade that's so famous in the Pacific Northwest and coastal sections of British Columbia.

Eastern Canada, when the EPO goes sharply negative, it's time to watch out – there's a predisposition to colder air from Siberia infiltrating Eastern Canada (Winter 2013-2014 *Shudder*).

This is often called cross polar flow (a type of atmospheric flow where cold air advects (transports) over the pole, (often originating in Siberia).  The air remains near the source region's ambient temperature while undergoing very little moderation (warming of an air mass). By the time the frigid air arrives in Central Canada bone-chilling cold can ensue. 

  • The red arrow below references and highlights a textbook example of cross polar flow with an anomalous high pressure ridge (red) over Alaska and the Yukon similar to what we've been seeing for portions of this winter (2014-2015).
  • Often our cold outbreaks in Vancouver occur when this ridge retrogrades (a system moving west while typical storm direction is to the east. Often fully mature cyclones will retrograde and exhibit this reverse motion characteristic as they strengthen and occlude (cold front catches up to a warm front).
    • We may have to keep a close eye on a retrograding ridge by month's end, but large uncertainty exists as of now.

Often systems have trouble infiltrating a blocking ridge (a relatively stationary area of high pressure), and air is forced either above or below the ridge. This case is beautifully illustrated by Weather Network meteorologist Michael Carter.

Courtesy: Michael Carter

Number 1 (circled above), illustrates the trouble Pacific storms and disturbances have when they encounter such prolific blocking. Instead of the consolidated conveyor belt of storms hammering B.C, we get a split flow (a flow pattern that can feature two weaker jet streams, a polar and subtropical variety) situation.

The case above is almost reminiscent of a rex block, or a type of blocking that has an upper trough underneath an upper level ridge.

EPO (+)

Image: Of note, the quasi-stationary low in the Gulf of Alaska favors a strong jet stream and zonal flow across Canada. Favoured by ski hills for building snow and Canadian's sensitive to the cold, it works wonders with warming the hearts of many (literally).

Here's what has transpired with the EPO over the past several days (think: Up, up, up...):

EPO: Has trended sharply positive over the past few days (blue line is verified data), and it's forecast to remain mildly positive until the 20th when a probable shift back into negative territory is likely

Another source/comparison (yellow box below) using another method and looking at climatology/analogs as well; judging by the two visual sources sandwiched between this text, the positive phase will be brief. Some have speculated that the anomalous warm waters in Gulf of Alaska and coastal regions may be partially to blame for the persistent ridging seen on a regular basis.

Do the ensembles agree?

Ensemble: A weather model/computer program that is run many times over to simulate specific weather events (some more likely than others).

Yellow: Ridge Red: Troughs


The GEM ensemble output above highlights the high confidence that ridging will once again resume the typical location snuggling up to coastal B.C, or what we've seen for the past 18 months or so.

The image below should remind you of the image above; troughs and ridges often paint with great accuracy where we're most likely to see above or below seasonal temperatures.


Using the multi-ensemble approach, computer models show temperatures running about 1°C above climate normals with a persistent negative EPO and the subsequent ridging a possible cause. Several other long term climatology forecasts and indices are hinting at similar solutions.

Multi-model ensemble hinting at above normal temperatures for a large portion of Western Canada.

If you're hoping for beautiful blue sunshine I have a disclaimer, though.

Dirty ridging likely

What does this mean? How can a ridge be dirty? Isn't high pressure sunny and beautiful?

Well, unlike the beautiful high pressure systems Vancouver is known for in the summer, low cloud/fog/stratus have a very tough time burning off with the lower angle of the sun and less range in diurnal (daily temperature cycle) heating and cooling. Sometimes systems and regions of moisture can be injected into regions of the ridge, bringing trace amounts of precipitation to YVR.

Inversions (temperatures warming with height), are also possible in this type of weather pattern, so temperatures in the alpine may skyrocket — something to watch next week.

Temperatures here, give a simulation of alpine temperatures (1500 metres) late next week (with temperatures further south towards Puget Sound approaching double digit warmth).

Q: Vancouver? Can the effects of an -EPO be felt at such a localized level?

A:Yes, and local ski-hills need to be prudent after the system on Sunday and protect their snow base with the appropriate grooming and mountain practices.

Mountain snow forecast

Another 20-30 cm + of fresh powder is possible through Sunday night, but it looks like the only significant snow in the near future:

Take a look for further assurance. When the EPO trends negative, all hopes of an active storm track dry up...

I'm not saying the EPO will remain negative throughout the duration of the winter. There will be heavy rains, strong winds, and high elevation snows to come, but the snow pack as an average will likely measure up below normal for many regions around the South Coast this winter season.

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