Wednesday, October 10, 2012

Weather Whys Wednesday: Get in the Know on the EPO

One thing seems fairly certain in the coming week or two: The cold snap we are in the midst of probably will not last to end the month. You can see the Day 8-10 Euro vs. GFS output here. Generally, the Euro outperforms the GFS (though at times, it too can be imperfect). But what you're seeing in that 8-10 day window is generally a zonal type flow. This is a trademark look of a positive phase of the EPO, or Eastern Pacific Oscillation. And you can see that as we go into much of next week, we're dominated by a +EPO look on the models. So in general, this has a pretty strong correlation to US weather. So let's talk about it a bit.

-EPO Temperature Correlation Map for October
Invert colors (purple = warm, red = cold) for +EPO temps
Credit: NCEP/NCAR
What is it? Think of the EPO sort of like a Pacific cousin of the NAO. The EPO is a good measure of what the upper wind flow is doing and tends to correlate well to downstream impacts. When the EPO is in a positive phase, you typically see troughing or zonal winds on the West Coast, which translates to the Lower 48 being "flooded" by Pacific air. Anyone who is a weather enthusiast, especially with regard to winter, knows that this does not bode well for snow and cold. These sorts of patterns are dominated by fast flow, often some weather systems get embedded in the flow and it can be almost a "dirty" type pattern, with cloud cover and mild temps. Alternatively, you can occasionally get fair weather in that sort of pattern this time of year, but I tend to view it as more of a cloudy, mild to occasionally warm sort of pattern. When the EPO shifts to a negative phase, this can lead to a ridge in the Gulf of Alaska, which is generally good for a trough downstream somewhere, specifically in the Eastern US, and as a result, cold air.

Why does it matter? A lot of people tend to neglect the EPO as a major consideration in forecasting, as much laboring will be done over the AO or NAO...the more discussed and relatable correlations to Eastern US cold. But given that the EPO is upstream from here, it tends to be a decent gauge and have a big impact on what may occur in our neighborhood. In fact, the NWS in Detroit, which often puts together very detailed and forward thinking seasonal outlooks, discussed the relationship at length a few years back.

So when you see it getting warmer next week, know that the EPO helped to figure out it was coming, and you may want to start paying closer attention the EPO as the fall transitions into the winter.