When forecasting the weather, a meteorologist takes a look at not only small-scale features affecting a state or region, but also larger-scale patterns affecting entire continents, or even the whole hemisphere! It is these larger patterns that steer the smaller systems that bring our day-to-day weather. This means we need to get a full picture of the atmosphere before making a forecast, especially more than a few days in advance. The northern hemisphere is huge; however, there are a number of indices, or “teleconnections”, that can help meteorologists quickly dissect what the large-scale weather pattern looks like.
The AO: Arctic Oscillation
The arctic oscillation, or AO tells us if there is unusually high pressure or low pressure over the polar region of the northern hemisphere. A positive arctic oscillation features low pressure or a trough in the jet stream close to the pole, and high pressure / ridging in the jet stream in the mid-latitudes; this is a generally mild pattern for the eastern United States. A negative arctic oscillation occurs when there is high pressure or ridging over the pole, which supports troughs / lower pressure in the mid-latitudes, a generally colder and more active pattern for the central and eastern United States. The Arctic Oscillation is generally positive in this image, with low pressure (denoted by the blues) over the pole and higher pressure / ridging (shown in the reds) over the mid-latitudes, including the eastern U.S.
The NAO: North Atlantic Oscillation
The North Atlantic Oscillation, or NAO tells us if there is unusually high or low pressure over the North Atlantic. This is similar to the AO, but has a much more focused domain that has big impacts for the East Coast of the U.S., and even the U.K. on the other side of the pond. The NAO is positive when there’s low pressure over Greenland, and is negative when there’s high pressure over Greenland. High pressure over Greenland tends to slow down or “block” storms as they move off the East Coast, favoring colder and stormier conditions over the eastern U.S. In the case above, the NAO is positive, shown by all of the blues over Greenland.
The PNA: Pacific / North American Pattern
The Pacific / North American Pattern, or PNA describes the orientation of troughs (dips) and ridges (bulges) in the jet stream from south of the Aleutian Islands (in Alaska) to the West Coast of the U.S. A positive PNA features a trough over the Aleutian Islands and ridge in the jet stream along the West Coast; a negative PNA, as seen in the image above, features a ridge south of the Aleutian Islands and trough on the West Coast. A positive PNA features colder weather east of the Great Plains, and in this instance the opposite is occurring.
BONUS: The EPO (East Pacific Oscillation)
The East Pacific Oscillation, or EPO tells us if there is a ridge in the jet stream into Alaska or a trough / dip in the jet stream over Alaska. A negative EPO features a ridge over Alaska and favors very cold weather being forced south into the central and eastern U.S. The lead image features a trough over Alaska, shown by the blues, which favors warmth over the central and eastern United States.
To sum it up: The pattern shown at the top of the page is a model forecast valid for January 10th. EVERY index or teleconnection we look at is in the wrong phase for cold weather over the central / eastern United States. Not surprisingly, mild weather has been occurring and big snow will be hard to come by. That's not to say it can't snow (i.e. this upcoming weekend, 1/18-1/19/2020). However, late January looks to be the turning point in this milder pattern, as cold and snow threats likely increase.