How anomalous high pressure developing over Greenland could signal the start of a cold and wintry pattern in NYC

When it comes to the weather in the Northern Hemisphere, you’d better keep your head on a swivel. If you’re not paying close enough attention to what’s going on around you, you’ll fall behind the forecast very quickly. In this case, strong high pressure developing over Greenland will serve as a warning shot that a colder and more wintry weather pattern will arrive in NYC by mid-December.

The weather in Greenland may seem irrelevant at first, but it has a surprisingly strong correlation to what goes on here in NYC. The most popular index that ties our weather together is known as the North Atlantic Oscillation, or NAO. The NAO is an index based on the surface sea-level pressure between the subtropical high and sub polar low.

In layman’s terms, the index monitors ongoing sea level pressures across the North Atlantic and Greenland. A positive NAO features lower pressures and a trough over the North Atlantic, often associated with warmer than normal weather in the NYC Area. A negative NAO features high pressures and a ridge over the North Atlantic, often associated with colder and stormier weather in NYC.

If you follow winter weather or snowfall trends in the NYC Metro Area, you know that a negative NAO is often synonymous with active patterns and snowfall potential. However, not all negative NAO events are made the same. Some are progressive, weak, or fast-moving. Others are strong, anomalous and lead to major pattern impacts.

Which leads us to today. Forecast model guidance over the past few days has come into agreement on a highly anomalous and possibly record-breaking -NAO developing over the North Atlantic and Greenland during the first week of December.

The modeled event is extremely impressive at multiple levels – for example, the ECMWF Ensemble suggests a 3 sigma anomaly event for both 500mb heights and sea level pressures. This is a huge and expansive Greenland ridge and negative NAO signal.

The forecast ridge moves westward into Greenland, towards the Davis Strait and eventually into Central Canada. There is excellent agreement on both the ECMWF and GFS ensemble data that this highly anomalous ridge will develop during the first week of December and persist through mid-month.

What the heck does this have to do with NYC?

When large ridges form over Greenland or the North Atlantic, the entire flow across the Northern Hemisphere is disrupted. Colder air which usually resides over the Arctic regions of Canada is shunted southward into the United States. High pressure sinks southward into the Central United States and eventually towards our area with colder air taking over.

Eventually, the pattern slows down considerably and elongates over Eastern Canada, which lies directly north of us. This puts us in a precarious position for winter weather – deep low level cold is waiting in our area, a high pressure is settled in to our north to keep in place, and the atmospheric flow has been slowed and blocked considerably by the ridging over Greenland to allow slow-moving and strong storms to form.

Need proof? Patterns similar the one being advertised on forecast models have been associated with memorable periods of wintry weather in our area in the past. In fact, a glance at the 500mb composite of all NYC snowstorms over 15″ leads us to a map that looks curiously similar to forecast model simulations for the middle part of December.

How will this all play out, and when?

In the short term, the weather pattern is likely to remain in flux. Temperatures will oscillate back and forth a bit over the next several days. The high latitude block and ridging over Greenland is forecast to develop during the middle part of next week – the first week of December.

From there, we’re very likely to see colder temperatures begin to shift towards the area as we approach December 10th. This will begin a period of below normal temperatures that could last through the holidays. We aren’t expecting any winter weather threats until around December 15th – it will take some time for the cold air to become established.

From mid-December onward, all bets are off. While the intricacies are still yet to be determined, the overall progression of the pattern across the hemisphere suggests that conditions will be much more favorable than usual for colder temperatures and possible winter storms.

Tl;dr: Stay tuned. When it comes to the weather in NYC, keep your head on a swivel. It’ll be very wintry around here before you know it.