Positive NAO Responsible for Snow Drought

Positive Phase of the North Atlantic Oscillation: Arctic Air Currently "Locked Up" in Alaska and Greenland
  • Last time the NAO Index was neutral was back in late October when parts of the DC Area received one to four inches of snow.
  • Since the late October snow storm the NAO index has been positive.
  • Forecast ensemble models (see graph below depicting red lines) predict the NAO index may go neutral or negative towards the end of January.
  • In order to get a major east coast snow storm, you must have blocking over Greenland (a massive area of High Pressure) found during a negative NAO oscillation.

Ski resorts are suffering in the Mid-Atlantic and New England (Northeastern United States):

Snow Drought Continues across portions of the Mid-Atlantic & southern Appalachians (Courtesy: NOAA)
Snow Drought continues across New England & New York State (Courtesy: NOAA)

Temperature Anomalies During a Positive NAO Index:

** Other factors contributing to the extremely mild start to winter has been the Arctic Oscillation (AO) index which has also been positive favoring strong westerlies over the high latitudes.  The lack of an amplified jet stream (lack of high latitude blocking/strong westerlies) has kept all the arctic air locked up in Alaska, extreme northern Canada and Greenland.

** Notice that the AO is beginning to drop drastically over the next several days indicative of the arctic blast heading to the Midwest, Mid-Atlantic and New England Monday through Wednesday (the end of the black line in the chart below shows the current AO index; the red line depicts where the multiple forecast models are predicting the index to be in the future).


3 thoughts on “Positive NAO Responsible for Snow Drought

  1. NAO oscillation is positive when an area of low pressure sits over Greenland and an area of High Pressure over the Canary Islands… making for a very fast, progressive, westerly flow (the jet stream remains in zonal flow and storms can’t slow down along the east coast)

    NAO oscillation is negative when an area of High Pressure sits over Greenland and an area of anomalous low pressure over the Canary Islands… this is called blocking (it’s like throwing a bolder in a fast moving stream) and it causes the flow in the atmosphere to buckle with a very amplified trough and ridge pattern. The much deeper troughs and ridges produce large mid-latitude cyclones that move slowly due to the blocking area of high pressure over Greenland. They are much slower to leave producing the famous Nor’easters that slam the Northeast Boston-Washington Megalopolis. “Winter’s Hurricane”.

  2. Tim,
    Awesome overview! I’m teaching now and am currently on a unit about meteorology. Could you add a dummies definition of NAO Index?

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