The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or 
This past winter, many in the middle Atlantic have stated that their most severe events (such as the President’s day weekend storm) occurred while the NAO was positive, this, while true is not what it may seem. The fact is the very cold area of high pressure which formed over northern New England is what made the president’s day storm possible. It supplied cold air at the surface and aloft so that as the precipitation moved northward, there was cold air available to result in a snow event, and allowed for the system to take a slightly suppressed track before re-developing off the east coast and then coming northward, favorable for dumping heavy amounts of snow in the major cities. Otherwise president’s weekend 2003 may have been little more than a heavy rain event like many during the 1997/98 winter. The same can also be said for the blizzard of ’83.
The high pressure to the north also allowed for the formation of a very impressive cold air damming situation east of the mountains, further precluding any changeover to mixed precipitation in the major cities, during the period in which the heaviest precipitation occurred. Some areas further to the south and east along the New Jersey coast, the Delmarva, and eastern portions of Maryland did see some sleet and freezing rain as the 850Mb low jumped to the Delmarva on the morning of the 17th.
Even though the event was characterized by little in the way of upper air and dynamical support, the interaction between the persistent sub-tropical moisture feed, intense easterly LLJ and the cold high to the north (allowed to remain in place thanks to the negative NAO) resulted in a record breaking snow event for the major cities. Recall, Boston broke its previous single storm record set during the blizzard of 78.
It was not until the NAO switched to positive was the high pressure area was allowed to move out of New England, and allow the weak area of coastal low pressure to move more northeastward and bring heavy amounts of snow to New England…instead of just moving directly east off of the Delmarva after re-developing.
In order for us to better understand the effects of both NAO phases, we must understand the forcing mechanisms particular to both phases of the North Atlantic Oscillation.
As we all know, the NAO has two phases, Positive and Negative.
In the Positive Phase, the Icelandic low and Azores high become particularly strong which results in a tightening of the pressure gradient across the Atlantic, therefore, the westerlies strengthen and lift northward between the Icelandic low and sub-tropical high, resulting in a storm track across the far north Atlantic. The flow across the Atlantic is mostly zonal, perpetuating the tendency for cold air to be drained off North America, resulting in mild…dry conditions. The effect is similar across Europe, warm dry conditions are prevalent across southern Europe and the Mediterranean, while mild, and wetter than normal conditions develop across northern Europe. Additionally, the very strong sub-tropical high can result in the strengthening of the trade winds across the tropical Atlantic, advecting significant amounts of Sahara dust westward across the tropical regions, in addition…the amount of ice flux increases across the Denmark strait, Greenland, and Norwegian seas.
Link to dataset:
http://www.cdc.noaa.gov/correlation/nao.data
500MB height in December with a Positive NAO:
http://www.cdc.noaa.gov/cgi-bin/Composi ... reate+Plot
500MB height in January with a Positive NAO:
http://www.cdc.noaa.gov/cgi-bin/Composi ... reate+Plot
500MB height in February with a Positive NAO:
http://www.cdc.noaa.gov/cgi-bin/Composi ... reate+Plot
On the other hand, in the negative phase, the strong Icelandic low and sub-tropical high weaken, which resulting in a slowing, and southward displacement of the westerlies across the Atlantic. at times, the Icelandic low and sub-tropical high can become very weak, allowing blocking high pressure to form across the far north Atlantic (which can take on three different forms), the storm track becomes displaced to the south which would therefore allow storm systems to cross the north Atlantic at a lower latitude, producing radically different effects across eastern north America and Europe as compared to what is specific to the NAO positive phase. Additionally, a significantly decreased number of storm systems are noted across the Atlantic during periods where the NAO is negative. The southward displacement of the westerlies across the Atlantic directs the main storm track at southern Europe and the Mediterranean, resulting in below normal temperatures and above normal precipitation, while the highly amplified pattern directs Siberian air westward into Europe resulting in extreme cold. Similarly, on the North America side, a deep trough forms across the eastern part of the country which, suppresses the storm track, and promotes the invasion of extremely cold air into the region. During these periods, the PV can become displaced very far to the south specifically near Hudson’s bay, and/or significant low pressure can take up territory near Newfoundland (which is known as the 50/50 low).
It is the Negative phase of the North Atlantic oscillation in combination with a positive PNA pattern which contributes to the coldest weather pattern possible for the eastern half of the United States. Furthermore, the amount of ice flux across the Denmark Strait, Greenland Sea, and Norwegian Sea is reduced. The trade winds across the tropical Atlantic (easterlies) are also weaker, resulting in warmer than averaged conditions across those locations.
The three forms of North Atlantic blocking characteristic of the Negative phase of the North Atlantic Oscillation are defined as follows:
The first is the classic Greenland block, where the strongest above normal height center relative to normal is located across Greenland. The second manifestation of a negative NAO finds the greatest above normal height center located over the Davis Strait, Baffin Island, and Northeastern Quebec. Lastly, the above normal height center can be located as far east as Ireland and England, and would still count as a negative NAO.
500MB height in December with a Negative NAO:
http://www.cdc.noaa.gov/cgi-bin/Composi ... reate+Plot
500MB height in January with a Negative NAO:
http://www.cdc.noaa.gov/cgi-bin/Composi ... reate+Plot
500MB height in February with a Negative NAO:
http://www.cdc.noaa.gov/cgi-bin/Composi ... reate+Plot
Predictors:
There are many methods which one can use to predict the tendencies of the North Atlantic oscillation. The most common (and those which I use the most) are listed below:
IT SHOULD BE NOTED THAT WHILE, YES IT IS POSSIBLE TO PREDICT THE TENDENCIES OF THE NORTH ATLANTIC OSCILLATION ON A SHORT TERM AND SEASONAL SCALE WITH REASONABLE SKILL, THERE IS NO 100% GUARANTEED METHOD.
Atlantic Decadal/Multi-decadal Thermohaline Circulation:
The Atlantic Thermohaline circulation is manifested in the variation in strength of the northern hemispheric thermal gradient, and transportation of ocean energy. In periods where the ATC is weak, the difference in temperature between the Polar Regions and the tropics is greater than average, which results in an increase in the strength of the westerlies across the northern hemisphere, less even distribution of heat and energy resulting in a mostly positive NAO (and AO as well). When the ATC is in its strong cycle, oceanic heat and energy is dispersed more evenly…allowing the strength of the NH thermal gradient to weaken, slowing the westerlies and promoting a general tendency for a Negative NAO. Normally hurricane activity is increased during the ATC strong cycle and diminished in the weak cycle.
Short term variations in the ATC are common within the longer term warm (weak) and cold (strong) cycles.
Atlantic (tripole) SST configuration:
It should be noted that the ATC and Atlantic tripole SSTA configuration go pretty much hand in hand.
Research has shown that…periods in which the Atlantic SSTA tripole configuration features warm water in the far north Atlantic, and in the equatorial Atlantic, with cold water in-between to favor the Negative Phase of the NAO. During these periods…storm systems develop off the east coast and move northward to a point…then cut eastward instead of moving directly northward and deepening toward Greenland. This helps to pump heat northward resulting in ridging and building heights in the north Atlantic consistent with a negative NAO. This set-up is most consistent with the ATC strong cycle. When this signal is reversed…and cold water takes up position in the far north Atlantic and tropical region with slightly warmer water in the center…storm systems move directly northward toward Greenland as they deepen… lowering heights across the north Atlantic and resulting in a stronger Icelandic low (consistent with the Positive Phase of the NAO). This SSTA set-up is most commonly observed in the ATC weak cycle.
Solar Activity:
High geomagnetic activity released from Coronial holes created by recent solar flares on the sun when directed in the path of earth is drawn to the earth’s magnetic poles causing resulting in a weakening of the Aleutian low and strengthening of the Icelandic low…and the positive phase of the NAO. 10.7cm radio flux has more significant impacts on the mid-latitudes.
Stratospheric Temperature:
This idea presumes that a release of stratospheric heat over the Polar Regions will eventually work its way downward through the atmosphere resulting in warming…building heights and thermal ridging.
