Global Patterns: Influences on NC Snowfall

El Niño-Southern OscillationArctic & North Atlantic OscillationsPacific/North American PatternPacific Decadal OscillationInteractionsInfluences on SnowfallCurrent Conditions
Research at the SCO has revealed that North Carolina snowfall shares connections with global climate patterns on seasonal, monthly, and even daily time scales. North Carolina snowfall is dependent not only on favorable phases of individual global patterns, but also on interactions between the individually favorable patterns, along with other important trends that have significance on NC winter weather.

NAO Seasonal
Average snowfall departure from five NC Stations (yellow) and NAO index departure (shaded)
Seasonal Connections

On a seasonal time scale, NC snowfall departures are inversely related to departures in the NAO. In other words, a negative NAO is often observed with increased snowfall in North Carolina.

The graph on the right shows a comparison of seasonal snowfall departures from normal and NAO departures from normal. The two are negatively correlated (when one goes up, the other goes down) with a correlation coefficient r = -0.39. Although this coefficient is not extremely high (an r value of -1.0 would indicate a perfect negative correlation), it is important to remember that changes in the NAO are only one factor in North Carolina receiving snowfall, and not all negative NAO events are associated with an NC snowfall event.

With that said, knowing a season will have a negative NAO can be a good indicator of above-normal snowfall.

Monthly NC Snowfall/NAO Correlation Monthly Connections

Winter months in NC that average a negative NAO see a significant increase in snow days.

The chart to the right shows the average number of snow days (with ≥1 inch of snow) in central and eastern NC, broken down by NAO phase.

Excluding March, winter months that averaged a negative NAO experienced at least a 25% increase in the number of snow days compared to the 52 year average, while positive NAO months had at least a 25% decrease in snow days.

Daily Connections

On a daily timescale, it appears that trends in the NAO can be used to detect enhanced potential for winter weather in NC. When the NAO has been in a negative phase (allowing cold air to become entrenched across the region), and then begins to increase towards a neutral phase, it appears that significant NC snowfalls become more likely.

Similarly, Kocin and Uccellini (2004) found that Mid-Atlantic snowfalls are common during a "receding NAO" (transition from negative to neutral NAO phase).

How does this work? During a negative NAO event, a strong upper-level low pressure system (in weather terms, a 500 mb polar vortex) is often located near the northeast US or Newfoundland. The position of this cyclone helps drain very cold air from Canada into the eastern U.S. As the polar vortex pulls out (and the NAO transitions toward neutral), if jet stream interactions have resulted in surface cyclogenesis (often along the Gulf Coast/southern U.S. region), the resultant surface low is able to track northward along the east coast of the U.S., often intensifying along the Gulf Stream.

You can see this for yourself by interacting with the chart below. The red line indicates the NAO index and the blue bars show daily snowfall in Raleigh during the January 2000 storm. Hover over a data point in the chart to view the map of 500 mb heights and sea level pressure for 12Z (8 am/et) on that date.

So what do these maps show? Notice the deep upper-level trough (indicated by the blue and white colors extending into the Southeast US) over the east coast around January 21, which then began to lift as the NAO shifted toward neutral. By January 24, a surface low began to strengthen as it crossed over Florida and moved onto the warm waters of the Gulf Stream. The upper-level energy provided by a shortwave trough allowed the low to rapidly intensify on January 25, resulting in a record-setting 20 inch snow in Raleigh.
500 mb heights (shaded) and sea level pressure (contoured)

Seasonal NC Snowfall/NAO CorrelationNAO Wintertime Frequency

Additional research at the SCO has revealed that the NAO appears to go through a full-phase cycle approximately every 20 days. This was determined by performing spectral analysis on daily NAO indices and hourly air pressure data from Raleigh from June 2008 through November 2010. Both indices showed an energetic frequency about every 20 days (0.05 per day).

Pattern Interactions

Global patterns do not occur independently; they occur on overlapping timescales, and have important interactions that can magnify or diminish the effects of an individual pattern.

For example, the warm (positive) phase of the ENSO cycle, better known as El Niño, typically results in a more active southern jet stream, which ultimately leads to increased precipitation across the southeastern U.S. during the winter. When a negative NAO is in place during an El Niño winter, cold, Arctic air is transported towards the southeastern U.S. with enhanced precipitation potential due to the El Niño effect, and research at the SCO has found that the number of snow days in NC increase significantly in all four winter months.

The chart below shows the average number of snow days (with ≥1 inch of snow) in central and eastern NC, broken down by NAO and ENSO phase.

Daily NC Snowfall/NAO Correlation

Our results found that a negative NAO combined with a positive ENSO phase (El Niño) resulted in the most snow days on average, with an increase of 25% (or more) in snow days for all four winter months.

A positive NAO combined with a negative ENSO (La Niña) resulted in the greatest decrease in average snow days. This is due to a lack of cold air (results of a typical positive NAO), and less active subtropical jet stream (results of a typical La Niña).

NAO had the most significant impact on snow days. Even in winter months that featured La Niña conditions (typically warm and dry), combined with a negative NAO, only February saw a decrease in snow days, which suggests that the NAO has a more direct influence on NC snowfall than ENSO. The reason behind this is that the NAO directly impacts the large scale atmospheric pattern over the eastern U.S. on a daily timescale, whereas the ENSO pattern indirectly effects the eastern U.S. atmospheric pattern by altering global circulations, and does so on monthly to seasonal timescales.