Why was last year so snowy? Part II
Winter outlook preview
(Please welcome Wes Junker, who will serve as Capital Weather Gang's winter weather expert moving forward. Read Wes's full bio. This is the second in a series of articles leading up to our 2010-2011 winter outlook, which will be published Thursday)
By Wes Junker, Winter Weather Expert
Yesterday, I discussed how El Nino played a key role in establishing a storm track and moisture feed for generating last winter's historic snows in the mid-Atlantic. But I concluded by emphasizing El Nino did not lead to the epic snow totals by itself.
The help from the north last year came from the Arctic Oscillation (AO) and its cousin the North Atlantic Oscillation (NAO). Both these fluctuations have two phases and they are predicated by the differences in the pressure between the polar vortex and the pressures farther south in the mid-latitudes.
POSITIVE AO/NAO: SNOW REDUCER
When the two indexes are positive and the pressures at high latitudes are lower than those at mid-latitudes, the increased pressure differential between the polar region and the mid-latitudes leads to an increase in the strength of the low level winds that blow from west to east in the mid-latitudes. This causes weather systems to typically move faster. Also, having higher pressure to the south of a high latitude low keeps a southerly component to the wind which prevents Arctic air from being swept southward into the U.S. since air flows from higher to lower pressure.
Importantly, note that a positive AO and NAO index is usually associated with above normal temps across much of the U.S. including the mid-Atlantic States (see below, the warm colors on the bottom two panels indicate areas where the surface temperatures are usually above normal with a positive AO and NAO)
NEGATIVE AO/NAO: SNOW ENHANCER
The opposite is true when the two oscillations are in the negative phase. Pressures then are above normal across the higher latitudes and lower to the south which leads to the winds having a northern component more often than when the opposite is true. Therefore, more Arctic air masses enter the U.S. and temperatures tend to be colder than normal during the negative phase.
The colder air over the Southeast and mid-Atlantic regions leads to a stronger than normal temperature difference between the warmer air over the oceans and the colder air over the land. The anomalously strong temperature differences (gradient) along the coast helps fuel more frequent "Nor'easters".
Last winter, the AO and was more than 4 standard deviations below normal, the NAO was more than 1 standard deviation from normal indicating that the negative phases of each oscillation were unusually strong.
The colder than normal air across the mid-Atlantic States and Ohio Valley and the increased thermal gradient along the coast are two reasons why negative NAO winters tend to be snowier than periods when the oscillation is in its opposite phase. A negative NAO also increases the chances of getting a major snowstorm.
Paul Kocin and Louis Uccellini in their book, Northeast Snowstorms Volume 1, investigated thirty snowstorms that produced at least 10 inches of snow. Twenty-two of the thirty storms occurred with a negative NAO, only 5 were associated with a positive index. They also found a negative correlation between the NAO index and the seasonal snow across the Northeast (including Washington and Baltimore).
Another study done at Rutgers University indicates that there is a 55% increase in snowfall at Reagan National Airport (DCA) when the NAO is negative versus when it is positive. However, the increase in snowfall is 347% when the NAO is strongly negative versus strongly positive (see table below for other locations). Last year both the NAO and AO were extremely negative during the snowstorms (see below)
The December 19th and Feb 5th snowstorms are great examples of how El Nino and a negative NAO can act synergistically to produce a major snowstorm. Last winter, both storms leaped onto the list of Washington's top 10 heaviest snowstorms. The figure below compares the December snowstorm forecast pattern (on left) to the February storm (on right). Note that the above normal pressure (also known as heights, warm colors) are at high latitudes on both with a high in the vicinity of Greenland. Below normal pressures (or heights) are found to the south with a low near or just off the Canadian Maritimes. The configuration is a classic negative AO and NAO.
The high pressure system around Greenland (closed upper level anticyclone) acts as a block that helps hold the intense low near the Canadian Maritimes in place. Note how the lines over New England on both forecasts become closer together as you move from west to east. Meteorologists call such a look confluent flow which is usually an area where air molecules are converging which causes the weight of the atmosphere above the earth's surface to increase. This causes the surface pressures across the northeast to rise and stay higher than the pressures farther south with the low.
When the surface pressure is high to our north and northwest, cold air continues to feed into the area as the surface low is forced to our south. The confluence across New England is one mechanism for keeping the cold air damming across the Metro area making it more likely to be an all snow event rather than one that mixes or changes to rain. A perfect track taking the storm across the south to near Cape Hatteras also was optimal for keeping cold air in place during both storms.
Both maps also show another area of below normal heights across the south. That feature is more likely to occur with an El Nino than during other years. That feature is our southern stream storm that gives us moisture and brings a surface low towards the east coast while the negative NAO helps provide adequate cold air for snow.
The February 1983 blizzard, another top 10 snowstorm during another El Nino winter, had the same basic NAO look with a distinct southern stream wave.
Random luck comes into play in a couple of ways. While it's not unusual to have a season where the NAO and AO are negative for much of the winter, much of the 1960's featured a predominantly negative oscillation, having both indexes so strongly negative for such long stretches is rare and the reasons why may be due to random chance or serendipity. The Arctic oscillation reached record negative levels last winter. Why these levels were so negative is anyone's guess.
Also, while the phases of the southern and northern oscillations favored above normal snow, there have been other years when both were in the same phases that produced significantly less snow than last winter. How much snow is produced by an individual storm is partly governed by luck. Both these storms were associated with impressive plumes of moisture that came from very low latitudes making them extremely wet.
Later in February and March, the heavy snow was confined to our north because the timing and the location of the various players were a little out of phase for the DC area to get a big storm. During most years, that is the norm. The timing of the system in December and again in early February were perfect for getting major snowstorms last winter. We were lucky, or unlucky depending on your point of view about snow.
| November 2, 2010; 11:25 AM ET
Categories: Capital Weather Gang, Snowmageddon, Winter Storms
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