A detailed look at this winter's biggest players
Exploring La Nina & its interplay with the NAO
Last week, I wrote about how El Nino and the negative phase North Atlantic Oscillation (NAO) optimally came together to produce last winter's historic snows.
The pattern has flipped this year to a moderate-to-strong La Nina, characterized by colder than normal sea surface temperatures across the east and central Pacific (caused by stronger than normal easterly trade winds). These conditions are expected to continue through the winter - almost certainly leading to less snow than last winter in the mid-Atlantic. But how much less? It significantly depends on the phase of the NAO, a major factor in our snow potential, and a big wild card.
In our winter outlook, we walked through some of this but if you want to understand the full range of possibilities for the upcoming winter you can dig deeper. Let's take a really close look at how this moderate-to-strong La Nina may influence the evolution of this winter's weather while factoring in the curve balls the NAO might throw...
The typical La Nina pattern has several characteristics that have a marked impact on the weather over the United States.
There is usually a strong polar jet stream (purple wiggly line above) with a lack of a storm track across the South. The position of the primary ridge (or peak in the jet stream) across North America is usually farther west (shown all the way up over Alaska in the above image) than during an El Nino season. This places a second upper-level ridge (follow the purple line) across the South and Southeast (more subtle, beneath the arrow in the image above) which leads to a storm track that forces low pressure systems to track to the northeast towards the Ohio Valley and Great Lakes Region before reforming off the Northeast coast.
Such a storm track has two impacts for the mid-Atlantic region. One is that the southerly low-level winds to the east of a storm will tend to pull warmer than normal air northward into the region as a storm moves to the north and west of the Washington/Baltimore area. The track of the low also keeps the heavier precipitation to our north and west.
The impact is exacerbated during a strong La Nina like the current one.
Normal conditions across the Southeast and mid-Atlantic are more pronounced during strong events than during weak ones. Across the Ohio Valley region, the precipitation surplus increases compared to average during the stronger events (see below). Also note that during strong events, the Pacific Northwest and northern Rocky Mountain region often are wetter than normal.
When the ridge gains amplitude (gets bigger) and extends northward into Alaska (as shown in the top image), cold Arctic highs tend to develop and then settle southward into the West or southeastward into the northern Plains depending on the ridge position. This tends to lead to below normal temperatures on average across the Northern Plains.
Some of this cold air makes it into the East especially during the weaker La Nina events causing the weather across the Northeast and portions of the mid-Atlantic to alternate between colder and warmer than normal conditions. While these temperature fluctuations are also present during strong La Nina events, they are less frequent and, across the mid-Atlantic region on average, there are more warm periods than cold ones.
You can also see from the figure above that the strength of the La Nina seems to matter in terms of temperatures with the strong La Nina year averaging quite a bit warmer than the weaker La Nina years.
For that reason, we'll be watching the evolution of this year's event closely for any signs of weakening or signs that a surge of tropical convection might be getting farther east than the norm during a strong La Nina event. When more convection gets to near the dateline, it can sometimes produce changes in the weather pattern for the East.
The combination of an increased number of Arctic air masses and an active jet stream that often impinges upon the Pacific Northwest leads to the potential for increased snowfall across the western U.S, especially the Pacific Northwest and Northern Rockies compared to normal.
Across the East, snowfall averages a little below normal across the mid-Atlantic but also averages a little above normal over portions of the Northeast, especially northern New England.
As I discussed last week the Arctic and North Atlantic Oscillations have strong impact on the temperatures across the U.S during winter.
National Airport has a 55% percent increase in snowfall during a negative NAO versus a positive one. When the NAO is strongly negative versus strongly positive, the increase is over 300% (http://www.cep.rutgers.edu/~oman/NAO.htm).
Therefore, any winter forecast needs to try and guess the most likely state of these northern oscillations (NAO and AO) which are much trickier to forecast than the forecast of an El Nino or La Nina.
Note the large temperature differences across the Plains into the mid-Atlantic region as the AO switches signs. Those differences make this year's Capital Weather Gang Winter Outlook a lower-confidence forecast than last winter's because there is less certainty about the AO and NAO.
Last year, there were a number of factors suggesting that the AO might be negative for much of the winter. This year, those same factors are no longer present which normally might lead one to think that this year the AO index would be positive.
However, there are also other factors that suggest the NAO (and AO) might be trending toward a period when winters are more likely to have a negative NAO than positive. There may be a decadal cycle now trending to a more negative NAO. The uncertainty in what the AO and NAO might do make this year's forecast a tougher than normal one.
Below is a table showing the snowfall during all the La Nina years since 1949-1950 ranked according to the Multivariate ENSO Index (MEI; note the MEI is negative for La Nina and would be positive for El Nino), an index that takes into account not only sea surface temperatures but also winds. The years highlighted in red had the North Atlantic Oscillation index average positive during each of the four months December-March. That's typically a phase that favors warm temperatures over the U.S. and is the warm, less snowy phase for D.C. The years highlighted in blue were negative NAO years - more favorable for snow and cold in the mid-Atlantic.
During strong La Nina years (the top seven in the table) there seems to be less chance of having a strongly negative NAO while enhancing the chances of a positive NAO. Also years having a strong La Nina (like this year) average less snow than the years with a weaker La Nina (the years below 1964-1965) though the sample size is probably too small to allow one to draw any meaningful conclusions.
Of the strong La Nina winters, December and March were the two coldest months with temperatures averaging near normal. January and February both averaged 2.5 degrees above normal but with considerable monthly differences as a few years had a cool January followed by a very warm February and vice versa.
Of the twelve years when the MEI was below -1.0 (for the strong events), the average snowfall was 10.3 inches. 75 percent of the strong La Nina years ended the season with less than 12 inches of snow. The heaviest seasonal snowfall was 17.1 during those years and it occurred during a year when the NAO averaged being negative each of the four months December-March. However, one snowstorm can really tip a year's seasonal snowfall. Parts of Southern Maryland received 15-18 inches of snow on January 25, 2000 while DCA only received 9.3 inches from the storm. If you shifted that storm track northwestward a little, Washington D.C. might have ended the season with well over 20 inches of snow for the season, a total well above average. Hence the uncertainty in winter outlooks, especially for snow.
While the Capital Weather Gang is expecting snowfall across the area to average a little below normal and temperatures to average a little above normal, the forecast is far from certain. We'll be monitoring the AO and NAO this winter along with any changes in the position of the upper level ridge in the Pacific that might lead to a colder and snowier pattern than is usually associated with La Nina.
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