Network News

X My Profile
View More Activity
The new Washington
Post Weather website
Jump to CWG's
Latest Full Forecast
Outside now? Radar, temps
and more: Weather Wall
Follow us on Twitter (@capitalweather) and become a fan on Facebook
Posted at 10:00 AM ET, 11/11/2010

A detailed look at this winter's biggest players

By Wes Junker

Exploring La Nina & its interplay with the NAO

* Nice weekend ahead: Full Forecast | Cold coming? | Winter Outlook *

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.

la-nina-jet.jpg
Weather patterns associated with La Nina.

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.

strong-v-weak-nina.jpg
Winter precipitation and temperature anomalies (or departures from average) in a strong La Nina (bottom) and a weak La Nina (top).

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.

nina-anomalies.jpg
Winter snowfall anomalies (departures from average) during La Nina.

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.

ao-lanina.jpg
La Nina temperature anomalies (blue indicates colder than normal, orange indicates warmer than normal) during different phases of the Arctic Oscillation

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.

We'll be watching closely what happens during this coming winter using these two sites: NAO Predictions and AO Predictions.

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.

mei-lanina.jpg
Snowfall during recent La Nina winters in D.C. by La Nina strength (indicated by the MEI index), with the strongest La Ninas at the top. Blue highlighted years had a negative NAO, red highlighted years had a positive NAO.

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.

By Wes Junker  | November 11, 2010; 10:00 AM ET
Categories:  Local Climate, Winter Storms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   Del.icio.us   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: High pressure spells carefree weather
Next: A touch of color at the Capitol

Comments

As I posted a few days ago, woolly caterpillars seem to be in deep disagreement about the coming winter, more so than in recent years[when they generally forecast the same type of boring mild winter down here by the Potomac].

Posted by: Bombo47jea | November 11, 2010 11:45 AM | Report abuse

I appreciate reading these details - it's almost as if understanding the source(s) of uncertainty somehow helps remove it. Thanks, Wes!

Posted by: --sg | November 11, 2010 3:02 PM | Report abuse

--sg

Thanks,

There are other uncertainties that are less easy to understand that could help screw up a forecast especially the monthly temperature forecasts. One is the Madden-Julian Oscillation (MJO) which can help flip the pattern from warm to cold in the east. It's predicated on the location and movement of the convection in the equatorial Indian and Pacific Oceans. During MJO events convection shifts from the Indian OCean just off the coast of Africa to the Central Pacific usually in 30-60 days. However, during strong La Ninas the MJO is not usually much of a player. However, it was in 2007-2008 (a strong La Nina) so despite the fact that it's looking like we're going to be in a moderate to strong nina this winter, it still is possible the MJO could play a role in nudging the pattern from warm to cold and vice verse this winter. Unfortunately, our understanding of the MJO has a long way to go and how the MJO impacts on the pattern is not always the same making using it as a forecast tool really tough.

http://www.cpc.noaa.gov/products/precip/CWlink/MJO/MJO_1page_factsheet.pdf

Another is random chance which sometimes can lead to changes in the position of the ridge over North America and the eastern Pacific.

Posted by: wjunker | November 11, 2010 3:35 PM | Report abuse

--sg & Bombo47jea: glad you two could read the article. I am happy to have Wes' understanding of the atmosphere now on the Capital Weather Gang team! It was quite a great, thorough overview, wasn't it?

Posted by: Camden-CapitalWeatherGang | November 11, 2010 3:46 PM | Report abuse

what level of influence and/or uncertainty is introduced by warmer global temperatures and increased water vapor relative to your analogue years?

Posted by: heatmiser | November 11, 2010 4:03 PM | Report abuse

heatmiser,

That's a good question. CPC offers a look at La Nina Climo along with the trend and it's biggest impact is over the central U.S. which is warmer than the La Nina composites.

http://www.cpc.noaa.gov/products/precip/CWlink/ENSO/composites/lanina.djf.temp.gif

I'm not sure how Matt weighted it in the final forecast. I don't think an awful lot as how the overall global temperature (quite warm) may not have that great an impact on specific region. For example, last winter globally was warm but over Europe and parts of Asia and the southeast U.S were quite a bit colder than normal because of the negative NAO.

Posted by: wjunker | November 11, 2010 4:44 PM | Report abuse

For folks (like me) who love looking ahead, this post is great.

That said, if the record in a strong La Nina year, even with a strong negative NAO is 17", it seems unlikely statistically that we'll come near that - correct?

Posted by: josh28 | November 11, 2010 4:46 PM | Report abuse

josh28

not necessarily because I've only looked at all the La Ninas since 1950 and there were strong ones before 1950 that had more snow. My sample size is pretty small so I don't think you can safely draw that conclusion, especially when 1999-2000 could have been much bigger with a slight change in storm track.

Posted by: wjunker | November 11, 2010 5:27 PM | Report abuse

Great article, Wes, of the many factors coming into play with winter forecast. As you well know, the devil is in the details when it comes to the particular circumstances that might lead to scenarios compatible with the possibility of individual snow events.

In particular, the sign and magnitude of the AO is a critical player. Unfortunately there is little skill in predicting the AO beyond a few days ahead, especially in transitions from positive to negative and the reverse.


Posted by: ensemblemean | November 11, 2010 6:01 PM | Report abuse

Great article! Very informative!

Posted by: Snowlover2 | November 11, 2010 6:56 PM | Report abuse

@wjunker Thanks for the response, but it brings up another question - what's an appropriate sample size? As a non-scientist, I would have thought the data since 1950 would be reliable as a guideline, unless the argument is that we're heading into a longer cycle of a certain type of weather that doesn't correlate with the past 50-60 years of data? How do climatologist determine what date sample size is relevant (or, relevant _enough_) to baseline a seasonal prediction?

Posted by: josh28 | November 11, 2010 8:23 PM | Report abuse

Josh28,

I'm not a statistician so I could be wrong. I think the problem is not that the records only go back to 1950 but that there have only been 12 years with MEI below 1 during that period. I think you'd like a larger sample size than that to be able to have a lot of confidence in saying that the snowfall this year is unlikely to be less than 17 inches. Personally, I'd forecast less than 17 as the median of all years is around 12" and we average a little less than normal during a La Nina.

Posted by: wjunker | November 11, 2010 10:10 PM | Report abuse

ensemblemean,

I actually think there are some times when you can have a decent idea of what the AO might do more than a a couple of days in advance. Last year was a good example, once the AO tanked in December and we also had the fairly strong stratospheric warming event that helped weaken the polar vortex even more, it was likely that the AO would stay negative for an extended period. Baldwin and Dunkerton have written a couple of papers on the northern annular mode and have suggested it may have some forecast utility in helping forecast the AO. However, during most years, I think you are right, especially a year like this one when the polar vortex is likely to be stronger than normal due to the La Nina. My thought on the AO probably aren't in the mainsteam.

Posted by: wjunker | November 11, 2010 10:26 PM | Report abuse

Wes,

In regards to Nina/NAO interaction this winter, I'm seeing Dec. as the big money-maker for below normal temps and snowfall in D.C., with a -NAO overcoming the Nina for most of the month. After that, Jan. and Feb. look to be rather uneventful for the area. What do you think our month-to-month snowfall distribution this year?

Given the Nina climo. for March, do you think we could see a late-season snowstorm if the cold air can make it down at the right time?

Thanks,
- Mark

Posted by: MarkEllinwood | November 12, 2010 9:52 AM | Report abuse

Wes, ensemblemean is me, SteveT; still trying to track down how user name got changed

You are correct that last year was exceptional in the persistence/recurring negative AO, and hence its above average degree of predictability.

I don't necessarily buy into the stratospheric warming argument. It may well be that the tropospheric blocking highs(==> negative AO) may be a cause or independent of the warm stratosphere and/or weakening polar vortex.

But, as you say, this year is likely a totally different story, with much more uncertainty in forecasts of the AO

Posted by: ensemblemean | November 12, 2010 10:27 AM | Report abuse

Mark

As Ensemble mean (Steve Tracton) will tell you, I'm not a long range forecaster. The climo for stronger La Nina winters is for Dec and March to be the two coldest months relative to normal but there is considerable year to year fluctuations so calling for Dec to be cold with a negative NAO is a semi-educated WAG. To be honest, there isn't much skill in forecasting individual months much in advance. The nina snow stats indicate there is no big preference for when the heaviest snow would fall. 1973-1974 it fell in Dec, 1999-2000 in Jan. I guess I'm saying I'm not good enough to say when the heaviest snow is likely to occur this year.

Posted by: wjunker | November 12, 2010 4:37 PM | Report abuse

+1 - excellent analysis Wes.

As for AO predictability; I'd suggest the presence of a prevailing la Nina hi-amplitude ridge over AK might could favor SSW events ==> prolonged -AO b/c it's the vertical propagation of these ridges into the ignorosphere that causes the PV to weaken/slow/reverese ==> -AO.

Working against such a scenario is the current westerly QBO phase...which 1) is just now getting under way (so it'll be around all winter) and 2) favors a stronger PV ==> +AO. QBO determines the character of the early winter...leading to a colder and more stable polar vortex in December and January during its west-phase.

The solar cycle influences the latter part of winters when a clear difference is observed between periods of high and low solar activity. During high solar activity the winters...which will not the case this go'round...in the west phase of the QBO tend to be disturbed and are often connected with Major Midwinter Warmings.

Pays your money.
Takes your chances.

Posted by: toweringqs | November 18, 2010 9:47 PM | Report abuse

+1 - excellent analysis Wes.

As for AO predictability; I'd suggest the presence of a prevailing la Nina hi-amplitude ridge over AK might could favor SSW events ==> prolonged -AO b/c it's the vertical propagation of these ridges into the Ignorosphere that causes the PV to weaken/slow/reverse ==> -AO.

Working against such a scenario is the current westerly QBO phase...which 1) is just now getting under way (so it'll be around all winter) and 2) favors a stronger PV ==> +AO. QBO determines the character of the early winter...leading to a colder and more stable polar vortex in December and January during its west-phase. The solar cycle influences the latter part of the winters when a clear difference is observed between periods of high and low solar activity. During high solar activity the winters...which will not the case this go'round...in the west phase of the QBO tend to be disturbed and are often connected with Major Midwinter Warmings.

Pays your money.
Takes your chances.

Posted by: toweringqs | November 18, 2010 9:48 PM | Report abuse

The comments to this entry are closed.

 
 
RSS Feed
Subscribe to The Post

© 2012 The Washington Post Company