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Posted at 10:30 AM ET, 12/21/2009

How did this happen?

By Matt Rogers

Blame the North Pole and the equator

* Christmas Eve storm possibility: Full Forecast *
* View CWG-user snowfall map | NWS snowfall reports (view map) *

Recent Arctic Oscillation index. Courtesy NOAA.

Before this weekend, the biggest snowstorm in Washington's December history was a mere 12 inches in 1932. So how did 2009 manage to upset the record in a very big way?

An unusual convergence of factors stretching from the North Pole to the equator conspired to set up this powerful storm. And these factors could continue to play a prominent role for the rest of our winter!

Let's start up north -- way north. Over the past few weeks, a pattern has been developing over the Arctic Circle region that flushes cold air away from the North Pole and toward the midlatitudes (where we live). This pattern, known as "high-latitude blocking," is essentially a large area of high pressure in the upper atmosphere over far northern latitudes. The high pressure pushes air downward, and as that air near's the Earth's surface it's forced outward and away, sending cold air south toward us.

As of right now, this high-latitude blocking pattern (also known as the negative Arctic Oscillation, or -AO) is the strongest on record for December. This -AO pattern has been so strong that it has sent above-normal snows to Siberia and China, big cold and snow to Europe (London and Copenhagen recently), and lots of cold into the U.S. after a very mild November.

So we can blame the -AO for our cold weather. But another factor was also key to our record-breaking snow.

A well-known meteorological nemesis named El Niño gets the blame for a very moist atmosphere that "juiced up" the storm. El Niño refers to warmer-than-normal waters in the central to eastern Pacific Ocean along the equator. This year's El Niño is the strongest since the 2002-2003 winter (remember that winter's President's Day storm?).

Those warm waters tend to feed moisture into a southern branch of the jet stream (known as the sub-tropical jet stream), which adds fuel to storm systems over North America, especially in the southern U.S. and along the Eastern Seaboard. There is significant evidence that our weekend powerhouse was fueled by El Niño. In fact, according to reliable records back to 1950, this is the strongest El Niño with the most negative Arctic Oscillation we have ever seen in the month of December. This powerful convergence of weather influencers appears to have bred such an impressive record-setting event.

The other facet to this story is that as of right now, neither feature shows any signs of ebbing. That means the potential exists for more powerhouse storms as we trek our way into the typically more snowy part of the winter: January and February.

One more thing... Meteorologists look at a measurement known as Outgoing Longwave Radiation (OLR), which shows us where lots of clouds and rain are essentially reducing the amount of Earth's radiation that is sent back into space. In the graphic below, from the Australian weather service, the blue-to-pink regions are those areas where OLR is very low (very wet). This graphic shows the average of OLR over the past three days. I drew arrows on the graphic to show the trail of low OLR from the El Niño region in the central Tropical Pacific right up into the southern U.S. where the same storm that dumped the snow on us caused flooding on Friday. It's evident that this particular storm was fueled by sub-tropical moisture fed by those warm Tropical Pacific waters.

By Matt Rogers  | December 21, 2009; 10:30 AM ET
Categories:  Winter Storms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: Cold, dry, and officially wintry
Next: PM Update: Melt, freeze, repeat...


saw on the radar last night an image of the storm exiting off the east coast and it looked like it formed an eye...was really wild, it looked so much like a tropical hurricane

Posted by: asimo | December 21, 2009 10:39 AM | Report abuse

Intersting OLR graphic showing the tropical connection into the Gulf of Mexico.


Posted by: toweringqs | December 21, 2009 10:54 AM | Report abuse

watch out for seevere icing on xmas day. The cold air is going to hold its ground. The ice stituation has to be monitered

Posted by: snowlover3 | December 21, 2009 11:02 AM | Report abuse

I agree snowlover3. Whenever we have (a) snow on the ground and (b) high pressure over New England, the guidance can struggle to pick up on classic cold air damming. It is a big concern of mine too...especially from DC westward along the edge of the Appalachians.

Posted by: MattRogers | December 21, 2009 11:08 AM | Report abuse

asimo, be on the lookout for a post this week covering items like that.

Posted by: Ian-CapitalWeatherGang | December 21, 2009 11:24 AM | Report abuse

asimo...can you post a link to that picture of the storm? thanks.

Posted by: crozet | December 21, 2009 11:30 AM | Report abuse

Road conditions here in AA County are slowly improving but schools on the Broadneck Penninsula still have not been touched. Makes me wonder if schools will closed again on Tuesday....CWG, will you post a "School Cast" for tomorrow?...By the way...great job during the storm!

Posted by: jcmcgrath1969 | December 21, 2009 11:39 AM | Report abuse

Hi Matt, interesting post. What causes high-latitude blocking? Also you say the -AO index is the lowest on record for December? What then contributed to the even lower temps we had back in 1989 when I recall we were pretty much below freezing the entire month of December.

Posted by: Thundershock | December 21, 2009 11:48 AM | Report abuse

I remember a storm many years ago that tracked to the west of the Great Lakes when we had very deep snow cover in the DC area. The NWS issued flood warnings for DC for heavy rain combined with melting snow. Ultimately, the temperature never went above freezing and we had a major sleet and ice storm. Meanwhile, cities west of the mountains like Pittsburgh had just rain with temperatures above 50 degrees. I'm not saying we are getting a big ice storm later this week, but it's something watch. By the way, we can forecast cold air damming much better now.

Posted by: Kevin-CapitalWeatherGang | December 21, 2009 11:53 AM | Report abuse

The moisture flow is having an even bigger effect on the Gulf Coast.

Posted by: CapitalClimate | December 21, 2009 11:55 AM | Report abuse

Thundershock, yes, you are correct that 1989 was much colder in December. In fact, it was the coldest December on record nation-wide. The main trigger was a very strong West Coast ridging pattern that extended well up into Western Canada. That +PNA pattern was impressive, but the AO did contribute at a weaker level (about -0.6 that month). So far, Dec 2009 is tracking as the lowest AO on record, but we are not done the month yet and the current champion, Dec 2000, had a strong finish to the month.

Posted by: MattRogers | December 21, 2009 1:08 PM | Report abuse

Great post, Matt. Thanks.

Posted by: Ann-CapitalWeatherGang | December 21, 2009 1:10 PM | Report abuse

Can we get a school cast any time soon?

Posted by: skinsfan214 | December 21, 2009 1:21 PM | Report abuse

Saturday's storm looked like a Miller A originating in the Florida Gulf area and it tracked just right for max. snow in our area.

Historically, the track is very similar to that of the famous "Knickerbocker" storm of 1/28/1922. Storms which originate in that area on this particular track will need to be picked up and monitored very closely in the future [regardless of what AccuWeather says...Joe Bastardi is BRAGGING today!!!]

Posted by: Bombo47jea | December 21, 2009 1:35 PM | Report abuse

This post confirms my worst fears, although I was predicting a harsh winter based mostly on the amount of precipitation we've had this year? Roughly how many such storms should we expect this winter? And to what extent do you think global warming might be involved?

Posted by: pitchtorobert | December 21, 2009 2:10 PM | Report abuse

With all due respect to Matt, I believe it's a mistake to tie this storm primarily to El Nino. As I explained for the major storm the week before last( here ), conditions that "lock in" El Niño via the distribution of atmospheric heating associated with tropical convection have yet to develop. The tropical jet and moisture stream is most likely incidental, not a consequence of El Nino. One sure bet signature of El Nino will be development of a strong and persistent positive PNA. Additionally, like the case I described last week, the snow storm can be traced back to the down stream influence of disturbances moving off the Asian continent into the western Pacific.

The NAO was a key player and a fortuitous circumstance for everything to come together. But it is relatively short term phenomena with little predictive skill beyond a few days in advance, and therefore it may or may not ebb before end of the year.

Posted by: SteveT-CapitalWeatherGang | December 21, 2009 2:17 PM | Report abuse

"AccuWeather says...Joe Bastardi is BRAGGING today!" they should, and hats off to the posters posting here early last week, that a big east coast snowstorm looked favorable for the weekend. you guys nailed this storm, lets see who was it? i think tuesday, that first mentioned blizzard. once the storm was a sure winner, CapitalWeatherGang was all over it, and did a great job.

Posted by: deveinmadisonva | December 21, 2009 2:23 PM | Report abuse

If these Arctic and El Nino factors persist, what are the changes of another significant storm like the one we just saw? I can say with reasonable certainty that in my 32 years of life I have never seen two blizzards in one winter...that would make this snow lover happy :)

Posted by: authorofpoetry | December 21, 2009 2:35 PM | Report abuse


Very unlikely we'd get another storm like this in the same winter. This was a 1 in 10-15 year type event.

Posted by: Jason-CapitalWeatherGang | December 21, 2009 2:39 PM | Report abuse


re: schoolcast -- it's dependent on the school system. some are already closed. For those that aren't, I'd say 2 or 3 apples.

Posted by: Jason-CapitalWeatherGang | December 21, 2009 2:46 PM | Report abuse

Thanks, Jason. As a Government contractor at the NIH, blizzards seem to be the only way we get days off...if there is anything I can do weather wise to ensure we get another significant storm, let me know!

Posted by: authorofpoetry | December 21, 2009 2:53 PM | Report abuse

Hey Steve, we have noticed this year that the southern branch systems have tended to verify on the wetter side and there is a strong correlation between a wet South and El Nino events. So, as I noted, it was not the ONLY reason, but I do not believe it was NOT a coincidence that we just had the biggest storm since our last notable El Nino winter (02-03). The satellite data confirms the connection to the above normal waters of the Central Tropical Pacific. Matt Ross predicted a snowy/cold winter based on guidance that included El Nino and so far, so good.

Posted by: MattRogers | December 21, 2009 5:05 PM | Report abuse

cant find a picture of the eye I was speaking of with this storm, I am not real familiar with how to retrieve historical satellite images..did find a picture on wikipedia of a blizzard in 2006 that effected the affected some parts of the region and there was an eye offshore near the jersey coast...looking forward to the posts covering this type of stuff later this week

Posted by: asimo | December 21, 2009 6:12 PM | Report abuse


Sorry, I just can't agree with you. The southern jet may appear El Nino like, but appearances can be misleading. The balance of the usual features with El Nino are still missing. If one looks at the history of the southern jet, it is clear to me that it evolved independent of El Nino. Coincidence with 2002-2003? Yes I think so! Besides, a majority of the top 15 snowstorms in the Metro region occurred in non-El Nino years and none occurred in December even in El Nino winters.

According Climate Prediction, the nominal impact of El Nino on weather over North America is not great until the late winter and this El Nino is not expected to be different.

Posted by: SteveT-CapitalWeatherGang | December 21, 2009 7:00 PM | Report abuse

Steve, what can be misleading about the obvious moisture feed with that storm from the warmer-than-normal central tropical Pacific (El Niño location)? This is not a typical El Niño, by the way. It is more west-based than any we have seen since reliable records back to 1950. Therefore, El Niño climatology will be challenged.

Posted by: MattRogers | December 22, 2009 10:03 AM | Report abuse

To authorofpoetry: You are obviously too young, but the winter of 1960-61 had THREE major snowstorms....starting with the very cold 12/11/60 east coast blizzard, the 1/19/61 (I think) Kennedy Inaugural storm and the mid-February one.

Admittedly, they were not all blockbusters in terms of depth (here). The first and last attained max strength farther northeast, but they were all major east coast nor'easters.

Don Lipman, Capital Weather Gang

Posted by: Weatherguy | December 22, 2009 10:28 AM | Report abuse

Posted by: toweringqs | December 23, 2009 9:33 AM | Report abuse

Record snow storms in December. Global Warming just like Al Gore warned. Here is the cure:

Posted by: ge123 | December 23, 2009 11:17 AM | Report abuse

A video of what the weather was like before Al invented the internet and global warming.

Posted by: ge123 | December 23, 2009 11:23 AM | Report abuse

Hi Matt,

This blog also refers to a record December negative AO. What was your data source? I've looked online for a complete archive of daily AO values but can't find one.

Posted by: StuOstro | December 23, 2009 5:30 PM | Report abuse

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