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Posted at 10:45 AM ET, 11/ 1/2010

Why was last year so snowy? Part I

By Wes Junker

Winter outlook preview

Why was last year so snowy? Part II

By Wes Junker, Winter Weather Expert

(From the editor: Please welcome Wes Junker, who will serve as Capital Weather Gang's winter weather expert moving forward. Wes worked for 30 years an operational meteorologist at the National Weather Service and has written many scholarly articles on forecasting snow and extreme precipitation. Read Wes's full bio.)

(Over the next several days, Wes and other CWG contributors will recap last winter and put it into context. On Thursday, CWG will release its 2010-2011 Winter Outlook)

cwg_Junker.jpgLast winter was the snowiest on record for much of the mid-Atlantic region from Washington D.C. to Philadelphia, Pa. Baltimore shattered its old record with 77" compared to the 62.5" from the 1995-1996 season. Washington received 56.1" of snow which broke the snowfall record of 54.4" that was previously set in 1899.

Three of the heaviest 10 snowstorms on record for Baltimore and two of the top 10 for Washington DC occurred last winter. In retrospect, a few readers probably have wondered why last year was so snowy compared to other recent years.

This two part article will attempt to address that question.

Several atmospheric oscillations were aligned in a manner to increase the chances of receiving more snow that normal. The El Nino Southern Oscillation (ENSO) is the most recognizable of the group.

Schematic of La Nina (left) and El Nino (right) Source: NWS Climate Prediction Center.

ENSO has two phases (El Nino and La Nina) typically taking two to four years to transition from one phase to the other. During the two phases, there are large scale differences in the atmospheric circulation patterns across the tropical Pacific that have a marked impact on trade winds.

During the cold phase of ENSO (La Nina), enhanced easterly trade winds across the Eastern Pacific lead to upwelling off the coast of South America and the winds blow the surface water away from the land mass. Cooler waters from below the surface rise and replace the warm surface waters. This colder water gradually shifts westward with time which helps keep the eastern Pacific water temps cold, the warmest waters in the equatorial Pacific are then located in the far western Pacific as is most of the convection.

However, during an El Nino event, the trade winds weaken and at times are replaced by westerly winds which lead to less upwelling and warmer than normal waters along the equator from South America westward to at least the date line. The changes in the atmospheric circulation pattern and warmer water temperatures across the eastern and central equatorial Pacific help produce more convection than normal across the region (see diagram below for how the location of the convection shifts between phases).

Wintertime impacts of El Nino (top) and La Nina (bottom) in North America. Source: NWS Climate Prediction Center.

The differences in the location of the convection along the equator force changes in the circulation patterns at mid-latitudes, where we live. Below is a schematic representation of the differences in flow patterns and how they affect North America.

During an El Nino there often is a split in the jet stream with a very strong subtropical Pacific Jet that extends from the Pacific across the southern U.S. Waves in the atmosphere along this southern jet stream help to keep the storm track across the south leading to wetter and colder weather across the south into the southern mid-Atlantic region. Such a storm track increases the chances for a storm to take the perfect snow storm track for the Baltimore/Washington area. It takes a low across the south and then off the North Carolina coast near or just inland of Cape Hatteras.

However, El Nino years often lock the polar jet stream across Canada which prevents arctic air from penetrating into the U.S. resulting in warmer than normal temperatures across the northern tier of states.

Snowfall departure from average (or anomaly) during El Nino. Blue shades indicate more than average snow, brown/yellow/red shades indicate less than average snow. Source: NWS Climate Prediction Center.

Because of the favorable storm track, El Nino storms are more likely to be significant snowstorms than storms during a La Nina winter. Therefore, El Nino winters tend to average more total seasonal snowfall than normal across the south and mid-Atlantic (see figure to right) even though the frequency of days when Washington area receives measurable snow is not that different from an average winter.

Despite on average having more snow than other winters, El Nino winters can have a lack of Arctic air, especially during strong El Nino years when the equatorial Pacific is unusually warm and relatively mild Pacific air floods the country.

For example, during two strong El Nino winters (1972-1973 and 1997-1998) Washington recorded only 0.1" of snow, the lowest seasonal totals of any years since records have been kept starting in 1888. While El Nino raises the potential for snow, the phase is no guarantee that a season will be snowy. Cooperation with other more northerly atmospheric weather patterns or oscillations and a little serendipity is needed to get an exceptionally snowy winter.

Tomorrow, I'll discuss how these northern oscillations set up to support an exceptionally snowy winter last year.

By Wes Junker  | November 1, 2010; 10:45 AM ET
Categories:  Capital Weather Gang, Snowmageddon, Winter Storms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Freeze watch for entire metro region tonight
Next: Southeast rainfall more variable as climate warms


This winter, we shouldn't get 30 or more inches of snow. However the conflict between La Nina Arctic outbreaks and warm air from the enhanced ridge to our southeast should give us our share of weather excitement. Sharp variations in temperatures and shifts in precipitation type will make for highly variable weather regimes.

Possible weakening of the La Nina regime as we approach the critical Presidents' Day heart of winter period could set the stage for one sizable snowstorm or snow/mix/rain event during mid-February. In addition, winter could drag well into March if the La Nina regime weakens by then.

Those watching sea surface temperatures ought to check the critical "Nino 3.4" region in the Central Pacific south of Hawaii and west of the Galapagos. Right now the Nino 3.4 area is substantially cooler than normal, the earmark of a rather strong La Nina. Besides influencing the weather, the La Nina is good for the Peruvian fertilizer industry, which depends on the cool-water anchovy fishery and guano deposited on offshore islands by seabirds which feed on the anchovies for its livelihood.

Our last La Nina winter was that of 2008/09, a winter marked by generally dry weather, frequent Arctic outbreaks, and not much snow, but plenty of non-accumulating flurries, one of which [12/31/2008] manifested as a thundersnow shower with light accumulation which vanished in an hour or two.

Posted by: Bombo47jea | November 1, 2010 11:59 AM | Report abuse

The record snows weren't confined to the Wash, DC/Baltimore/Philly corridor. Beckley, WV had an amazing 130". I wish we had gotten last winter snowfall totals for Davis/Thomas, WV, because I recall Bob Ryan mentioning that one of his friends (a retired meteorologist?) who lives there measured 240". Pittsburgh, PA and Asheville, NC didn't set all-time records, but both cities came close.

Posted by: JerryFloyd1 | November 1, 2010 12:35 PM | Report abuse

P.S. Welcome, Wes Junker. When the flakes start flying in DC, CWG traffic goes bonkers.

Why are professionals who study and report on the weather known as meteorologists, whereas those who study meteors are astronomers (or is there a specialized term for meteor experts?).

Posted by: JerryFloyd1 | November 1, 2010 12:42 PM | Report abuse

Great post, Wes. The CWG community will surely value your contributions in the future... Wes is one of the best out there, all!

Posted by: Ian-CapitalWeatherGang | November 1, 2010 1:03 PM | Report abuse

I'm also excited to have Wes on-board - welcome to the team!

Posted by: Camden-CapitalWeatherGang | November 1, 2010 2:44 PM | Report abuse

JerryFlood1, A meteor is anything falling from the sky and a raindrop, snow flake or ice pellet is a hydrometeor. I don't know what you call someone who studies other types of meteors.

Ian, Camden, thanks for the kind words. I'm looking forward to making more contributions this winter.

Posted by: wjunker | November 1, 2010 6:10 PM | Report abuse

Woo hoo! I cannot believe the CWG has a meteorologist who is an expert in winter weather! That's what I want to be went I graduate from high school! Thank you Wes Junker for making my dreams come true! WOW! I cannot believe this! Okay, breathe.....Woo hoo!

I am a die hard winter weather fan. If there is a blizzard, you will always find me outside standing in 3 feet of snow walking around as if it was 85 and sunny. I will definitely want to read more of your stuff Mr. Junker. WOW! I cannot believe this! We have our own Winter Weather Expert! Here's to lots of snow this winter!

Posted by: Yellowboy | November 1, 2010 6:10 PM | Report abuse

Wes, welcome and thank you for this extremely interesting post! I'm looking forward to Part 2.

Posted by: --sg | November 1, 2010 10:07 PM | Report abuse

Yellowboy, It's nice to see such excitement about the weather. When I was in high school, I too wanted to be a weatherman/meteorologist.

Posted by: wjunker | November 2, 2010 8:39 AM | Report abuse

Welcome, Wes! Loved reading this and look forward to more in the near future! Please tell me we'll all have a white Christmas! :)

Posted by: eunicedelrosario | November 2, 2010 2:45 PM | Report abuse

Enjoyed your insight, Wes! :)

Posted by: Rcmorgan | November 2, 2010 3:55 PM | Report abuse


I can't tell you much about Christmas this early. Like you, I'd like a white one.

Posted by: wjunker | November 2, 2010 8:39 PM | Report abuse

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