wpostServer: http://css.washingtonpost.com/wpost2

Most Read: Local

Posted at 10:51 AM ET, 11/03/2011

Washington D.C. winter weather forecast 2011-2012: average temperatures, more snow than last winter


After a rare October snowstorm, some of you may think we are releasing our outlook a bit too late. But for those of you not quite ready to plunge into winter, a nice chunk of fall still awaits. Plenty of days in the 50s and 60s (and perhaps even 70+) linger before we settle into winter.

How does the October 29 storm affect our outlook? In short, it doesn’t. The snow accumulated counts towards seasonal totals, but for most of the immediate region, only tacks on a trace to two inches. For most of the region, most or all of the cold season’s snow has yet to fall.

For snow lovers hoping for a repeat of the epic 2009-10 winter, you will probably be disappointed.  This winter is much more likely to be similar to last winter, when 10.1” fell, than the record-setting 56.1” of 2009-10. 

But we don’t imagine this winter will be a dud either. 

It’s unlikely to be as cold as last winter, but we are not favoring a warm winter either.  Furthermore, we have a good chance of matching or exceeding the snow totals of last winter at all three airports. 

When we look back at this winter, we doubt it will be extraordinary in any measure, but the devil is in the details.  While another event as unusual and shocking as our October storm may not repeat, there are sure to be plenty of surprises.

Related content: Live chat with Matt Ross (lead author) at 2 p.m. | Contest: How much snow this winter? | Winter outlook: A snow point/counterpoint

Keep reading for the detailed outlook and methodology. And if you haven’t already, join us on Facebook and follow us on Twitter.

OUTLOOK DETAILS

While advances have been made in seasonal forecasting, there is still a great deal of uncertainty and limited skill in developing these outlooks. This is a low-confidence forecast, especially the overall snowfall estimate, where one big storm (or lackthereof) could make or break the projections provided below...

Temperatures

Overall Temperatures for December through February (relative to 1981-2010 normals):

We favor near average temperatures but would lean below rather than above if we had to choose.

Monthly temperatures (relative to 1981-2010 normals)

December: one degree colder than average
January: normal to one degree colder than average
February:  one to two degrees warmer than average

We believe cold air (relative to average) is more likely to come early in the winter than late in the winter.

Note that the individual month temperature predictions are less reliable than for the whole season . A cold or warm pattern lingering a week too long or ending a week short can greatly alter a monthly average. 

The uncertainty is also magnified due to the fact our region is rarely in the core of warm or cold temperature anomalies (i.e. areas where it’s much colder or warmer than average), especially in a La Niña winter (see below for further explanation) like the upcoming. Quite often, we are caught between two competing air masses.  Usually, one of the air masses wins out in the end. But in months that end close to normal, it is hard to predict on which side of normal we will fall.  But we are not shirking our responsibility to offer our inclination, and we will grade ourselves after winter is over with a critical eye.

Snowfall

We are unlikely to get as many measurable snow events as last winter, when we got a surprising 12.  But we expect that, on average, the snow events we do get will pack more of a punch.  As a result, we think chances are good that we will match or surpass last winter’s seasonal snow totals. 

Throughout the last two winters, we have seen an unusual number of big storms strike the mid-Atlantic and Northeast.  Last winter, we tracked several events that showed a lot of potential within days or even hours of the event, but only one event of significance panned out (January 26, Commutageddon). 

We may be due to catch a blow from a storm or two that targets the cities to our north.  But the likelihood of a weaker southern jet stream (often characteristic of La Niña) greatly reduces the chances of receiving one of those historic-type storms that hit us flush in 2009-10. So for those of you hoping for a blockbuster storm, we can’t rule it out as the recent trend is in our favor, but we think it more likely that our biggest event tops out in the 5-10” range. 

Overall, we can reasonably expect two storms of 3-4”+ for most of the region.  We favor the remaining snowfall to be rounded out by several smaller events in the 1”-3” range or smaller, though localized higher amounts are always possible in some events. 


The probability of different seasonal snow totals at Reagan National Airport. There is roughly a 50/50 chance of 12” or more (or less).
* Our snowfall projection covers October through April (1981-2010 normals in parentheses):

Overall: Slightly below normal

Reagan National Airport (DCA): 10-14” (14.5”)

Dulles Airport (IAD): 15-19” (22.0”)

BWI: 16-20” (20.2”)

Fairfax/Loudoun/Montgomery counties: 14-24”

Arlington/Alexandria/PG/DC counties: 10-18”

* Projections include what fell in the October 29 event. Therefore, the higher end of the ranges, particularly for Loudoun and Montgomery counties, are intended for those who received 2”+ in the storm.

METHODOLOGY

Below are the factors that we have deemed most important in determining conditions for this upcoming winter. No single factor tells the whole story, nor are the correlations between current/past conditions and future conditions always strong. But we have chosen factors that in the past have proven to at least have some predictive value. And when considered collectively, they help paint a picture of what we believe is most likely to happen this winter.

No two winters are alike, but we expect this winter to share some similarities with the winters of 1934-35 (snowfall: 31.4”), 1938-39 (13.6”), 1943-44 (7.6”), 1944-45 (7.8”), 1954-55 (6.6”), and 1956-57 (14.2”).  These analogs helped to form the basis of our temperature and snow predictions because the weather in those years most closely matched the factors below:


A visualization of the current La Nina showing below average temperatures (shaded in blue) over the equatorial Pacific. ( NOAA )
Equatorial Pacific Ocean

A La Niña event that got underway in the summer of 2010 waned a bit this summer.  However, it has been showing signs of re-strengthening since August.  La Niña is indicated by anomalously cold sea surface temperatures in the equatorial Pacific ccean.  In our region, La Niñas, particularly moderate to strong events, are often associated with dry, warm winters without much snow.  This is usually because of two primary factors:

1) The frequent presence of a southeast ridge.  It is a persistent area of high pressure near Bermuda that both keeps us in warmer air masses and pushes the storm track to our north and west. 

2) A dominant northern jet stream and lack of a subtropical jet.  Prevalent storm tracks along the northern branches of the jet stream typically cut to our west and/or redevelop as coastal storms to our north and we are left either warm and rainy, or dry.

La Niñas also tend to produce highly variable temperatures characterized by lots of warm thaws followed by Arctic plunges and back to warmth again. 

Usually in weak to moderate La Niña events,  the warm outbreaks are more common than the cold outbreaks in the moderate events and vice versa in the weaker events.  In both, the cold outbreaks are typically dry.  Not all La Nina’s are the same and there are other factors that drive our weather.  However, we expect a weak to moderate La Niña event this winter that starts to wane in January/February. 

If La Niña tends toward the weaker end, we can expect temperatures to likely lean colder than predicted, and perhaps a bit more snow as well.  Conversely, if it ends up stronger than forecast, we may be somewhat warmer with less snow.  We believe that the chances of a weaker event are higher than a stronger event.  Consequently, if we are wrong with our temperature and snow predictions, it is more likely that we went too warm and not snowy enough. 

North Pacific Ocean

The Pacific Decadal Oscillation (PDO) is a measurement of the intensity and location of sea surface temperature anomalies in the North Pacific. When it is strongly positive it often correlates with a cold and stormy pattern for the mid-Atlantic. When it is sharply negative, it often, but not always, tends toward warm and dry.  Measured monthly, it also oscillates in predominant cycles that can last 2-3 decades. From the late 1940s to mid-1970s we were in a predominantly negative PDO cycle, and from the mid-1970s to late 1990s a predominantly positive PDO cycle.  It is likely that we are in a negative decadal phase of the PDO that started in 2007.  

During La Niña events, the PDO tends negative and during El Niño events it tends positive.  We expect the PDO to average negative this winter.  We are probably in a long term negative phase and during a La Nina event it is highly unlikely that the PDO will act counter to the current decadal phase.  It has been sharply negative this fall and we expect it to be somewhat to strongly negative throughout winter.  A negative PDO often supports a persistent area of low pressure in the Gulf of Alaska.  This is usually not a good sign for snow lovers as an area of low pressure farther west over the Aleutian Islands is the preferred setup for wintry weather in our region.


A time series of the North Atlantic Oscillation since 2002. It has tended to run strongly negative over the last two winters. ( NOAA )
North Atlantic Oscillation (NAO)

The NAO is technically a measurement of the differences in air pressure over the North Atlantic Ocean.  However, it is often indicated by either an upper level low pressure area (positive phase) or upper level high pressure area (negative phase) over or near Greenland.  A negative NAO in the winter months strongly correlates with a cold and stormy pattern in our region.  This was the major factor in our historically snowy winter of 2009-10.  High pressure over or near Greenland helps push the storm track further south and east, often creating storm tracks that are cold and snowy for our region. 

It is likely that we are currently in the first few years of a long term negative phase of the NAO.  It has averaged negative for 29 out of the last 36 months.  This could be the saving grace for snow lovers.  Good periods of high latitude blocking can sometimes negate or offset the negative signals in the Pacific.  “Cashing in”, or failing to do so, during these periods is often the key to how much snow we receive. 

We believe that the NAO will average negative overall this winter, but probably not decidedly so. Collectively, our analogs indicate a positive height anomaly (higher than average pressures) near Greenland consistent with a negative NAO.  However, if we continue the trend of the last two winters and it averages more sharply negative, it may result in a colder and more snowy forecast than we are indicating. 

Summer/Fall Pattern

This summer was another brutal one for D.C., including our warmest summer month on record (July).  However the most significant hallmark of this summer was not in our backyard, but rather a severe and often historic drought in the southern plains and Texas.  This correlated with extreme heat over that region for most of the summer, which usually had little problem penetrating our backyard.  In fact, most of the eastern two thirds of the U.S. had a warm/hot summer. 

Late summer and fall has seen a different pattern settle in the continental United States.  In addition to the incredible amount of precipitation in the mid-Atlantic and Northeast, we have a seen a core of cold temperature anomalies in the Midwest in September which made their way into the Dixie region in October.  These factors were among those considered in choosing analog years.

FREQUENT QUESTIONS

What are other forecast outlets predicting for the coming winter?

* AccuWeather expects near average temperatures, with a cold start to winter and mild ending, similar to our outlook. On the other hand, it is predicting slightly above average snowfall, whereas we’re predicting near to below average snow. The differences are subtle.

* NOAA does not offer a detailed forecast for our region. Its generalized outlook gives equal chances of above/below average temperatures and precipitation.

* The only broadcast meteorologist to release an outlook for the region so far is Doug Hill of WJLA. He is calling for temperatures near to slightly below average and 10-15 inches of snow, which is very similar to our outlook.

How did last year’s winter outlook verify?

In a detailed self-assessment, we graded our outlook a B- or C+. We forecast slightly too much snow and temperatures were somewhat colder than forecast

What are the farmers’ almanac predictions?

The are actually two farmers’ almanac.

* The Old Farmer’s Almanac predicts: “Winter will be drier than normal, with near-normal temperatures but above-normal snowfall.”

* The Farmers’ Almanac predicts: “...above-normal temperatures are expected across most of the southern and eastern U.S” and “A very active storm track will bring much heavier-than-normal precipitation from the Southern Plains through Tennessee into Ohio, the Great Lakes, and the Northeast. Because of above normal temperatures, much of the precipitation will likely be rain or mixed precipitation, although, during February, some potent East Coast storms could leave heavy snow, albeit of a wet and slushy consistency.”

We do not believe the farmers’ almanac outlooks are skillful.

RELATED LINKS

Past winter outlooks

2010-2011 winter outlook
2010-2011 winter outlook live chat
2010-2011 winter outlook recap
2009-2010 winter outlook
2009-2010 winter outlook recap
2008-2009 winter outlook
2008-2009 outlook live chat
2008-2009 winter outlook recap
2007-2008 winter outlook
2007-2008 winter outlook recap
2007-2008 winter outlook
2006-2007 winter outlook
2005-2006 winter outlook

Matt Rogers, Jason Samenow, Wes Junker, Ian Livingston, Greg Postel, and Steve Tracton contributed to this outlook.

By  |  10:51 AM ET, 11/03/2011

Categories:  Latest, Winter Storms, Capital Weather Gang

 
Read what others are saying
     

    © 2011 The Washington Post Company