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Posted at 10:10 AM ET, 11/13/2012

Winter outlook 2012-13 for Washington, D.C.: near normal cold and snow


Here we are again on the precipice of winter. Only three weeks separate us from average high temperatures in the 40s that will last almost 12 weeks. Thus, it is time for our annual winter outlook.

The easiest prediction is that it will be colder and snowier than last “non-winter” which had near record seasonal warmth and low snowfall. But we do not expect this to be an epic snowy winter.

Link: Competition - How much snow do you think will fall?

Over the last 25 years, winters have delivered either feast or famine for snowfall in the Washington, D.C. metro region. Except for the few blockbuster winters of 1995-96, 2002-03, and 2009-10, the overwhelming majority of winters featured normal to below normal snowfall. We expect this winter to fall into that majority. That doesn’t mean it won’t come with its share of inclement weather.

Generally we can expect 6-7 accumulating (more than a trace) snow events this winter with around 4 events of an inch or more. The outlying suburbs to the north and west may experience a couple additional events.

We can’t yet say whether our wintry events will occur during the work week at rush hour creating havoc, or, whether they’ll occur over the weekend, proving little more than a nuisance. The impact of winter storms is strongly related to exact temperatures during a storm and the storm’s timing. Those details will have to wait.

The most we can say is that we favor the highest impact events to occur in our chilliest months, January and February, especially the latter. We’re calling for February to be our coldest month with respect to normal and also our coldest outright. December and January should be on the warm side of normal.

We don’t think odds are high for a massive, crippling “Snowmageddon”-like storm, but a 6-10 inch (or modestly higher) storm is certainly not out of the question.

Beyond that, forecasting this winter is trickier than normal, but here are our detailed ideas....

OUTLOOK DETAILS

Temperatures

Overall temperatures for December through February (relative to 1981-2010 normals): Slightly above average (Near average to one degree above average)

Monthly temperatures (relative to 1981-2010 normals)

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

Snowfall


Odds of different snowfall amounts in Washington, D.C. The most likely range is shaded in red.

Our snowfall projection covers November through April (1981-2010 normals in parentheses):

Overall: Slightly to somewhat below normal

Reagan National airport (DCA): 10-12” (15.4”)
Dulles airport (IAD): 16-20” (22.0”)
BWI airport: 15-18” (20.1”)
Fairfax/Loudoun/Montgomery counties: 14-24”
Arlington/Alexandria/Prince George’s county/D.C.: 10-16”

Caveats

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 the lackthereof) could make or break the projections provided.

Note that monthly 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.

Also, our region is rarely in the core of areas where it’s much colder or warmer than average. Quite often, we are caught between two competing air masses. Usually, one of the air masses wins out. But in months that end around normal, it is hard to predict on which side of normal we will fall. However, we are not shirking our responsibility to offer our inclination, and we will grade ourselves after winter is over with a critical eye.

FREQUENT QUESTIONS

What are other outlets forecasting for our area?

* NOAA: Equal chances of above or below average temperatures and precipitation.
* AccuWeather: Above average snow.
* Old Farmer’s Almanac: Colder than normal temperatures, near normal snow.
* Farmers’ Almanac: Colder than normal, above average snow.
* Joe Bastardi: Colder than normal, above average snow.

Was the recent cold, stormy weather a predictor of upcoming wintry conditions?

Weather in late October and early November took us on a wild ride: colder than average temperatures, Superstorm Sandy and its incredible mountain snows, and then the nor’easter which brought some sleet to the D.C. metro area and unusually early season snow in New Jersey and points north. Rather than a sign of things to come, we think this spell of inclement wintry weather was more likely a false alarm.

We still have yet to get any measurable winter precipitation at our three local airports and our average high remains around 60 degrees.

How have your winter outlooks performed in past years?

We have been doing winter outlooks since 2005-2006 and have evaluated ourselves after the fact the last five winters. We’ve generally been in the ballpark giving ourselves an average grade around C+ in that span, though we’ve missed important details here and there. Our best forecast preceded the Snowmageddon winter (2009-2010) when we said “Overall, we find chances for a large snowstorm of 8-12”+ are much higher than normal this coming winter.” Our worst outlook was last winter: we called for near normal temperatures and it was five degrees warmer than normal.

You can review our past outlooks and evaluations by referring to the links at the bottom of this post.

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 1946-47, 1968-69 and 1996-97 - our so-called “analog years.” These analogs helped to form the basis of our temperature and snow predictions because the governing weather patterns those years are similar to what we’re seeing this year. The patterns we evaluated in identifying analogs are discussed below.

Tropical Pacific Ocean

After a La Niña event - characterized by cooler than normal waters in the tropical Pacific - that spanned almost two years, it faded this summer and we began to see the slow formation of a potential El Niño event. Or so we thought. Remember, El Niño events are associated with warmer than average water in the tropical Pacific and can fuel snowy East Coast storms.


The Southern Oscillation Index is an indicator of a La Niña (red) and El Niño (blue) conditions. When the magnitude is within -0.5 and 0.5, it is an indicator of neutral conditions. (NOAA)

The development of El Niño has sputtered, and its formation is greatly in doubt. We are in the period when the vast majority of El Niño events strengthen and we are seeing the opposite. In fact, just last week the National Weather Service called off its El Niño forecast and changed it to a prediction of a neutral winter - neither El Niño or La Niña.

Neutral winters occur about 30 percent of the time. Though of those neutral winters, around half show a noticeable tendency toward an El Niño or La Niña event but fall a little short.

We expect neutral conditions this winter, but there is a good chance that we will show somewhat of a tendency toward an El Niño event without ever officially getting there. One of our analogs, 1968-69, was a weak El Niño event, while the other 2 were neutral winters - 1946-47 tended toward a weak El Niño and 1996-97 tended toward a weak La Niña. The winters prior to all three analogs were borderline or weak La Niña’s not dissimilar to the winter of 2011-12.

Since El Niño and La Niña, especially stronger events, are often one of the greatest influences on how winter will evolve, formulating an outlook on neutral conditions is especially tricky. Without El Niño/La Niña calling the shots, other factors that are typically less predictable take on a more prominent role. These factors, some of which we discuss below, sometimes have opposing impacts on our weather. So whatever factors “win out” will determine what kind of winter we experience.

North Pacific Ocean

The Pacific Decadal Oscillation (PDO) is a measurement of the intensity and location of sea surface temperature differences from normal 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. We are now in a negative decadal phase of the PDO that started in earnest in 2007.


Chart of the Pacific Decadal Oscillation (PDO) over time (NOAA)

Currently we have had 28 consecutive months with a negative PDO, though just in the last month or so, we have seen signs of it moving toward a more positive state. Often an El Niño will help “force” the PDO to go positive. However since we do not expect an El Niño event this winter, we expect the PDO to default to its current decadal state and average negative for the winter, though perhaps not as markedly so as we have recently experienced.

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. But because the negative PDO may be a little weaker this winter, we expect that patterns driven by the PDO will be less stable and more changeable than they might be in a more sharply negative state like last winter.

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 - sometimes called a “blocking high” helps push the storm track further south and east, often creating storm tracks that are cold and snowy for our region.


Chart of the North Atlantic Oscillation (NAO) over time (NOAA)

It is likely that we are currently in the first few years of a long term negative phase of the NAO. After an 8 month stretch of a positive NAO including last winter that went counter to our long term phase, we have experienced a negative NAO in the averages for 6 consecutive months now, with it being sharply negative in October. 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 higher than average pressures near Greenland consistent with a negative NAO.

Late summer/fall pattern

Over the last three months average out, we have seen areas of upper level high pressure northwest of the Aleutians toward Siberia, and near Greenland which is consistent with a negative NAO. Additionally, we have seen a persistent area of lower pressure over the center of the country with warmer than normal temperatures in New England and the Desert Southwest.

In our backyard we have been warmer than normal for the August-October period. November started out quite cold, and after our brief warm spell, there are signs that the cold will beat out the warmth over the next couple weeks. These factors were among those considered in choosing analog years, though none of our analog years either collectively or individually were ideal matches for all of the above factors.

Other factors

We also considered, but gave less weight to, the phase of the Quasi-biennial Oscillation (QBO) which is in a negative state, and Northern Hemispheric snow cover which is above normal. The current state of both these factors loosely correlates to a -NAO this winter.

WINTER OUTLOOK VIDEO

On October 15, I briefed the D.C. Council on my preliminary thoughts about what to expect this winter. My ideas have not changed much since that time. You can watch my testimony here.

RELATED LINKS

2011-2012 winter outlook
2011-2012 winter outlook recap
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, Wes Junker, and Jason Samenow contributed to this outlook.

By  |  10:10 AM ET, 11/13/2012

Categories:  Capital Weather Gang, Winter Storms, Latest

 
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