January 30, 2010 at the Reflecting Pool. (Ian Livingston)

Despite a taste of September last week when we experienced five straight days with highs above 70, the inevitable comes every year, and often without much warning.  Our average high temperatures plummet from the mid-60s at the beginning of November to the mid 40s in mid-December.

There is no stopping the onset of winter, so we might as well discuss it. As is our tradition here, we present you with our annual winter outlook.

Unlike our last two winters, we expect a warmer than normal winter with below average snow.  But below average snow doesn’t necessarily mean dry. We favor a stormier than normal winter. However, expect many of the storm events to produce rain or mixed precipitation rather than snow.

Less brutal cold, more rain than snow

We expect some cold shots, especially in the second half of winter, but nothing like the punishing blows we experienced the last two winters.

Our cold snaps are likely to be short in duration and to produce temperatures only 5 to 10 degrees below normal. Mild spells will occur as well, and we can’t rule out hitting 60 degrees a handful of times, perhaps even in January.

Mostly, we expect run-of-the-mill winter air masses with temperatures running near or a few degrees above normal. Theses types of air masses are more likely to support rain rather than frozen precipitation when storms come through.

Some snow and somewhat increased odds of a blockbuster

Still, we should get our share of snow this winter. We’re not predicting a lot, but we also aren’t anticipating virtually snowless winters like in 2011-12 or 1997-1998.


(Jordan Tessler/Capital Weather Gang)

Even though we are expecting below average snow and not that many events — perhaps four to six around the metro region — the chances of a crippling event such as Snowmageddon or the Blizzard of 1996, while always slim (10-15 percent in a given winter), will be somewhat elevated this year (around 25 percent).

[History shows a strong El Niño may mean huge snow for D.C. this winter or almost none]

Because we are expecting an active winter with storms approaching from the south and west, it is possible the ingredients for a bigger storm could come together when we do get our cold shots. The most likely time period for a big storm would be February.

However, as blockbuster snowstorms are still a low probability in any given winter, chances are we won’t get a blockbuster event. Several small to medium events is a more probable scenario with no single storm exceeding 4-8 inches.

The general numbers and narrative only tell us so much.  Although warmer than normal and less snowy than normal may sound great to many of you (and not so much to others), we will more than likely have our share of delays, closings, and tricky forecasts.  So buckle your seat belts.

Below, see our specific temperature and snowfall forecasts, answers to common questions, and the scientific rationale for the outlook.

The Capital Weather Gang's seasonal forecaster Matt Ross shares his winter outlook with D.C.'s Committee on Transportation and the Environment. (Council of the District of Columbia)

Outlook by the numbers

Temperatures

Overall, we expect temperatures for December through February (relative to 1981-2010 normals) to be somewhat (2 degrees) milder than average:

  • December: 3 degrees warmer than average
  • January: 2 degrees warmer than average
  • February: 1 to 2 degrees warmer than average

Snowfall

Our snowfall projection covers November through April (1981 to 2010 statistics in parentheses). Overall, we expect modestly below normal snowfall:

  • Reagan National Airport (DCA): 10 inches (compared to a 15.4 inch average, 11 inch median)
  • Dulles Airport (IAD): 16 inches (compared to a 22.0 inch average, 16 inch median)
  • BWI: 12 inches (compared to a 20.1 inch average, 15 inch median)
  • Fairfax, Loudoun, Montgomery counties: 12-20 inches
  • Arlington, Alexandria, Prince George’s County, and the District: 10-14 inches

Caveats

Though 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.

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. Furthermore, it takes only one big snowstorm for us to near or exceed our seasonal average.

Common questions

Does the mild start to November help predict the mild winter?

Historically, there’s no strong correlation between the weather in November and the rest of the winter. However, we may see the pattern we’ve seen early this month become familiar with generally warmer than normal temperatures and occasional storminess developing to our south

What are other outlets forecasting for our area?

Of the outlooks we’ve reviewed, they are split between a relatively mild winter and a snowy winter

(Note: the Old Farmer’s Almanac is calling for a cold and snowy winter and the Farmers’ Almanac predicts snowy and unseasonably cold conditions. However, we do not consider these outlooks credible because of a lack of transparency in their methodology.)

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 nine winters. We’ve generally been in the ballpark, giving ourselves an average grade around a B- or C+ in that span, though we’ve had notable triumphs and misses.

Our best forecast preceded the record-breaking Snowmageddon winter (2009-2010) when we said: “Overall, we find chances for a large snowstorm of 8-12 inches or more are much higher than normal this coming winter.” Our outlook for last winter (2014-2015) was also quite successful as we correctly called for it to be cold with somewhat above normal snow.

Our worst outlooks were for the winter of 2011-2012 and 2013-2014. In 2011-2012, we called for near normal temperatures and it was 5 degrees warmer than normal. Two winters ago (2013-2014), we called for a warm winter with slightly below normal snow and it was cold with snow more than twice average.

Outlook rationale

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 past conditions and future conditions — which we used to inform the outlook — always strong. But we have chosen factors that in the past as a guide have proven to have at least 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  1877-78, 1982-83 and 1997-98. These analogs helped to form the basis of our temperature and snow predictions because the weather in those years had some similarities to the factors below.

Tropical Pacific Ocean

We are currently experiencing a very strong El Niño event which should continue to strengthen over the next month before gradually weakening throughout the winter. El Niño is a phenomenon in the equatorial Pacific Ocean in which sea surface temperatures are warmer than normal.


Visualization of El Nino (null.earthschool.net)

This El Niño event is incredibly robust. It is likely the second strongest event in well over 100 years, only slightly trailing 1997-98.  However, unlike 1997-98, this event is Pacific basin-wide, meaning El Niño conditions extend from the South American coast almost all the way to the international date line. 1997-98 may have been a stronger overall event, but it was concentrated further east.

While we favor this winter to be warmer than normal, the strength of this current El Niño in the western flank is one factor that may help prevent it from being a repeat of the very warm (relative to normal) and snowless winter of 1997-98. El Niño events that are more east-based like 1997-98 can lead to conditions in the North Pacific conducive to warmer air masses in the U.S.

One thing that most moderate-to-strong El Niño events have in common, and this one should be no different, is an active subtropical jet stream. An active subtropical jet stream is indicated by storm systems entering the California coast and traversing across the southern half of the United States. Often these storms will come up the east coast. But even when they don’t, the D.C. area is frequently far enough south in latitude to be affected. It is because of this that we think this winter will be wetter than normal.

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, conditions often, but not always, tend warm and dry.


Time series of the Pacific Decadal Oscillation. The phase is currently positive. (NOAA)

Currently we are in the midst of a very strong positive PDO period. The PDO has been positive for 21 consecutive months, and we expect that to continue throughout the winter.

A positive PDO often correlates with upper level high pressure or a ridge over western Canada (and sometimes up to and north of Alaska) and a downstream area of low pressure or trough over the southeastern United States. A ridge in western Canada is a good mechanism for delivering cold into the eastern United States, but the cold air has to be available. That is one of the biggest question marks this winter.

Will the cold air source in Alaska and Siberia be available for us to tap?

Last winter, we had an extensive area of high pressure over and near Alaska that helped to deliver very cold air masses into the United States. This winter we think Alaska will be a battleground. Often, stronger El Niño events correlate with a broad area of low pressure near or over the Aleutians that extends into Alaska.  The further north and east this area is, the more likely colder air masses will be hard to come by.

We think warmer waters in the western El Niño regions (mentioned earlier) in combination with the very positive PDO may help to keep this persistent area of low pressure far enough west that we will be able to tap into some colder air masses on occasion.  However, this El Niño is so strong that warmer Pacific air masses will probably be more of the norm.

Arctic Oscillation (AO) and North Atlantic Oscillation (NAO)

The AO is a measurement of surface air pressure at the high latitudes over and north of Greenland. Pressures lower than normal indicate the positive phase, and pressures higher than normal the negative phase.

During the positive phase of the AO, cold air is characteristically locked up over the Arctic by a strong polar vortex and the mid-latitudes tend to be mild (left). In the negative phase of the AO, the polar vortex becomes disturbed, and cold air outbreaks become more likely over the mid-latitudes, including the United States (right).

Jet stream pattern during the positive AO (left) and negative AO (right) (University of Washington)
Jet stream pattern during the positive AO and negative AO. (University of Washington)

NAO, the AO’s cousin,  is technically a measurement of the differences in air pressure over the North Atlantic Ocean. 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. Often, though not always, the AO and NAO share the same phase, especially when averaged over the course of the winter.

A negative AO in the winter months often correlates with a cold pattern in our region, and supports winter storms when other factors align with it, particularly when we have a negative NAO as well. This was the major factor in our historically snowy winter of 2009-10.

High pressure over Greenland or high latitude blocking helps push the storm track farther south and east, often creating storm tracks that are cold and snowy for our region. Though a negative AO and NAO combination far from guarantees a cold and snowy period, our chances of a meaningful snow event are much greater than without it.

On the other hand, a positive AO and NAO combination typically supports a warmer pattern, with a storm track that will often go to our west. This is not always the case, as we averaged a positive AO and NAO the last two winters and still managed cold and snowy weather because of other factors being so strongly in our favor, plus a little bit of luck.

Because of the strong El Niño event this winter and the mild air it typically introduces, it will be very hard to get any meaningful snow events without a negative AO and NAO.

[Two geeky charts show why forecasting this winter’s snowfall is so difficult]

We think the AO and NAO will average positive this winter, but not significantly so. Over the last two winters we had a positive AO in the averages, and the NAO hasn’t averaged negative since the winter of 2010-11. While we believe this positive trend will persist this winter, sometimes El Niño correlates with high latitude blocking patterns. Even if both indices average positive over the whole winter, we expect to have periods this winter in which both are negative. It is during these periods when we should accrue most of our seasonal snowfall.

Past winter outlooks and evaluations

2014-2015 winter outlook
2014-2015 winter outlook recap
2013-2014 winter outlook
2013-2014 winter outlook recap
2012-2013 winter outlook
2012-2013 winter outlook recap
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

(Jason Samenow contributed to this post)

More from the Capital Weather Gang

Expert: Ramp-up in Siberian snow cover hints at cold winter for eastern U.S.

Local persimmons, acorns and hickory nuts are forecasting a harsh winter in D.C.