Capital Weather Gang 2014-2015 winter outlook for Washington, D.C.: Colder and snowier than normal

Gulls at the Tidal Basin. The mounds of snow in the foreground blanket paddle boats which soon will be filled with tourists. (Kevin Ambrose)

SUMMARY

Our average high temperature, still around 60 degrees in early November, will plummet to the mid-40s in a mere 6 weeks. In fact, we’ll get a taste of that kind of December chill starting Thursday. 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.

Get the full experience.Choose your plan

While we are predicting similar cold – on average – to last winter, we’ll get there with more consistency, but less intensity. In other words, we do not expect cold outbreaks to pack the same punch as last winter, but at the same time, we don’t believe our warm thaws will persist as long. The cold pattern will be more stable, but less severe.

We had a whopping 13 accumulating snow events at Reagan National Airport last winter. We’re not thinking that many this year. Closer to 8 accumulating events is a reasonable expectation, with a few more the farther you get outside of the Beltway to the north and west.

The chances of a crippling event such a Snowmageddon or the Blizzard of 1996, while always slim, will be somewhat elevated this year. A widespread event of 6-10 inches is more likely — perhaps a 50-50 chance. As always, some of our events will be of the smaller variety (fewer than 3 inches). But in a colder than average winter, which we expect, the chances are higher that small events could carry an impact more significant than their totals.

We’re forecasting January to be our snowiest month, more likely to be focused on the latter half when our climatology is more favorable for snow. Keep in mind that our average seasonal snowfall is skewed by a small handful of blockbuster winters like 2009-10, 2002-03, and even to a lesser extent, last winter. Our median snow is quite a bit less than our “normal” snow. That is why we can get a winter with just slightly above normal snow, and yet amounts are greater than we attain in most winters. This should be one of those winters: only slightly above normal snowfall, but about 150 percent of our median.

In summary, we expect a colder than average winter, with more snow than we usually receive, but we are less likely to be discussing incredible cold shots like we did last winter. The discussion this winter is more likely to be focused on the duration and persistence of the cold. And we don’t need a crippling blizzard to have plenty to discuss in the snow department. We will have our share of closings, delays, and tricky forecasts. If you love warmth and hate snow, this probably won’t be your type of winter.

See below for more specifics and details.

OUTLOOK DETAILS

Temperatures

Overall, we expect temperatures for December through February (relative to 1981-2010 normals) to be somewhat (1 to 2 degrees) below average:

December: average to 1 degree colder than average
January: three degrees colder than average
February: one to two degrees colder than average

Snowfall

Our snowfall projection covers November through April (1981-2010 statistics in parentheses). Overall, we expect slightly above normal snowfall:

Reagan National Airport (DCA): 16 inches (compared to a 15.4 inch average, 11 inch median)
Dulles Airport (IAD): 25 inches (compared to a 22.0 inch average, 16 inch median)
BWI: 21 inches (compared to a 20.1 inch average, 15 inch median)
Fairfax, Loudoun, Montgomery counties: 20-30 inches
Arlington, Alexandria, Prince George’s County, and the District: 16-22 inches

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.

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.

FREQUENT QUESTIONS

What are other outlets forecasting for our area?

Of the outlooks we’ve reviewed, most are predicting a colder and snowier than normal winter.

* National Weather Service: Equal chances of above or below normal temperatures, and a slightly enhanced chance of above normal precipitation.
* AccuWeather: Somewhat colder and snowier than normal – very similar to our outlook
* Dave Tolleris, WxRisk: Somewhat colder than normal, snowier than normal
* WeatherBell Analytics (Joe Bastardi): Colder, snowier than normal
* Two Baltimore meteorologists: Colder and snowier than normal
* Old Farmer’s Almanac: Cold, snowy
* Farmers’ Almanac: Crisp and stormy

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 7 winters. We’ve generally been in the ballpark giving ourselves an average grade around a 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”+ are much higher than normal this coming winter.”

Our worst outlooks were for the winter of 2011-2012 and last winter. In 2011-2012, we called for near normal temperatures and it was five degrees warmer than normal. Last winter, we called for a warm winter with slightly below normal snow and it was cold with snow more than twice average.

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 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 that have proven 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 1939-40, 1969-70, 1976-77, and 1993-94. Other analogs that informed our outlook to a lesser extent include 1952-53, 1958-59, 1977-78, 1979-80, 1992-93, 2003-04, 2004-05, 2006-07.

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 have medium-high confidence we are going to experience an El Nino event. El Nino is a phenomenon in the equatorial Pacific ocean in which sea surface temperatures are warmer than normal.

We expect this El Nino event to be on the weaker side. In fact, we may not even reach official El Nino status, which requires conditions to persist for a certain duration. But that is splitting hairs. We are currently on the brink of El Nino conditions, that could become established for a couple months, or even beyond.

Not all weak El Nino events are alike. Some are associated with winter warmer than normal, some colder. Some tend toward the snowier side, and some not so snowy. So we believe it will be a colder winter, not because of El Nino alone, but when combined with the other factors we discuss.

In terms of snow, while weak El Nino events are not usually very prolific seasonal snow producers, they very rarely yield super low seasonal snowfall, which is partly why we think we have a 90 percent chance of exceeding 6 inches this winter. If we end up not getting an official El Nino event, it really doesn’t change much to our outlook, perhaps a tad less snow.

If El Nino is stronger than we are expecting, while our temperature forecast wouldn’t change much, we would likely get more snow than we are forecasting. More robust El Nino events tend to have a stronger southern branch of the jet stream, and thus better chances of a significant storm system coming from our south and southwest. The kind that give us our big snowstorms. In terms of what could go wrong with our outlook, that is our single most vulnerable area. Should El Nino unexpectedly strengthen to a moderate event, expect snow totals to exceed what we are predicting.

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, the PDO 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, but that doesn’t mean every winter will have a negative PDO.

We are currently in a period that is running counter to the predominant or default phase. In fact, we have had a positive PDO for 9 consecutive months now.

Often an El Nino will help “force” the PDO to go positive or in our case, stay positive. Given the positive period we have been in for 2014 so far, and the expected El Nino event, we expect the PDO to average positive this winter, though not strongly so. 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 U.S. This pattern tends to be favorable for cold and snow, especially when aligned with a negative Arctic Oscillation and North Atlantic Oscillation (see below).

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 U.S. (right).

The AO’s cousin, the NAO 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. While a negative AO and NAO combination far from guarantees a snowy period, our chances of a meaningful snow event are much greater than without it.

Due to the combination of our October pattern in the North Pacific, the budding El Nino event, and the incredible extent and advance of Eurasian snowfall this fall, we believe the AO will be somewhat to sharply negative this winter, and a significant driver of our cold and sometimes snowy pattern. We expect the NAO will average negative over the winter, as well.

Matt Rogers, Wes Junker, Jason Samenow, and Ian Livingston offered input into this outlook.