November 7, 2017 at 6:30 AM
The inevitable arrival of winter comes every year, and often without much warning. After being in a persistently warm pattern since mid-September, there are finally signs of winter — a brief cold snap is predicted later this week. In a mere month, average high temperatures fall into the 40s — a far cry from the present near 60.
With the official start of winter (Dec. 1) less than a month away, we present our annual winter outlook.
More ‘winter’ than the last one, but nothing extreme
To the delight of many and the despair of some, last winter was very warm with very little snow. A meager 3.4 inches fell at Reagan National Airport and 7.3 inches at Dulles International. But this winter without much winter followed three straight featuring above-average snow.
If anything, Washington winters have proven wildly variable in recent years. This winter is sure to take on its character, but will it be memorable? We think it will end up pretty ordinary but should deliver more of a punch than last winter, which closed six degrees warmer than average.
Temperatures should be noticeably colder than last year, though still slightly above average. We also expect more snow, though a little less than the average of 15.4 inches at Reagan National Airport.
Multiple snow events, but chances for a blockbuster lower than normal
Overall, we think we’ll have five to seven accumulating snowfall events for the immediate D.C. metro area, with a couple more in the outer suburbs. This doesn’t include dustings or ice events, as we are likely to have some of those, too. Most of our wintry weather will come from clippers and storms that cut to our west, though we can never rule out a classic nor’easter.
Clippers are typically moisture-starved and only drop a dusting to a couple of inches of snow.
When storms take a track to our west, most of what usually falls is rain. However, when there is enough cold air in place before the storm, it starts as snow before usually changing over to a mix or just rain. These are tricky forecasts, and we think we’ll have our share of them this winter.
Winter gets going in January
While we’ll have plenty to talk about this winter, we will still have our typical stretches of dry “boring” weather, particularly in December.
Our best chance of feeling the full weight of winter is in January, when we might see some persistent colder and stormier periods, and we think the month as a whole will end up slightly colder than normal.
Overall, we expect temperatures for December through February (relative to 1981-2010 normals) to be slightly (near one degree) milder than average:
Our snowfall projection covers November through April (1981-2010 statistics in parentheses). Overall, we expect slightly below average snowfall, though near the median:
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. Furthermore, it takes only one big snowstorm for us to near or exceed our seasonal average.
What are other outlets forecasting for our area?
Most outlooks issued so far are, on balance, broadly consistent with our expectations of a winter that is pretty close to normal. In short, here are what some other weather outlets are predicting for our region:
The National Weather Service, which issued its winter outlook Oct. 19, leans toward warmer than average conditions but said it is unlikely to be as warm as the past two winters. It also hinted at somewhat less snow than normal in the Mid-Atlantic. We agree.
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 for the past 11 winters. We’ve generally been in the ballpark, giving ourselves an average grade around a B- or C+, although we’ve had notable triumphs and misses.
Last winter’s forecast had mixed success. We called for slightly below normal snow and slightly above normal temperatures, and snowfall ended up much below normal and temperatures were much above normal. We gave ourselves a grade of C for the outlook.
Our best winter 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 the winter of 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 winters 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 the average.
Below are some, though not all, of the factors that we considered 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 1984-1985 and 2005-2006. These analogs helped to loosely 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 the infancy stages of a weak La Niña event. La Niña is indicated by anomalously cold sea-surface temperatures in the equatorial Pacific Ocean.
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.
Usually in weak La Niña events, while we experience frequent and often brief oscillations from warm to cold and back again, the cold outbreaks are typically dry. However, not all La Niñas are the same, and there are other factors that drive our weather.
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, trend warm and dry.
We are currently in the midst of a persistent positive PDO period. According to one popular datatset the PDO has been positive for 45 consecutive months. Though there are currently signs that this lengthy positive PDO period is waning. Even though La Niña often lends itself to a negative PDO, we expect the PDO to average neutral or slightly positive this 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 if the cold air is available to tap.
However, La Niña is usually indicated by a trough over western Canada. That is one of the bigger question marks. Despite a neutral or slightly positive PDO regime, will typical La Niña conditions win out?
At this time we are favoring a mean trough over western Canada with a mean ridge over the southeast. However, we still expect the pattern to oscillate throughout winter, with no permanent setup.
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. 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.
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 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.
We think the AO will average neutral to slightly negative this winter, with the NAO near neutral or even slightly positive. Consequently, while the predominant high-latitude pattern will be amenable to cold outbreaks at times, the upper-level pattern near and over Greenland might act as a mitigating factor, both in terms of cold air and storm track, which we expect to mostly be to our west. That said, we think there will be a window, perhaps lasting a couple weeks, when both indices link up and go sharply negative, enhancing our chance for snow events. We think this is most likely to occur in January.
Summer and fall pattern
While not approaching record levels, we did have a classic hot D.C. summer, averaging about one degree above normal. Although we experienced a cooler period from late August to mid-September, the past six weeks have been persistently above normal. October finished more than 5 degrees above average, ranking third warmest on record. While correlations between summer-fall and the upcoming winter are weak, we did consider the months leading into winter in selecting analog years for this outlook.
Capital Weather Gang’s Jason Samenow contributed to this outlook.
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