2008-09 Capital Weather Gang Winter Outlook
Does D.C. snow continue its decline?
For local snow lovers, it's the sorry truth: average seasonal snowfall has been dropping and dropping fast over the past 120 years. Around the turn of the 20th century, Washingtonians would typically shovel about 23" of snow (averaged over a 15-year period) per season. In the last 15 years, we've averaged a paltry 14" a year. The long-term trend is to drop about 1" every 15 years. At the current rate of decline, single-digit seasonal snow totals will become the norm in many of our lifetimes when they used to be the exception.
Keep reading for more on our snow starvation, and our outlook for snow and cold this winter. Stay with us all winter long for the latest forecast -- rain, snow or shine -- including accumulation maps, storm timelines, SchoolCast and more.
The decline in snow hasn't been entirely smooth. The 15-year average spanning 1919-1933 (of 13.7") was actually lower than the last 15 years. But in those 15 years, snowfall exceeded 15" seven times compared to just three times in the past 15 years.
Our recent snow drought becomes more depressing for snow enthusiasts when we remove the blockbuster snow seasons of 1995-1996 and 2002-2003 (when 46" and 40" fell, respectively) from the 15-year average. In the remaining 13 years, we've averaged a lowly 8". And not once in those 13 years did we exceed 18" -- the 120-year average. The graph below shows the feast or famine situation with snow in recent years, with a heavy bias towards famine.
In stark contrast to recent times, our grandparents and great grandparents feasted on the white stuff from 1904-1918 with a 15-year average of 25" (the highest analyzed). Even taking away the two snowiest seasons from that period, Washington still averaged about 20" a year and in 10 of those remaining 13 years, snowfall exceeded the long-term mean of 18".
The diminishing snow totals of late could be attributed to a number of factors: the urban heat island effect (i.e. the warming effects due urbanization), global warming, natural climate swings, and/or changes in observing locations (the official observing location at Reagan National Airport moved several times prior to 1941). It's difficult to say which of these is most important. But one thing is sure for D.C. snow lovers (myself included), the trend is not our friend.
Will we buck the trend this coming season? More than half of you who voted in our poll think we will. What does Capital Weather Gang think?...
While significant advances have been made in seasonal forecasting, there is still a great deal of uncertainty. So please keep in mind this is a low-confidence forecast, especially the overall snowfall estimate, where one big storm can make or break the forecast.
We expect this winter to be colder and snowier than last winter, and perhaps our coldest in several years. Though a historic winter, in terms of cold and snow, is unlikely. Here's how we see it:
Snowfall: Slightly below normal, but around the median
*National Airport (DCA): 12-14" (avg: 15.2"; median: 12.1")
*Dulles Airport (IAD): 18-20" (avg: 21.5"; median: 17.6")
*Baltimore-Washington Airport (BWI): 16-18" (avg: 19.2"; median: 15.5")
*Fairfax/Loudoun/Montgomery counties: 16-22" (avg: ~20-26")
*D.C./Arlington/Alexandria/Prince George's County: 12-18" (avg: ~15-20")
Overall 2008-09 Winter Temperatures: Normal to +1°F above normal
*December: -2°F to -4°F (below normal)
*January: Normal to +2°F
*February: +2°F to +4°F
Note: We did not include March in the temperature forecast, as here in the D.C. area the month tends to be more like spring than winter. Nevertheless, we do occasionally see snow events in March and even April, and thus the snow forecast is for the entire season (November-April).
HOW DID WE MAKE OUR FORECAST?
Analog Method Explained
There are several popular methods used to craft a seasonal outlook. The prevailing one for this outlook is the analog method. This entails looking at conditions leading up to and expected during this winter, and then finding past years, or analog years, that featured similar conditions. The idea being that winter 2008 will be similar to past winters that saw similar conditions leading up to and during the season.
The analog method is far from scientific. First, one has to determine which conditions are important and which are not, which is no easy task and very subjective. Second, even when you do come up with analog years, the associated winters may be quite different from each other. Therefore, analogs should be viewed collectively as general guidance for the upcoming winter and nothing more.
The following factors were among those that we deemed most important in identifying analog years...
Equatorial Pacific Ocean
Although many factors go into a seasonal outlook, the state of the equatorial Pacific is perhaps the most important. Last winter we had a classic moderate to strong La Nina event. A La Nina -- the opposite of El Nino -- is characterized by sea-surface temperatures in the Pacific Ocean along the equator that are colder than normal for an extended period of time.
The tell-tale signs of a meaningful La Nina event were evident throughout last winter, including a strong area of high pressure off the southeast Atlantic coast, which frequently put the D.C. metro area in the warm sector of clashing air masses. This, combined with a lack of high-latitude high-pressure systems, forced the predominant storm track to our west. And there was a volatile but progressive weather pattern. As a result, while we saw frequent shifts from warm to cold and back again, the warmth dominated, leaving us with our warmest and least snowiest winter since 2001-02.
As we approach this winter, the equatorial Pacific is in a neutral state, meaning we are neither in a La Nina nor an El Nino event, though some leftover effects of the La Nina event earlier this year have been apparent in the pattern this autumn. Future El Nino/La Nina (ENSO) state is hard to predict. Signs point toward a neutral winter, though very weak La Nina conditions may develop later in the winter. Winters where ENSO is neutral are especially hard to forecast for, as factors other than the Pacific can often carry greater significance. However, we did pay particular attention to past years in which neutral or weak La Nina conditions followed a moderate/strong La Nina.
This summer was another hot one for D.C., averaging almost two degrees above normal, thanks to a very hot June. However, this wasn't the case for the continental U.S.; outside the coasts, it was a cool summer for much of the heart of the country.
This fall started out quite warm with a hot September followed by a warm first half of October. However, a major shift in the pattern occurred in mid October, leaving most of the country, absent the west coast, with normal to below-normal temperatures for the month. We are seeing a similar pattern in November, as a very warm start for much of the country looks to be followed by a significantly colder-than-normal second half of the month, particularly in the East.
Based on the above factors, the following winters emerged as analogs for this upcoming winter, some carrying greater weight than others...
In chronological order (along with seasonal snowfall): 1934-35 (31.4"), 1943-44 (4.6"), 1945-46 (21.6"), 1974-75 (12.8"), 1985-86 (15.4"), 1989-90 (15.3"), 1996-97 (6.7") and 2000-01 (7.4")
The analog winters for this upcoming one tend to be quite volatile. While we don't expect the pattern to shift as frequently as last winter, winter 2008-09 may be defined by extended periods of significant warmth and cold. An extreme example of this type of winter is one of our analog years, 1989-90. That winter featured a late-November snowstorm, followed by one of our coldest Decembers on record. Snow was on the ground from Thanksgiving to Christmas. Then, a complete pattern flip led to a very warm and snowless January and February, followed by a record heat wave in mid- March. But winter wasn't quite done as we saw several accumulating snow events in late March and early April.
The extremes will be a bit more muted this winter, we think, but we are likely to see some dramatic pattern changes throughout the season.
Other factors considered include persistence (what have recent D.C.area winters been like?) and climatology (the long-term averages for winter temperatures and snowfall in the D.C. area). Over the past 15 winters in D.C., only five featured below-normal temperatures and only three finished with normal to above-normal snowfall (see snowfall graph of last 15 years above). The last winter to do both was 2002-03, and we have now had four consecutive winters with above-normal temperatures and below-normal snowfall.
Our more recent history has been even more skewed toward warmth, as only six of the last 30 months have featured below-normal temperatures. The last was this past May. There are likely multiple causes for this phenomenon, but suffice to say, getting a cold month has not been an easy task. While this didn't factor into our selection of analog years, it was a factor in issuing our final forecast numbers.
More Snow Stats
Least snow winters--
30-yr DC snow average (1979-2008): 15.3"
30-yr DC snow average (1949-1978): 16.7"
30-yr DC snow average (1919-1948): 16.3"
30-yr DC snow average (1889-1918): 23.7"
60-yr DC snow average (1949-2008): 16"
60-yr DC snow average (1889-1948): 20"
120-yr DC snow average: 18"
How'd we do with last year's winter outlook? See our recap here.
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