Outside of the December 5 "clipper storm" that brought 3-5" to most of the DC metro area, the metro region has seen little to no snow. And it's been warm. December finished over two degrees above normal at Reagan National Airport (DCA) for DC's eighth consecutive month with at or above normal temperatures. As we approach what is statistically our coldest period of winter (January 9th - January 23rd), we are basking in spring-like conditions with temperatures in the 60s. Will it ever snow again?
Before we assume we'll cruise to a relatively snow-free winter based on the paltry snow totals to date, let's consider our climatology. Historically, the DC area does not receive a whole lot of snow in November and December. November and December combined only average 2.2" of snow per winter, or only about 15% of the area's seasonal norm of 15"-16". As the clipper storm on December 5th officially dumped 2.6" at Reagan National Airport, we actually slightly exceeded our average for the early winter season. Sitting here in early January, we still have nearly 80% of our snow statistically ahead of us, so snow lovers should actually be encouraged, right?
Well, we know that statistics rarely tell the whole story when it comes winter. For most folks, experience trumps statistics, and as snow lovers who have seen nary a flake in over a month, we are frustrated. It doesn't matter that we are performing OK by the numbers. We are roasting in early January for the fourth year in a row, and prospects seem grim. The only thing that will save us is not statistics, but a change in the pattern.
We are in the midst of a moderate La Nina event, our first moderate event in eight years (1999-2000 winter). La Nina is an oceanic phenomenon in which the sea surface temperatures in the equatorial Pacific Ocean run 1-2 degrees C below normal. This is the opposite of the more widely known El Nino, when such temperatures run above normal.
La Nina produces some predictable effects on our weather here on the East Coast, and this winter is performing according to form. While weak La Nina events are more amenable to snow and cold, and strong events to little snow and warmth, we are in a battleground during most moderate events. This La Nina event has maintained its strength heading into early 2008. One of the telltale signs of La Nina, the southeast ridge, has been a sometimes dominating factor through early winter.
The southeast ridge is a ridge of high pressure off of the southeastern coast, often centered over Bermuda, which is responsible for warmer than average weather in the Southeast up through the Mid-Atlantic. The southeast ridge has kept our region a battleground between cold and warm weather regimes during the early part of winter. Places that are very susceptible to the influence of the ridge include Raleigh, N.C., which recorded a December that was almost six degrees above normal with no snow. On the flip side, the ridge's warm air has not reached locations well to the north in southern New England, while steering storms in that direction. Boston, for example, recorded one of its snowiest Decembers on record. So what does that mean for us and the prospect of snow?
Well, first we need the southeast ridge to relax. While its presence in a weaker form can help force snow-producing storms up the coast, so far the ridge has been too powerful. While cold air has occasionally beaten down the ridge enough to make it down here from Canada, storm tracks have mainly been north and west of us, giving plenty of snow to the interior northeast and New England, but mainly keeping us in the warm sector with cold rain. The relaxation of the ridge will allow even more penetration of cold air and potentially shift the storm track to our south and east, a traditional snow maker for us during winter. What else do we need to happen?
It is probably pretty evident that in our area, we need several ingredients to come together to usher in a snowier pattern, not just one. This isn't New England. Nobody panics at the prospect of snow quite like Washingtonians, and that is partly due to its relative rarity. Some other ingredients, but not all, that make up the classic snow pattern for the region include the North Atlantic Oscillation (NAO), the 50-50 low, and the Polar Vortex.
While you don't need every ingredient in place to get snow here, the more you have in place the better. Let's start with the NAO. The NAO is traditionally measured by the difference in pressure between Iceland and the Azores. When this index is negative it is often indicated by a block of high pressure over Greenland. When this block is strong and well-placed, it tends to lock the cold air in place over the East Coast, which is critical for snow in our region. The negative NAO often operates in conjunction with another helpful snow ingredient, the 50-50 low.
The Newfoundland, or 50-50, low, so named because it often occurs at 50N latitude and 50W longitude, is an upper level low pressure system with closed circulation that spins off the coast of Newfoundland. Coupled with the Greenland block as described above, the 50-50 low keeps cold high pressure from sliding out to sea and helps force storms to cut below our area and up the coast, often resulting in whiter rather than wetter conditions for our area. But what if we don't have any cold air in place to lock in? This is where the polar vortex, essentially our cold air supplier, plays a major role.
The polar vortex is an upper level area of low pressure which often resides near Hudson Bay in Canada. The closer it gets to the U.S., the more cold air it brings from Canada. A strong polar vortex diving south towards the U.S. provides critical cold air for a snowy pattern in our region. But when the vortex is really weak or far away (displaced on the other side of the north pole), there is not enough cold air to support snow in our region. So far this winter, the polar vortex has not been well positioned to consistently provide the necessary cold air for lots of snow.
While snow lovers are certainly frustrated that a snowy pattern with the potential to bring several storms to the region has yet to materialize, there is hope. In our last moderate La Nina event (1999-2000) we saw a 2-3 week period of lots of snow and cold (despite an otherwise warm, snow-lacking winter). There are indications that toward the middle to end of January, the ingredients for a snowier pattern may come together, although snow lovers in our area will always be better off keeping expectations low. There is plenty of winter left to get a more snow favorable pattern in place, but the truth remains a lot needs to come together for a snowy pattern in the metro area. Let's just hope it isn't in late March or April when it happens. I think we all can agree that the last thing we want is a delayed spring.