Did it seem like March was not only cold, but also very windy? That’s the truth: D.C. experienced the windiest March in over 30 years. And five of the past seven Februarys have ranked among the 15 windiest in Reagan National Airport’s recorded weather history.
While historical data shows March and winters in general have been trending less windy over the long term, average speeds are ticking up again. The tendency toward more frequent winter blocking patterns is helping to enhance these winds and – if we are indeed entering a period of stronger blocking – may continue to keep speeds elevated in the future.
The average March wind speed (using peak gusts or fastest five-second wind speeds) since 1953 clocks in at 27.2 mph. In March 2013, the average speed was 30.5 mph, the highest number since 1981’s 32.3 mph. Among all March’s in DCA history, the 30.5 mph speed in 2013 ranks 8th highest. See the table to the right.
Note that I’ve included the corresponding March NAO value in the table. The NAO, or North Atlantic Oscillation, is a climate pattern that indicates the strength of high pressure or blocking (warmth) over Greenland and surrounding areas in the Arctic. A negative NAO value means there is significant blocking (warming) in place across these regions which typically, in wintertime, forces frigid air over the eastern U.S.
Remarkably, the NAO was negative during each of the 10 windiest March’s on record. This suggests that blocking, which forces the jet stream (and cold air) farther south, enhances winds in our region. The upper low pressure system that characteristically sets up and spins over southeast Canada in this pattern sends waves of energy southward into the Northeast and Mid-Atlantic. These waves induce strong gusts at times, as well as the occasional snow shower and graupel we saw last week. And, sometimes, a dynamic storm system can track close to the region, creating a strong pressure gradient (large difference in pressure over short distances) and drawing high winds into D.C. as it pulls away.
At the jet stream level (30,000 feet or so), the negative NAO pattern supports two streams of stronger winds: one running westerly across the North Atlantic Ocean and into eastern Canada, and the other extending eastward from the southern U.S. through the central Atlantic. D.C., which lies at the intersection of these two enhanced air streams, contends with higher winds from the upper low to the north and the stormier pattern to the south, both thanks to the blocking pattern.
The long-term trend of March wind speeds is pointing down, but it seems to follow the direction of the NAO index. Most of the windiest Marches occurred in the three decades spanning the 1950s and 1970s – a period during which winter NAO values averaged mostly negative. From the 1980s through the late 2000s, the NAO has averaged positive, aligning with several of the least windy Marches on record; these include years like 1973 and 1990 (23.2 mph averages), along with 1995 and 2003 (23.6 mph).
It’s not only the negative NAO that signals a windier period; consider, also, the negative AO and its influence on D.C. winter winds. The AO, or Arctic Oscillation, is another pattern that reveals the state of high pressure and blocking in the high latitudes. A negative AO tells us that blocking – in some cases strong – has developed over and around the North Pole.
Winter (again, defined as the period from November-March) wind speeds (again, using peak gusts or fastest five-second wind speeds) have averaged 25.4 mph over the 61-year span since 1952-53. For the winter just completed (2012-13), the average speed was 25.7 mph. Over the five-winter period from 2006-07 to 2010-11, the average speed was 26.2 mph, an increase from the 1997-98 to 2005-06 period, within which speeds averaged 24.6 mph. Average speeds from that same 2006-07 to 2010-11 period, however, fall short of matching the 27.1 mph average achieved early in the record (1952-53 to 1959-60). Even here, we can observe that the higher winds recorded from the 1950s-1970s generally align with the negative AO. The inverse argument – that the lower speeds observed from the 1980s-2000s align with the positive AO – is also valid.
A look at the top 10 windiest winter seasons buttresses the negative AO argument. The table below shows the average speeds recorded in each of these seasons along with the corresponding average November-March AO value.
The pattern at 500 mb (or roughly 20,000 feet above ground level) vividly shows the blocking ridge – and associated very warm air compared to normal – as well as the cold trough over the Eastern U.S. This setup steers low pressure systems – some of which develop powerfully – along or just off the East Coast.
At a higher level (250 mb), the wind field again features enhanced flow over the North Atlantic Ocean westward into eastern Canada, and across the Southern U.S. eastward into the central Atlantic. The interplay between the Quebec upper low and persistent storminess to the south and east places D.C. in prime position; strong winds may blow in from an approaching storm, or we will get the backlash from said storm (high pressure arrives in its wake, increasing the gradient and, hence, strengthening winds). If the storm track is well offshore, the Mid-Atlantic is still “stuck” within a larger-than-normal gradient caused by the Canadian low and high pressure to the north and west.
This past winter’s bitterly cold wind chills were not kind to the ears and hands. If we have indeed entered a cycle of stronger blocking patterns, high winds – and the occasional cold outbreak – will continue to grab headlines over the next 25-30 years. D.C. won’t vie with Chicago for “The Windy City” moniker, but Washingtonians might want to stock up on earmuffs and gloves.
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A word about the wind data analyzed for this analysis. The airport’s period of record dates back to 1945, but there are large gaps in this data through October 1952; therefore, this analysis starts with the winter of 1952-53. I define winter as the five-month period from November through March for the purposes of this study, since some of the strongest winds occur in November and March in concert with a transitioning jet stream.
As for the type of wind data I studied, Reagan’s records show only the peak wind gust speed in uninterrupted form through January 1998. The peak wind gust speed data disappear beginning in February 1998, so, from that point on, I compiled the data for fastest five-second wind speed (which comes closest to imitating the peak gust speed).