The polar vortex and its connection to climate change

The bitterly cold Arctic air that spilled into the Tri-State last week, and again this morning, runs counter to the message we've been hearing from meteorologists and climate scientists around the world: 2016 is on track to become the hottest year on record. While most people understand fully that it can occasionally get cold, despite an overall warming pattern, it still is a curiosity. If the Earth is getting warmer, where is the unusually cold air coming from?

The answer lies with the Polar Vortex. This weather phenomenon is certainly nothing new-- it has existed as long as Earth has had an atmosphere-- but has only gained widespread recognition outside meteorology circles in recent years. This term is used for the frigid air that semi-permanently resides over both the North and South Pole. The Polar Jet Stream keeps this cold, dense air contained to the high latitudes through most of the year. The strong winds of the Polar Jet are created in part by the contrast in temperatures between the high latitudes and mid-latitudes. When the temperature gradient from north to south is weakened, the jet-level winds respond by slowing down. Slower winds are a weaker barrier and are easier for a polar airmass to penetrate (see video).

In the Northern Hemisphere, the Polar Vortex sits over the Arctic Ocean, which is a much less stable surface than at the South Pole, where the landmass of Antarctica encompasses nearly the entire Antarctic Circle.

The Arctic Ocean has shown its susceptibility to climate change, especially in recent decades. Sea ice, which typically covered the ocean's surface year-round during the first half of the 20th Century, has been melting more thoroughly than in decades past. The thinning sea ice is not just a sign of milder ocean temperatures beneath; it also exacerbates the warming process because snow and thick ice reflect the sun's rays, whereas thin ice and ocean water absorb much more solar radiation, making the Arctic ocean and surrounding air that much milder. Though the entire  Earth has seen a rise in average temperature since the beginning of the 20th century, the change is starkest north of the Arctic Circle, as seen in the picture below.

Global average temperature anomaly in 2015. Red and orange areas have the highest positive temperature anomaly. (Courtesy of NASA)

This has profound effects on the Polar Jet's strength. The strength of the jet stream is directly proportional to the temperature gradient, or change in temperature over a given space. So when the Arctic gets disproportionately warmer than the rest of the Northern Hemisphere, the gradient weakens, and so does the Polar Jet. Weaker winds make it easier for Arctic air to head south. So, a weak Polar Vortex can bring a strong Arctic outbreak to the continental United States. 

If global temperatures continue to rise, or even if they remain steady, we can expect that the Polar Vortex will become more unstable, since Arctic sea ice extent continues to shrink to new record lows.  http://svs.gsfc.nasa.gov/4494 This could result in even more frequent and dramatic temperature swings than the 33-degree drop in temperature we endured between Sunday afternoon and Monday morning. 

Snowiest March in decades! Is winter over yet?

An image from Front Royal, VA
during the March 17th storm.

An image from Spotsylvania, VA
during the March 3rd storm. 

The St. Patrick's Day snowstorm we just endured will go down in the record books as the biggest storm on that date in DC history. The previous St. Patrick's Day record was only 1.9" of snow. While this storm easily broke the record with 3.9" on the holiday itself, the real wallop was with the storm total. We got 7.2" of snow at Reagan National from Sunday night into Monday morning! This storm, plus the early-March snow and ice, bring us to a grand total of 11" for March 2014. This is the snowiest March on record since... can you believe it... 1960?!?! It's been half a century since DC has had this much snow, this late in the season!

"SnowPatricksDay", as it was dubbed, seems to be the last straw for many Washingtonians, even those who normally love the snow. Our seasonal snowfall total is now almost exactly double the 30-year average, at 30.3" (The average is 15.4"). However, this number is still not high enough to crack the top 10 snowiest winters in DC. So now the question is, will we get one or two more snowstorms that push us over the edge, and into the top 10?

While it is possible, of course, that we could get another big snow storm or two, it's highly unlikely. As of today, we are exactly nine inches short of creating a tie for the 10th-snowiest winter in DC (39.3" in 1910-1911). A snowfall of nine inches or more in DC has only happened twice this late in the season in recorded history (in 1942 and in 1891). And these storms were extreme outliers; on average, after March 17th, we only get 0.2" of additional snowfall for the entire season.

April is notoriously sparse for snow-lovers. The most recent April snow at Reagan National happened in 2007, but it was a paltry 0.4" of accumulation. Since 1888, there have only been nine April snowfalls of 1" or more, with the most recent one happening in 1924! Yep, that was quite awhile ago. We can only expect accumulating snowfall (more than a trace) at Reagan National about once every ten years. Add in trace snowfall events, and the number only increases to two out of every ten years. That means, most of the time, we do not see any snowflakes in the month of April in Washington. By stark contrast, DC gets accumulating snow nine out of every ten Marches, and we have never had a March in recorded history without at least a trace of snowfall in the month. So, the frequency of snow events rapidly declines from March to April, and so does the amount of snow in each event!

One last thing... a snowy March doesn't increase the odds of getting snow in April. Since 1888, there have been 24 Marches with at least 6" of snow. Only three of those Marches were followed by snow in April of more than a trace. Statistically, it means that there is no correlation. This winter has been surprising at every turn, though, so maybe we'll get one last surprise to finish out this sneakily snowy season!

Long-range forecasts are "Snow Joke"

There has been a LOT of buzz on social media and around the watercooler over the past couple days about the potential for a major storm next weekend. No, not Super Bowl weekend like the Farmer's Almanac predicted, but the following weekend. I hope that this talk raised your eyebrows and your level of suspicion about the accuracy of such a forecast. 

The biggest problem with this forecast is that it is way too far in advance. The rumor mill began on January 29th, which is 12 days prior to the supposed arrival of this storm. On that date, the European computer model's long-range output showed a massive storm for the Mid-Atlantic region. But that is just one run from one computer model; it's not nearly enough information to make any sort of prediction! It's like seeing a photograph from a city block away, through a frosted pane of glass, and saying that your view of the photo is crystal-clear. I'm going to show you a few maps so you can really visualize how much the forecast can change, based on small changes in the model output!

First of all, the forecast will change from one model run to the next. A model run is each instance that the forecast model receives new data ("initial conditions" from satellite and radar data, weather balloons, and other sources). Most forecast models are run 4 times per day. 

The GFS, or Global Forecast System, is the primary model used and maintained by NOAA. Last year, the GFS received the first of two upgrades, which will boost its computing power tremendously. (You can learn more about the upgrades here.) Even with the upgraded computing power, the model still has wide variations from one model run to the next! Take a look at the different forecasts from the GFS model, valid for 12Z Sunday. The map shows the GFS's prediction for precipitation during the 6-hour period leading into 7am on Sunday, February 9th. 

These two model runs happened 6 hours apart from each other, and check out the huge difference in the placement of the precipitation! The same computer forecast model came up with two very different ideas for Sunday morning's precipitation potential. 

Now, let's compare two forecast models against each other: the GFS and the ECMWF (the European model). This time, I'll use the same model initialization time, but I'll still show you the same forecast time of 7am on Sunday, February 9th. 

Here, the difference is even bigger! The European model keeps us dry during the early morning hours... a huge contrast to the approximately 1.00" of liquid precipitation over the same 6-hour period that the GFS model is predicting. This model variability is extremely common in the long-range, which is why we professional meteorologists don't put any stock in any extreme weather predictions that they might produce! 

I want to make something clear... I'm not saying that a big storm is impossible in the 2nd weekend of February. All I'm saying is that it's way too early to make any sort of prediction. Hopefully, now that you've looked at the highly variable model data with me, you can see why it's too early to make such a forecast, and you understand why there aren't any reputable weather sources that are making such a bold prediction! 

Understanding the new findings on climate change

Note: This blog post originally appeared on wusa9.com on October 1st, 2013. Go to wusa9.com/weather for more blog posts from the First Alert weather team of Erica Grow, Topper Shutt and Howard Bernstein. 

The Intergovernmental Panel on Climate Change issued its full report on Monday, detailing the latest findings, assessments, and predictions about our current global climate system and how it will change in the future. This is the fifth such report from the IPCC, a colossal joint effort between scientists specializing in a broad range of disciplines, all sharing their knowledge, research and data to make this collaboration possible.

Before I delve into any findings in the report, it should be mentioned that scientists are famous (or notorious) for not using definite statements. As a meteorologist, for instance, I abhor saying that there's a 100% chance or 0% chance or something happening in the weather. In this IPCC report, the scientists refer to the level of confidence they have with phrases such as "virtually certain" and "medium confidence". All of the confidence-defining words and phrases are defined within percentage ranges. Suffice it to say, they take this stuff very seriously. In this article, I italicized the confidence terms used in the Report, just like they did.

Of course, when trying to make statements of confidence, these scientists were up against some big barriers. Weather records only extend back as far as a city, airport, or weather observation station has been in place. Gathering climate data from before weather instruments were widely used, or even before they were invented, can be tricky. But there is "paleo-evidence", if you will, in the form of ice core samples from glaciers, data from tree rings, rock layers in the geological record, and other data sources. So, we can't know what the high temperature was in Washington DC on today's date 100,000 years ago, but we can tell what the gas composition was of the atmosphere, and what type of plant and animal life existed. Scientific data is advanced enough today that we can use these pieces of evidence to see what the climate was like, dating back hundreds of millions of years. It's this inter-disciplinary collaboration within the scientific community that makes this report possible.

Most of the findings can be summed up as a strengthening of confidence in the conclusions that were previously drawn in earlier IPCC Reports. I'll focus on the findings for our part of the world- the midlatitudes of the Northern Hemisphere. In the report, the Panel talks about the long-term cooling trend, which had been in progress for about 5,000 years in the Northern Hemisphere. The report says, with high confidence, that this cooling trend has been reversed in the past 50 years or so, in the mid-to-high northern latitudes (places like the USA, Canada, Russia, and Europe). This is the part of the globe which has the most land mass, which may be part of the reason that the trend reversal is so evident here. But they also say in the report that the data collection is the most thorough in this part of the world, adding to their confidence in the data and findings.

Another big conclusion from the Report is that it is likely that atmospheric circulations have shifted poleward since the 1970s. What does this mean? Circulations, such as the jet stream, are now traveling at a higher latitude on average than they did 40 years ago. The jet stream serves as a superhighway for storm systems, so a different storm track will bring more precipitation to areas that are accustomed to drier weather, and vice versa. Since the jet stream also divides airmasses of different temperature and moisture levels, a shift in its pattern could affect growing seasons, impact water usage, and steer tropical systems in abnormal paths.

The Report also concludes with very high confidence that glaciers and ice sheets around the world are continuing to shrink. And, in one of those weird quirks of thermodynamics, the ice hasn't been able to melt as fast as the temperature has changed. This means that, even if Earth's average temperature were to suddenly become static, the ice would continue melting.

Scientists from 39 countries around the world were involved in the IPCC report in some way. On Monday, five of the key players took part in a Google hangout to provide their insights. They said that the newest report raises confidence in the data supporting human-influenced climate change from 90% in the last report to 95% in this latest report. The increased confidence is mainly because of better technology for assessing ice sheets at the Earth's poles. According to Eric Rignot of the Jet Propulsion Laboratory, the Report represents "multiple lines of research converging on the same answer", resulting in such a high level of confidence. As far as the short-term effects are concerned, Dr. Ralph Kahn of NASA Goddard said that "wet places are likely to get wetter, and the dry places are likely to get a bit drier".

It's very important to remember that our climate has ALWAYS changed. You've heard of the Ice Ages, right? And when dinosaurs roamed Earth and grew to unfathomable sizes, our planet was in the warmest pattern of its history. However, today's global climate is influenced by human activity. The scientific community at large is virtually certain of it.

This blog post really just scratches the surface; there's a ton of data assessments and findings in the Report. If you'd like to check out the research for yourself, the technical summary is available here (warning: It's LONG, and it's very scientific). Or, you can check out the IPCC's page.

Humberto... Stunningly close to a record

This morning, the National Hurricane Center issued an advisory stating that Humberto, a tropical system in the Atlantic Ocean, had officially become a hurricane, with winds reaching above the 74mph (65kt) threshold. There is nothing remarkable about this hurricane. It's a Category 1 storm, it isn't a threat to land over the next 5 days, and it is forecast to be a hurricane for a short amount of time. I guess it's fitting, because this storm is just barely missing out on having its own unique place in history!
If Humberto had gained strength just a little bit later in the day, it would have set a new record for the latest first hurricane in a calendar year during the satellite era. What do I mean about the satellite era? As I mentioned in an earlier article, the first weather satellite wasn't even launched until 1961. That means we couldn't see tropical systems from above, and it's pretty easy to spot most tropical storms on satellite imagery because of their circular structure and rotation (see picture below).

A classic hurricane structure is evident when viewed from above,
using modern satellite imagery. (Courtesy of USAToday)

It's not just the satellite technology that helps us spot storms. Also, the system of using reconnaissance aircraft to study hurricanes was first used in the 1940s, which is decades after we started keeping official records of tropical storms and their impact on the United States. The aircraft's structure, and its data collection methods, are much more sophisticated now than they were back in the '40s. Today, we fly airplanes into developing systems to get accurate measurements for central air pressure, core wind speed, and other parameters. It's possible that, because of our more advanced technology, there are storms that were out there in the ocean and were never acknowledged as being tropical storms or hurricanes!
Based strictly on the data we have, the "official" record for the latest hurricane belongs to Hurricane Alice, going all the way back to 1954. As mentioned above, this is before the satellite era. Alice became a Tropical Storm on December 30th, and a hurricane on December 31st! The Atlantic Basin almost got through an entire calendar year without a single named storm... as far as we know.

Is the tornado risk growing in the USA?

 So far, 2013 has been a pretty calm year for tornadoes. It might not seem that way, because of the disasters in the Oklahoma City area in May, and our own tornado outbreak in the DMV back in June. But, surprisingly, this year's tornado count is actually below average. This is illustrated in the graph below, which I got from the Storm Prediction Center's website. 

Tornado counts, based on local storm reports from weather offices around the country

As you can see, the tornado count is below normal, and it's also way below the count for 2011, which was arguably the worst year for tornadoes in American history. Here's a map of the tornado reports for 2011: 

2011 tornado reports, courtesy of NOAA. Triangles are color-coded for a tornado's
strength; lines indicate long-track tornadoes.

In comparison, the tornado reports were a lot sparser 50 years earlier. 

1961 tornado reports, courtesy of NOAA. Triangles are color-coded for a tornado's
strength; lines indicate long-track tornadoes.

Tornadoes are reported via eyewitness accounts by trained storm spotters, and/or through rotation signatures on Doppler radar. Tornadoes are then confirmed via damage assessments, relying on a path of destruction to determine if a funnel touched down. So here's the problem-- how do we know that 2011 was the worst year for tornadoes? There are a few factors that should be taken into consideration. 

Not as many people lived in the Midwest and the Plains states in 1961 than in 2011, so there were fewer people who could potentially see a developing funnel. More people means more buildings, so it's also possible that it's easier to see and assess tornado damage now than it was 50 years ago, simply because there are more buildings and structures that could be damaged by the storm's winds. 

Technology improvements are a big factor. In 1961, the weather radar was primitive compared to the tools we have today, and 1961 also happens to be the year when the first weather satellite was launched into space. Before then, we could not look at storm systems from above. (I can't even comprehend what it was like to make a forecast without looking at satellite data!) 

Finally, it has occurred to me that the "interest factor" should not be ignored! As technology has improved and weather coverage has expanded in the media, people's interest in the weather has skyrocketed. The phenomenon of the storm chaser is not new, but its recent popularity has led to a huge increase in the number of storm spotters and other people who follow thunderstorms in anticipation of a developing funnel. The increased number of tornadoes could be partially attributed to a larger number of trained storm spotters, amateur weather watchers, and everyone in between! 

The Fall Forecast Looks Much Drier in the DC Metro

Well, how's this for unfortunate timing? Now that meteorological summer is over and the kids are back in school, our weather pattern is turning much more pleasant. The long-range forecast looks sunnier and drier, in stark contrast to the soggy summer weather we had to deal with.
As I talked about in a previous post, the summer was a bit cooler than normal, and much wetter. If we're just considering meteorological summer- which is the months of June, July and August- we had 5.3" more precipitation than in a typical 3-month summer period. Statistically, the summer didn't look that cool on paper- our average temperature was 0.57 above normal- but that number takes both the high and the low temperature for the day into account. Since it was so humid this summer, our overnight low temperatures were much milder than average. The humidity not only prevented temperatures from cooling down overnight, but it also kept us from getting too terribly hot during the day. The humidity is partially responsible for our small number of days with 90+ temperatures (only 23 days in 3 months). As a result, most of the summer was very warm and uncomfortably humid.
Of course, summer is typically more humid than fall, but this pattern shift in the forecast is more than just a seasonal variation! We're also finally seeing a breakdown of the huge ridge out west, which gave portions of the Rocky Mountains one of the hottest and driest summers on record. An upper-air map from September 1st shows the ridge in place out west, and our hot and humid pattern from the Labor Day weekend:

Model data, such as the chart below from the European model, suggests that the ridge will subside a bit and slide to the east. Here's a look at the forecast upper-air map for this upcoming Sunday: 

Notice how the ridge is now in the center of the country! When a ridge is situated just to our west, it blocks strong and/or moisture-laden cold fronts from moving through the DC Metro. A general northwest flow will keep the humidity nice and comfortable, and should lead to some nice, crisp fall nights. The ridge is expected to slowly progress eastward during the month of September. If that happens, we can expect a period of dry weather with temperatures above normal. I think we'll see lots of 80s for high temperatures in September, with only a scant few days with highs in the 70s. The 90s that we had to begin the month might be it for this year! Let's hope I didn't speak too soon. :-)