Figure 2: Snapshot of a typical North American winter jet stream
Ice storm 98: Meteorological factors
As with any loss event – natural or manmade – a number of conditions linked by a precise chain of events leads to a destructive end. In and of themselves, each factor is harmless. Take one out, and the event doesn’t occur – at least not with the same force. But combine them in a certain way, and there’s havoc. With Ice Storm 98, it appears that the meteorological cause of the storm had its roots in the year’s record El Niño.
El Niño The 1997/98 El Niño event began in the spring of 1997. Instrumentation placed on buoys in the Pacific Ocean began recording abnormally high water temperatures off the coast of Peru. Over the spring and early summer, the strength of these anomalies grew. Eventually they became so large that quite early on, the 1997 event had already become the strongest in the 50-plus years of accurate data gathering.
Meteorologists had been projecting that overall, the year’s El Niño was going to be good news for Canada. The 1997/98 event marked the twelfth El Niño in 50 years. Nine meant warmer than usual temperatures for the country. The phenomenon was anticipated to produce a milder winter and a drier, earlier spring – at least from British Columbia through Ontario and southern and central Quebec. Drier-than-normal conditions were expected from the B.C. interior through the Prairies and into the Great Lakes Basin. Though forecasters never maintained that every winter day was going to be good, they didn’t predict an event of the magnitude experienced between January 4 and 10.
Jet stream Weather experts point to El Niño as a contributing factor to the ice storm because the eastward shift in warm Pacific water created by the phenomenon upsets the atmosphere’s energy balance. Warm ocean water pumps moisture and wind energy into the upper atmosphere, creating huge thunderstorms. As these storms move east over the Gulf of Mexico, they help change the jet stream. Changes to the jet stream, in turn, move storms on atypical paths upsetting normal patterns of wet and dry weather.
The jet stream is a narrow band of 160 to 300 kilometre-an-hour winds located anywhere from eight to 15 kilometres up into the troposphere of the mid-latitudes and subtropical regions of the Northern and Southern Hemispheres. Flowing eastward in a semi-continuous band around the globe, the jet stream is created by changes in air temperature where cold polar air moving towards the equator meets warmer equatorial air moving towards the poles.
Figure 3: Snapshot of a typical North American winter jet stream in an El Nino year
Because everything in the Earth’s atmosphere constantly changes – heat, cold, highs, lows, and inversions of air – the jet stream meanders, twists and turns like someone attempting to push a rope. In the winter months of a typical year, there can be two major jet streams in the northern hemisphere (see Figure 2). First is the polar or Arctic jet stream, created by a polar front that separates the main body of colder, drier air in the north from warmer, moister air to the south. During major cold outbreaks this polar front dives south over the United States, taking the Arctic jet stream with it. The warm and moist subtropical jet stream (which tends to develop during the winter) is formed by air movements in tropical and subtropical regions. Both the polar jet stream and the subtropical jet stream help to develop and steer storms and disturbances.
In the winter months of an El Niño year the jet stream often does something completely different (see Figure 3). As it moves eastward over the Pacific Ocean toward the United States the single jet stream often splits into two arms – roughly at California. The north jet moves in a north-easterly direction up mainland British Columbia, hooks east over northern B.C., Alaska and the Yukon and meanders over the Northwest Territories until it runs out to sea off Labrador and Newfoundland. The south jet flows eastward over Southern California, Texas and Florida, often generating powerful Gulf storms. For a large part of late fall and early winter 1997/98, the jet stream over North America did just this. As a result, large areas of Canada basked in milder temperatures as much of the country was south of the north stream. Conversely, the southern U.S. experienced some of the wettest weather in years.
This is a textbook depiction of what happens to the jet stream during an El Niño year and it appears that, for the most part, North America followed the book – that is until early January, 1998.
Recipe for disaster: Imported In the days prior to the ice storm, the north jet was not flowing over Canada’s sub-Arctic as it had been for weeks prior (see Figure 3 – North jet stream). Rather when it reached central Canada it dipped south, taking a path that is not typical for the Arctic jet during an El Niño winter (see Figure 4-1). As a result, in early January much of central Canada, Ontario and Quebec were above the Arctic jet, locking large portions of these areas in colder temperatures. The south jet (see Figure 4-2) was following the typical pattern – moving east over Southern California, Texas and Florida, releasing large amounts of precipitation and causing deadly flooding in some states. But instead of flowing out to sea, the south jet hit a ‘brick wall’ in the form of a large high pressure cell stalled over Bermuda (see Figure 4-3). This cell was so powerful it wasn’t going to move for anything.
With nowhere to go, the south jet made a detour and headed north on a path just to the west of the Appalachian mountains (see Figure 4-4). Gulf disturbances generally follow the Atlantic Gulf Stream in a north-easterly direction, heading for a ‘graveyard of storms’ off Iceland where they die. Those that do travel up the U.S. mainland usually flow east of the Appalachians rather than west. So in early January normal El Niño patterns took a brief respite, setting the stage for what would soon become the worst natural catastrophe in Canadian history.
Figure 4: Meteorological factors which contributed to the Eastern Canada Ice Storm of 1998
Recipe for disaster: Domestic At the same time that the warm, moist south jet barreled northward from roughly the Texas Panhandle, a large stationary Arctic high pressure area over Hudson Bay forced a cold, shallow air mass into the Ottawa/St. Lawrence Valley (Figure 4-5). Given the V-shaped topography of the land in this corridor, it is quite normal for cold air to sit in the area for extended periods. What is not normal is for a collision to take place between such a cell and a mild, extremely wet air mass travelling up from the southern U.S. But this is exactly what happened.
As a general principal, cold air is heavier than warm. So when the warm, wet mass from the south met the cell hanging over parts of eastern Ontario and Quebec; the cold, heavy air stayed put and the moist lighter airmass moved upward – resting on top of the cold mass. As is common with a warm, wet airmass, it began releasing precipitation. In this case, the moisture fell in the form of rain.
Freezing precipitation Meteorologically, there is a fine line between rain, freezing rain, sleet, ice pellets, wet snow, snow and snow pellets. Only a minute difference in temperature can mean the difference between one form of precipitation falling and not the other.
In the most gentle liquid precipitation, known as drizzle, the droplets are very fine and closely spaced. Larger drops form rain. Sometimes raindrops fall through an inversion, in which the lower layer of air is cold enough to freeze them into ice pellets. Ice pellets come in two types. The first type of ice pellet, known as sleet, is formed by the freezing of small raindrops as they fall. In the situation in which sleet occurs, a layer of air having temperatures above zero degrees Celsius overlies a colder layer near the ground. The second type, known as ice crystals, is formed at greater heights when frozen precipitation melts as it falls through a warm layer of air. The drops formed then freeze while falling through the cold air below.
More serious from the standpoint of damaging weather are conditions that cause ice storms. In these situations the vertical temperature distribution is similar to that which produces sleet, but the subfreezing air at the ground is not deep enough or the raindrops are too large for the drops to freeze as they fall. Falling rain in winter is often at such a critical temperature that contact with any surface that is below zero degrees Celsius – streets, sidewalks, buildings, trees, wires, poles, pylons – immediately turns the water to ice. In these circumstances, raindrops are said to have been ‘supercooled.’ The veneer of ice is very smooth, facilitating layer after layer of ice buildup depending on how long the precipitation falls.
In the case of Ice Storm 98, rain drops falling through the warm air mass (which was about +5 degrees Celsius) met the cold air mass which was anywhere between -3 to -7 degrees Celsius and less than 500 metres thick. The droplets – at first about +5 degrees – were then supercooled to about -2 degrees. If the air mass hanging over the Ottawa/St. Lawrence Valley was colder, the droplets would have turned into ice pellets before they hit the ground, causing considerably less damage. (Pellets don’t load on trees, power lines, hydro poles or pylons.)
To add insult to injury, there wasn’t much wind in the Ottawa/St. Lawrence Valley at the time of the storm. And though this may have been a blessing in some ways (high winds may have brought down more trees, poles and pylons) the lack of wind could have pushed the cold cell out of the region much sooner. This meant that some areas of Eastern Canada (such as Montreal) got over 80 hours of freezing rain – almost 20-times the norm for a freezing rain event. The total hours are a result of what has been described by Meteorologist as a ‘conveyor of water’ from the south.