Weather: Blame the jet stream for all that summer rain

Sunday, October 5, 2008

This summer's rainfall broke the previous record by more than 10 per cent.
But despair not. Projections are for a drier than normal fall

by HENRY HENGEVELD

The summer of 2007 set a new central Ontario record for minimum precipitation. This past winter almost set a new record for maximum seasonal snowfall. Now, already by mid-August, a new record has been set for maximum total precipitation for the June through August summer season.

Welcome to Ontario's erratic weather!

Because summer precipitation tends to be highly variable in space as well as time, only a few areas in Ontario actually set new records. Most notable was the region around the Pearson International Airport, where 388 millimetres of rain fell between June 1 and Aug. 18.

This was enough to surpass the previous summer record set in 1986 by more than 10 per cent – with almost two weeks of summer yet to go!  Other notable wet spots included Sarnia, where July precipitation amounts were unprecedented in more than 120 years of weather history.

While precipitation levels in most of the rest of Ontario were less extreme, most areas experienced well above average amounts. By comparison, other traditionally wet spots in Canada were much drier. My two sons in coastal B.C. chortled with glee when they advised us that "wet" Vancouver had only received 69 millimetres during the same period. In fact, the only region in the country to receive more precipitation than Pearson was Quebec City.

You can blame the jet stream for all that rain. Forecasters at Environment Canada note that this summer the jet stream, which influences the movement of weather systems through our region, was displaced further south than normal. This allowed unstable air masses to sweep regularly across the south and central parts of the province, increasing the risk of showers and thunderstorms.

The new precipitation record, however, didn't cause the usual laments and hand-wringing that often comes with extreme weather events. That's partly because most of the rain came in relatively short bursts interspersed with periods of sunny skies. 
Also, even though at least a trace of rain fell on more than half the days, more than 60 per cent of the total rainfall occurred during seven intense downpours – two in June, four in July and one in early August – which each dropped between 25 and 51 millimetres of rain. As a result, the amount falling during the other rain days was brief and relatively modest.

In fact, despite all that rain, the total amount of sunshine remained close to average. The relatively uniform temperatures also made the rainy weather more tolerable. On average, temperatures stayed within one degree of normal, with only nine "hot" days in excess of 30 C and eight "cool" days when maximum temperatures remained below 20 C. 

There were a lot of reasons to smile about the abundant and frequent rainfall. Vegetation – crops, trees, gardens, lawns, wild flowers – all seemed to thrive. Most fruit crops were reported to be larger than average, although some producers indicated that a bit more sunshine and heat units would have been nice. Irrigation systems were, for the most part, left idle in the fields. Streams and ponds, many of which had dried out during last year's drought, were now filled to the brim.

While water levels in the upper Great Lakes remained well below normal, those for Lake Erie had bounced back to normal levels by early August. Lake Ontario's were almost 20 centimetres above the norm. Forest fires reported across the province were at their lowest level in more than 50 years. With the regular cleansing of the air by falling raindrops, there were only 10 smog alerts in southern Ontario by mid-August, most coinciding with the hot days in early June and mid July.

The wet weather was also a boon for the electrical energy sector. Lower air conditioning demands, as well as conservation measures, helped create a drop in electricity consumption. Meanwhile, high water levels increased hydro-electrical production. With the added benefit of increased production of energy from wind turbines and slightly enhanced nuclear power generation, this was one summer where Ontario's electrical system had more than adequate energy to meet demands.

However, too much of a good thing can also create problems. With the heavy rain events, municipal storm sewer systems were taxed to their limits – and sometimes beyond.  Toronto storm sewer authorities noted that their system would need an additional $1 billion expansion beyond that already planned if it were to cope effectively with such large pulses of runoff on a regular basis.

Because of the lack of dry days, some farmers also had difficulty getting their hay and winter wheat crops harvested. There was also concern that the high humidity could trigger rust, mould and mildew in corn and wheat crops. 

Experts note that more precipitation records may well be set before the year is over. For starters, this could become the wettest year ever. When the summer's high rainfall is added to the past winter's heavy snow fall, total precipitation in much of the province has already exceeded what we normally receive during an entire year.

At Pearson Airport, for example, precipitation since the first of January had already reached 728 millimetres by mid-August. By comparison, normal precipitation for this location over a full 12 month period is only 685 millimetres. Should precipitation during the remainder of the year be normal, the annual total for 2008 would come to 968 millimetres – virtually tied with the existing record high of 971 millimetres set in 1977. Hence, even slightly above average precipitation would be enough to set a new benchmark for Pearson. If the current seasonal forecast for this autumn proves to be accurate, that appears unlikely. Environment Canada projections are for drier than normal conditions for all but northern Ontario for the September through November period.

Of course, when it comes to weather, one never knows! BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.