Get ready for 3 seasons in one day this weekend, details
Friday, April 8, 2016, 12:17 PM - Friday into Saturday will feel like summer in B.C. and Alberta, winter in Ontario, and mid-spring in the Atlantic.
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And no, I’m not exaggerating.
Forecast daytime highs on Friday across B.C. and Alberta will be comparable to climatological means for July. Take Calgary, for example: with a forecast high of 25˚C, this would exceed July’s monthly mean maximum temperature of 23.2 degrees Celsius. Evidently, it would also be almost fifteen degrees higher the seasonal mean high temperature for this time of year.
In Ontario, meanwhile, Saturday is already shaping out to be distressingly reminiscent of January. With Toronto’s present forecast high of -2˚C, this would make it colder than January’s monthly mean maximum temperature of -1.5˚C.
To better put this temperature in perspective, a forecast high for Saturday in Toronto of -2˚C would also be eleven degrees colder than the forecast high for Whitehorse, YK, four degrees colder than Nain, NL – the northernmost permanent settlement in Newfoundland and Labrador – and astonishingly is most comparable to Kuujjuak, QC – the largest northern village in Nunavik, QC, located at 58˚N.
The only region that seems to be matching the calendar is Atlantic Canada. Here, daytime highs for Friday and Saturday in the Maritimes average to be around 10˚C, which is identical to April’s climatological mean maximum temperature for Fredericton. While St. John’s will be a bit below seasonal to start the weekend, much of the island will see temperatures in the mid-to-upper single digits.
So how are three simultaneous seasons possible?
As with many cases of temperature contrasts across the country, an amplified jet stream is a major contributing factor.
The image above shows Friday - Saturday’s forecast jet stream across the country. Here we see three distinct features: a sharp ridge over western Canada; a deep trough over central Canada, and a ridge over Atlantic Canada. When a jet stream has these distinct ridges and troughs, meteorologists refer to it as being amplified – in contrast to a mostly flat jet stream that would be referred to as zonal.
Amplified jet streams are not uncommon across Canada. The winter of 2013/2014, as well as February 2015 were both dominated by this phenomenon; so much so in fact that a now-dreaded pairing of words began to enter mainstream vocabulary: the infamous polar vortex.
As you may have already experienced first-hand, areas under the influence of a ridge in an amplified jet stream usually experience much milder temperatures, as warm air is favourably ushered up from the south – such as the case of B.C. and Alberta last week, for example. By contrast, areas in a trough are subject to cold air infiltrating from the north - a major contributing factor to the early-April snowstorm in Ontario this past Sunday and Monday.
The regions in-between these troughs and ridges, such as Saskatchewan and Quebec, can experience drastically different conditions from one side of the province to the other. Take Saskatchewan as an example: Friday’s forecast high in the eastern city of Yorkton is -2˚C, while in the west Swift Current’s is +15C. By Saturday, however, the province as a whole warms up as the ridge shifts east.
So whether this temperature story across the country gives you reason to rejoice or sob quietly to yourself, rest assured that this too shall pass. While the rest of the month is by no means a certainty, there are some preliminary indications that a more zonal jet will develop by the end of April; if it does, expect milder temperatures by comparison in the east, and closer to seasonal values across the west.