Sunday, February 12, 2006

Column 13: White Juan (originally posted January 4, 2006)

On February 18-19, 2004, a most memorable winter storm smacked the east coast, giving unconfirmed reports of almost 100 cm of snow to the Halifax area. Almost everything shut down; everything being covered in snow that sometimes flew sideways because of the 60 to 80 km/h winds.
"White Juan", so named because of the actual Hurricane Juan which had hit the same area barely 5 months prior, is the stuff of legends.

Why, though, did it happen this way? Why was it so severe? Why did it have so much snow in it? I will attempt to answer these questions in this column.

Whenever gauging the amount of snowfall, we consider the snowfall rate and duration. In this storm, the snowfall rate wasn't atypical--3 to 4 cm/hour, which I personally have seen in Halifax. In some storms, the snowfall rate has been reported as high as 8 to 10 cm/hour. (In snowsuqalls off the Great Lakes, snowfall rates can also approach 10 cm/hour, but that's another column.)
In this storm, then, the key wasn't the snowfall rate but the duration. As outlined below, the regular snowfall rates persisted over a much longer time, leading to this most impressive snowfall.

To start off the heavy snowfall, White Juan bombed. A meteorological "bomb" is defined as a storm that deepens by 24 mb in a 24-hour period. This storm did that and more, bombing from 996 mb to 959 mb in 24 hours, a drop of 37 mb. Whenever a storm experiences such deepening, it is always accompanied by very strong upward motion and, as a result, heavy precipitation.
Conveniently, this storm bombed at about the time when the upper low (500 mb) was pretty much collocated with the surface low. Whenever this happens, the surface low is said to be "captured" and its forward speed slows to a crawl. If this happens during a heavy snowfall rate, then the heavy snow happens over a longer-than-normaal timeframe.

The actual meteorological conditions forming this storm weren't all that uncommon: because of the Gulf Stream off the coast and a relatively cold airmass on the continent, a semi-permanent baroclinic zone (front) was set up. Because of the availability of the Gulf Stream, moisture was copious. All that was needed was a disturbance in the upper atmosphere to act on the surface front, and a low pressure system would be born, just off the coast of Cape Hatteras. This upper disturbance came about in mid-February, perhaps a little deeper than usual, inducing explosive deepening of the surface low.
As per most conceptual models of such systems, moisture was streamed northward ahead of the system, gradually being lifted, causing cloud and precipitation. Because of the cold air at the surface, the precipitation fell as snow (and freezing rain and ice pellets.)
The generally accepted swath of heaviest snow in a "Cape Hatteras Low" is a specific distance northwest of the surface low, a distance I can't recall and was unable to find in my research for this column. But suffice it to say that the optimal distance was reached; add in that the storm was bombing as it approached, inducing heavy snowfall, and the fact that it got captured as it was at this optimal distance, and you have the setup for an abnormally heavy snowfall.

Snowfalls of (kind of) this type are not uncommon over the east coast. Usually one or two storms per year have snowfalls in the 30 to 50 cm range. What made this one different was the fact that, at the height of the storm, it slowed its forward movement, putting Halifax in the area of heaviest snowfall for 24 to 30 hours, instead of the usual 6 to 12.

I love snow. I only wish I had been in Halifax at the time, so I could have taken in this storm.

For more information about this storm, check out Chris Fogarty's storm summary at this link or Environment Canada's summary at this link.


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