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Old 12-20-2004, 11:12 PM
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I came across some good information on unstable weather hazards with a section on winter storms at: http://www.msc-smc.ec.gc.ca/educatio...weather_e.html

Some of what appears at that link is reproduced below:

"Basic understandings - Winter storms
Winter Storm Development

1. Winter storms are large-scale disturbances associated with low-pressure areas called mid-latitude cyclones.
2. Winds blow counterclockwise as seen from above (in the Northern Hemisphere) around the centre of the low pressure system.
3. Winter storms occur when warm, humid air interacts with colder air along the frontal boundary separating the two air masses. The two contrasting air masses provide energy, which permits the storm to intensify.
4. Winds speeds increase as the storm strengthens. The warm, moist air is lifted upward, producing widespread areas of cloudiness and precipitation along the frontal surfaces in the vicinity of the developing cyclone.
5. The normal lifetime of a winter cyclone is about three to five days.
6. Steered by the direction of the upper air flow, winter storms tend to move from west to east.

Winter Storm Hazards

7. Winter storms produce strong winds, heavy precipitation (rain, freezing rain, or snow) and cold temperatures. Hazardous winter weather includes freezing rain, snow, blizzards, and bitterly cold temperatures.
8. Cold temperatures feel more extreme when there is wind. The wind chill factor combines the effect of both temperature and winds to determine the cooling rate and the equivalent temperature with no wind.
9. Environment Canada issues a variety of severe winter weather watches and warnings to alert the public to the approach of winter storm conditions.
10. Winter storm dangers include being stranded outside while exposed to the elements, having breakdowns in transportation systems due to accidents, and losing access to basic necessities and services.



Narrative - Winter storms
What is a Winter Storm?

A winter storm is a large-scale disturbance, often hundreds of kilometres across, associated with a low-pressure system (called a cyclone) that develops along a front during the cooler part of the year. Winter storms can produce strong winds, heavy precipitation (rain, freezing rain, ice pellets or snow) and cold temperatures.
What Causes Winter Storms?

Winter storms occur when relatively warm, humid air interacts with colder air along the frontal boundary separating the two air masses. Initially, the front is slow-moving or stationary. The formation and evolution of a deep low pressure centre and the associated circulation is referred to as cyclogenesis. It is a complex process involving upper level divergence as well as near-surface processes. The two contrasting air masses provide energy to this rotating system, permitting the storm to intensify with time. Wind speeds increase as the storm strengthens. The warm, moist air is lifted upward, producing widespread areas of clouds and precipitation along the frontal surfaces in the vicinity of the developing cyclone.



Surface map depiction of a winter storm system, showing the low pressure centre (L), cold and warm fronts, and a accompanying weather

Surface map depiction of a winter storm system, showing the low pressure centre (L), cold and warm fronts, and a accompanying weather (legend gives explanation of symbols)
Structure and Movement of a Winter Storm

The figure (Illustration of the vertical structure of a winter storm) below displays the structure of a mature stage of a winter storm, with major features noted at both the surface and at upper levels (6 to 10 km). The primary storm is associated with the surface position of the low (L) centre and the accompanying cold and warm fronts. At upper levels, the storm normally is associated with an upper level trough, a low pressure region which forms a distinct southerly "dip" in the upper air flow, which generally lags the surface low pressure centre. The surface high (H) pressure behind the cold front brings colder temperatures, clear skies and fair weather. The entire storm system moves(arrow on surface chart labelled Movement) with the upper level steering wind currents (noted by the direction of the arrow of the jet stream).

Illustration of the vertical structure of a winter storm.

Illustration of the vertical structure of a winter storm. The bottom view depicts the surface features, and the top view shows upper level features of the system (adapted from Ahrens)
Weather Accompanying Winter Storms

Some of the hazardous weather conditions that accompany winter storms area:

Heavy snow
snowfalls greater than 15 cm in 12 hours usually cause significant problems.
Blowing Snow
wind-driven snow that reduces visibility and causes significant drifting
Blizzards
winds exceeding 50 km/h with snow and blowing snow reducing visibility to near zero; high wind chills.
Snowsqualls
narrow bands of very heavy snow that blow in off the Great Lakes, Gulf of St. Lawrence, and other bodies of water. Heavy snow and near zero visibilities are generally associated.
Ice pellets
raindrops that freeze before reaching the ground. This normally occurs when the rain forms along a warm front and descends through a layer of air with temperatures just below freezing.
Freezing Rain
rain falls through a layer or onto a surface with a temperature just below freezing, causing a layer of ice to form on the surface.



Most of the hazardous weather associated with winter storms occurs in the vicinity of the low pressure centres and along the frontal systems. Warmer, moister air is lifted over the frontal systems producing widespread areas of cloudiness and precipitation. Freezing rain is often observed just ahead of the warm front, as the rain falls through colder air below. Snow occurs further north of the freezing rain area and especially in the area to the north of the cyclone where there is a deeper layer of colder air through which the precipitation falls. The prime area for blizzard conditions occurs in the immediate vicinity of the cyclone where there often is heavy snow and the strong winds rotate about the storm centre.
Winter Weather Advisories and Warnings

Environment Canada issues a variety of special weather statements, advisories, watches and warnings to alert the public to the approach of winter storm conditions.

A special weather statement / winter weather advisory is issued when winter weather conditions are expected to cause significant inconveniences.

A winter storm watch is a “heads up” that winter severe weather is expected in the near future.

A winter storm warning is issued when severe winter weather conditions have begun or are about to begin in a given area.

A blizzard warning is issued when snow and strong winds will combine to produce blinding snow (near zero visibility), deep drifts, and life-threatening wind chill.
What is Wind Chill and the Wind Chill Index?

Wind chill is the cooling effect of the wind in combination with low temperatures. When it is windy, we feel colder because our skin temperature is lower. This sensation of cold is what the wind chill index quantifies: as such, the index is not a real temperature and is expressed without units, even though it is based on the Celsius temperature scale.

Environment Canada's wind chill index estimates the temperature which, with a wind of about 5 km/h, would give your face a sensation of cold similar to that caused by the actual temperature with the wind. Wind chill also estimates the risk of your getting frostbite (a severe injury caused by cold), according to these approximate thresholds:

* Wind chill below -25: risk of frostbite in prolonged exposure.
* Wind chill of -35: frostbite possible in 10 minutes (warm skin, suddenly exposed; shorter time if skin is cool at the start).
* Wind chill of -60: frostbite possible in less than 2 minutes (warm skin, suddenly exposed; shorter time if skin is cool at the start).



Chart of Wind Chill.


Major Health Hazards


The major health problems associated with winter storms are overexposure and overexertion. Overexposure implies that parts of the body are not properly protected from the cold temperatures and/or strong winds, leading to frostbite or hypothermia. Overexertion results from the strain of working too hard in cold temperatures, and can lead to heart failure."


There are some good graphics in that article that are worth reviewing directly.
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