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Snow, freezing rain and
Arctic temperatures
Reprinted from
Issue 4/2013
Natural Hazards in North America
Snow, freezing rain and
Arctic temperatures
Winter can be unpleasant – even for companies. What sorts of scenarios you have
to expect in the USA and Canada and how
you can prepare your company.
Mark Bove
Encompassing several types of weather systems and
hazards, winter storms are one of the few perils that
impact every region of the United States and Canada.
The term “winter storm” is a bit misleading, however,
as the mid-­latitude and polar cyclones that generate
these events occur year-round. But it is during the
winter months that these storms are at their strongest, when temperature gradients between the tropics
and polar regions are at their maximum. The types of
hazards produced by winter storms vary greatly by
region, but all can produce extensive insured property
damage, most notably from high winds, very cold
temperatures, and excessive frozen and liquid precipitation.
Genesis and characteristics
Winter storms, also known as extratropical cyclones,
rely on temperature gradients between different air
masses to develop. The larger the contrast between
the air masses, the stronger the storm can become.
Since temperature gradients in the mid-latitudes are
much larger during winter than summer, extratropical
cyclones are more powerful during the winter months.
The formation of an extratropical cyclone begins with
two air masses, one warm and moist, the other cold
and dry. The winds from these air masses can cause
perturbations in the boundary between them, and
some of these perturbations can grow into a centre of
low pressure. When an area of low pressure develops,
it forces warm air ahead of it northward and cold air
behind it southwards. This movement of air masses
forms warm and cold fronts, generating rain, thunderstorms, and frozen precipitation at these boundaries.
The earth’s Coriolis force makes the storm rotate, giving the cloud distribution of the extratropical cyclone
a comma-shaped appearance.
Winter storms can paralyse virtually every
means of transport and block overland routes in
particular for days on end.
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Several dozen winter storms impact North America
every year, though only a few cause severe damage.
However, severe winter storms have a much greater
geographical footprint than other meteorological perils. Frontal systems associated with a winter storm
can stretch for thousands of miles, causing a variety
of weather along its length. Heavy rains and severe
thunderstorms can develop along a cold front’s southern section, spawning tornadoes and hail, while a
large swath of snow, sleet, and freezing rain can occur
further north. Dam­aging winds may develop around
the low-pressure centre, par­ticularly if the storm is
trailed by a strong high-pressure system. As the
storms move, the impacted area can stretch for over a
thousand miles before the storms weaken or head out
to sea.
Hazards
Winter storms produce several hazards that are
unique to the peril. These include:
Snow
Snow is frozen precipitation composed primarily of
small, hexagonal ice crystals. Snow occurs when precipitation de­­velops in an atmospheric environment
that is below freezing during its entire descent. Surface temperatures can be slightly above freezing during snow, but it will likely not accumulate on the
ground. A blizzard is one of the most severe types of
snowfall events: a combination of gale-force winds
and falling and blowing snow that reduces visibility to
a quarter mile (400 m) or less.
Sleet
Precipitation composed of frozen raindrops is called
sleet. Sleet occurs in a winter storm when there is a
deep layer of freezing air near the earth’s surface and
a layer of warmer air above it. The warmer air causes
precipitation to start off as rain, but when the raindrops enter the colder layer below, they freeze in midair before reaching the ground.
Freezing rain
Arguably the most hazardous form of winter precipitation, freezing rain occurs when there is a very shallow layer of freezing temperatures near the earth’s
surface, but a large column of warmer air above it.
Like with sleet, the warmer air aloft causes precipitation to fall as rain. However, unlike sleet, the shallow
layer of cold air near the surface does not give the rain
enough time to freeze before reaching the ground.
Instead, the rain freezes after coming in contact with
frozen surfaces, glazing whatever it touches in ice. A
large freezing rain event with heavy ice accretions is
often called an ice storm.
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Freeze
Unlike other winter storm hazards, deep freeze events
are not caused by low-pressure systems, but by
strong areas of arctic high pressure that often move
into a region after a winter storm has passed through.
These very cold air masses can cause temperatures to
remain far below freezing for several days.
Other hazards not exclusive to winter storms can be
generated as well. These include winds that can reach
hurricane force in intensity and excessive rainfall that
can trigger flooding and landslides. Winter storms
also often cause the development of severe thunderstorms along their frontal boundaries that can generate tornadoes, straight-line winds, and heavy hail.
Regional winter storm impacts
Though the impacts of winter storms vary widely
across North America, different regions of Canada
and the United States typically see similar hazards
from event to event. Generally speaking, these are the
dominant winter storm hazards by region:
Alaska and Arctic Canada
Severe winter conditions are typical in this region,
including bitter cold temperatures and months of
accumulating snowfall, though heavier snowfalls typically do not occur in the middle of winter, as cold air
temperatures limit the amount of moisture the air can
hold. Hurricane-force winds are also a threat in
coastal regions. Further south, winter storms in the
Alaska panhandle commonly bring high winds and
intense rains along the coast, with heavy snowfalls at
higher elevations.
Pacific coast states and British Columbia
Along the west coast of North America, winter storms
are primarily strong wind and severe rain events in
coastal re­­gions, with heavy snowfalls at higher elevations. Occasionally, heavier snows can fall at lower
elevations where major cities are located, particularly
in the Pacific Northwest region. Torrential rains from
winter storms in Southern California often trigger
devastating mudslides and flooding as well.
Intermountain West
Heavy snowfall is the primary hazard over the Intermountain West. Colder temperatures at higher altitudes are quite common, but arctic outbreaks of frigid
air typically cannot pass over the mountain ranges of
the region, and instead funnel into the Great Plains.
Northern Great Plains
In the northern sections of the Great Plains of North
America (north of about 37° northern latitude), snow
and arctic air outbreaks are the primary winter storm
hazards. Other forms of frozen precipitation can also
occur, especially toward the southern edge of the
region. High winds are a common threat from powerful winter storms, but are typically not as severe as
winds associated with coastal storms.
A nor’easter destroyed two
beach houses at Saco,
Maine, in April 2007.
Southern Great Plains
For the southern Plains, heavy rainfall and severe
thunderstorms are the most common hazards associated with winter storms. Due to their location, heavy
snow events are rare, but severe ice storms can occasionally paralyse the region. Cold air outbreaks are
also possible, as arctic air masses move southward
out of Canada.
Southeast United States
Similar to the southern Great Plains, heavy rainfall
and severe thunderstorms are the typical winter
storm hazards in the southeast. Thanks to ample
moisture from the Gulf of Mexico, severe freezing rain
events are possible, but heavy snowfalls are infrequent. Freezes can reach this region as well, and even
southern Florida is not immune to occasional subfreezing temperatures. Developing coastal storms in
the Gulf can also produce high winds, but these are
typically not as strong as seen along the eastern seaboard.
Northeast United States and Canada
The northeastern United States and Canada are
exposed to all forms of frozen precipitation, particularly heavy snowfalls and high winds associated with
powerful coastal winter storms called nor’easters.
Arctic outbreaks of cold air and severe freezing rain
events are also possible, as are heavy rain events
when temperatures are not cold enough to support
snow.
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Natural catastrophe statistics for North America
Damaging winter storms from 1980 to 2011
Winter storms comprise nor’easters, snowstorms and
blizzards. They can occur – despite their name – year-round
and include various hazards: wind, snowfall, ice and freeze.
Number of damaging events
20 Number
16
12
The average number of loss-rele­vant
winter storms has tripled in the
second half of the last 32 years
compared to the first. However, this
number has remained fairly stable
in the last 17 years, with some winters
standing out.
Source: Munich Re,
NatCatSERVICE
8
4
0
1980 1985 1990 1995 20002005 2010
Overall and insured losses (in 2011 values)
5 US$ bn
4
3
2
Overall losses
Insured losses
1
Source: Munich Re,
NatCatSERVICE
0
1980 1985 1990 1995 20002005 2010
6
Like the number of damaging events,
the overall and insured losses do not
reveal a distinct trend over the past
decades. High loss peaks are mostly
produced by individual extreme
storms. For large events the insured
portion of losses tends to be lower.
Munich Re Topics Risk Solutions 4/2013
Damage
Frozen precipitation can damage property in several
different ways. While light accumulations of freezing
rain, sleet, and snow will typically not damage buildings, they can cause damage to automobiles by creating hazardous driving conditions that often lead to
traffic accidents. Heavier accumulations of snow and
sleet can damage buildings in two ways. The first, ice
damming, occurs when snow and ice prevent melt
water from draining from a rooftop. The water can
back up under shingles, damaging the roof structure,
insulation, and drywall below. Very large snow loads
can lead to partial or total roof collapses, particularly
on flat roofs with little drainage and few underlying
supports. However, thanks to good regional building
codes that consider snow loads, instances of roof collapse are relatively rare, even in heavy snowfall events.
Large accumulations of freezing rain, however, can
create much greater and more widespread amounts
of damage than snow and sleet. Freezing rain adds a
tremendous amount of additional weight to the
objects it collects on, and often leads to tree limbs
and power lines snapping. In severe events, the
glazing can become several centimeters (more than
one inch) thick and can bring down larger structures,
like high-voltage transmission towers.
Aside from being a risk to health and agricultural
interests, pro­­longed cold air outbreaks can inflict
property damage by rupturing water pipes. Since
water expands when it freezes, extended sub-freezing
temperatures can cause improperly insulated pipes to
burst, flooding buildings with water which can freeze
to ice.
In addition to severe freezing rain events, other winter
storm hazards have the potential to cause as much, if
not more, property damage. Heavy rainfall associated
with winter storms can cause severe flooding, particularly if the rain contributes to snowmelt or the ground
is frozen. Heavy rains can also cause landslides on
unstable terrain, particularly in California. High winds
in winter storms tend to cause a large amount of
indir­ect wind damage via tree and power line damage,
but this is generally lighter in severity than wind damage seen in tropical cyclones.
Loss potentials
Despite the number of different hazards and large
geographic scope of winter storms, loss potentials are
typically not as severe as those from tropical cyclones
and thunderstorm perils.
Since 1980, the largest insured winter storm loss was
US$ 1.75bn (Can$ 2.2bn), resulting from the 1993
“Storm of the Century” nor’easter that generated hurricane-force winds along the US east coast and left
over 1 foot (30 cm) of snow from Georgia to Maine.
While indeed a severe event, insured losses from
thunderstorm outbreaks and hurricane landfalls have
greatly exceeded this amount several times over the
past 40 years. Winter storm losses over the ten-year
period 2002–2011 averaged US$ 1.8bn (Can$ 1.8bn)
per year. During the same period, losses from tropical
cyclones averaged US$ 15.3bn (Can$ 15.6bn) and
US$ 9.3bn (Can$ 9.5bn) from thunderstorms (figures
in 2011 dollars).
The roof of the Metrodome
in Minneapolis collapsed in
December 2010 following a
severe winter storm.
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Even though, historically, winter storm losses have
been relatively low, there is potential for much greater
losses from future events. The most likely scenario
regarding a major, multi-billion-dollar insured loss
would be similar to the 1998 ice storm (freezing rain)
in Ontario, Quebec, and northern New England.
During the event, several centimeters of freezing rain
fell on the area, causing trees, power lines, and highvoltage transmission towers to collapse, leaving parts
of the region without power for weeks. US$ 950m
(Can$ 1.4bn) in insured losses was incurred in Canada, particularly around Montreal, and an additional
US$ 200m (Can$ 293m) in the United States.
Although this was the worst winter storm loss in
Canadian history, a similar event over densely populated regions of the United States such as the northeast corridor or over Midwestern cities like Chicago
and Detroit could cause losses several times greater
than the 1998 event, potentially exceeding US$ 10bn
(Can$ 10bn).
Our Expert
Mark Bove, meteorologist in
Underwriting/Risk Accumulation
at Munich Re, specialises in the
modelling of natural catastrophe risks
in the United States.
[email protected]
How to protect your equipment
from freezing weather
Any interruption of electrical power during cold weather
is a freeze hazard. In addition, ice and snow from winter
storms can cause severe equipment and property damage
during the storm and from flooding when the temperatures rise. The engineers at Hartford Steam Boiler recommend that you plan ahead and consider taking these steps
in advance of a storm:
Building closures. Make sure all doors, windows, shutters
and dampers that can be closed are in place and secured
to minimise heat loss.
Snow and ice removal. Check snow removal equipment
and have contracts or arrangements with reliable snow
removal contractors.
>> Our publication “Severe
Weather in North America”
sheds light on the physical
principles that give rise to
­natural hazard phenomena,
explains their occurrences
and effects, and analyses
the resulting loss events.
If you want to order the
publication, please contact
your client manager.
>> For more information about
natural hazards, risks and
insurance, see
www.munichre.com/
touch/naturalhazards
Collect and circulate a list of emergency telephone
numbers. Distribute this emergency information to everyone in the organisation who needs it. Have a backup communications strategy, such as using mobile phones and
laptop computers and tablets, in case power and regular
phone service is lost.
Heating systems. If backup electrical power is not avail­
able, make arrangements in advance to obtain portable
heating units which do not require electrical power.
Protect piping. Piping systems which could freeze must
be checked. Be sure that heat tracing is energised. Be prepared to supply backup power to electrical heat tracing
systems or drain the piping. Pay particular attention to
sprinkler fire protection systems. Any change in the readiness status of your sprinkler systems should be reviewed
by your local fire department.
Anticipate flooding. If the premises may be flooded as a
result of severe cold weather or storms, take precautions.
Move exposed equipment or stock to alternate locations if
possible. Check operation of pumps or other dewatering
equipment. Do not energise equipment which has been
flooded until it is properly cleaned, dried out, and until
insulation has been tested.
>> F
or more information visit
www.hsb.com/HSBGroup/Weather_Alerts.aspx
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Munich Re Topics Risk Solutions 4/2013
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