To complement the Daily Summary for Tuesday, 13 April 1999
WINTER STORM TERMINOLOGY
To inform and alert the public to impending adverse winter weather
conditions, the National Weather Service issues various event
specific winter weather advisories, watches and warnings depending
upon the event and the locale. Some of the following (given in
alphabetical order, not severity) include:
- Blizzard warning -- issued to warn the public of a
potentially life threatening winter storm condition with sustained
winds or frequent gusts to 35 mph or higher, combined with considerable
falling and/or blowing snow, which reduces visibility to less
than 0.25 mile (400 meters). These conditions are expected to
last for at least 3 hours.
- Blowing snow advisory -- issued to alert the public
of the possibility that wind-driven snow (the condition of blowing
snow) will reduce visibility enough to hamper travel. Visibility
conditions are not expected to deteriorate sufficiently to issue
a blizzard warning.
- Freezing rain (or freezing drizzle) advisory -- issued
to alert the public of the potential for freezing rain or freezing
drizzle of sufficiently light intensity and that ice is not expected
to form on all exposed surfaces.
- Heavy snow warning -- posted to warn the public of
the potentially life-threatening snowfall accumulations that are
expected to exceed specific values that may vary according to
locale; typically 4 inches or more in 12 hours or 6 inches or
more in 24 hours or less. Winds are expected to be light (less
than 10 mph) and no freezing rain or ice pellets are expected.
- Ice storm warning -- posted to warn the public of a
potentially dangerous winter weather situation with significant
ice (glaze) accumulations are usually 1/4 inch or greater during
a freezing rain episode. As a result, walking and driving is expected
to become extremely dangerous.
- Snow advisory -- used to alert the public to an anticipated
snowfall that will cause inconvenient conditions but not a threat
to life or property if caution is exercised. The exact criteria
for snowfall amounts depend on the locale, if the expected snow
were to occur at the beginning of the snow season or after a prolonged
interval between snow events. Typically, the anticipated amounts
for a snow advisory are less than for a heavy snow warning.
- Wind chill advisory -- posted to inform the public
that wind- chill equivalent temperatures are generally expected
to reach -20 degrees F or colder for a sustained time interval.
Wind speeds are expected to be at least 10 miles per hour. Mitigating
circumstances such as strong sunshine and acclimation in cold
climates, may require colder thresholds.
- Wind chill warning -- issued to warn the public that
wind-chill equivalent temperatures are expected to reach 50 degrees
below zero or lower for a sustained time interval.
- Winter storm warning -- issued to warn the public that
heavy snow and some strong winds are imminent or currently occurring,
perhaps in combination with ice pellets (sleet) and/or freezing
rain or freezing drizzle. Winter storm warnings are usually issued
for up to a 12-hour duration, but can be extended to 24 hours.
A warning is used for winter storm conditions posing a threat
to life or property.
- Winter storm watch -- issued to alert the public that
conditions are favorable for development of a significant winter
storm that could affect the area, with hazardous weather elements
such as heavy snow and/or blizzard conditions, or significant
accumulations of freezing rain or ice pellets (sleet). Watches
are usually issued 12 to 48 hours in advance of the expected event,
even if the exact path of the storm has not be determined.
- Winter weather advisory -- issued to advise the public
of the expectation of a mixture of precipitation, such as snow,
ice pellets (sleet), and freezing rain (or freezing drizzle).
Advisories are conditions less serious than warnings that cause
significant inconvenience and, if caution is not exercised, can
be a threat to life and property.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.