DATASTREME SUPPLEMENTAL SUMMARY
To complement the Daily Summary for Tuesday, 13 November 2001
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 outlook -- issued to alert those segments of the public
who need extra lead-time that hazardous winter weather may develop within 3 to
5 days. However, uncertainty as to the exact timing and the location of the
event remains.
- 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, 2001, The American Meteorological Society.