During the course of the
winter season, a variety of inclement weather conditions occur which can impact
the safe and orderly flow of traffic. The following is a general description of
terms associated with winter weather and road icing conditions:
- Freezing Rain Advisory: Freezing rain/drizzle is
light and ice accumulations are less than 1/4 inch.
- Winter Weather Advisory: This is an all-encompassing term that
replaces the old snow advisories, sleet advisories, blowing/drifting snow
advisories, and freezing rain/drizzle advisories. It is used when a
mixture of precipitation is expected such as snow, sleet and freezing rain or
freezing drizzle, but will not reach warning criteria but will not reach
warning criteria. Used when sleet or snowfall amounts are expected to be
less than 3 inches in a 12-hour period, or freezing rain/drizzle is light and
ice accumulations are less than 1/4 inch.
- Special Weather Statement:
Used to highlight weather events that can have a high impact but fall below
advisory or warning criteria. Used when wind-driven snow intermittently reduces
visibility to 1/4 mile or less. Travel may be hampered. Strong winds create
blowing snow by picking up old or new snow.
- Winter Storm Watch: Issued when conditions
are favorable for the development of hazardous weather elements, such as heavy
snow or sleet, blizzard conditions, significant accumulations of freezing rain
or drizzle, or any combination thereof which meet or exceed local Winter Storm
Warning Criteria. Watches are usually issued 12 to 48 hours in advance of a
Winter Storm event.
- Winter Storm Warning:
Issued when hazardous winter weather conditions are imminent or very likely,
including any occurrence or combination of heavy snow, wind-driven snow, sleet,
and/or freezing rain/drizzle. Winter Storm Warnings are issued for
expected sleet and snowfall amounts of 4 or 6 inches or more in 12 hours, or 6
inches or 8 inches or more in 24 hours. It is also used when damaging ice
accumulations are expected during freezing rain situations; walking and driving
becomes extremely dangerous, and ice accumulations are usually 1/4 inch or
greater. Finally, it is also issued with sustained winds or frequent
gusts of 35 miles per hour or greater and considerable falling and/or blowing
snow reducing visibility to less than 1/4 mile. These conditions are
expected to last at least three hours. Usually issued 12 to 24 hours before the
event is expected to begin.
Inclement Winter Weather contains
conditions where pavement temperature is at 32 degrees or above with
light rain or snow creating icy bridge decks or light ice on roadways.
Accumulation of 2” or less.
Winter Storms consist
of snowfall with temperatures near or just below freezing with no
anticipated drop in temperature; little to no drifting of snow. Accumulation
between 2” – 4”.
Winter Storms consist
of snowfall with temperatures at freezing and dropping; moderate
drifting of snow. Accumulation between 4” – 6”
Winter Storms consist
of significant freezing rain or snow with temperatures falling and winds
creating major drifts resulting in blocked roadways. Accumulation of 6” or
are dangerous winter
storms that are a combination of blowing snow and wind resulting in very low
visibilities. While heavy snowfalls and severe cold often accompany blizzards,
they are not required. Sometimes strong winds pick up snow that has already
fallen, creating a ground blizzard.
- Blizzard Warning: Issued for sustained or gusty winds of 35 mph or more, and
falling or blowing snow creating visibilities at or below ¼ mile; these
conditions should persist for at least three hours.
- Snow Flurries: Light snow falling for short durations. No
accumulation or light dusting is all that is expected.
- Snow Showers: Snow falling at varying intensities for
brief periods of time. Some accumulation is possible.
- Snow Squalls: Brief, intense snow showers accompanied by
strong, gusty winds. Accumulation may be significant. Snow squalls are best
known in the Great Lakes region and may persist for many hours and produce 6
inches or more of snow in 12 hours or less.
- Blowing Snow: Wind-driven snow that reduces visibility and
causes significant drifting. Blowing snow may be snow that is falling and/or
loose snow on the ground picked up by the wind in quantities that horizontal
visibilities are reduced to less than seven statute miles.
road icing in rare
cases, icy roads can be created from man-made sources. Automatic sprinkler
systems, sump pumps and water main breaks have been known to cause accidents on
nearby roads during subfreezing temperatures.
rainwater and/or groundwater occur
when previously-fallen rainwater freezes onto roadways after temperatures drop
below freezing. Runoff can drip or flow off of cliffs and hillsides, covering
roads with a sheet of ice.
Rain: Rain that falls
onto a surface with a temperature below freezing. This causes it to freeze to
surfaces, such as trees, cars, and roads, forming a coating or glaze of ice.
Even small accumulations of ice can cause a significant hazard.
- Sleet: Rain drops that freeze into ice pellets
before reaching the ground. Sleet usually bounces when hitting a surface and
does not stick to objects. However, it can accumulate like snow and cause a
hazard to motorists.
ice storm is a storm which results in the accumulation of at least .25”
of ice on exposed surfaces. They create hazardous driving and walking
conditions. Tree branches and powerlines can easily snap under the weight of
- Wind Chill Warning: Issued when wind chill temperatures
are expected to be hazardous to life within several minutes of exposure. Issued
when wind chill temperatures are expected to be minus 25 degrees Fahrenheit.
- Wind Chill Advisory: Issued when wind chill temperatures
are expected to be a significant inconvenience to life with prolonged exposure,
and, if caution is not exercised, could lead to hazardous exposure. Wind Chill
temperatures are expected to be in the range of minus 10 degrees Fahrenheit to
less than or equal to minus 24 degrees Fahrenheit.