Weather Terms and Warning/Advisory Criteria


Contents:

Watches, Warnings, and Advisories

Weather Terminology
     Precipitation Probablities
     Sky Conditions
     Wind
General Weather Terms


WATCHES, WARNINGS AND ADVISORIES  

 

The National Weather Service has developed a multi-tier concept for forecasting all types of hazardous weather. These are:

Outlook - A hazardous weather outlook is issued daily to indicate that a hazardous weather or hydrologic event may occur in the next several days. The outlook will include information about potential severe thunderstorms , heavy rain or flooding, winter weather, extremes of heat or cold, etc., that may develop over the next 7 days with an emphasis on the first 24 hours of the forecast. It is intended to provide information to those who need considerable lead time to prepare for the event.

Watch - A watch is used when the risk of a hazardous weather or hydrologic event has increased significantly, but its occurrence, location or timing is still uncertain. It is intended to provide enough lead time so those who need to set their plans in motion can do so. A watch means that hazardous weather is possible. People should have a plan of action in case a storm threatens and they should listen for later information and possible warnings especially when planning travel or outdoor activities.

Warning - A warning is issued when a hazardous weather or hydrologic event is occurring, imminent or likely. A warning means weather conditions pose a threat to life or property. People in the path of the storm need to take protective action.

Advisory - An advisory is issued when a hazardous weather or hydrologic event is occurring, imminent or likely. Advisories are for less serious conditions than warnings, that cause significant inconvenience and if caution is not exercised, could lead to situations that may threaten life or property.


 

WEATHER TERMINOLOGY

PRECIPITATION PROBABILITIES  

Technically, the probability of precipitation (PoP) is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount (.01 inch or more) of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Forecasts are normally issued for 12-hour time periods. Descriptive terms for uncertainty and areal coverage are used as follows:

PoP Expressions of
Uncertainty
Areal
Coverage
0% none used none used
10% none used isolated
20% slight chance isolated
30-50% chance scattered
60-70% likely numerous
80-100% none used none used

The following terms of duration imply a high probability (80-100%) of occurrence;
brief, periods of, occasional, intermittent, frequent.

SKY CONDITION  

Term Predominant or Average
Sky Condition
Cloudy 95 to 100% opaque cloud cover

Mostly Cloudy or Considerable Cloudiness

70 to 95% opaque cloud cover
Partly Cloudy or Partly Sunny 30 to 70% opaque cloud cover
Mostly Clear or Mostly Sunny 5 to 30% opaque cloud cover
Clear or Sunny 0 to 5% opaque cloud cover
Fair Less than 40% opaque cloud cover, no precipitation and no extremes of temperature, visibility, or wind.

WIND  

Sustained Wind Speed Descriptive Term
0 to 5 mph light or light and variable
5 to 15 mph none used
10 to 20 mph none used
15 to 25 mph breezy (mild weather) or brisk, blustery (cold weather)
20 to 30 mph or 25 to 35 mph windy
30 to 40 mph or 35 to 45 mph very windy
40 to 73 mph high, strong, damaging or dangerous winds


GENERAL WEATHER TERMS  

Cold air funnel - A funnel cloud or (rarely) a small relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold. They are much less violent than other types of tornadoes.

Crest - The highest level of a flood wave as it passes a point.

Degree Day - Gauges the amount of heating or cooling needed for a building using 65 degrees as a baseline. To compute degree days, the average temperature for a day is taken and referenced to 65. An average temperature of 50 yields 15 heating degree days, while an average temperature of 75 would yield 10 cooling degree days.

Dew Point - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming pressure and moisture content are constant).

Doppler Radar - Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar).

Downburst - A strong downdraft from a thunderstorm resulting in an outward burst of damaging winds on or near the ground. Downburst winds are often 50 to 100 mph and in a few cases, 100 to 150 mph. They can do as much damage as a small tornado.

  • Microburst - a small downburst affecting an area less than 2 1/2 miles in diameter with peak winds lasting generally less than five minutes.
  • Macroburst - a large downburst affecting an area greater than 2 1/2 miles in diameter with peak winds generally lasting five minutes or longer.

Flood Stage - The level or stage at which a stream overflows its banks or the stage at which the overflow of a stream begins to cause damage.

Freezing Rain/Freezing Drizzle - Rain or drizzle which falls in liquid form and freezes on impact with cold surfaces to form a glaze on the ground and exposed objects.

Enhanced Fujita Scale or EF-Scale - A scale of wind damage intensity in which wind speeds are inferred from analysis of wind damage. All tornadoes and most other windstorms are assigned a numerical rating from this scale according to the most intense damage caused by the storm.

F0 (weak) 65-85 mph, light damage
F1 (weak) 86-110 mph, moderate damage
F2 (strong) 111-135 mph, considerable damage
F3 (strong) 136-165 mph, severe damage
F4 (violent) 166-200 mph, devastating damage
F5 (violent) Over 200 mph, incredible damage

Funnel Cloud - A rotating column of air, extending from a towering cumulus or cumulonimbus cloud, that is not in contact with the ground.

Gustnado - A gust front tornado. A small, weak, short-lived tornado that occurs along a gust front - the leading edge of a thunderstorm, caused by rain-cooled air flowing out from the storm's downdraft. A gustnado is often visible as a debris cloud or dust whirl.

Hail - Precipitation in the form of lumps of ice that form during some thunderstorms.

Heat Index - The apparent temperature that describes the combined effect of high temperatures and high levels of humidity, which reduces the body's ability to cool itself.

Lake Breeze - A local wind blowing from the cooler surface of Lake Michigan to warmer inland areas, usually during the afternoon in spring and summer.

Lake Effect Snow - Snow that occurs to the lee of the Great Lakes when cold air moves across relatively warm waters of the lakes.

Rain/Showers - Rain is a nearly steady and uniform fall of precipitation over an area. Showers are intermittent and/or scattered convective rainfall of varying intensity.

Seiche - An oscillation of the surface of southern Lake Michigan usually caused by a squall line moving rapidly south-southeast down the Lake. Rapid changes in the lake level pose a serious threat to marinas and people on piers and breakwaters.

Relative Humidity - The ratio of the amount of water vapor actually present in the air to the greatest amount possible at the same temperature.

Sleet - Sleet or ice pellets are solid grains of ice formed from the freezing of rain or the refreezing of melted snow, which bounce off the ground and other objects.

Squall line - A line of thunderstorms or squalls which may extend over several hundred miles.

Tornado - A violently rotating column of air, from a thunderstorm, in contact with the ground.

Waterspout - In general, a tornado occurring over water. Specifically, it refers to a small, relatively weak rotating column of air over water beneath a towering cumulus or cumulonimbus cloud. They occasionally occur over Lake Michigan in late summer with unusually cold air aloft.

Wind Chill - An apparent temperature that describes the combined effect of wind and low temperature on exposed skin.


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