Weather Glossary
The glossary is broken down into five basic areas for easier understanding:
SPRING/SUMMER WEATHER TERMS
It is very important that the difference between a WATCH or WARNING be understood. The term WATCH implies that people should be alert for the possibility of severe weather and have a plan of action in case a storm threatens.
When a WARNING is issued by the National Weather Service, this means that severe weather has been detected by radar, observed by trained storm spotters (SKYWARNTM), or has been forecast for a specific area. People in the path of the storm are expected to take action to protect life and property when the term WARNING is heard.
Following is a list of short-term watches, warnings, and advisories that the National Weather Service issues and the criteria used for issuing them:
- TORNADO WATCH - Conditions are favorable for the development of tornadoes in and close to the watch area. Watches are usually in effect for several hours, with 6 hours being the most common.
- TORNADO WARNING - Tornado is indicated by radar or sighted by storm spotters. It is very important the warning include where the tornado was and what towns will be in its path.
- SEVERE THUNDERSTORM WATCH - Conditions are favorable for the development of severe thunderstorms in and close to the watch area. Watches are usually in effect for several hours, with 6 hours being the most common.
- SEVERE THUNDERSTORM WARNING - Issued when a thunderstorm is producing or will produce hail 3/4 of an inch or larger in diameter and/or winds equal or exceed 58 miles an hour. Again, it is very important to include where the storm was, what towns will be affected and the primary threat associated with the storm.
- URBAN/SMALL STREAM FLOOD ADVISORY - Alerts the public to flooding which is generally only an inconvenience (not life-threatening) to those living in the affected area. Issued when heavy rain will cause flooding of streets and low-lying places in urban areas. Also used if small rural or urban streams are expected to reach or exceed bankfull. Some damage to homes or roads could occur.
- FLASH FLOOD WATCH - Indicates that flash flooding is a possibility in or close to the watch area. Those in the affected area are urged to be ready to take action if a flash flood warning is issued or flooding is observed.
- FLASH FLOOD WARNING - Signifies a dangerous situation where rapid flooding of rivers, small streams, or urban areas occurs. Very heavy rain that falls in a short time period can lead to flash flooding, depending on local terrain, ground cover, degree of urbanization, degree of man-made changes to river banks, and initial ground or river conditions.
FALL/WINTER WEATHER TERMS
The following advisories will be used to alert the public of situations that may cause difficult, but not impossible traveling conditions.
- SNOW ADVISORY - Used for 12 hour snowfalls of 2-3 inches in the Columbia basin, 3-5 inches in the valleys and central Oregon Mountains, and 6-11 inches Blue Mountains and east slopes southern Washington Cascades. Used for 24 hour snowfalls of 3-5 inches in the basin, 5-9 inches in the valleys and central Oregon Mountains, 6-11 inches Blue Mountains, and 12-17 inches in the east slopes of southern Washington Cascades.
- WINTER WEATHER ADVISORY - Used when a mixture of precipitation is expected such as snow, sleet and freezing rain or drizzle.
- FREEZING RAIN or FREEZING DRIZZLE ADVISORY - Will be used with discretion and only during times when the intensity of freezing rain or drizzle is light.
- BLOWING SNOW ADVISORY - Used when wind-driven snow reduces visibility enough to hamper travel. Strong winds create blowing snow by picking up old or new snow.
The following terms warn the public of more serious winter weather situations that may cause impossible traveling conditions and could pose a threat to life and property.
- WINTER STORM WATCH - Issued when conditions are favorable for the development of hazardous weather elements such as heavy snow and/or blizzard conditions, or significant accumulations of freezing rain or sleet. These conditions may occur singly, or in combination with others. Watches are usually issued 24 to 48 hours in advance of the event(s).
- WINTER STORM WARNING - Issued when more than one of the following is likely to cause life threatening conditions: heavy snow, wind, freezing rain/drizzle, sleet. Winter Storm Warnings are usually issued for up to a 12 hour duration, but can be extended out to 24 hours. Event specific warnings may be issued if the forecaster is confident that nothing else will occur.
- HEAVY SNOW WARNING - Issued for 12 hour snowfalls of at least 4 inches in the Columbia Basin, 6 inches in the valleys and central Oregon Mountains, and 12 inches in the Blue Mountains and east slopes of southern Washington Cascades. Issued for 24 hour snowfalls of at least 6 inches in the basin, 10 inches in the valleys and central Oregon Mountains, 12 inches in the Blue Mountains, and 18 inches east slopes of southern Washington Cascades.
- BLIZZARD WARNING - Issued for winter storms with sustained winds or frequent gusts to 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 3 hours.
- ICE STORM WARNING - Issued when damaging accumulations of ice are expected during freezing rain situations, with walking and driving becoming extremely dangerous. Significant ice accumulations are usually 1/4 inch or greater which may cause power lines and large tree branches to break.
- SLEET WARNING - Issued when accumulations of sleet, to a depth of 1/2 inch or more, are expected. This is a relatively rare event.
NON-PRECIPITATION WEATHER TERMS
The following terms are used to make the public aware of weather situations that are not necessarily associated with precipitation, but need to be highlighted.
- FOG ADVISORY - Used when dense fog covers a significant area and reduces visibility to less than 1/4 mile. May be called a DENSE FOG ADVISORY.
- FROST ADVISORY - Issued in growing season to indicate formation of widespread frost.
- BLOWING DUST ADVISORY - Used for visibility 1/4-3/4 mile due to blowing dust.
- FREEZE WARNING - Used during the growing season when temperatures are expected to drop well below freezing over a large area, regardless of whether frost forms or not.
The following terms are used to warn the public of non-precipitation events that could be a threat to life or property.
- WIND CHILL WARNING - Used when wind chill factors are expected fall below 20 degrees below zero.
- HIGH WIND WATCH - Issued 12-36 hours in advance of non-convective sustained winds of 40 mph and gusts 58 mph or stronger.
- HIGH WIND WARNING - Sustained winds of 40 miles per hour or greater are expected to last for 1 hour or longer. Also used if winds of 58 miles per hour or greater are anticipated for any duration.
- DUSTSTORM WARNING - Used for visibility less than 1/4 mile due to blowing dust.
PRECIPITATION PROBABILITIES: WHAT DO THEY MEAN?
Following are precipitation probabilities used in National Weather Service forecasts and a brief explanation of each. Technically, the Probability of Precipitation (PoP) is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Measurable precipitation is defined as equal to or greater than .01 inch or .2 mm. Normally, the period of time is 12 hours, unless specified otherwise.
PoP Percent Expressions of Equivalent Areal Qualifiers
- Uncertainty (convective only)
- 20 percent slight chance isolated
- 30-40-50 percent chance scattered
- 60-70 percent likely numerous (or none used)
- 80-90-100 percent (none used) occasional, periods of, (or none used)
GENERAL WEATHER TERMS
Following are weather terms frequently used by the National Weather Service. All temperatures are in Fahrenheit.
- ADVISORY - Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience and, if caution is not exercised, could lead to situations that may threaten life and/or property.
- DEGREE-DAY - Gauges the amount of heating or cooling needed for a building using 65 degrees as a baseline. To compute heating/cooling degree-days, the average temperature for a day is taken and referenced to 65 degrees. An average temperature of 50 yields 15 heating degree-days while an average of 75 would yield 10 cooling degree-days. Electrical, natural gas, power, and heating, and air conditioning industries utilize heating and cooling degree information to calculate their needs.
- To compute growing degree days one would use a reference of 50 degrees. Every degree that the average temperature is above 50 becomes a growing degree day. Agricultural-related interests use growing degree days to determine planting times.
- DOPPLER - (NATIONAL WEATHER SERVICE DOPPLER WEATHER RADAR) - A new Weather Surveillance Radar (WSR-88D) system developed in 1988. About 120 systems are installed at Weather Forecast Offices. An additional 24 systems are at Department of Defense (Air Force Bases) sites. This powerful and sensitive Doppler system generates many useful products for meteorologists, among them: standard reflectivity "echoes," wind "velocity" or atmospheric air motion pictures, and areal 1-hour, 3-hour, or storm-total precipitation images.
- DOWNBURST - A strong downdraft, initiated by a thunderstorm, that induces an outburst of damaging staringht-line winds on or near the ground. Downbursts may last for anywhere from a few minutes in small scale microbursts on up to 20 minutes in larger, longer living macrobursts. Wind speeds in downbursts can reach 150 mph, or squarely in the range of a strong tornado!
- FLASH FLOOD - A dangerous and sudden flood that threatens lives and property and usually occurs after heavy rain. May also occur after an ice jam breaks up or after a dam fails.
- FLOOD STAGE - The level at which a river causes flooding.
- FUNNEL CLOUD - A rapidly rotating column of air that does not touch the ground.
- FREEZING DRIZZLE, FREEZING RAIN - Describes the effect of drizzle or rain freezing upon impact on objects that have a temperature of 32 degrees or below.
- HAIL - Precipitation in the form of balls or lumps usually consisting of concentric layers of ice. A thunderstorm is severe when it produces hail 3/4 of an inch or larger in diameter.
- HEAT INDEX - The apparent temperature that describes the combined effect of high temperatures and high levels of humidity.
- HEAVY SNOW - A significant and unusual accumulation of snow. Twelve hour snowfalls of at least 4 inches in the Columbia Basin, 6 inches in the valleys and central Oregon Mountains, and 12 inches in the Blue Mountains and east slopes of southern Washington Cascades. Twenty-four hour snowfalls of at least 6 inches in the basin, 10 inches in the valleys and central Oregon Mountains, 12 inches in the Blue Mountains, and 18 inches east slopes of southern Washington Cascades.
- PROBABILITY OF PRECIPITATION (POP) - The chance for at least .01 inch of rain or .01 inch liquid equivalent of frozen precipitation at any one location.
- RAIN - Indicates a nearly steady and uniform fall of precipitation over an area for several hours, as opposed to the term "showers" which implies intermittent and scattered rainfall of a more unstable, convective nature.
- SEVERE THUNDERSTORM - A thunderstorm that produces either of the following: winds of 58 miles an hour or greater (these speeds can result in structural or trees damage), hail 3/4 of an inch in diameter or larger, or a tornado.
- SLEET - Describes solid grains of ice formed by the freezing of raindrops or the refreezing of largely melted snowflakes. These grains usually bounce upon impact with the ground or pavement.
- TORNADO - A violently rotating column of air that is touching the ground. The visible cloud portion of a tornado may not extend all the way to the gound.
- WATCH - Alerts the public to the possibility of severe weather, or some other hazardous weather element. It is intended to provide enough lead time so those who need to set their plans in motion can do so.
- WARNING - Issued to warn the public that a hazardous weather element is imminent or has a very high probability of occurrence.
- WATERSPOUT - A tornado that occurs over water.
- WIND CHILL - An apparent temperature that describes the combined effect of wind and low air temperatures on exposed skin.
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