NOAA National Weather Service Weather Forecast Office

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Contents:

Watches, Warnings, and Advisories
Hazardous Weather
     Severe Local Storms
     Winter Weather
     Other Hazards
Weather Terminology
     Precipitation Probablities
     Sky Conditions
     Wind
General Weather Terms

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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.

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HAZARDOUS WEATHER  

The hazardous weather warning section of this guide is divided into three types of hazardous weather/hydrologic events.

1. Severe Local Storms - These are short-fused, small scale hazardous weather or hydrologic events produced by thunderstorms, including large hail, damaging winds, tornadoes, and flash floods.

2. Winter Storms - These are weather hazards associated with freezing or frozen precipitation (freezing rain, sleet, snow) or combined effects of winter precipitation and strong winds.

3. Other Hazards - Weather hazards not directly associated with thunderstorms or winter storms including extreme heat or cold, dense fog, high winds, river flooding and lakeshore flooding.

SEVERE LOCAL STORMS  

Tornado Watch - Conditions are favorable for the development of severe thunderstorms and tornadoes in and close to the watch area. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for four to six hours.

Tornado Warning - Strong rotation in a thunderstorm is indicated by Doppler radar or a tornado is sighted by skywarn spotters. These warnings are issued on a county by county basis by the local Weather Forecast office in Chicago/Romeoville.

Severe Thunderstorm Watch - Conditions are favorable for the development of severe thunderstorms in and close to the watch area. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for four to six hours.

Severe Thunderstorm Warning - A severe thunderstorm is indicated by Doppler radar or sighted by skywarn spotters. A severe thunderstorm contains large damaging hail, 3/4 inch diameter or larger, and/or damaging winds of around 60 mph or greater. These warnings are issued on a county by county basis by the local Weather Forecast Office in Chicago/Romeoville.

Flood Watch - Conditions are favorable for flash flooding in and close to the watch area. These watches are issued by the Weather Forecast Office in Chicago/Romeoville and are usually for the first 12 to 24 hours of a forecast.

Flash Flood Warning - Flash flooding is occurring, imminent or highly likely. A flash flood is a flood that occurs within 6 hours of excessive rainfall and that poses a threat to life and/or property. Ice jams and dam failures can also cause flash floods. These warnings are issued on a county by county basis by the local Weather Forecast Office in Chicago/Romeoville and are generally in effect for up to 6 hours.

Flood Warning - General or areal flooding of streets, low-lying areas, urban storm drains, creeks and small streams is occurring, imminent, or highly likely. Flood warnings are issued for flooding that occurs more than 6 hours after the excessive rainfall. These warnings are issued on a county by county basis by the local Weather Forecast Office in Chicago/Romeoville and are generally in effect for 6 to 12 hours.

Special Marine Warning - A warning to mariners on southern Lake Michigan, of hazardous thunderstorms or squalls with wind gusts of 34 knots or more, hail 3/4 inch diameter or larger, or waterspouts.

WINTER STORMS  

Winter Storm Watch - Conditions are favorable for hazardous winter weather conditions including heavy snow, blizzard conditions, or significant accumulations of freezing rain or sleet. These watches are issued by the Weather Service Forecast Office in Chicago and are usually issued 12 to 36 hours in advance of the event.

Winter Storm Warning - Hazardous winter weather conditions that pose a threat to life and/or property are occurring, imminent or likely. The generic term, winter storm warning, is used for a combination of two or more of the following winter weather events; heavy snow, freezing rain, sleet and strong winds. The following event-specific warnings are issued for a single weather hazard:

• Blizzard Warning - Sustained winds or frequent gusts of 35 mph or greater, considerable falling and/or blowing snow reducing visibility frequently to 1/4 mile or less for a period of three hours or more. There are no temperature criteria in the definition of a blizzard but freezing temperatures and 35 mph winds will create sub-zero wind chills.
• Heavy Snow Warning - Snowfall of 6 inches or more in 12 hours or less, or 8 inches or more in 24 hours or less.
• Lake Effect Snow Warning - Lake effect snowfall of 6 inches or more in 12 hours or less, or 8 inches or more in 24 hours or less.
• Ice Storm Warning - Accumulations of 1/4 inch or more of freezing rain.
• Heavy Sleet Warning - Accumulations of 1/2 inch or more of sleet.

Winter Weather Advisory - Hazardous winter weather conditions are occurring, imminent or likely. Conditions will cause a significant inconvenience and if caution is not exercised, will result in a potential threat to life and/or property. The generic term, winter weather advisory, is used for a combination of two or more of the following events; snow, freezing rain or drizzle, sleet, blowing snow. The following event-specific advisories are issued for these single weather hazards:

• Snow Advisory - Snowfall of 3 to 5 inches.
• Lake Effect Snow Advisory - Lake effect snowfall of 3 to 5 inches.
• Freezing Rain Advisory - Accumulations of less than 1/4 inch of freezing rain or freezing drizzle.
• Sleet Advisory - For accumulations of less than 1/2 inch of sleet.
• Snow and Blowing Snow Advisory - Sustained winds or frequent gusts of 25 to 35 mph accompanied by falling and blowing snow, occasionally reducing visibility to 1/4 mile or less.

OTHER HAZARDS  

Wind Chill Warning - Extreme wind chills of -30^oF or colder.
Wind Chill Advisory - Dangerous wind chills of -20^oF to -30^oF.

Excessive Heat Warning - Extreme values of the heat index.
Chicago metro area:

1. 3 consecutive days with maximum heat index 100 to 105^oF with at least 85% sunshine on two of the days, or minimum heat index 75^oF or greater each day, or
2. Two consecutive days with the maximum heat index 105 to 110^oF, or
3. 1 day with the maximum heat index 110^oF or greater.

Remainder of northern Illinois and northwest Indiana:
Maximum heat index of 110^oF or greater and minimum of 75^oF for two or more consecutive days.

Heat Advisory - Maximum heat index of 105 to 110^oF with a minimum of 75^oF for two or more consecutive days. (Heat Advisories not issued for the Chicago metro area.)

High Wind Warning - Sustained winds of 40 mph or greater for a duration of one hour or longer or gusts to 58 mph or greater.
Wind Advisory - Sustained winds of 30 mph or greater or gusts to 45 mph or greater for a duration of one hour or longer.

Dense Fog Advisory - Widespread dense fog reducing visibility to less than 1/4 mile.

Flood Warning (river flood)- A warning for specific communities or areas along a river where flooding is imminent or occurring. Flood warnings normally give specific crest forecasts.

Freeze Warning - Widespread temperatures at or below 32^oF during the growing season. A freeze may occur with or without frost. A hard freeze occurs with temperatures below 28^oF. Growing season is based on the average date of first and last freeze at O'Hare. It begins April 17 and ends October 27.

Frost Advisory - Widespread frost during the growing season. Frost generally occurs with fair skies and light winds.

Lakeshore Warning - Lakeshore flooding that is occurring or is imminent in the next 12 hours, which poses a serious threat to life and/or property. A seiche warning is issued for rapid and large fluctuations in water level in Lake Michigan usually caused by a strong line of thunderstorms moving rapidly southeast across the lake.

Blowing Snow Advisory - Widespread blowing snow with winds 25 to 35 mph, occasionally reducing visibility to 1/4 mile or less.

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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

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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. Fujita Scale or F-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) 40-72 mph, light damage F1 (weak) 73-112 mph, moderate damage F2 (strong) 113-157 mph, considerable damage F3 (strong) 158-206 mph, severe damage F4 (violent) 207-260 mph, devastating damage F5 (violent) 261-318 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.