Advisory - The event is generally not life-threatening if precautions are taken. Issued for first or second forecast period.

Blizzard - A storm where the following conditions prevail for three hours or more: sustained winds of 50 mph or more and considerable falling/blowing snow frequently reducing visibility to less than 1/4 mile. Wind speeds of greater than 75 mph are required over the Sierra Nevada. There is no temperature requirement but if the temperature falls below 20 degrees, the life- threatening wind chill will be highlighted.

Blizzard Warning - This is a statement issued when blizzard conditions are imminent or occurring.

Blowing/Drifting Snow Advisory - Up to the forecaster's discretion.

Breezy - Sustained winds of 15 to 25 mph.

Broken clouds - clouds which cover 6/10 to 9/10 of the sky.

Chance - A 30 to 50 percent probability of measurable precipitation occurring.

Cirrus - High clouds composed of ice crystals.

Clear - Less than 1/10 cloud coverage.

Cloudy - Between 9/10 and 10/10 cloud cover.

Coastal Flooding - Inundation of coastal areas from waves and storm surge.

Coastal Waters - Waters out to 60 nautical miles (nm).

Considerable Cloudiness - Between 7/10 and 8/10 cloud cover.

Cumulonimbus Cloud - A cloud with great vertical extent producing rain, hail, strong winds, lightning, thunder and sometimes tornados. Also called a thunderstorm.

Cumulus Cloud - Puffy domed clouds with flat bases.

Dambreak Flooding - Dambreaks present a unique flood warning problem. Dams may erode and empty relatively slowly. In these situations, when river forecasts are available, flood warnings and flood statements are used. Dams may also fail catastrophically during a heavy rainfall event or earthquake, resulting in a very dangerous flash flood-type situation. For these events, Flash Flood watches, warnings and statements will be used. Similar action may be required for the failure of certain levees.

Dense Fog/Smoke/Volcanic Ash Advisory - Widespread visibilities of 1/8 mile or less.

Dew point - The temperature to which air must be cooled for dew to form.

Downburst - A strong downdraft from a thunderstorm which induces damaging winds on or near the ground.

Drizzle - Small water drops with diameters of .2 to .5 millimeters.

Dust storm / Sandstorm Warning - This statement is issued when widespread dust storm/sandstorm conditions are occurring or imminent. The conditions must persist for several hours.

El Nino - A major warming of the tropical Pacific waters. Causes shifts in normal weather patterns.

Fair - Less than 4/10 cloud cover with no extremes in temperature, humidity, visibility or wind.

Few - Showers covering less than 15 percent of an area.

Flood - The inundation of a normally dry area caused by an increased water level in an established watercourse, such as a river, stream or drainage ditch; or may also be the ponding of water at or near the point where the rain fell. Flash Flood - A flood which is caused by heavy or excessive rainfall in a short period of time, and occurring generally less than 6 hours after the causative event. A dam failure may also cause a flash flood depending on the time period of the break.

Flash Flood Warning - A flash flood warning is a public warning issued by local NWS offices. It is site specific, focusing on communities, streams or areas where flooding is imminent or in progress. Persons in the warning area shall be advised to take necessary precautions immediately. Larger, main stem rivers usually are not included in Flash Flood products.

Flood / Flash Flood Statement - The office issuing a flood/flash flood watch or warning will issue follow-up statements to keep the public fully informed of the most current information. Statements are used to remove geographical areas covered by the original flood or flash flood watch/warning when flooding is no longer a threat or has ended in that area. A statement will not be used to extend the length of time or add area to a watch/warning. In those cases, a new watch or warning will be issued.

Flood / Flash Flood Watch - This product is used to inform the public and cooperating agencies that current and developing conditions are such that there is a threat of flooding, but the occurrence is neither certain nor imminent. Persons in the watch area are thereby advised to check flood action plans, keep informed and be ready to take necessary actions if a warning is issued or flooding is observed.

Flood Warning
Main Stem River Flooding
River Statement
Flood Statement - Main stem river flooding refers to flooding of gauged and forecasted rivers. In Northwestern California, the gauged rivers are:
Smith, Trinity, Klamath , Redwood Creek, Mad , Mattole, Navarro, Eel, Van Duzen , Russian. Flooding of all other smaller rivers and streams in the area will be covered by Small River and Stream Flood products, Flash Flood products or Urban/Small Stream Flood products.

Flood Stage - The stage at which over bank flows are of sufficient magnitude to cause considerable inundation of land and roads and/or threat of significant hazard to life and property.

Freezing Rain (or Drizzle) - Rain or drizzle that freezes immediately upon striking an object. Formed by liquid rain falling through a sub-freezing layer of air and becoming super-cooled.

Freezing Rain/Drizzle/Sleet Advisory - No accumulation requirement, up to the forecaster's discretion.

Funnel Cloud - A rotating column of air forming a pendant from a thunderstorm with the circulation not reaching the ground.

Gale - Wind speeds from 34 to 47 knots.

Ground Fog - Fog formed by cooling of the air near the ground. Also known as radiation fog and Tule fog.

Gust - A brief, sudden increase in wind speed. The speed is at least 10 mph greater than the average wind.

Hail - Precipitation in the form of hard lumps of ice.

Heavy Snow -

 Mountain Areas
• Below 7000 ft - 8 inches in 12 hrs / 12 inches in 24 hrs
• Above 7000 ft - 12 inches in 12 hrs / 18 inches in 24 hrs


Non-Mountain Areas

• 4 inches in 12 hrs / 6 inches in 24 hrs

Heavy Surf Advisory - Issued for fast moving deep water waves which can result in big breaking waves in shallow water (the surf zone). Typically, issued when deep water significant swell heights reach 18 feet or greater.

High Wind -

 Below 7000 feet
• Sustained winds of 40 mph or more and/or gusts of 58 mph or more continuing for an hour or more.


Above 7000 feet

• Sustained winds of 50 mph or more and/or gusts of 70 mph or more continuing for an hour or more.

High Wind Watch - This is a statement outlining the possibility of high winds.

High Wind Warning - When high winds are imminent or occurring and are expected to continue for several hours, a high wind warning will be issued.

Hurricane - Severe tropical cyclone with wind speeds of greater than 64 knots.

Ice Storm Warning - Freezing rain/drizzle or sleet accumulating 1/4 inch or more.

Inversion - A condition where temperature increases with height. The atmosphere is very stable under the inversion to very little mixing with higher levels.

Isolated - Showers covering less than 15 percent of an area.

Jet Stream - Fast high level winds contained in a narrow band. Separates polar air from subtropical air.

Knot - One nautical mile per hour (1.15 mph).

Likely - A 60 to 70 percent chance of measurable precipitation occurring.

Macroburst - A downburst more than 2 1/2 miles in diameter and lasting 5 to 20 minutes.

Microburst - A small downburst, less than 2 1/2 miles in diameter and lasting 2 to 5 minutes.

Monitor Stage - The stage at which initial action must be taken by concerned interests (livestock warning, removal of equipment from lowest overflow areas or simply general surveillance of the situation). This level may produce overbank flows sufficient to cause minor flooding of low-lying lands and local roads.

Mostly Sunny - Between 1/10 and 3/10 cloud cover.

Mostly Clear - Same as mostly sunny.

Numerous - Showers covering more than 54 percent of an area.

Offshore Waters - Waters from 60 nm to 250 nm.

Orographic Lift - The forcing of air over terrain. Can produce clouds or precipitation.

Overcast - Greater than 9/10 cloud cover.

Partly Cloudy - Between 3/10 and 7/10 cloud cover.

Partly Sunny - Same as partly cloudy.

Relative Humidity - The amount of water vapor in the air as compared to what the air can hold at saturation.

Scattered Clouds - Between 1/10 and 5/10 cloud cover.

Scattered Showers - Showers covering 25 to 54 percent of an area.

Seas - Combination of swell and wind waves.

Severe Thunderstorm - A thunderstorm with winds of 58 mph (50 knots) or more and/or hail 3/4 of an inch in diameter or larger.

Severe Weather Statement - This product is issued by offices with warning responsibility for reporting severe weather, to cancel all or part of a warning and to provide follow-up information on a warning in effect.

Short-Term Forecasts - A short term forecast covering the next three to six hours will be issued when weather conditions warrant. Generally, the forecast will discuss current thunderstorms, showers, fronts, fog and wind. If these phenomena become more of a threat to life or property, appropriate watches/warnings/advisories will be issued.

Sleet - Frozen raindrops that fall as ice pellets. Formed by liquid rain falling through a deep sub- freezing layer of air and freezing before striking the ground.

Slight Chance - A 20 percent chance of measurable precipitation.

Small River and Stream Flooding - A flood of an un gauged river or stream. A more serious type of flooding than the urban and small stream type. Road, bridges and areas near the river/stream are threatened by the flood waters. People in the area must take protective measures.

Small River and Stream Flood Watch
Small River and Stream Flood Warning
Small River and Stream Flood Statement - The North Coast area contains several un-gauged rivers and streams that can pose a serious flood threat. When flooding is imminent or occurring on these small rivers, streams and creeks, a Small River and Stream Flood Warning will be issued. A Small River and Stream Flood Watch may be issued if there is a flood threat beyond 12 hours. Small River and Stream Flood Statements will be issued to update information in the Watch or Warning.

Snow Advisory -

Mountain Areas
 
• Above 7000 feet 6 inches in 12 hrs or 9 inches in 24 hrs
• Below 7000 feet 4 inches in 12 hrs or 6 inches in 24 hrs

 
Non-Mountain Areas
 
• 2 to 4 inches in 12 hrs or 4 to7 inches in 24 hrs

Special Weather Statement - The special weather statement is issued by NWS offices with warning responsibility and is used to pass along information on non-severe weather events and the status of watches.

State Forecast - The State Forecast is written by the Forecast Office and covers all of Northern and Central California. It is meant to be a broad overview of the general weather conditions in the area.

Storm (marine) - Wind speeds greater than 47 knots.

Storm Surge - A rise of the sea level preceding a storm due to the wind and low atmospheric pressure.

Swell - Wind generated waves that have traveled out of the generation area, of regular and longer duration than wind waves.

Tornado - A violently rotating column of air, usually forming a pendant, from a thunderstorm with the circulation reaching the ground.

Tornado / Severe Thunderstorm and Flash Flood Warning - If flash flooding is expected to accompany a tornado of severe thunderstorm, a combined warning may be issued. If one part of the warning expires before the other, a statement will be issued explaining which part has expired and which part is still in effect.

Tornado / Severe Thunderstorm Watch - This product is prepared by the National Severe Storms Forecast Center in Kansas City, Missouri. The watch statement combines information of interest to both public and aviation sectors and is divided into five sections.

Tornado / Severe Thunderstorm Warning - This product is issued by the Northern California NWS office with warning responsibility for the affected area. A warning will be issued when a reliable report is received or if radar indicates that severe weather is occurring or is imminent.

Tsunami - Seismic sea wave caused by an earthquake, undersea landslide or volcanic eruption. Typically arrives onshore as a series of surges.

Tropical Depression - Tropical cyclone with wind speeds between 20 and 34 knots.

Tropical Disturbance - Tropical cyclone with wind speeds less than 20 knots.

Tropical Storm - Tropical cyclone with wind speeds between 35 and 64 knots.

Urban and Small Stream Flooding - Flooding of small streams, streets or low-lying areas, such as railroad underpasses and urban storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening.

Very Windy - Sustained winds of 30 to 40 mph.

Virga - Precipitation that evaporates before reaching the ground.

Wall Cloud - A local and often abrupt lowering of a thunderstorm base into a low-hanging accessory cloud. Diameter is usually 1 to 4 miles. Rotating wall clouds usually precede tornado development.

Warning - The event is imminent or occurring. Issued for the first period of the forecast, i.e. "Today" or "Tonight".

Warning Stage - Changed to Monitor Stage. The stage at which initial action must be taken by concerned interests (livestock warning, removal of equipment from lowest overflow areas or simply general surveillance of the situation). This level may produce overbank flows sufficient to cause minor flooding of low-lying lands and local roads.

Watch - The event is expected to occur in the second or third period of the forecast, i.e. tomorrow or tomorrow night.

Waterspout - Essentially, a tornado over water.

Widely Scattered - Showers covering 15 to 24 percent of an area.

Widespread - Showers covering nearly all of an area. Used primarily for emphasis.

Wind Advisory -

Below 7000 feet-
 
• Winds of 35 to 39 mph or gusts from 39 to 57 mph.


Above 7000 feet-

 
• Winds of 49 mph or more and/or gusts from 79 mph.

Windy - Sustained winds of 20 to 30 mph.

Wind Chill Advisory - Wind chills of less than -20.5F at or below 4000 feet or less than -30.5F above 4000 feet.

Wind Chill Temperature - The apparent temperature felt by exposed flesh from the effects of wind and temperature. . The method of computing is as follows:

Winter Storm Watch - This is a statement that highlights the possible occurrence of one or more of the following events: blizzard conditions, heavy snow, snow where it is rare, significant freezing rain or sleet. May also be issued for the first significant snow of the season, even if the expected conditions don't quite meet the advisory/warning criteria.

Winter Storm Warning - As the winter weather event becomes more certain and imminent, a warning will be issued. The warning has the same criteria as the winter storm watch. If only one weather condition is expected, a warning for just that event will be issued. For example, only a heavy snowfall is forecast to occur so a Heavy Snow Warning would be issued.

Winter Weather Advisory - The Winter Weather Advisory is similar to the Winter Storm Warning in that the actual product issued may be event specific. These events are significant but generally not life-threatening. Some of the events that an advisory can be issued for include: snow, wind, fog, blowing dust/sand, blowing/drifting snow, wind chill and so on.