The National Weather Service uses outlooks, watches, warnings, and advisories to describe hazardous weather, according to the National Weather Service Denver/Boulder Weather Forecast Office. The office lists the criteria it uses for Colorado along and east of the Continental Divide. The page explains thresholds by hazard type and, in some cases, by terrain.
How the National Weather Service categorizes alerts
The National Weather Service says it will inform the public about developing hazardous weather through outlooks, watches, warnings, and advisories. The Denver/Boulder office presents definitions and criteria for those products. The thresholds on the page are used to explain the products in that forecast area.
Where these criteria apply
According to the National Weather Service Denver/Boulder office, the watch, warning, and advisory criteria on the page apply to Colorado along and east of the Continental Divide. Heavy snow criteria for eastern and central Colorado are described as representative values applied over a large geographic area. Several products distinguish between the mountains and foothills and lower elevations.
Winter watches and warnings
The National Weather Service says a winter storm watch is issued when winter storm conditions are possible within the next three days, but the timing, intensity, or occurrence may still be uncertain. A winter storm watch for blizzard conditions is issued when blizzard conditions are possible in the next 12 to 36 hours. A winter storm warning is issued when heavy snow is occurring or will develop in the next 36 hours, and the heavy snow may be accompanied by wind greater than 15 mph and blowing snow.
Blizzard, snow squall, wind chill, and dust storm warnings
A blizzard warning in lower elevations is issued when, for at least three hours, sustained winds of 35 mph or greater combine with considerable falling and/or drifting snow to lower visibility to less than one-quarter mile, according to the National Weather Service. In the mountains and foothills, a blizzard warning is issued for the same snow and visibility conditions but with winds in excess of 50 mph at higher elevations. A snow squall warning is a short-fused warning, normally 30 to 60 minutes, for an intense, short-lived burst of heavy snowfall that can quickly reduce visibility to less than one-quarter mile, create sudden whiteout conditions, and produce slick or hazardous roads. A wind chill warning is issued for wind chills of at least minus 25 degrees on the plains, and minus 35 degrees in the mountains and foothills. A dust storm warning is issued when widespread or localized blowing dust is or is expected to reduce visibility to one-quarter mile or less, and sustained winds of 25 mph or greater are usually required.
High wind watches and warnings
The National Weather Service states that a high wind watch is issued when high wind conditions are expected to develop in the next 12 to 36 hours. It may also be issued late in the first forecast period, six to 12 hours ahead, if potential exists but there is some uncertainty. A high wind warning is issued for sustained winds of 50 mph for at least one hour or gusts to 75 mph for any duration in the mountains, foothills, and nearby adjacent plains including Fort Collins, Boulder, and western Denver suburbs; and for sustained winds of 40 mph for at least one hour or gusts to 58 mph for any duration at lower elevations away from the foothills.
Freeze, fog, frost, and blowing dust advisories
A freeze watch is issued when freeze conditions are possible in the next 12 to 36 hours. A freeze warning is issued during the growing season when widespread temperatures are expected to drop to below 32 degrees. A dense fog advisory is issued when fog will reduce visibility to one-quarter mile or less. During the growing season, a frost advisory is issued when temperatures are expected to drop to between 32 and 35 degrees on clear calm nights. A blowing dust advisory is issued when blowing dust reduces visibility to between one-quarter mile and one mile.
Winter weather and wind chill advisories
A winter weather advisory is issued when general snow accumulations are expected between four and eight inches in 12 hours in the mountains and foothills, and between three and six inches in 12 hours at lower elevations, according to the National Weather Service. It is also issued when falling snow with blowing snow causes travel problems due to lower visibility; when wind-blown snow will occasionally reduce visibility and create a hazard for travelers; or for freezing drizzle or a mix of precipitation types, such as snow and sleet, that will impact travel conditions. A wind chill watch is issued when wind chill warning criteria are possible in the next 12 to 36 hours, and a wind chill advisory is issued on the plains when wind and temperature combine to produce wind chills of minus 18 degrees to minus 25 degrees, and in the mountains and foothills when wind and temperature combine to produce wind chills of minus 25 degrees.
Snow amounts used on this page
The National Weather Service lists representative heavy snow criteria for eastern and central Colorado. In the mountains, eight inches of snow in 12 hours or 12 or more inches in 24 hours meet the criteria. At lower elevations, six inches in 12 hours or eight or more inches in 24 hours meet the criteria.