Special Weather Statements: Regional Definitions and Usage
Weather alerts are critical tools for public safety, providing timely information about atmospheric hazards. Among these, the Special Weather Statement serves as a versatile communication tool used by meteorological agencies to notify the public about hazards that may not meet the strict criteria for a formal warning or advisory, but still require attention due to their potential impact on safety and travel.
Special Weather Statements in the United States
In the United States, Local Weather Forecast Offices (WFO) of the National Weather Service (NWS) utilize Special Weather Statements to alert the public of specific hazards. These statements are typically used when a threat is approaching or exists below the threshold of a formal warning or advisory, or when no specific alert product code exists for the particular phenomenon.
These alerts often cover imminent hazardous convective weather—weather resulting from convection, such as thunderstorms—expected within a one- to two-hour window. They are also used for major events forecast beyond a six-hour timeframe, including dense fog, winter weather events, or substantial temperature shifts.
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Supplementary and Short-Term Alerts
Special Weather Statements frequently provide supplementary information to other hazardous weather products. Examples include alerts for black ice, short-duration heavy snow, lake-effect snow bands that briefly reduce visibility, or heavy rainfall that is not expected to cause flooding. They may also be issued when heat index or wind chill values approach advisory criteria for a brief period, or for localized blowing dust when wind speeds remain below advisory levels.
Sub-Severe Thunderstorms and Fire Weather
Since July 2021, following the discontinuation of the separate significant weather advisory product, these statements have become the primary tool for alerting the public to sub-severe thunderstorms. These are storms characterized by sustained winds or gusts of 40–57 mph (64–92 km/h) and/or hail smaller than one inch (2.5 cm) in diameter, often accompanied by continuous lightning or funnel clouds that are not expected to become tornadoes.
Additionally, these statements are used for other short-term impacts such as snow squalls, bursts of freezing rain, or heat bursts from decaying thunderstorms. They may also be issued for possible fire weather conditions (typically at least an enhanced risk) or to clear specific counties from severe weather watches. While they can occasionally update current conditions, this is usually handled by the Short-Term Forecast product. In very rare instances, an Emergency Alert System (EAS) activation may be requested.
Special Weather Statements in Canada
The Meteorological Service of Canada employs Special Weather Statements through its regional offices. In the Canadian context, these are issued for weather events that are unusual or those that cause general public concern and inconvenience and cannot be sufficiently described within a standard weather forecast.
These statements are written in a free style and cover a variety of widespread events, including:
- Arctic outflows: Cold air masses moving from the Arctic toward lower latitudes.
- Alberta clippers: Fast-moving low-pressure systems that originate in the lee of the Canadian Rockies.
- Coastal fog banks.
- Areas with potential thunderstorm development.
- Strong winds, such as chinooks and les suetes winds.
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Furthermore, Canadian Special Weather Statements may reflect active warnings in effect near the United States border to ensure regional awareness.
Key Facts
- US Usage: Used by the NWS for hazards below warning/advisory criteria or those without a specific product code.
- Sub-Severe Criteria: In the US, this includes winds of 40–57 mph and hail under one inch.
- Timeframes: US statements often target imminent threats (1–2 hours) or major events beyond 6 hours.
- Canadian Usage: Issued by the Meteorological Service of Canada for unusual events or those causing general public inconvenience.
- Canadian Examples: Includes Alberta clippers, Arctic outflows, and chinook winds.
Summary of Regional Differences
| Feature | United States (NWS) | Canada (MSC) |
|---|---|---|
| Primary Purpose | Hazards below warning/advisory levels or lacking specific codes. | Unusual events or those causing general public concern. |
| Common Examples | Sub-severe thunderstorms, black ice, lake-effect snow. | Alberta clippers, Arctic outflows, coastal fog. |
| Typical Timeframe | 1–2 hours (imminent) or >6 hours (major events). | Not specified; based on event nature. |
| Style/Format | Structured alert product; rare EAS activation. | Free style writing. |
Frequently Asked Questions
What is a sub-severe thunderstorm?
A sub-severe thunderstorm is a storm that produces sustained winds or gusts between 40 and 57 mph (64–92 km/h) and/or hail smaller than one inch in diameter, along with frequent lightning or funnel clouds that are not expected to become tornadoes.
How do Special Weather Statements differ from warnings?
Special Weather Statements are generally used for hazards that do not meet the specific technical criteria required to trigger a formal warning or advisory, but still pose a risk or cause significant inconvenience.
What are Alberta clippers?
Alberta clippers are fast-moving low-pressure weather systems that originate in the lee of the Canadian Rockies and are often the subject of Special Weather Statements in Canada.
Can a Special Weather Statement trigger an emergency alert?
In the United States, an Emergency Alert System (EAS) activation can be requested for a Special Weather Statement, although this occurs only on very rare occasions.
What types of winter weather trigger these statements in the US?
They are used for events such as black ice, short-duration heavy snow, lake-effect snow bands that reduce visibility, and dense fog.