What triggers a SPECI special weather report to be issued?

A SPECI is automatically issued by ASOS whenever a key weather element crosses a predefined safety threshold between scheduled hourly METARs. Common triggers include visibility dropping below 3 statute miles, a ceiling crossing through 3,000 ft or 1,500 ft, a thunderstorm beginning or ending, freezing precipitation onset, or a significant wind shift. The thresholds align with VFR, MVFR, and IFR flight rule boundaries.

Does AWOS issue SPECI reports the same way ASOS does?

No—this is one of the most important distinctions in automated weather systems. AWOS generates a METAR every 20 minutes and provides one-minute observations (OMOs) by radio and phone, but it does not issue SPECIs. Only ASOS automatically generates and disseminates SPECI reports when weather element thresholds are crossed.

What does the AUTO designator in a METAR or SPECI mean?

AUTO indicates the observation was made entirely by an automated system with no human weather observer or certified tower controller involved. Some weather phenomena that require human judgment—such as certain precipitation types or blowing obstructions—may be missing from an AUTO report. Augmented observations, where a human adds oversight to the automated system, do not include the AUTO designator.

SPECI Special Weather Reports and What Triggers Them

Every hour, airports across the United States issue a routine Aviation Routine Weather Report, the METAR, summarizing current surface conditions. But weather rarely waits for the top of the hour. When conditions deteriorate rapidly—or improve suddenly after a period of low visibility—pilots and dispatchers need updated information immediately. That is precisely the role of the SPECI, formally known as the Aviation Selected Special Weather Report.

The SPECI uses the exact same code format as the METAR, so any pilot who can decode a METAR can instantly read a SPECI. The critical difference is timing: a SPECI is issued between scheduled hourly observations whenever one or more weather elements cross a predefined threshold that is operationally significant to aviation safety. Understanding what triggers a SPECI, which automated systems generate them, and how to interpret the data is a core competency for every certificate level.

The SPECI in Context: METAR vs. SPECI

Both the METAR and the SPECI satisfy World Meteorological Organization (WMO) and International Civil Aviation Organization (ICAO) requirements for surface meteorological reporting. The United States follows those international standards while also applying certain domestic modifications permitted under ICAO rules. The code structure—station identifier, date/time, wind, visibility, weather, sky condition, temperature/dewpoint, altimeter, and remarks—is identical for both report types. The only structural difference is the report type designator at the very beginning of the message: METAR for a scheduled hourly observation and SPECI for a special report.

Because most modern preflight briefing tools and weather applications now display decoded plain-language versions alongside the coded text, some pilots overlook the raw code. The FAA Aviation Weather Handbook cautions that it is still important for all pilots to know how to decode the METAR/SPECI in case a source does not provide a decoded version—an examiner will almost certainly test this ability.

What Triggers a SPECI

The Automated Surface Observing System (ASOS)—the nation's primary surface weather observing network, operated jointly by the National Weather Service, the FAA, and the Department of Defense—is programmed to detect significant changes and automatically issue a SPECI when conditions exceed preselected weather element thresholds. A classic example provided in FAA-H-8083-28B is a visibility decrease to less than 3 statute miles. Other common threshold crossings that generate a SPECI include:

Visibility crossing through 3 SM, 2 SM, 1 SM, or ½ SM (in either direction—improvement can also trigger a SPECI).
Ceiling height crossing through 3,000 ft, 1,500 ft, 1,000 ft, or 500 ft AGL, as well as the development or dissipation of a ceiling below 3,000 ft.
Tornado, funnel cloud, or waterspout beginning or ending.
Thunderstorm beginning or ending on the airport or in the vicinity.
Freezing precipitation or freezing fog beginning or ending.
Hail beginning or ending.
Squall occurrence (a sudden increase in wind speed of at least 16 knots, rising to a sustained speed of 22 knots or more, lasting at least one minute).
Wind shift of 45° or more with a sustained wind speed of 10 knots or more before and/or after the shift.
Rapid pressure change meeting specific criteria in the remarks section.

The thresholds are carefully chosen to align with IFR, MVFR, and VFR flight rule boundaries, as well as with instrument approach minimums commonly encountered in the national airspace system. A SPECI issued because a ceiling just dropped through 1,000 ft, for example, directly affects whether a flight can execute a non-precision approach at that airport.

ASOS vs. AWOS: A Critical Operational Difference

Here is one of the most heavily tested distinctions in FAA written examinations: ASOS issues SPECIs; AWOS does not.

ASOS detects significant changes and disseminates both hourly observations and special (SPECI) reports automatically. It also broadcasts computer-generated voice observations via FAA ground-to-air radio and telephone, making real-time data available to aircraft in the vicinity of the airport.

AWOS, while similar to ASOS in many respects, generates a METAR only at 20-minute intervals and does not report SPECIs. AWOS provides one-minute observations (OMOs) by phone or radio, but these are not SPECIs in the formal sense. If a pilot is relying on weather from an AWOS-equipped airport and conditions deteriorate rapidly between 20-minute reports, there will be no automatically issued special report in the national distribution network—a significant limitation compared to ASOS coverage.

AWOS comes in six capability tiers, from AWOS-A (altimeter only) through AWOS-4 (full suite including freezing rain). Regardless of tier, none issue SPECIs. Federal AWOS units are owned and maintained by the FAA; non-Federal units are owned by the site operator. Both share the same no-SPECI limitation.

Automated, Augmented, and Manual Observations

When an ASOS station is operating without human oversight, the report includes the designator AUTO in the body of the message. These fully automated observations have a known limitation: certain weather phenomena that require human judgment—such as snow depth, ice pellets in some configurations, or blowing snow—may not appear in the report unless a certified human observer or tower controller is present.

At select airports, human observers provide oversight and augmentation to the automated system. These are called augmented observations. The human observer adds weather elements beyond the automated system's capabilities and flags operationally significant phenomena. Crucially, augmented reports do NOT contain the AUTO designator. When you see AUTO in a METAR or SPECI, it means no human oversight is in the loop—so certain phenomena may be missing. When AUTO is absent, either a human-only or an augmented observation was made.

Time Averaging and the Recency of Data

A common misconception is that an automated observation—especially a one-minute OMO—reflects truly instantaneous conditions. It does not. ASOS and AWOS substitute time averaging for the spatial averaging a human observer would perform by scanning the sky and horizon.

Sky condition (cloud cover) is evaluated from sensor data gathered over the 30-minute period ending at the time of the observation.
All other elements (visibility, wind, temperature, pressure, etc.) are based on sensor data within 10 minutes or less of the observation time.

Importantly, the averaging algorithms are intentionally asymmetric: they are designed to report deteriorating conditions much faster than improving conditions. This conservative bias protects safety—a sensor will quickly reflect a visibility dropping into IFR, but may take up to 10 minutes to confirm that dense fog has fully dissipated and VFR has returned. Pilots should keep this in mind when departing after a period of low visibility; the OMO showing 10 SM may still be working through that 10-minute improvement window.

Key Numbers and Rules

ASOS issues SPECIs automatically when thresholds are crossed; AWOS issues no SPECIs (20-minute METARs only).
A SPECI visibility threshold example: visibility drops below 3 SM.
Sky condition averaging window: 30 minutes.
All other elements averaging window: 10 minutes.
Fog (FG) reported when visibility is less than 5/8 SM; mist (BR) or haze (HZ) for 5/8 SM to less than 7 SM.
BR vs. HZ distinction: if temperature-dewpoint spread is 4 °F (≈2 °C) or less, report BR; otherwise HZ.
Wind direction in national distribution (FSS, internet, FIS-B) is in true degrees; local dissemination (radio, telephone) is in magnetic degrees.
ASOS sky condition reporting limit: up to 12,000 ft AGL.
AUTO in a report = fully automated, no human observer; no AUTO = human-observed or augmented.

Common Test Traps

AWOS issues SPECIs—FALSE. AWOS generates METARs every 20 minutes and OMOs every minute, but it never generates a formal SPECI. Only ASOS does.
AUTO means the data is less current—WRONG framing. AUTO means no human observer; data currency follows the same 30/10-minute averaging rules as any automated station.
Confusing the averaging windows: sky condition uses 30 minutes; everything else uses 10 minutes. Exams love to swap these numbers.
Wind direction: true vs. magnetic. Pilots hear magnetic on the radio and telephone; they see true north on FIS-B and internet sources. Applying the wrong reference to a wind correction calculation is a common trap.
Assuming improving conditions are reported instantly: The algorithms report deterioration quickly but are slow to confirm improvement—a SPECI showing clear skies may lag reality by nearly 10 minutes.