The METAR remarks section occasionally surfaces encoded strings that fall outside standard pilot training curricula, and the code "T2 SET" represents exactly this category of edge-case observation. Standard instruction on METAR remarks covers the T-group as the precise temperature and dewpoint group — a nine-character string such as T02030175 that extends the resolution of reported temperature and dewpoint to tenths of degrees Celsius. What the Reddit post describes, however, is a distinct and separate entry, suggesting "T2 SET" carries an independent meaning unrelated to the precision temperature encoding that most training resources address. The failure of conventional online research to surface a definitive answer points to a remark generated by an automated station system operating under a secondary or maintenance-tier reporting protocol not widely documented in publicly accessible pilot references.
Automated Surface Observing System (ASOS) and AWOS installations in the United States transmit remarks drawn from Federal Meteorological Handbook No. 1 (FMH-1), but they also append internally generated diagnostic and sensor-status codes that are not always fully described in pilot-facing publications. "T2" in some ASOS contexts refers to a secondary or redundant temperature sensor — ASOS units typically operate with multiple sensor inputs for quality assurance — while "SET" may indicate that the secondary sensor was activated, accepted as authoritative, or transitioned into service, possibly because the primary sensor (T1) failed a quality check or was taken offline for maintenance. This kind of sensor-state reporting is primarily useful to NWS technicians and station managers rather than end users, but it can appear in the remarks block of a transmitted METAR before being stripped by downstream data processing systems.
For working pilots and CFIs, the practical implication is that ASOS and AWOS-generated remarks do not always map cleanly to any single published decoding guide. The FAA's Aeronautical Information Manual, the Aviation Weather Services advisory circular, and even FMH-1 itself do not exhaustively enumerate every sensor-status remark that an automated station may generate. Pilots operating under Part 91, 91K, or 135 who encounter unfamiliar METAR remarks during preflight planning should treat unrecognized codes as noise rather than actionable weather data, while noting that the presence of such codes — particularly those suggesting sensor transitions — may be a subtle indicator that observation quality at that station is in a transitional state, warranting cross-checking against nearby METARs, PIREPs, or a direct call to a flight service specialist.
The broader context here is that aviation weather infrastructure has undergone significant automation over the past three decades, with the ASOS network expanding to cover thousands of airports that previously had no surface observation capability at all. That automation brings with it a parallel expansion of machine-generated diagnostic output that was never designed for pilot consumption. NWS and FAA have generally done a reasonable job filtering that output, but edge cases persist — particularly at smaller airports where maintenance cycles may differ, software versions may lag, or sensor anomalies are more common due to lower traffic and observation validation rates. Instructors and check airmen working with students or new hires in actual IFR environments would do well to use encounters like this one as teaching moments: the METAR is a formatted product with defined syntax, but it is also a live data stream from fallible hardware, and cryptic remarks are occasionally a window into the machinery behind the weather product rather than the weather itself.