4.2 Weather Reports, Forecasts & Preflight Briefings

Weather Reports, Forecasts & Preflight Briefings

Thorough preflight weather analysis is mandatory under 14 CFR 91.103, which requires every pilot in command to become familiar with all available weather information prior to beginning a flight. For Sport Pilots, interpreting surface observations, aerodrome forecasts, and inflight advisories correctly ensures safe VFR navigation and prevents accidental non-instrument flight into IMC.

Surface Observations: Decoding METARs

A METAR (Meteorological Aerodrome Report) is an aviation routine weather report issued hourly at individual airports, typically between 50 and 55 minutes past the hour. When rapid, significant weather changes occur, a SPECI (Special Report) is issued.

METAR reports use standardized international alphanumeric codes. Consider the following sample METAR report:

METAR KPAO 161853Z AUTO 14008G15KT 10SM TSRA BKN025OVC040 18/16 A2992 RMK AO2 SLP132

Decoding this METAR line-by-line reveals:

• Type of Report: METAR — Standard hourly observation report.
• Station Identifier: KPAO — Palo Alto Airport, California (ICAO code format starting with 'K' for contiguous US).
• Date and Time: 161853Z — Issued on the 16th day of the month at 1853 Zulu (UTC).
• Modifier: AUTO — Automated observation generated without human observer intervention.
• Wind Vector: 14008G15KT — Wind from 140° True at 8 knots, gusting to 15 knots. (Note: METAR wind directions are ALWAYS given in degrees True North).
• Visibility: 10SM — Flight visibility of 10 Statute Miles.
• Present Weather: TSRA — Thunderstorm (TS) with moderate Rain (RA).
• Sky Condition: BKN025 OVC040 — Broken ceiling at 2,500 feet AGL and Overcast layer at 4,000 feet AGL. (Cloud heights are reported in hundreds of feet AGL).
• Temperature / Dewpoint: 18/16 — Temperature is 18°C, Dewpoint is 16°C.
• Altimeter Setting: A2992 — Barometric pressure setting of 29.92 inHg.
• Remarks: RMK AO2 SLP132 — Automated station with precipitation discriminator (AO2), Sea Level Pressure 1013.2 hPa (SLP132).

When temperature and dewpoint converge to within 3°C (5°F), moisture condenses rapidly, creating high risk for ground fog, low stratus clouds, or reduced surface visibility.

Terminal Aerodrome Forecasts (TAF)

A Terminal Aerodrome Forecast (TAF) is a concise statement of expected meteorological conditions within a 5 nautical mile (NM) radius around an airport runway complex. TAFs are issued four times daily (at 0000Z, 0600Z, 1200Z, and 1800Z) and are valid for a 24- to 30-hour period.

Examine the sample TAF forecast below:

TAF KKOA 161720Z 1618/1718 21010KT P6SM FEW030 SCT050 FM162200 15006KT P6SM BKN030 TEMPO 1702/1706 3SM TSRA BKN015CB

Line-by-line TAF decoding:

• Header & Valid Period: TAF KKOA 161720Z 1618/1718 — TAF for Kona Airport (KKOA), issued 16th at 1720Z, valid from 1800Z on the 16th to 1800Z on the 17th (24 hours).
• Initial Period: 21010KT P6SM FEW030 SCT050 — Wind 210° True at 10 knots, visibility greater than 6 Statute Miles (P6SM), Few clouds at 3,000 ft AGL, Scattered at 5,000 ft AGL.
• From Group: FM162200 15006KT P6SM BKN030 — Rapid change starting at 2200Z on the 16th (FM162200): wind shifts to 150° at 6 knots, visibility P6SM, Broken ceiling at 3,000 ft AGL.
• Temporary Group: TEMPO 1702/1706 3SM TSRA BKN015CB — Temporary weather changes expected between 0200Z and 0600Z on the 17th: visibility dropping to 3 SM in thunderstorms and rain, broken cumulonimbus clouds (CB) at 1,500 ft AGL.

Inflight Advisories: AIRMETs, SIGMETs & Convective SIGMETs

Inflight weather advisories warn pilots of hazardous weather en route. They are classified into three levels of severity:

Preflight Briefing Types & FSS Services

Pilots obtain official preflight weather briefings through Flight Service Stations (FSS) by calling 1-800-WX-BRIEF or utilizing 1800wxbrief.com. FSS provides three distinct types of briefings based on departure timing:

1. Standard Briefing: Requested when no previous weather briefing has been obtained for a planned flight departing within 6 hours. Provides a complete weather overview.
2. Abbreviated Briefing: Requested to update specific items (such as a delayed departure time or updated METAR) when a standard briefing has already been completed.
3. Outlook Briefing: Requested when the proposed flight departure is 6 or more hours in the future. Provides forecast trends for planning.

A Standard Preflight Briefing follows a structured sequence:

1. Adverse Conditions (hazardous weather, SIGMETs, TFRs)
2. VFR Flight Not Recommended (VNR recommendation if ceiling/visibility is marginal)
3. Synopsis (fronts and pressure systems)
4. Current Conditions (METARs)
5. En Route Forecast
6. Destination Forecast (TAFs)
7. Winds and Temperatures Aloft
8. Notices to Air Missions (NOTAMs) and ATC Delays

Inflight Weather Resources & FIS-B Limitations

In flight, pilots communicate with FSS on the national radio frequency 122.2 MHz or local Remote Communications Outlets (RCO). Pilots can also tune to Automated Weather Observing System (AWOS) or Automated Surface Observing System (ASOS) frequencies near airports.

Modern Electronic Flight Bags (EFBs) receive real-time weather via Flight Information Service-Broadcast (FIS-B) over 978 MHz ADS-B In. However, pilots must understand FIS-B NEXRAD radar limitations:

• Transmission Latency: FIS-B radar displays can be 5 to 15 minutes old by the time they render on cockpit screens.
• Strategic vs. Tactical: NEXRAD imagery is strictly a strategic routing tool for avoiding broad weather systems miles away. It must NEVER be used for tactical penetration of thunderstorm gaps, as a cell may have moved or intensified significantly during the transmission lag.