chapter 7 Interpreting Weather Data and Graphic Weather Products — Study Notes

Forecasting Process

  • The forecast is not perfectly precise, but accuracy has been improving; seven-day outlooks are increasingly reliable (averaged) compared to older two-day forecasts.
  • Forecasting drives decisions across many sectors (Gardening to Airlines) and relies on observers, data, and computer models.
  • FORECASTING METHODS (various approaches; can be used alone or in combination)
    • Persistence Forecast: what is happening now will continue for a short time; accurate only for a few hours; simple observational basis (e.g., sunny morning likely to stay sunny for the morning).
    • Trend Forecast: assumes weather systems moving in one direction/speed will continue unless interrupted; e.g., a cold front moving toward an airport at 30 mph might pass in 2 hours if unchanged.
    • Climatological Forecast: uses long-term regional averages (historical climatology); e.g., San Francisco historically has less than one rainy day per month in summer; reliable where day-to-day conditions vary little.
    • Analogue Forecast: compares current weather patterns to past similar patterns to guide future conditions; can predict temperatures and related variables, but exact replication is rare.
    • Meteorological Forecast (expert judgment): forecaster uses scientific knowledge of atmospheric processes to produce predictions; benefits from experience and data interpretation; often provides 6–12 hour insights.
    • Numerical Weather Prediction (NWP): computers solve mathematical equations that relate atmospheric conditions to other variables; forecast models run iteratively (very near term like 5–10 minutes) and feed back into the system to project ahead; increasingly integral to modern forecasts (e.g., national/global models).
  • PROCESSING AND DISSEMINATION OF WEATHER DATA
    • Observations plus past conditions are compiled, analyzed, and transformed into weather charts, graphics, and text products.
    • Observations are gathered from land stations, ships, radiosondes, aircraft, satellites, radar, etc.; data are relayed to World Meteorological Centers, national centers (e.g., NCEP in the U.S.), and then to national weather service facilities.
    • Forecasts and products are disseminated through national weather services, private sector channels, and aviation-specific outlets.
  • KEY TRENDS AND LIMITS
    • Forecast accuracy improves with technology, but long-range forecasts remain less reliable than short-term ones.
    • Nowcasts (short-term, high-resolution forecasts) are common for imminent weather events.
    • Chaos theory (sensitive dependence on initial conditions) implies small initial differences can diverge outcomes over time, constraining long-range precision.

Printed Reports and Forecasts

  • Printed reports summarize observed conditions and forecast expectations; examples include METARs, SPECI, TAFs, FA (aviation area forecast), and winds/temps aloft (FD).
  • METAR (Aviation Routine Weather Report) – surface weather observations in a standard format; two types:
    • METAR: hourly observation.
    • SPECI: non-routine (special) aviation weather report for significant changes.
  • METAR structure (typical US example):
    • TYPE OF REPORT (METAR or SPECI) and STATION IDENTIFIER (ICAO code; K prefix for US airports).
    • TIME OF REPORT (day and Zulu time).
    • MODIFIER (e.g., AUTO for automated station; COR for corrected report).
    • WIND INFORMATION: wind direction and speed, gusts (e.g., 08020G38KT).
    • VISIBILITY: prevailing visibility; RVR may follow visibility (R36L/2400FT).
    • WEATHER: precipitation/obscuration phenomena (e.g., TS for thunderstorm; SH for showers).
    • SKY CONDITION: cloud amount and height (e.g., SCT008, OVC012CB).
    • TEMPERATURE/DEWPOINT: e.g., 20/18 (°C).
    • ALTIMETER: altimeter setting with A prefix in inches of Mercury (e.g., A2995 means 29.95 inHg).
    • REMARKS: RMK fields with additional information (e.g., TSB24RAB24, SLP134).
  • Prevailing visibility vs Runway Visual Range (RVR)
    • Prevailing visibility: greatest distance at which objects are visible in most of the sky; reported in statute miles (SM).
    • RVR: what a pilot would see down the runway; reported as RwyNumber/Value (e.g., R36L/2400FT); within METAR, RVR can be appended after prevailing visibility or as a separate value.
    • If visibility varies by sector, sector visual ranges may be reported in remarks.
  • Sky condition and ceiling concepts
    • Ceiling: AGL height of the lowest broken (BKN) or overcast (OVC) layer, or the vertical visibility (VV) into an obscuration.
    • Cloud cover octas: FEW (1/8–2/8), SCT (3/8–4/8), BKN (5/8–7/8), OVC (8/8); SKC if no clouds (CLR in automated reports).
    • When more than half the sky is covered, a ceiling exists.
  • Obscurations and weather phenomena
    • Obscurations: FG (fog), BR (mist), FU (smoke), HZ (haze), DU (dust), VA (volcanic ash); VC indicates obscurations in the vicinity.
    • Weather phenomena codes include RA (rain), TS (thunderstorm), SH (shower), FZ (freezing), PL (ice pellets), SN (snow), IC (ice crystals), etc. Each type can be listed with multiple phenomena in order of predominance.
  • Remarks (RMK) and coded remarks
    • RMK marks start of remarks; B (begin) and E (ending) times indicate when phenomena begin/end within the hour; examples: TSB24 (thunderstorm began at 24 minutes past the hour).
    • Remarks also include extra data such as SLP (sea level pressure), wind data, or other observations not in the main groups.
  • Radar weather reports (SDs) and PIREPs
    • SDs (Radar Weather Reports) describe precipitation areas, echo types, movement, and tops/bases when significant.
    • PIREPs (UA) are pilot weather reports; pilots are encouraged to report in-flight weather conditions, including bases/tops of clouds, in-flight visibility, icing, wind shear, turbulence.
  • Terminal Aerodrome Forecasts (TAF)
    • Valid for ~24 hours; issued four times daily (0000Z, 0600Z, 1200Z, 1800Z).
    • TAF content mirrors METAR-like codes, including wind, visibility, weather, and sky conditions; includes forecast change groups:
    • FM (from): permanent changes starting at a time.
    • BECMG: gradual change within a time window.
    • PROB: probability (e.g., PROB40 0407 indicates a 40% chance between 0400–0700Z).
    • TEMPO: temporary fluctuations lasting up to an hour.
    • NSW: no significant weather (after a period with active conditions).
  • Aviation Area Forecasts (FA)
    • Covers 18-hour forecast with a synopsis and VFR clouds and weather section (by state/region).
    • Includes precursors such as ceilings, visibilities, winds, weather phenomena; 12-hour CF and 6-hour outlook extensions (Alaska/Hawaii differ slightly).
  • Warnings and outlook products
    • Convective Outlook (AC): two 24-hour periods (Day 1 and Day 2) forecasting thunderstorm activity and severities (Slight, Moderate, High); includes See Text guidance.
    • Severe Weather Watch Bulletins (WW): outline areas of possible severe weather/tornadoes; updates as needed.
    • AIRMETs (WA): issued every 6 hours for weather hazardous to small aircraft (moderate icing, moderate turbulence, low ceilings/poor visibility, mountain obscuration); identifiers Sierra (IFR), Tango (turbulence/low-level winds), Zulu (icing, freezing levels).
    • SIGMETs (WS): hazardous weather (non-convective) such as severe icing, severe turbulence, CAT, dust/sand storms, volcanic ash; valid 4 hours (6 hours for hurricanes).
    • Convective SIGMET (WST): hazardous convective weather (tornadoes, lines of thunderstorms, embedded storms, hail ≥ 3/4 in, gusts ≥ 50 knots); issued for eastern/central/western U.S.; typically valid 2 hours; issued every hour with updates.
    • HIWAS (Hazardous In-Flight Weather Advisory Service): continuous broadcast of AIRMETs, SIGMETs, convective SIGMETs, urgent PIREPs on selected VORs.
    • EFAS (Flight Watch): en-route weather advisory service (HF/low-freq) at 122.0 MHz below 18,000 ft MSL; real-time weather advisories and radar assistance.
    • TIBS (Telephone Information Briefing Service): automated preflight briefings via telephone; typically preliminary briefing tool; not a substitute for an FSS brief.
    • DUATS (Direct User Access Terminal System): weather briefing and flight plan filing via computer; requiring current medical certificate.
  • In-flight weather information
    • EFAS/Flight Watch, HIWAS, TWEB, and ATIS/ASOS/AWOS interfaces continue to provide updates during flight.
  • Graphic weather products overview
    • Surface Analysis Chart: surface pressure patterns and observations; horizon-level weather features; valid at chart time; includes station models for many reporting points.
    • Weather Depiction Chart: overview of favorable/adverse weather; simplifies to VFR/MVFR/IFR; not a complete en-route forecast; based on METARs.
    • Radar Summary Chart: precipitation locations, types, movement, and echo tops/bases; updated ~35 minutes past each hour; radar can show lines of storms and hazardous areas but does not depict all clouds.
    • NEXRAD (Doppler Radar): capable of detecting movement and velocity; improves storm tracking and forecast capability.
    • Satellite Weather Pictures: visible and infrared (IR) imagery; visible shows cloud presence/shape; IR shows cloud tops and heights via temperature differences; 30-minute refresh, except at night for visible imagery.
    • U.S. Low-Level Significant Weather Prog Chart: 24-hour forecast up to 24,000 ft; four-panel chart with surface weather and significant weather information; upper panels show turbulence and freezing levels.
    • Convective Outlook/Prog Charts and VAFTAD: forecast convective activity and volcanic ash dispersion; VAFTAD provides ash concentration forecasts for planning, not a substitute for SIGMETs.
  • Sources of weather information (SOURCES OF WEATHER INFORMATION)
    • Preflight sources: Flight Service Station (FSS) and AFSS, telephone briefing, TIBS, DUATS, internet, and private providers.
    • In-flight sources include EFAS, HIWAS, TWEB, HIWAS, EFAS (Flight Watch), and AWOS/ASOS/ATIS with updates.
    • Internet sources (NWS, ADDS) provide METARs, TAFs, satellite and radar imagery, NCWF, etc.; caution about currency of internet data.
  • Practical tools and safety notes
    • Always cross-reference multiple sources for accurate planning; radar is not a mirror of cloud cover; doppler radar highlights precipitation and movement, not cloud heights.
    • In-flight, update weather as needed and communicate significant changes via appropriate channels (PIREPs, HIWAS, EFAS, FSS).