11.2 Significant Weather Charts (SIGWX) & Inflight Advisories
Key Takeaways
- High-Level Significant Weather (SIGWX) charts (FL250 to FL630) depict forecasted jet stream cores (isotachs >= 80 kts with altitude of maximum wind), clear air turbulence areas, cumulonimbus (CB) clouds and tops, tropopause heights, volcanic ash, and tropical cyclones.
- On SIGWX charts, CB cloud areas are enclosed by scalloped lines indicating spatial coverage (ISOL, OCNL, FRQ, EMBD) with bases below FL250 marked as 'XXX' and tops in flight levels; all depicted CB areas automatically imply severe turbulence and severe icing.
- AIRMETs (WST/WA) are scheduled 6-hour advisories covering operational weather of lower severity than SIGMETs: Sierra (IFR / mountain obscuration), Tango (moderate turbulence, sustained surface winds >= 30 kts, non-convective LLWS), and Zulu (moderate icing and freezing levels).
- Non-convective SIGMETs (WS) are unscheduled 4-hour advisories (6 hours for hurricanes and volcanic ash) issued for severe icing, severe or extreme turbulence or CAT, duststorms/sandstorms reducing visibility < 3 SM, and volcanic ash clouds.
- Convective SIGMETs (WST) are valid for 2 hours and issued for severe thunderstorms (surface winds >= 50 kts, hail >= 3/4 in, tornadoes), lines of thunderstorms >= 60 NM affecting >= 40% of length, embedded thunderstorms, or large storm areas covering >= 40% of 3,000 sq NM.
Significant Weather Charts (SIGWX) & Inflight Advisories
Core Airline Transport Principle: High-altitude airline operations require synthesizing prognostic chart products with real-time inflight hazardous weather advisories. High-Level Significant Weather (SIGWX) charts provide the strategic synoptic picture of upper-level turbulence, jet stream dynamics, and deep convection from FL250 to FL630, while AIRMETs, SIGMETs, and Convective SIGMETs establish mandatory tactical rerouting and avoidance imperatives.
1. High-Level Significant Weather (SIGWX) Chart Architecture (FL250 to FL630)
High-Level SIGWX charts are produced jointly by the World Area Forecast Centers (WAFC Washington and WAFC London) under ICAO and WMO mandates. They are issued four times daily (valid at 0000Z, 0600Z, 1200Z, and 1800Z) as 24-hour prognostic charts covering upper-air airspace from FL250 to FL630 (pressure levels $250\text{ hPa}$ to $70\text{ hPa}$).
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| HIGH-LEVEL SIGWX CHART (FL250 - FL630) |
| |
| Features Depicted: |
| 1. Jet Stream Cores & Isotachs (Wind speeds >= 80 knots) |
| 2. Clear Air Turbulence (CAT) Areas (Moderate & Severe) |
| 3. Cumulonimbus (CB) Clouds (Coverage, Base 'XXX', and Tops in FL) |
| 4. Tropopause Spot Heights & Troughs / Ridges ([FL]) |
| 5. Tropical Cyclones (Hurricanes, Typhoons) with System Names |
| 6. Volcanic Eruption Sites & Ash Cloud Boundaries |
| 7. Intertropical Convergence Zone (ITCZ) Convective Boundaries |
| 8. Solar Radiation Hazards (Cosmic Radiation / Solar Flare Alerts) |
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2. High-Level SIGWX Symbology & Decoding
Jet Stream Cores and Wind Isotachs
Jet streams on SIGWX charts are represented by a heavy solid black line with directional arrowheads showing the core axis. Winds are only depicted when core velocities reach or exceed 80 knots.
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| JET STREAM SYMBOLOGY ON SIGWX |
| |
| FL340 |
| \ 260 / 420 |
| ==========\==============\=============================> |
| \ \ |
| (Pennant: 50kt) (Barbs: 10kt each; Half: 5kt) |
| |
| - Wind Speed: Depicted by standard pennants (50 kts) and barbs (10 kts). |
| Example above: 2 pennants (100 kt) + 3 barbs (30 kt) = 130 KNOTS. |
| - Core Altitude: Number indicates altitude of maximum wind (FL340). |
| - Vertical Extent: '260/420' indicates winds >= 80 kts exist from FL260 |
| up to FL420 (vertical thickness of the 80-knot isotach core). |
| - Double Hatch Marks (//): Indicate a 20-knot change in core wind speed. |
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Clear Air Turbulence (CAT) Polygons
- Symbology: Enclosed by heavy dashed lines or polygonal boundaries.
- Labeling: Inside or adjacent to the polygon, the turbulence symbol (moderate $\Lambda$ or severe $\text{\char94}$) is accompanied by the vertical boundaries in flight levels: This indicates moderate-to-severe CAT extending from FL310 up to FL390.
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| CUMULONIMBUS (CB) COVERAGE CLASSIFICATION |
| |
| Symbology: Enclosed within SCALLOPED LINES. |
| |
| Coverage Descriptor Spatial Percentage Operational Implication |
| ----------------------------------------------------------------------- |
| ISOL CB (Isolated) < 50% coverage Individual, discrete cells. |
| Visual/radar avoidance easy. |
| |
| OCNL CB (Occasional) 50% to 75% coverage Well-separated cells with |
| clear gaps between storms. |
| |
| FRQ CB (Frequent) > 75% coverage Continuous line or mass of |
| cells with little/no separation|
| |
| EMBD CB (Embedded) Any coverage Concealed within stratiform |
| clouds, haze, or darkness. |
| |
| * Vertical Extent Format: Base / Top (e.g., 'XXX / 480'). |
| - 'XXX' means the cloud base is below the chart floor (below FL250). |
| - '480' indicates CB cloud tops reach FL480 (48,000 ft MSL). |
| * MANDATORY RULE: Every CB area depicted automatically implies SEVERE |
| TURBULENCE and SEVERE ICING within the convective column. |
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Tropopause Spot Heights
Tropopause heights are depicted in rectangular boxes across the chart (e.g., [380] represents a tropopause height of FL380 / 38,000 ft MSL). Tropopause high centers are marked with H (e.g., H [450]) and low centers (tropopause depressions associated with cold upper-level troughs) are marked with L (e.g., L [290]).
Volcanic Eruptions & Tropical Cyclones
- Volcanic Eruptions: Marked by a stylized trapezoidal volcano icon with the name of the volcano, latitude/longitude coordinates, eruption date/time, and a reminder box directing crews to check active Volcanic Ash Advisories (VAA) and SIGMETs.
- Tropical Cyclones: Depicted by standard hurricane / typhoon symbols with storm name, position, central pressure, and maximum surface winds.
3. Inflight Weather Advisories: The Hierarchy of Hazardous Weather
In the National Airspace System, inflight weather advisories notify flight crews of hazardous meteorological conditions that develop or are forecast to develop enroute. Advisories are structured in a clear operational hierarchy based on severity, validity window, and affected aircraft classes.
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| INFLIGHT WEATHER ADVISORY HIERARCHY & COMPARISON |
| |
| Advisory Type Validity Issuance Schedule Target Hazards |
| ----------------------------------------------------------------------- |
| AIRMET (WA) 6 Hours Scheduled every Moderate hazards; |
| / G-AIRMET 6 hrs (0245Z, etc.) critical to light ac,|
| G-AIRMET: 3-hr snapshots operational impact|
| for transport jets. |
| |
| SIGMET (WS) 4 Hours Unscheduled / As Severe non-convective|
| (Non-Convective) (6 hrs for Conditions Warrant hazards dangerous to |
| Hurricanes/Volcanoes) ALL aircraft types. |
| |
| Convective SIGMET 2 Hours Hourly at H+55 Severe thunderstorms,|
| (WST) (or 'NONE' issued) squall lines, embedded|
| CBs, large storm mass|
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4. AIRMETs (WA / G-AIRMET): Categories & Operational Thresholds
AIRMETs (Airmen's Meteorological Information) are routine advisories valid for 6 hours. While primarily intended for general aviation and light aircraft, AIRMET conditions (such as low-level wind shear and freezing levels) heavily influence Part 121 air carrier terminal approach and departure planning.
- Graphical AIRMETs (G-AIRMETs): Modern graphical representation issued every 6 hours with 3-hour time-step snapshots valid for up to 12 hours.
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| THREE DISTINCT AIRMET CLASSIFICATIONS |
| |
| [AIRMET SIERRA] IFR Conditions & Mountain Obscuration |
| - Widespread ceilings < 1,000 ft and/or visibility |
| < 3 statute miles affecting > 50% of an area. |
| - Extensive mountain obscuration caused by clouds, |
| fog, precipitation, or haze. |
| |
| [AIRMET TANGO] Turbulence, Strong Surface Winds & LLWS |
| - Moderate turbulence across any altitude layer. |
| - Sustained surface winds of 30 KNOTS or greater. |
| - Non-convective low-level wind shear (LLWS) below |
| 2,000 feet AGL. |
| |
| [AIRMET ZULU] Icing & Freezing Levels |
| - Moderate icing across specified flight levels. |
| - Freezing level contours (surface freezing level |
| and multiple aloft freezing level heights). |
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5. Non-Convective SIGMETs (WS): Severe Atmospheric Threats
A SIGMET (Significant Meteorological Information) is an unscheduled inflight advisory concerning weather phenomena of such severity that it is hazardous to all aircraft, regardless of size, type, or certification.
Validity Windows
- Standard Non-Convective SIGMET: Valid for 4 hours.
- Volcanic Ash Cloud or Tropical Cyclone (Hurricane) SIGMET: Valid for 6 hours.
Non-Convective SIGMET Criteria
A SIGMET is issued whenever any of the following occur or are forecast to occur across an area of at least 3,000 square miles:
- Severe Icing not associated with thunderstorms.
- Severe or Extreme Turbulence not associated with thunderstorms.
- Clear Air Turbulence (CAT) of severe or extreme intensity.
- Widespread Duststorms or Sandstorms lowering surface or flight visibility to less than 3 statute miles.
- Volcanic Ash Clouds.
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| SIGMET IDENTIFICATION & NOMENCLATURE |
| |
| - Identified by an ALPHABETIC PHONETIC NAME and ISSUE NUMBER. |
| - Example: SIGMET OSCAR 2 (Second issuance of the 'Oscar' weather system).|
| - Phenomena within each FAA region use designated letter series: |
| * Northeast (BOS): ALFA through FOXTROT |
| * Southeast (MIA): GOLF through LIMA |
| * North Central (CHI): MIKE through ROMEO |
| * South Central (DFW): SIERRA through UNIFORM |
| * Rocky Mountain (SLC): VICTOR through WHISKEY |
| * Pacific Coast (SFO): XRAY through ZULU |
| - Volcanic ash SIGMETs are numbered sequentially under specific series. |
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6. Convective SIGMETs (WST): Convective Hazard Rules
Convective SIGMETs (WST) are issued for severe convective phenomena across the contiguous United States. They are issued hourly at H+55 minutes for three geographic regions (Eastern [E], Central [C], and Western [W]), valid for up to 2 hours.
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| CONVECTIVE SIGMET (WST) MANDATORY CRITERIA |
| |
| A Convective SIGMET is issued for any of the following 4 criteria: |
| |
| 1. SEVERE THUNDERSTORMS: |
| - Surface winds >= 50 KNOTS. |
| - Hail at the surface >= 3/4 INCH in diameter. |
| - Tornadoes, funnel clouds, or waterspouts. |
| |
| 2. LINES OF THUNDERSTORMS (SQUALL LINES): |
| - A line of thunderstorms at least 60 MILES LONG. |
| - Thunderstorms affecting at least 40% of the length of the line. |
| |
| 3. EMBEDDED OR SEVERE THUNDERSTORMS: |
| - Embedded thunderstorms occurring in stratiform clouds or haze. |
| - Severe thunderstorms affecting an area for MORE THAN 30 MINUTES. |
| |
| 4. LARGE CONVECTIVE AREAS (RADAR INTENSITY): |
| - Thunderstorms covering at least 40% OF AN AREA OF AT LEAST 3,000 SQ |
| STATUTE MILES, with heavy or extreme radar reflectivity |
| (VIP Level 4 [41-45 dBZ] or greater). |
| |
| * MANDATORY TRANSPORT OPERATING RULE: |
| Any Convective SIGMET ALWAYS implies SEVERE OR GREATER TURBULENCE, |
| SEVERE ICING, and LOW-LEVEL WINDSHEAR. |
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Outlooks Attached to Convective SIGMETs
Each hourly Convective SIGMET bulletin includes a 2- to 6-hour Convective Outlook defining regions where severe convective development is expected, allowing Part 121 dispatchers and flight crews to plan proactive long-distance routing deviations around developing squall lines.
On a High-Level Significant Weather (SIGWX) prognostic chart (FL250–FL630), an area enclosed by scalloped lines is labeled 'OCNL CB XXX / 450'. What does this decode to?
Which of the following atmospheric conditions meets the mandatory criteria for issuing an unscheduled Non-Convective SIGMET (WS)?
A line of severe thunderstorms is detected along a cold front. Under which specific threshold will the National Weather Service issue a Convective SIGMET (WST)?