5.3 Significant Weather Prognostic Charts (SIGWX)
Key Takeaways
Read the actual SIGWX product’s region, vertical layer, legend, and forecast time; current products are not all identical to legacy figures.
Traditional high-level CB coverage uses ISOL <4/8, OCNL 4/8–6/8, and FRQ >6/8; EMBD concerns cloud/haze concealment.
On traditional high-level charts, XXX can indicate CB bases below the chart’s lower boundary.
A boxed tropopause 450 represents approximately FL450; it does not guarantee turbulence or a safe fixed offset.
Cruise and diversion weather must both be assessed with products covering their actual altitude/time profiles.
Significant Weather Prognostic Charts
SIGWX products describe forecast significant weather within their displayed vertical, geographic, and time coverage. They support strategic flight planning and briefing; they are not a tactical clearance to penetrate weather or a complete substitute for observations, advisories, radar, satellite, and PIREPs. Read the chart header and legend before applying a memorized symbol to a particular product.
Current products and legacy examination figures
The FAA Aviation Weather Handbook discusses low-level significant-weather progs, WAFS information, and traditional international high-level charts. Traditional high-level charts commonly use FL250–FL630; other layers and newer WAFS products have their own coverage and forecast times. Do not claim every current SIGWX product has one universal range or a single 24-hour validity. A legacy examination figure should be interpreted using its actual printed legend and times.
The World Area Forecast System (WAFS) supplies international aviation forecasts through the World Area Forecast Centers. Use the appropriate region and flight phase. A cruise-level chart does not fully cover a diversion descent to an airport, and a surface flight-category depiction does not establish the entire enroute icing or turbulence profile.
Low-level flight categories
Low-level progs can show areas of IFR, MVFR, and VFR conditions, turbulence, and freezing levels within their stated coverage. The standard flight-category thresholds use the worse of ceiling and visibility:
| Category | Ceiling and/or visibility relationship |
|---|---|
| IFR | Ceiling below 1,000 ft and/or visibility below 3 SM, including lower categories where shown |
| MVFR | Ceiling 1,000–3,000 ft and/or visibility 3–5 SM, when no lower category controls |
| VFR | Ceiling above 3,000 ft and visibility above 5 SM |
A forecast ceiling of 800 feet with visibility 6 SM still supports an IFR category; favorable visibility does not override the lower ceiling. The meteorological category is not itself an airline dispatch minimum. Apply the actual destination, alternate, and operating-rule tests separately.
Cumulonimbus depiction
Traditional high-level charts emphasize cumulonimbus (CB) because of its associated aviation hazards. The FAA handbook describes ISOL as less than four-eighths coverage, OCNL as four-eighths to six-eighths, and FRQ as more than six-eighths. EMBD identifies concealment in cloud layers or haze; night by itself is not the definition of embedded CB.
On the traditional high-level product, isolated CB is depicted when embedded in clouds or concealed by haze. A chart that omits an isolated nonembedded cell does not prove that no convection exists. Check the other current products and observations. The CB label implies associated thunderstorm hazards including turbulence, icing, hail, and lightning; do not interpret it as harmless cloud coverage.
Vertical limits and XXX
Read top and base values in the specified flight-level notation. In the traditional FL250–FL630 high-level chart, XXX can identify a CB base extending below the chart's lower boundary. It does not mean that a cloud top automatically exceeds FL630. A hypothetical CB annotation with top 450 and base XXX therefore indicates a top near FL450 and a base below that product's lower boundary; consult the legend and lower-level products for the actual base.
Turbulence areas and other hazard layers also require their proper upper/lower values and severity symbols. A top/base fraction is a chart-notation convenience, not a mathematical quotient that computes severity. Confirm whether an altitude is in flight levels, hundreds of feet, or another unit before comparing with a proposed aircraft profile.
Jet streams and tropopause
Jet axes, flight-level labels, wind barbs/pennants, and associated annotations describe forecast wind structure. A pennant commonly represents 50 knots and a full barb ten knots; read the product legend for all symbols. An annotated layer associated with an 80-knot isotach describes wind structure, not a guarantee that severe CAT exists throughout that layer.
Tropopause heights are commonly shown in boxed values in hundreds of feet on traditional charts. A boxed 450 corresponds to about FL450. A lowered tropopause or sharp change can support a jet/shear assessment, but it does not by itself diagnose turbulence or guarantee a safe altitude a fixed 4,000 feet away. The tropopause is a variable transition region, not an impermeable boundary preventing all overshooting convection.
Other significant phenomena
The actual product may depict volcanic eruption information, tropical cyclones, or other significant hazards. A volcano symbol is a prompt to consult current volcanic-ash advisories, SIGMETs, and dispersion information, not proof that every shaded cloud is ash or that no ash exists elsewhere. Regional tropical products provide center motion and weather information, while surrounding rainbands can extend far outside a center-position annotation.
Interpret each phenomenon with its source, validity, and movement. Forecast polygons cover anticipated weather with uncertainty; the airplane's passage time and altitude must be compared with the forecast, and its diversion profile may cross a different hazard layer. A static screenshot without its timestamp is not adequate evidence for a current operational decision.
Worked cruise and diversion check
A fictional flight plans FL350 through a forecast turbulence layer FL310–FL390. The dispatcher examines a lateral reroute and alternate cruise levels using current turbulence guidance, PIREPs, aircraft limits, ATC availability, and fuel. Do not prescribe FL320 as automatically clear when it lies inside the stated layer. A proposed lower route must also clear terrain and remain within approved performance and weather limits.
Now evaluate an engine-out or decompression diversion. The applicable approved profile may descend through cloud and icing below the main high-level chart. Add the relevant lower-level products, ETOPS alternate forecast window where applicable, and actual field/approach information. A suitable cruise corridor does not automatically make every diversion safe.
Briefing and continued assessment
Explain the chosen route, the significant chart features, assumptions, uncertainty, and the source/time for updates. Provide safety information under § 121.601 and reassess after changes in aircraft status, route, ETA, or weather. Use the chart as one part of a coherent operational weather analysis. The practical-test objective is an authorized, defensible flight and contingency plan, not recital of chart symbols without applying them.
Surface Prognoses and Depiction Across Time
A surface analysis represents an observed/analyzed past or current state; a surface prognostic chart forecasts a future state. Read issue time, valid time, region, legend, and units before applying either. A forecast pressure center and fronts help estimate where a low, cold front, or precipitation zone will be when the flight reaches that area. Compare consecutive valid times rather than treating one picture as stationary weather. Tight isobar spacing supports a pressure-gradient/wind assessment, but friction, terrain, and local circulation affect the actual runway wind.
For a hypothetical evening arrival, an earlier analysis has the cold front west of the destination while the arrival-valid surface prognosis places it east of the field. The dispatcher anticipates a possible frontal wind shift and post-frontal temperature change, then checks the destination TAF, observations, and runway performance rather than declaring a specific wind or ceiling from frontal position alone. If successive forecasts disagree about timing, preserve alternate and fuel options that address that uncertainty.
Traditional weather-depiction charts emphasize observed ceiling/visibility categories; contemporary GFA flight-category layers offer a related operational view with their own observed/forecast timing. Neither product is a blanket dispatch authorization. Surface pressure/precipitation prognoses and lower-level flight-category forecasts complement high-level SIGWX: they inform takeoff, destination, and alternate conditions that a cruise-layer chart cannot show. For an engine-out diversion, match each product to the diversion’s actual altitude and possible landing time.
Official References
Source check: 2026-10-12. 2026 FAA Aviation Weather Handbook. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.
On a Low-Level Significant Weather Prognostic Chart, which meteorological thresholds delineate the areas contoured as IFR and MVFR?
IFR requires ceiling less than 500 feet and visibility less than 1 statute mile; MVFR requires ceiling 500 to 1,500 feet and visibility 1 to 3 statute miles
IFR requires ceiling less than 2,000 feet and visibility less than 5 statute miles; MVFR requires ceiling 2,000 to 4,000 feet and visibility 5 to 7 statute miles
IFR requires ceiling less than 1,000 feet AGL and/or visibility less than 3 statute miles; MVFR requires ceiling 1,000 to 3,000 feet AGL and/or visibility 3 to 5 statute miles
IFR is defined solely by precipitation type (moderate rain or snow), whereas MVFR is defined solely by continuous fog coverage
Why do traditional high-level SIGWX charts emphasize CB clouds?
All cloud layers exceeding 5,000 feet in vertical thickness are depicted regardless of cloud type
Cirrus cloud shields exceeding 50% coverage are the only clouds plotted because of their impact on high-altitude visibility
Stratocumulus and altocumulus layers are highlighted to assist flight crews with optical celestial navigation
CB depiction highlights significant convective hazards within the chart’s stated coverage
On a traditional high-level SIGWX chart, a boxed tropopause value of 450 means what?
A forecast tropopause height near FL450, using the chart’s hundreds-of-feet convention
Maximum cleared altitudes for RNAV routes in hundreds of feet MSL
Radar altimeter correction factors in tens of feet for Oceanic airspace
Freezing level heights plotted in tens of meters above ground level
When Cumulonimbus clouds are labeled with the descriptor 'EMBD' on a High-Level SIGWX chart, what operational hazard does this communicate to the dispatcher?
Convective cells that have decayed into ordinary stratiform rain showers with no active lightning
CB cells concealed within cloud layers or haze, limiting visual identification
Thunderstorm cells restricted entirely within Class B terminal airspace boundaries
Supercell storms containing confirmed tornadic vortices touching the surface
Sections you finish are checked off in the contents.