Significant Weather and Graphical Forecasts

Text forecasts are good at saying what will happen at a place; charts are better at showing where the hazards lie along a route. Significant weather charts and the graphical products that have grown out of them put the jet streams, turbulence, icing, storms and low cloud on a map, so that a pilot can see at a glance which parts of the route and which levels to avoid.

Under ICAO Annex 3 these charts come in two families: high-level charts for airliner cruising levels, produced for the whole world by the World Area Forecast Centres, and low-level charts for light aircraft, produced by each state. The United States adds its own interactive products, above all the Graphical Forecasts for Aviation (GFA), together with flight categories and convective outlooks that US examinations cover in detail.

Prognostic and fixed-time charts

An analysis chart shows the weather as observed at a given time. A prognostic chart, or prog, shows the weather forecast for a later time. Almost every significant weather chart is a prog.

Most are also fixed-time charts: they show the situation at a single moment, such as 1200 UTC, and not over a period as a TAF or a GAMET does. WAFS significant weather charts are issued every 6 hours, usually 18 to 24 hours before their valid time. A flight at 1500 UTC therefore falls between the 1200 and 1800 UTC charts, and the pilot has to interpolate, allowing for the movement of fronts, jets and storm areas in between.

High-level significant weather charts

The high-level SIGWX chart is produced by the two World Area Forecast Centres, London and Washington (see meteorological services and briefings). It covers the levels used by jet transport, from FL250 up to FL600; older charts and much exam material give the top as FL630. The WAFCs also issued medium-level charts for FL100 to FL450 for limited regions. Since late 2024 they have also issued a new-generation WAFS SIGWX forecast as a single data set from FL100 to FL600.

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All heights on the chart are flight levels, and wind speeds are in knots. The main features and their symbols:

On these charts ISOL means individual cumulonimbus covering less than 50 % of the area, OCNL well-separated cells over 50 to 75 %, and FRQ little or no separation over more than 75 %. EMBD means the cells are embedded in layer cloud and cannot be seen. By convention a cumulonimbus area also implies hail, moderate or severe turbulence and moderate or severe icing, which are not drawn separately.

The chart shows where the problems were expected when it was drawn; the SIGMETs in force give the latest picture. A crew reads both, together with the upper wind charts for the jet's position and speed (see jet streams).

Low-level significant weather charts

Light aircraft need a different chart. The ICAO low-level significant weather chart typically covers the surface to FL100 and is produced by each state's meteorological service, alongside or instead of the GAMET area forecast. It gives, area by area, the visibility and weather, the cloud, the freezing level and hazards such as icing and turbulence.

On these low-level charts cloud bases and tops are in feet above mean sea level unless marked otherwise, not above ground, because the pilots who use them compare them with terrain heights, minimum safe altitudes and the freezing level, all given above sea level. XXX in place of a cloud top means the cloud extends above the top of the chart.

UK Form 215 and 415

The United Kingdom's low-level chart is Form 215, known as F215, with Form 415 as its night version. Wavy lines divide the chart into lettered areas, subdivided as B1, B2 and so on when conditions change only slightly within them. For each area a table at the side gives the surface visibility, weather, cloud and the altitude of the 0 °C isotherm.

Form 215 uses its own coverage definitions:

Form 214 and 414 are the companion spot wind and temperature charts (see surface and upper air charts). France publishes its equivalent as the TEMSI chart, a fixed-time chart of the significant weather and cloud masses for low-level flights. French AIRMET information is communicated in the form of TEMSI charts, and in metropolitan France these low-level area forecast charts are issued every 3 hours from 0600 to midnight UTC, a difference from Annex 3 recorded in the French AIP (see SIGMET, AIRMET and weather warnings).

Graphical Forecasts for Aviation and area forecasts

Under ICAO rules the area forecast for low-level flights is issued either as text, the GAMET, or as a chart. The United States long used its own text area forecast (FA), a regional description of clouds and weather. In 2017 it was replaced for the lower 48 states by the Graphical Forecasts for Aviation (GFA) on aviationweather.gov. Text area forecasts remain for Alaska, Hawaii, the Gulf and the Caribbean.

The GFA is an interactive map. Its forecast tab offers layers for clouds and ceiling, visibility, precipitation and weather, thunderstorms, icing, turbulence, winds and temperatures, with a time slider from the current hour to 15 hours ahead; it is updated hourly. The observation tab overlays METARs, PIREPs, radar and satellite pictures. It also colours each airport by flight category, so the pattern of VFR, MVFR, IFR and LIFR along a route shows at a glance.

The United States also issues a low-level significant weather prognostic chart, the low-level prog, covering the surface to FL240 for 12 and 24 hours ahead:

• IFR and MVFR areas are outlined, with the line styles and colours given in the legend;
• areas of moderate or greater turbulence are enclosed by dashed lines, with top and base in hundreds of feet MSL: 240/120 means 12,000 to 24,000 ft, and a top with no base means from the surface or below the chart's lower limit up to that top;
• the freezing level is drawn as dashed contours labelled in hundreds of feet, with SFC where it reaches the ground.

The GFA and the separate freezing level graphics show the lowest altitude of the 0 °C isotherm in the same way. Where a warm layer aloft lies over sub-zero air near the ground, a freezing-level contour appears inside the area where the surface is already below freezing. That marks multiple freezing levels, the profile that produces freezing rain or ice pellets at the surface and severe clear ice aloft (see airframe icing).

Ceiling and visibility analysis

Between airports there are no observations, and a US TAF covers only the area within 5 SM of the airport. The Ceiling and Visibility Analysis (CVA) fills the gaps: it combines surface observations with satellite cloud data into a gridded estimate of ceiling and visibility, updated every few minutes. It shows whether low conditions are widespread or local, but it interpolates rather than measures, and it does not forecast. For a flight that will arrive later, it is used with the TAFs and the GFA.

Flight categories: VFR, MVFR, IFR and LIFR

US weather displays, including the GFA and METAR and TAF maps, group conditions into four flight categories. The ceiling and the visibility are assessed separately and the worse of the two sets the category.

These are planning categories, not legal minima: the legal VFR minima are in 14 CFR 91.155 and the approach minima on the chart. ICAO and EASA rules define no equivalent categories; European pilots compare the forecasts directly with the VMC minima of SERA and their own limits (see cloud cover and ceiling).

Using the charts in planning

A chart is read against the flight, not on its own. The first check is time: which chart, or which pair of charts, brackets the time the aircraft will be over each part of the route. The second is level.

• High level. Compare the jet core, the clear air turbulence boxes and the cumulonimbus tops with the planned cruise levels. The worst turbulence near a jet is usually on its cold, low-pressure side, and a cumulonimbus top at or above the cruise level means a deviation, not a climb.
• Low level. Set the freezing level, the icing layer and the cloud base against the minimum safe altitudes. With a freezing level of 5,000 ft, moderate icing in cloud from there up to tops at FL120 and a minimum en route altitude of 7,000 ft, an aeroplane without ice protection has no usable level in cloud on that route: every level from the minimum en route altitude up to the tops lies inside the icing band.
• Currency. A chart is hours old by the time it is used. SIGMETs, AIRMETs, the latest METARs and TAFs and pilot reports show whether the weather is following the forecast, and they take precedence where they differ.

Convective outlooks

The Convective Outlook (AC) from the Storm Prediction Center is a planning product covering the contiguous United States. For days 1 to 3 it outlines where thunderstorms are expected and grades the risk of severe storms in six categories: general thunderstorms (TSTM), marginal, slight, enhanced, moderate and high. Days 4 to 8 are given as probabilities. Day 1 is updated several times a day.

An enhanced risk means numerous severe storms are possible, a strong reason to plan alternatives, but the outlook is not a warning of storms in progress. On the day of the flight, pilots turn to the GFA thunderstorm layer, convective SIGMETs, radar and satellite (see thunderstorms).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.