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Publications / Information on the Graphic Area Forecast (GFA)

Detailed Information

Implementation Date: April 20, 2000

General

The graphic area forecast (GFA) consists of a series of temporally adjusted weather charts, each depicting the most probable meteorological conditions expected to occur below 400 Mb (24,000 ft.) over a given area at a specified time. The GFA is designed primarily to meet general aviation and regional air carrier requirements for pre-flight planning in Canada.

Issue and Valid Times

Graphic area forecast charts are issued four times daily, approximately ½ hour before the beginning of the forecast period. The GFA is issued at approximately 2330, 0530, 1130 and 1730 UTC and is valid at 0000, 0600, 1200 and 1800 UTC respectively. Each GFA issue consists of six charts; a near-term forecast, a 6-hour forecast and a 12-hour forecast depicting the clouds and weather (CLDS & WX) and the icing, turbulence & freezing level (ICG/TURBC/FRLVL) conditions. The 12-hour CLDS & WX chart also includes an IFR outlook for an additional 12-hour period.

Area of Coverage

There are seven distinct GFA areas or domains, covering the entire Canadian domestic airspace, over which Canada has the responsibility for the provision of air traffic control services. The following map illustrates the GFA coverage areas:

Units of Measure

Speeds in the GFA are expressed in knots (KT) and heights in hundreds of feet. Horizontal visibility is measured in statute miles (SM). All times are stated in universal coordinated time (UTC). A nautical mile (NM) scale bar is included in the legend to assist in determining approximate distances on the chart. All heights are measured above sea level (ASL) unless otherwise noted.

Abbreviations and Symbols (Also valid for LGF)

Symbols used in the GFA are consistent with those found on similar meteorological products already described in the AIP, such as the Significant Weather Prognosis chart. Only standard abbreviations are used in the GFA.

Layout

Each GFA chart is divided into four parts; Title Box; Legend Box; Comments Box and Weather Information Section.

Title Box

The Title Box includes the chart name, issuing office four-letter ID, name of the GFA region, chart type, the date/time of issue and the valid date/time of the chart. The Title Box is found at the upper right corner of the GFA.

In the following example, the Title Box indicates the GFA name (GFACN33) and that it is issued by the Canadian Meteorological Centre in Montreal (CWUL). The GFA region for the sample chart is ONTARIO-QUÉBEC and the type of chart is the Clouds and Weather (CLDS & WX) chart. The next section indicates the issue time of the GFA chart, which is 11:30 UTC on September 17, 1999. The last section states the valid time for the GFA chart which, in this example, is valid at 00:00 UTC on September 18, 1999.

Legend Box

The Legend Box includes weather symbols that may be used in the chart and a nautical mile scale bar to facilitate the determination of distances. Symbols used in the GFA are consistent with those used in the Significant Weather Prognosis chart. In the following example, symbols for thunderstorms (TS) and tropical storms are indicated in the legend box. The scale bar at the bottom of the Legend Box provides a measurement reference for nautical miles.

Comments Box

The Comments Box provides information that the weather forecaster considers important (e.g. formation or dissipation of fog, increasing or decreasing visibility, etc.). It is also used to describe elements which are difficult to render pictorially or, if added to the depiction, would cause the chart to become cluttered (e.g. light icing). The standard phrases, "HGTS ASL UNLESS NOTED" and "TCU, ACC AND CB IMPLY SIG TURBC AND ICG. CB IMPLIES LLWS" are also included in the Comments Box. An IFR Outlook, for an additional 12-hour period, is included in the Comments Box of the 12-hour GFA Clouds and Weather chart.

In this example, the forecaster has added two comments. The first indicates that the Fog/Mist will dissipate after 14 UTC. The second comment advises that stratocumulus ceilings will become scattered after 15 UTC.

The Comments Box of the 12-hour Clouds and Weather GFA chart also includes an IFR Outlook, for an additional 12-hour period, in the lower section of the box. The IFR outlook is always general in nature, indicating the main areas where IFR weather is expected, the cause for the IFR weather and any associated weather hazards. In the example given, IFR conditions due to low ceilings (CIG), rain (RA) and mist (BR) south of the St Lawrence Valley are forecast. Also, local IFR conditions are forecast due to an onshore (ONSHR) and upslope (UPSLP) northwesterly flow of air from James Bay (JAMSBA) and Hudson's Bay (HSNBA).

For meteorological purposes, the IFR Outlook is based on the following:

CATEGORY

CEILING

 

VISIBILITY

IFR

less than 1000 feet AGL

or

less than 3 SM

MVFR

between 1000 feet and 3000 feet AGL

or

between 3 and 5 SM

VFR

more than 3000 feet AGL

and

more than 5 SM

Weather Information

The Weather Information part of the chart depicts either a forecast of the clouds and weather conditions or a forecast of the icing, turbulence and freezing level conditions for a specified time.

Clouds and Weather Chart

The GFA Clouds and Weather chart provides a forecast of cloud layers and/or surface-based phenomena, visibility, weather and obstructions to vision at the valid time of that particular chart. Lines joining points of equal surface pressure (Isobars) are depicted at 4mb intervals. In addition, relevant synoptic features that are responsible for the depicted weather, are also depicted, with an indication of their speed and direction of movement at the valid time.

Synoptic Features: The motion of synoptic features, when the speed of movement is forecast to be 5 knots or more, will be indicated by an arrow and a speed value. For speeds less than 5 knots, the letters QS (quasi-stationary) are used. A low pressure centre moving eastwards at 15 knots with and associated cold front moving southeast at 10 knots would be indicated as follows:

Clouds: The bases and tops of forecast clouds, between the surface and 24,000 feet ASL, will be indicated on the GFA Clouds and Weather chart. The tops of convective clouds (i.e. TCU, ACC, CB) are indicated, even if they extend above 24,000 feet ASL. Cirrus clouds are not depicted on the chart. The cloud type will be indicated if considered significant, however, convective clouds such as CU, TCU, ACC and CB will always be stated, if forecast to be present.

A scalloped border encloses organized areas of clouds, where the sky condition is either broken (BKN) or overcast (OVC). An organized area of broken cumulus clouds based at 2000 ft. ASL with tops at 8000 ft. ASL would be indicated as follows:

In areas where organized clouds are not forecast, and the visibility is expected to be greater than 6 statute miles, a scalloped border is not used. In these areas, the sky condition is stated using the terms SKC, FEW or SCT. In the following example, unorganized scattered clouds are forecast based at 3000 ft ASL with tops at 5000 ft ASL.

When multiple cloud layers are forecast, the amount of cloud at each layer is based on the amount of cloud at that level, not on the summation amount. The bases and tops of each layer are indicated. For instance, a scattered layer of cumulus cloud based at 3000 ft ASL with tops at 5000 ft ASL and a higher overcast layer of altostratus cloud based at 10000 ft ASL with tops at 13000 ASL would be indicated as follows:

All heights are indicated in hundreds of feet (e.g. 2 means 200 feet, 45 means 4500 feet, etc.) above sea level (ASL) unless otherwise specified. Heights above ground level (AGL) are indicated by the abbreviation CIG (e.g. ST CIGS 5-10 AGL). A note to this effect is included in the Comments Box in the lower right hand corner of the chart.

Surface-based Layers: Surface-based layers are described in abbreviated plain English. The abbreviation OBSCD (obscured) is used to describe surface-based layers. The vertical visibility into surface-based layers is measured in hundreds of feet above ground level (AGL). Local obscured ceilings with a vertical visibility of between 300 and 500 ft AGL would be indicated as follows:

LCL OBSCD CIG 3 - 5 AGL

Visibility: The forecast visibility is measured in statute miles. When the visibility is expected to be greater than 6 statute miles, it is indicated as P6SM. A forecast visibility that is expected to vary between 2 and 4 statute miles with light rain showers would be indicated as:

2 - 4 SM - SHRA

Weather and Obstructions to Vision: Forecast weather is always included immediately after the visibility. Obstructions to vision are only mentioned when the visibility is forecast to be 6 statute miles or less. (e.g. 2-4SM -RA BR) Only standard abbreviations are used to describe weather and obstructions to vision. A dashed line is used to enclose areas of intermittent or showery precipitation. A solid line is used to enclose areas of continuous precipitation.

Isobars: Isobars, which are lines joining points of equal surface pressure, are included in the GFA Clouds and Weather chart at 4 millibar intervals.

Surface Winds: The speed and direction of forecast surface winds, with a sustained speed of at least 20 knots, are indicated by wind barbs and an associated wind speed value. Wind gusts are indicated by the letter "G", followed by the peak gust speed in knots. In the following example, the surface wind is forecast to be from the west (270o true) with a speed of 25 knots and a peak gust speed of 35 knots.

Icing, Turbulence and Freezing Level Chart

 

The GFA Icing, Turbulence and Freezing Level chart depicts forecast areas of icing and turbulence as well as the expected freezing level at a specific time. Included on the chart are the type, intensity, bases and tops for each icing and turbulence area. Surface synoptic features such as fronts and pressure centres are also shown. This chart is to be used in conjunction with the associated GFA Clouds and Weather chart issued for the same valid period.

Icing: Icing is depicted whenever moderate or severe icing is forecast for the coverage area. The bases and tops of each icing layer, measured in hundreds of feet above mean sea level, as well as the type of icing (e.g. "RIME", "MXD"-mixed, "CLR"-clear) will be indicated. Areas of light icing are described in the Comments Box. An area of moderate mixed icing, based at 2000 ft. ASL with a top of 13000 ft. ASL would be indicted as follows:

If icing is expected to be present during only part of the forecast period covered by the chart, the time of occurrence of the icing is indicated in the Comments Box.

Turbulence: Turbulence is depicted whenever moderate or severe turbulence is forecast for the coverage area. The base and top of each turbulence layer is measured in hundreds of feet ASL. If the turbulence is due to mechanical turbulence, low level wind shear, lee/mountain waves, a significant low level jet or in clear air, an abbreviation indicating the cause of the turbulence will be included (e.g. MECH, LLWS, LEE WV, LLJ or CAT). The following example indicates an area of moderate clear air turbulence (CAT) based at 8000 ft ASL with a top at 20000 ft ASL.

Freezing Level: Freezing level contours are indicated on the ICG/TURBC/FRLVL chart by dashed lines. The height of the freezing level is measured above sea level and the contour lines for the freezing level will be at 2500-foot intervals, starting at the surface. Modifications to the freezing level such as above freezing layers aloft, temporal changes, etc., are explained in the Comments Box for that chart.

GFA Amendments

The GFA is automatically amended by AIRMET bulletins whenever weather conditions, which are considered significant to aviation, have not been forecast and subsequently occur or have been forecast but do not occur. Each AIRMET will indicate which GFA is being amended. In addition, the GFA is automatically amended by SIGMET bulletins, even though it is not explicitly stated in the SIGMET itself.

GFA Corrections

The GFA will be re-issued in the event that one or more of the original GFA charts contains a significant error which, if left uncorrected, could result in an erroneous interpretation of the GFA. In this event, only the erroneous chart(s) is corrected and re-issued with an appropriate explanation in the Comments Box.

When re-issued, the correction code "CCA" is added to the first line of the Title Box to indicate the first correction, "CCB" for the second, "CCC" for the third, etc.

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