How to Read a GFA (Graphic Area Forecast)

Six charts, four times a day

The graphic area forecast covers Canada in seven regions, each with its own identifier such as GFACN33 for Ontario–Québec, for weather at or below 24,000 ft (TC AIM MET 4.1 and 4.3).

At each of the three valid times there are two charts: clouds and weather, and icing, turbulence and freezing level. The last clouds and weather chart also carries an IFR outlook for a further 12 hours in its comments box (TC AIM MET 4.2). Each chart is a snapshot at its valid time, not a forecast for a period.

The four boxes on every chart

Each chart has a title box, a legend box, a comments box and the weather information itself (TC AIM MET 4.6).

• Title box, upper right: the region (for example GFACN33, ONTARIO–QUÉBEC), the chart type, the date and time of issue, and the valid date and time (MET 4.7). Check the valid time first: it is the time the picture shows.
• Legend box: the weather symbols and a nautical mile scale bar (MET 4.8).
• Comments box: what is hard to draw, such as light icing or fog forming, and the standard phrases "HGT ASL UNLESS NOTED" and "CB TCU AND ACC IMPLY SIG TURB AND ICE." (MET 4.9).

Units are fixed: speeds in knots, heights in hundreds of feet, visibility in statute miles, times in UTC (TC AIM MET 4.4).

Reading a cloud label

On the clouds and weather chart a cloud is written as its amount, its type when it matters, and its top over its base, in hundreds of feet ASL (TC AIM MET 4.11). "BKN CU" with 80 over 20 is broken cumulus based at 2,000 ft ASL with tops at 8,000 ft ASL.

• A scalloped border encloses organized cloud that is BKN or OVC. Where there is no organized cloud and the visibility is more than 6 SM, there is no border and the sky is given as SKC, FEW or SCT.
• Two layers less than 2,000 ft apart get one combined amount followed by LYRS. Layers 2,000 ft or more apart are each written with their own amount.
• CIG and AGL mark heights above the ground: "LCA CIGS 3–5 AGL" is local ceilings of 300 to 500 ft above ground level.
• Cirrus is not shown. The tops of TCU, ACC and CB are shown even above 24,000 ft.

Choose a label in the figure and raise the ground under it. The cloud does not move, because its base is above sea level: over 1,500 ft terrain, a 2,000 ft ASL base is 500 ft above you.

Visibility, weather and how much of the area

Visibility comes next, in statute miles, then the weather: "2–4SM -SHSN" is 2 to 4 SM in light snow showers, and P6SM means more than 6 SM. An obstruction to vision such as BR or FG is named only when the visibility is 6 SM or less (TC AIM MET 4.11).

The areas are drawn by kind of weather:

A coverage word says how much of the area gets it (TC AIM MET 4.11, Tables 4.4 and 4.5):

The chart also draws isobars every 4 hPa from 1,000 hPa, surface winds of 20 kt or more with gusts as "G", and fronts and pressure centres with an arrow and speed, or STNR when they move at less than 5 kt.

The icing, turbulence and freezing level chart

Use it with the clouds and weather chart valid at the same time (TC AIM MET 4.12).

• Icing is drawn only when moderate or severe, with its base and top in hundreds of feet ASL and its type: RIME, MX (mixed) or CLR (clear). Severe icing has denser stippling. Light icing is described in the comments box, not drawn.
• Turbulence is drawn only when moderate or severe, with its base, top and cause: MECH, L LVL WS, LEE WV, L LVL JET or CAT. Surface-based turbulence is given in feet AGL, for example 30 AGL over SFC.
• Freezing level contours are dashed lines at multiples of 2,500 ft, labelled SFC, 25, 50, 75, 100 for the surface, 2,500, 5,000, 7,500 and 10,000 ft ASL. An above-freezing layer (AFL) is drawn as a closed area.
• A low-level jet is shown when its core is forecast at 50 kt or more, and may be shown from 35 to 45 kt when it brings significant turbulence or shear.

Between the charts, and when they change

For a time between two charts, move the fronts and pressure centres yourself: each 5 kt of motion is 30 NM in six hours, and the legend's scale bar measures it (TC AIM MET 4.2). Then check the latest observations, which show whether a system is moving faster or slower than forecast.

A SIGMET or AIRMET automatically amends the current GFA for the region its RMK names (TC AIM MET 4.13). A corrected chart carries CCA in its title box for the first correction, CCB for the second, and so on (MET 4.14).

Where candidates lose marks

• Bases and tops are ASL. Subtract the ground elevation to get the height above the terrain. Only CIG and AGL heights are above the ground.
• Top over base. The upper number is the top.
• Valid, not issued. The 1130 UTC issue shows 1200, 1800 and 0000 UTC.
• Light icing is in the comments box. Only moderate and severe icing and turbulence are drawn.
• Turbulence is on the icing chart, not on the clouds and weather chart.
• Visibility is in statute miles, and P6SM means more than 6 SM.
• An AIRMET or SIGMET changes the GFA. Read the ones in force for your region with it.

Practice

Original questions in the style of the Transport Canada written exams. Tap an answer to see why it is right or wrong.

1. Question 1 of 4

   A GFA is issued at about 1130 UTC. At what times are its charts valid?

   11200, 1800 and 0000 UTC21130, 1730 and 2330 UTC3From 1200 to 0000 UTC as one period41800, 0000 and 0600 UTC

   Tap an answer to check it.

2. Question 2 of 4

   The GFA clouds and weather chart shows BKN CU with 80 written over 20 above your route. The ground there is 1,500 ft ASL. How high above the ground is the cloud base?

   1500 ft22,000 ft38,000 ft46,500 ft

   Tap an answer to check it.

3. Question 3 of 4

   Light icing is forecast in a GFA region. Where does the GFA show it?

   1On the clouds and weather chart, beside the cloud label2As a blue stippled area on the icing, turbulence and freezing level chart3In the comments box4Nowhere: the GFA forecasts moderate and severe icing only

   Tap an answer to check it.

4. Question 4 of 4

   On a GFA clouds and weather chart, what does OCNL TCU mean?

   1Towering cumulus forecast for part of the period only2Towering cumulus over 26 to 50% of the area3Towering cumulus over 25% or less of the area4Towering cumulus over more than 50% of the area

   Tap an answer to check it.

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