MANAIR Manual of Standards and Procedures for Aviation Forecasts: chapter 7


7.1 Definition

Forecasts in chart or pictorial form depict, with greater clarity than verbal, the most probable forecast conditions over large areas. Forecast of significant enroute weather phenomena prepared as SIG WX charts are primarily designed to meet requirements for preflight planning.

7.2 Issuing centres

7.2.1 Canadian Meteorological Aviation Centres (CMAC)

The CMAC routinely issue a Graphic Area Forecast (GFA) in chart form as described in Chapter 4. Additionally, the CMACs routinely issues forecasts of significant weather (SIG WX) in chart form for the Canadian airspace and for the North Atlantic. These are detailed in 7.3.

7.3 CMAC prognostic charts

7.3.1 Canadian significant weather prognostic chart (CAN SIG WX)

7.3.1.1 Description

The chart is a depiction of forecast significant weather at mid-levels (700 hPa to 400 hPa or FL100 to FL240).

7.3.1.2 Issue and valid times

The charts are issued four times a day at approximately 0210, 0815, 1425 and 1940 UTC, and are valid at 12, 18, 00 and 06 UTC respectively. In other words, a chart is issued roughly 10 hours before its valid time.

7.3.1.3 Domain

The Canadian significant weather prognostic chart covers the seven GFA domains. Refer to 4.4 for details.

7.3.1.4 Units

Speeds are expressed in knots (KT) and heights in hundreds of feet. Mean sea level pressure is in hectopascals (hPa), and time is expressed in coordinated universal time (UTC).

7.3.1.5 Map projection and background

As per the Technical Regulations, Volume II, Meteorological Service for International Air Navigation, Section [C.3.3] 3.2.2 (WMO-No. 49), the map shall use the polar stereographic projection true at 60 degrees latitude north.

For clarity, the map background contains at least the following features:

  • provincial, federal and territorial boundaries
  • oceans, coastlines and major lakes
7.3.1.6 Content

The significant weather charts depict forecast surface positions of pressure centres and fronts, forecast areas of significant cloud, turbulence and icing, as well as forecast freezing levels. When applicable, information on tropical cyclones, volcanoes/volcanic ash and radioactive releases are also indicated.

1) Areas of clouds

Areas of clouds are indicated by scalloped lines. Cloud type, amount and thickness are indicated by NCC hh/hh, where:

  • N indicates the cloud cover as BKN or OVC;
  • CC indicates the type of cloud or may be replaced by LYR in cases of multiple vertical layers; and
  • hh/hh indicates the heights, in hundreds of feet ASL, of the top and of the base of the cloud. XX indicates a base below the 700 hPa level.
2) Turbulence

Turbulence is indicated by a symbol (see below) shown within a given area. A forecast of turbulence implies a greater than 50% chance of encountering turbulence somewhere within the area to which the symbol applies.

Symbol indicates moderate turbulence:

Symbol 1. See long description below.

Description of image

Moderate turbulence symbol, depicted as an upwardly pointing chevron.

Symbol indicates severe turbulence:

Symbol 2. See long description below.

Description of image

Severe Turbulence symbol, depicted as an upwardly pointing double chevron

  • hh/hh gives the heights of the top and of the base of the turbulent layer, in hundreds of feet ASL. XX indicates a base below the 700 hPa level.

Note: A forecast of CB, thunderstorms, TCU, or ACC automatically implies moderate or severe turbulence; in this case, no turbulence symbol is used.

3) Icing

Icing is indicated by a symbol (see below) appearing within a given area.
Symbol indicates moderate icing:

Symbol 3. See long description below.

Description of image

Moderate Icing symbol, depicted as a U shape with two lines crossing through the middle.

Symbol indicates severe icing:

Symbol 4. See long description below.

Description of image

Severe Icing severe symbol, depicted as a U shape with three lines crossing through the middle.

  • hh/hh gives the heights of the top and of the base of the icing layer, in hundreds of feet ASL. XX indicates a base below the 700 hPa level.

Note: A forecast of CB, thunderstorms, TCU, or ACC automatically implies moderate or severe icing; in this case, no icing symbol is used.

4) Freezing levels

The freezing levels are depicted by dashed lines drawn at intervals of 5,000 feet and labeled in hundreds of feet ASL.

5) Surface fronts

Surface fronts are depicted in the standard fashion.

6) Pressure centres

Low and high pressure centres are marked "L" or "H", as applicable, and values of the central pressure are labeled in hPa.

7) Tropical cyclones

Tropical cyclones are indicated using the appropriate symbol along with official name:

Symbol indicates a tropical storm:

Symbol 5. See long description below.

Description of image

Tropical Storm symbol, depicted as a circle

Symbol indicates a hurricane:

Symbol 6. See long description below.

Description of image

Hurricane symbol, depicted as a solid circle

Note: The forecast mean direction and speed of motion (in knots) of surface pressure centres, fronts and trowals are added if the speed is greater than five knots. The direction and speed are six-hour means centred at the valid time of the chart.

7.3.2 North Atlantic significant weather prognostic chart (NAT SIG WX)

7.3.2.1 Description

The chart is a depiction of forecast significant weather at low and mid-levels (SFC to 400 hPa or SFC to FL240).

7.3.2.2 Issue and valid times

These charts are valid at 00 UTC and 12 UTC and are transmitted at approximately 1315 UTC and 0115 UTC respectively.

7.3.2.3 Domain

The North Atlantic significant weather prognostic chart covers the north-eastern section of North America, the North Atlantic Ocean between latitude 30 and 70 degrees, including Greenland, and a section of Western Europe.

7.3.2.4 Units

Refer to 7.3.1.4.

7.3.2.5 Map projection and background

Refer to 7.3.1.5.

7.3.2.6 Content

Refer to 7.3.1.6.

7.4 Examples of forecasts in chart form

Figure 8. See long description below.

Description of image

Example of a significant weather prognostic chart. Map shows Canada in its entirety. Displayed on the maps are synoptic features (lows, highs, cold fronts, warm fronts and trowals) and their motion indicated by an arrow for the direction and a number for the speed in knots. Areas of significant cloud have a brown scalloped border. Inside is the coded description of the cloud and significant icing if required. Areas of significant turbulence are enclosed by a solid red line with the inside hatched by red diagonal lines with a negative slope. Inside is the coded description containing the required turbulence symbol with the type of turbulence indicated below. The base and top of the turbulence layer is indicated to the right of the symbol. Dashed bold black lines depict the freezing levels with the height (level) indicated by a white number on a black tag attached to the line. In the upper right corner of the chart, is a small box containing information about the chart: Environment Canada Environnement Canada with Canadian flag, P SIG WX/TMPS SIG 700-400 MB V 2012-05-26 00:00Z. UNLESS OTHERWISE INDICATED, CB, TCU, AND ACC IMPLY MODERATE OR GREATER TURBULENCE AND ICING. ALTITUDES IN HUNDREDS OF FEET. À MOINS D'INDICATION CONTRAIRE, CB,TCU, ET ACC IMPLIQUENT DE LA TURBULENCE ET DU GIVRAGE MODÉRÉS OU PLUS FORTS. ALTITUDES EN CENTAINES DE PIEDS.

Figure 8: Canadian significant weather prognostic chart

Figure 9. See long description below.

Description of image

Example of a significant weather prognostic chart. Map shows the North Atlantic Ocean in its entirety with eastern North American and Western Europe/Africa. Displayed on the maps are synoptic features (lows, highs, cold fronts, warm fronts and trowals) and their motion indicated by an arrow for the direction and a number for the speed in knots. Areas of significant cloud have a scalloped border. Inside is the coded description of the cloud and significant icing if required. Areas of significant turbulence are enclosed by a solid line with the inside hatched by diagonal lines. Inside is the coded description containing the required turbulence symbol with the type of turbulence indicated below. The base and top of the turbulence layer is indicated to the right of the symbol. Dashed bold lines depict the freezing levels with the height (level) indicated by a tag attached to the line. In the lower right corner of the chart, is a small box containing information about the chart: Environment Canada Environnement Canada with Canadian flag, P SIG WX/TMPS SIG SFC-400 MB V 2011-11-15 12:00Z. UNLESS OTHERWISE INDICATED, CB, TCU, AND ACC IMPLY MODERATE OR GREATER TURBULENCE AND ICING. ALTITUDES IN HUNDREDS OF FEET. À MOINS D'INDICATION CONTRAIRE,CB,TCU, ET ACC IMPLIQUENT DE LA TURBULENCE ET DU GIVRAGE MODÉRÉS OU PLUS FORTS. ALTITUDES EN CENTAINES DE PIEDS.

Figure 9: North Atlantic significant weather prognostic chart (NAT SIG WX)

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