ppl_04_e2
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Chart 1 (Form 215).
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Chart 2 (Form 415).
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CHAPTER 20
WORLD AREA FORECAST SIGNIFICANT WEATHER CHARTS
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Figure 20.1 An example of the World Area Forecast Significant Weather Chart.
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INTRODUCTION.
World Area Forecast System Significant Weather (WAFS SIGWX) Charts apply to pressure altitudes between 10 000 and 63 000 feet (Flight Levels 100 - 630) to provide information about significant weather at medium and high altitudes. Significant weather phenomena include the forecast locations of embedded cumulonimbus clouds, medium and severe clear air turbulence, moderate and severe icing, jet streams, and other important meteorological occurrences.
WAFS SIGWX charts are not strictly part of the PPL syllabus, and deal with flight levels at which the general aviation pilot does not generally operate during crosscountry flights, although even basic training aircraft are capable of flying at levels considerably higher than Flight Level 100. This chapter, therefore, makes only a brief study of the weather forecast charts available for medium and high altitudes, in order to introduce general aviation pilots, who may be considering moving on to professional flying, to medium level and high level weather forecasting.
HIGH LEVEL WAFS SIGWX CHARTS.
High level WAFS SIGWX charts cover the atmospheric level bounded by Flight Level 250 and Flight Level 630.
Figure 20.1 is an example of a high level WAFS SIGWX chart. The chart has a clearly defined legend which contains the following information: the name of the World Area
Forecast Centre (WAFC) which produced the chart, (either London or Washington), a reminder that the forecast is for a single, fixed time; the ICAO-designated letter used to identify the area covered by the chart; the range of flight levels through which the chart is valid; the time in UTC, and the date on which the chart is valid.
Figure 20.2 Clear Air Turbulence (CAT) Legends. This displays the degree of turbulence and forecast flight levels for specific numbered areas on the chart. The areas visible on this chart, and their corresponding numbers have been highlighted in red, for clarity.
WAFS SIGWX charts are produced, daily, every 6 hours, and are valid for 0000,
0600, 1200, and 1800 UTC. Since each chart is valid for one single fixed time only, it is the responsibility of the user to interpolate between charts to establish what weather conditions can be expected at intermediate times.
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WAFS SIGWX charts which are produced by the UK Met Office (WAFC London) have an additional legend, which highlights the flight levels between which Clear Air
Turbulence can be expected.
In the legend, shown in Figure 20.2, each Clear Air Turbulence (CAT) symbol has a number assigned to it. This number corresponds to the number attached to an area of CAT on the chart; these areas have been highlighted in red, and are enclosed by a dashed line.
MEDIUM LEVEL WAFS SIGWX CHARTS.
The legends which are assigned to the medium level WAFS SIGWX charts are similar to those on the high level charts, except for the text line which clearly highlights that the medium level charts are valid between FL 100 and FL 450.
THUNDERSTORMS.
Thunderstorms are not explicitly displayed on the charts as such; however, thunderstorm activity is assumed whenever cumulonimbus cloud is forecast. Thus, wherever the code CB is seen on the chart, it must be assumed that thunderstorm activity is present in those areas. These areas on the chart are delineated using a scalloped line, as shown in Figure 20.3. However, it is important to remember that only embedded cumulonimbus clouds are displayed on WAFS charts.
On a high level WAFSSWX
chart “CB” implies
moderate to severe icing and turbulence.
Figure 20.3 Possible Thunderstorms are indicated by the symbol CB. CB implies moderate to severe turbulence, lightning and hail. The enlarged area shows the symbols used to describe the type, and flight level, of the activity.
Included in the information about the cumulonimbus activity is an indication of the flight level at which the cumulonimbus cloud will be present. This information is included either adjacent to the chart, or within the scalloped area on the chart.
In the example shown in Figure 20.3, “ISOL” is used to indicate that any occurrences of cumulonimbus will be isolated within the scalloped area shown on the chart.
Other abbreviations that may be used on the WAFS significant weather charts are:
“OCNL,” indicating that cumulonimbus may be occasionally encountered within the
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area; “WDSPD” implies that cumulonimbus are forecast to be widespread within the scalloped area on the chart; and, finally, “FRQ” is used to imply that cumulonimbus are expected to be frequent within that area.
In the boxes next to the areas denoting cumulonimbus activity, are indications of the flight levels between which cumulonimbus cloud can be expected. If any of these flight levels are outside the height range of the chart, the symbol XXX is used. In
Figure 20.4, the top of the cumulonimbus activity is forecast for Flight level 410, but the bottom of the cumulonimbus activity is below the lowest level of the chart forecast. This is flight level 250 for the high level chart shown in Figure 20.4. In the example shown, the medium level WAFS SIGWX chart must be consulted, in order to discover the base of the cumulonimbus activity.
Figure 20.4 Isolated, embedded cumulonimbus with tops at FL410 and a base below the chart level.
TROPICAL CYCLONES AND HURRICANES.
Tropical cyclones are referred by a variety of names; however, if they become particularly severe, they are called hurricanes in the Atlantic Ocean , (the generic term being ‘tropical revolving storm’). Hurricanes are easily recognisable on significant weather charts by the symbol highlighted in Figure 20.5.
Figure 20.5 Tropical cyclones are called hurricanes in the Atlantic; they are areas of very intense cumulonimbus activity and present a major hazard to aircraft.
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The symbol is always accompanied by the World Meteorological Organisation name of the storm. Hurricanes used to be given exclusively women’s names, but, for reasons that have never been revealed, are now given men’s names too. In Figure 20.5, the hurricane has been named “Olga”.
Tropical cyclones are associated with intense areas of cumulonimbus activity, and they need to be very carefully monitored.
TURBULENCE.
Moderate, or severe turbulence is displayed on the WAFS SIGWX chart using the symbols shown in Figure 20.6.
The symbol for turbulence, as for other weather phenomena, are the same as the symbols used on the low-level forecast charts.
Wherever cumulonimbus is forecast, moderate to severe turbulence must be assumed to be active in the area. On the medium level significant weather charts, areas of moderate and severe turbulence are also associated with other types of medium level cloud. The example from a medium - level chart in Figure 20.7 shows turbulence around broken cumulus and altocumulus.
Figure 20.6 Only moderate to severe turbulence is forecast. For CB always assume moderate to severe turbulence.
The turbulence symbols are included within the “cloud” boxes assigned to every scalloped cloud area. See Figure 20.7.
Figure 20.7 This example, on a medium level chart, shows moderate turbulence from FL160 down to a level below the chart coverage which, vertically, is FL100.
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ICING
Figure 20.8 Only moderate to severe icing is forecast. For CB, always assume moderate to severe icing.
As previously described, the only cloud type shown by high level WAFS SIGWX charts, is cumulonimbus clouds. You may also recall that moderate or severe icing is also automatically assumed to be prevail in the vicinity of these clouds. However, on medium level charts, other cloud types are shown (see Figure 20.7).
Figure 20.9 Sand and dust storms are forecast only if the dust storm or sand storm is reducing visibility within the flight levels of the chart.
Moderate to
severe icing is always
assumed to be
present in and in the vicinity of cumulonimbus.
SAND AND DUST STORMS.
Widespread sandstorms or dust storms are highlighted on the significant weather charts only when these phenomena are forecast to obscure visibility significantly, between the flight-levels for which the chart is valid. The symbol which is used on the chart is identical for both phenomena, and is shown in Figure 20.9.
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CLOUD AREAS.
Figure 20.10 Cloud Areas - on High Level Charts only Cumulonimbus are shown. On Medium
Level Charts - any cloud which could lead to significant icing or turbulence.
The only clouds which are displayed on high level charts are embedded cumulonimbus; however, on the medium level charts, rather more cloud information is displayed. Whereas high level charts show only embedded cumulous clouds, medium level charts show more detail on the types of cloud that may be encountered. Any cloud types which are likely to have a significant icing or turbulence risk are clearly highlighted.
In the example in Figure 20.10, above, the cloud box refers to the scalloped cloud areas highlighted in red. From the symbols within the box, it can be seen that broken amounts of cumulus and altocumulus may be present; moderate turbulence can be expected between Flight Levels 250 and XXX, which is less than FL 100; moderate icing can be expected between Flight Levels 250 and 150.
FRONTAL SYSTEMS.
Significant frontal systems are highlighted on the WAFS SIGWX charts.
Standard frontal coding is used to indicate the surface positions of the fronts. The direction of travel of the front is shown by arrows, with the speed of movement of the front shown by small numbers next to the front. In Figure 20.11, there is a cold front moving eastwards (a) at a speed of 20 knots (b).
Figure 20.11 Only significant fronts are shown. Standard front coding is used to denote frontal type. Direction of movement is shown by small arrows, (a), with numbers next to them, giving speed in knots (b).
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