Chapter 27, Forecasts 27-37
Note: The colors used in the Low-Level SIGWX Charts may vary
depending on the website or service provider.
Figure 27-11. Low-Level SIGWX Chart Symbols
27.9.1.2.1 Flying Categories
IFR areas are outlined with a solid red line, MVFR areas are outlined with a scalloped blue line, and
VFR areas are not depicted (see Figure 27-12).
Figure 27-12. Low-Level SIGWX Chart Flying Categories—Example
27.9.1.2.2 Turbulence
Areas of moderate or greater turbulence are enclosed by bold, dashe d, brown lines (see Figure 27-13).
Turbulence intensities are identified by standard symbols (see Figure 27-11). The vertical extent of
turbulence layers is specified by top and base heights separated by a slant. The intensity symbols and height
information may be located within or adjacent to the forecasted areas of turbulence. If located adjacent to
Chapter 27, Forecasts 27-38
an area, an arrow will point to the associated area. Turbulence height is depicted by two numbers separated
by a solidus (/). For example, an area on the chart with turbulence indicated as 240/100 indicates that the
turbulence can be expected from the top at FL240 to the base at 10,000 ft MSL. When the base height is
omitted, the turbulence is forecast to reach the surface. For example, 080/ identifies a turbulence layer from
the surface to 8,000 ft MSL. Turbulence associated with thunderstorms is not depicted on the chart.
Figure 27-13. Low-Level SIGWX Chart Turbulence Forecast—Example
27.9.1.2.3 Freezing Levels
The freezing level at the surface is depicted by a blue, saw-toothed symbol (see Figure 27-14). The surface
freezing level separates above-freezing from below-freezing temperatures at the Earth’s surface.
Freezing levels above the surface are depicted by blue dashed lines labeled in hundreds of feet MSL
beginning at 4,000 ft using 4,000-ft intervals (see Figure 27-14). If multiple freezing levels exist, these lines
are drawn to the highest freezing level. For example, 80 identifies the 8,000 -ft freezing level contour
(see Figure 27-14). The lines are discontinued where they intersect the surface.
The freezing level for locations between lines is determined by interpolation. For example, the freezing
level midway between the 4,000 and 8,000-ft lines is 6,000 ft.
Figure 27-14. Low-Level SIGWX Chart Freezing Level Forecast—Example
Chapter 27, Forecasts 27-39
Multiple freezing levels occur when the temperature is 0 °C at more than one altitude aloft (see Figure
27-15). Multiple freezing levels can be forecasted on the Low -Level SIGWX Prog Charts in situations
where the temperature is below freezing (negative) at the surface with multiple freezing levels aloft.
The words “Multiple Freezing Levels Possible” and/or “Multiple Freezing Levels,”
and/or associated shading and hatched area do not appear on the chart.
Note: The colors used in this example are different from those used in other
examples.
Figure 27-15. Low-Level SIGWX Chart Multiple Freezing Levels—Example
In Figure 27-15, areas with multiple freezing levels are located on the below -freezing side of the surface
freezing level contour and bounded by the 4,000 -ft freezing level. Multiple freezing levels are possible
beyond the 4,000-ft freezing level (i.e., below 4,000 ft MSL), but the exact cutoff cannot be determined.
27.9.2 Mid-Level Significant Weather (SIGWX) Chart
The Mid-Level SIGWX Chart (see Figure 27-16) is a product of ICAO’s WAFS. The Mid-Level SIGWX
Chart is also known as the Medium -Level SIGWX Chart. The Mid-Level SIGWX Chart is planned to be
phased out in 2024 and replaced by new WAFS SIGWX forecasts.
The Mid-Level SIGWX Chart provides a forecast of significant en route weather phenomena over a range
of FLs from 10,000 ft MSL to FL450. The chart depicts a “snapshot” of weather expected at the specified
valid time. It can be used by airline dispatchers for flight planning and weather briefings before departure
and by flightcrew members during flight.
Chapter 27, Forecasts 27-40
Figure 27-16. Mid-Level SIGWX Chart—Example
27.9.2.1 Issuance
The AWC in Kansas City, MO, has the responsibility, as part of the WAFC Washington, to provide global
weather forecasts of SIGWX phenomena. The AWC issues a 24-hour Mid-Level SIGWX Chart, four times
daily, for the North Atlantic Ocean Region (see Table 27-11).
Table 27-11. Mid-Level SIGWX Chart Issuance Schedule
Issued (UTC) Valid (UTC)
0800 0000 (next day)
1400 0600 (next day)
2000 1200 (next day)
0200 1800
27.9.2.2 Content
The Mid-Level SIGWX Chart depicts numerous weather elements that can be hazardous to aviation. The
weather elements and their presentation are the same as in the High -Level SIGWX Charts
(see Section 27.9.3) except for the addition of non-convective clouds with moderate or severe icing and/or
moderate or severe turbulence. See Section 27.9.3.2 for details on these other weather elements.
Chapter 27, Forecasts 27-41
27.9.2.2.1 Non-Convective Clouds with Moderate or Severe Icing and/or Moderate or
Severe Turbulence
Areas of non-convective clouds with moderate or severe icing and/or moderate or severe turbulence are
depicted by enclosed (red) scalloped lines (see Figure 27-16). The type of icing (i.e., rime, clear, or mixed)
is not forecast.
Note: Cumulonimbus clouds are also depicted by enclosed (red) scalloped lines.
The identification and characterization of each area appears within or adjacent to the outlined area. If the
identification and characterization is adjacent to an outlined area, an arrow points to the appropriate area.
The identification box uses the standar d icing symbol (see Table 27-12). The vertical extent of the icing
layer is specified by top and base heights. When the bases extend below 10,000 ft MSL, they are identified
with XXX.
Table 27-12. Icing and Turbulence Intensity Symbols
Intensity Icing
Symbol
Turbulence
Symbol
Moderate
Severe
27.9.3 High-Level Significant Weather (SIGWX) Charts
High-Level SIGWX Charts (see Figure 27-17) provide a forecast of significant en route weather phenomena
over a range of FLs from FL250 to FL630. Each chart depicts a “snapshot” of weather expect ed at the
specified valid time. The vertical range is planned to change in 2024 to FL100 to FL600 with the planned
implementation of new WAFS SIGWX forecasts, which will have 3 -hour time steps (valid times) from
0 to 48 hours.
High-Level SIGWX forecasts are provided for the en route portion of international flights. These products
are used by airline dispatchers for flight planning and weather briefings before departure and by flightcrew
members during flight.
Chapter 27, Forecasts 27-42
Figure 27-17. High-Level SIGWX Chart—Example
27.9.3.1 Issuance
In accordance with the WAFS of ICAO, High-Level SIGWX forecasts are provided for the en route portion
of international flights.
Chapter 27, Forecasts 27-43
High-Level SIGWX forecasts are issued as a global data set in digital format by two WAFCs, one at the
NWS AWC and the other at the United Kingdom ’s Meteorological Office. Each center produces a global
data set of SIGWX that is then made available (displayed) in chart form for different areas of the globe.
These charts are available on the AWC’s website.
Corrections are issued for format errors or missing information. These charts are not amended.
Table 27-13 provides the issuance schedule.
Table 27-13. High-Level SIGWX Forecast Issuance Schedule
Issued (UTC) Valid (UTC)
0800 0000 (next day)
1400 0600 (next day)
2000 1200 (next day)
0200 1800
27.9.3.2 Content
27.9.3.2.1 Thunderstorms and Cumulonimbus Clouds
The abbreviation CB is only included where it refers to the expected occurrence of an area of widespread
cumulonimbus clouds, cumulonimbus along a line with little or no space between individual clouds,
cumulonimbus embedded in cloud layers, or cumulonimbus concealed by haze. It does not refer to isolated
cumulonimbus not embedded in cloud layers or concealed by haze.
Each cumulonimbus area is identified with CB and characterized by coverage, bases, and tops.
Coverage is identified as isolated ( ISOL) meaning less than 4/8, occasional ( OCNL) meaning 4/8 to 6/8,
and frequent ( FRQ) meaning more than 6/8 coverage. Isolated CBs can only be depicted when they are
embedded (EMBD) in clouds or concealed by haze. Occasional cumulonimbus can be depicted with or
without EMBD.
The vertical extent of the cumulonimbus layer is specified by top and base heights. Bases that extend below
FL250 (the lowest altitude limit of the chart) are encoded XXX.
Cumulonimbus clouds are depicted by enclosed (red) scalloped lines. The identification and
characterization of each cumulonimbus area will appear within or adjacent to the outlined area. If the
identification and characterization is adjacent to an outlined area, an arrow will point to the associated
cumulonimbus area.
On SIGWX charts, the inclusion of CB shall be understood to include all weather phenomena normally
associated with cumulonimbus ( e.g., thunderstorm, moderate or severe icing, moderate or severe
turbulence, and hail).
27.9.3.2.2 Moderate or Severe Turbulence
Forecast areas of moderate or severe turbulence (see Figure 27-18) associated with wind shear zones and/or
mountain waves are enclosed by bold yellow dashed lines. Intensities are identified by standard symbols
(see Table 27-12).
Chapter 27, Forecasts 27-44
The vertical extent of turbulence layers is specified by top and base heights, separated by a horizontal line.
Turbulence bases that extend below FL250 are identified with XXX.
Convective or thunderstorm turbulence is not identified.
Figure 27-18. High-Level SIGWX Chart Turbulence—Examples
27.9.3.2.3 Moderate or Severe Icing
Moderate or severe icing (outside of thunderstorms) above FL240 is rare and is generally not forecasted on
High-Level SIGWX Charts.
27.9.3.2.4 Jet Streams
A jet stream axis with a wind speed of more than 80 kt is identified by a bold (green) line. An arrowhead is
used to indicate wind direction. Wind change bars (double-hatched, light green lines) positioned along a jet
stream axis identify 20-kt wind speed changes (see Figure 27-19).
Symbols and altitudes are used to further characterize a jet stream axis. A standard wind symbol
(light green) is placed at each pertinent position to identify wind velocity. The FL is placed adjacent to each
wind symbol to identify the altitude of the jet stream core or axis.
Jet stream vertical depth forecasts are included when the maximum speed is 120 kt or more. Jet depth is
defined as the vertical depths to the 80 -kt wind field above and below the jet stream axis using FLs. Jet
depth information is placed at the maximum speed point only, normally at one point on each jet stream.
When the jet stream is very long and there are several wind maxima, then each maximum should include
forecasts of the vertical depth.