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5.19.1.1.2 Turbulence.
Areas of moderate or greater turbulence are enclosed by bold, dashed, brown lines (see Figure 5-60, Low-Level SIGWX Chart Turbulence Forecast— Example). Turbulence intensities are identified by standard symbols (see Figure 5-58). The vertical extent of turbulence layers is specified by top and base heights separated by a slant. The intensity symbols and height informat ion may be located within or adjacent to the forecasted areas of turbulence. If located adjacent to 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 5-60. Low-Level SIGWX Chart Turbulence Forecast—Example
5.19.1.1.3 Freezing Levels.
The freezing level at the surface is depicted by a blue, saw-toothed symbol (see Figure 5-61, Low-Level SIGWX Chart Freezing Level Forecast— Example). The surface freezing level separates above-freez ing 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-foot intervals (see Figure 5-61). If multiple freezing levels exist, these lines are drawn to the highest freezing level. For example, 80 identifies the 8,000-foot freezing level contour (see Figure 5-61). The lines are discontinued where they intersect the surface.
5-125 The freezing level for locations between lines is determined by interpolation.
For example, the freezing level midway between the 4,000 and 8,000 foot lines is 6,000 ft.
Figure 5-61. Low-Level SIGWX Chart Freezing Level Forecast—Example
Mu ltiple freezing levels occur when the temperature is 0 ºC at more than one altitude aloft. Multiple freezing levels can be forecasted on the Low-Level Significant Weather Prog Charts in situations where the temperature is below freezing (negative) at the surface with multiple freezing levels aloft.
5-126 Figure 5-62. Low-Level SIGWX Chart Multiple Freezing Levels—Example 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.
On the chart, 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 (see Figure 5-62, Low-Level SIGWX Chart Multiple Freezing Levels—Example).
5.19.1.2 Issuance.
Low-Level SIGWX Charts are issued four times per day by the NWS AWC (see Table 5-24, Low-Level SIGWX Chart Issuance Schedule). Two charts are issued; a 12-hour and a 24-hour prog. Both are available on the https://www.aviationweather.gov.
Table 5-24. Low-Level SIGWX Chart Issuance Schedule
Issuance Time ~1720Z ~2310Z ~0530Z ~0935Z Chart Valid Time 12-Hour Prog 00Z 06Z 12Z 18Z 24-Hour Prog
12Z 18Z 00Z 06Z
5.19.1.3 Use.
The Low-Level SIGWX Charts provide only an overview of selected aviation weather hazards up to FL240 at 12 and 24 hours into the future. More detailed and timely products should be used for actual flight planning.
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5.19.2 Mid-Level Significant Weather (SIGWX) Chart.
The Mid-Level Significant Weather (SIGWX) Chart (see Figure 5-63, Mid-Level SIGWX Chart—Example) 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 provides a forecast of significant en route weather phenomena over a range of flight levels from 10,000 ft MSL to FL450. The chart depicts a “snapshot” of weather expected at the specified valid time.
The Mi d-Level SIGWX Chart is available on https://www.aviationweather.gov.
Figure 5-63. Mid-Level SIGWX Chart—Example
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5.19.2.1 Content.
The Mid-Level S IGWX 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 paragraph 5.19.3) except for the addition of Non-Convective clouds with mode rate or severe icing and/or moderate or severe turbulence. See paragraph 5.19.3.1 for details on these other weather elements.
5.19.2.1.1 Non-Convective Cloud 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 5-63). The type of icing, i.e., rime, clear or mixed, is not forecast.
Note: CB 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 standard icing symbol (see Table 5-25, Icing and Turbulence Intensity Symbols). 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 5-25. Icing and Turbulence Intensity Symbols Intensity Icing Symbol Turbulence Symbol Moderate
Severe
5.19.2.2 Issuance.
The AWC in Kansas City, MO, has the responsibility, as part of the WAFC, Washington, to provide global weather forecasts of significant weather phenomena. The AWC issues a 24-hour Mid-Level SIGWX chart, four times daily, for the North Atlantic Ocean Region (NAT) (see Table 5-26, Mid-Level SIGWX Chart Issuance Schedule).
5-129 Table 5-26. Mid-Level SIGWX Chart Issuance Schedule Issued (UTC) Valid (UTC) 0800 0000 (next day) 1400 0600 (next day) 2000 1200 (next day) 0200 1800
5.19.2.3 Use.
The Mid-Level SIGWX Chart is used to determine an overview of selected flying weather conditions between 10,000 ft MSL and FL450. It can be used by airline dispatchers for flight planning and weather briefings before departure and by flightcrew members during flight.
5.19.3 High-Level Significant Weather (SIGWX) Charts.
High-Level SIGWX Charts (see Figure 5-64, High-Level SIGWX Chart—Example) provide a forecast of significant en route w eather phenomena over a range of flight levels from FL250 to FL630. Each chart depicts a “snapshot” of weather expected at the specified valid time. They are available on https://www.aviationweather.gov.
Figure 5-64. High-Level SIGW Chart-Example
ISSUED BY WAFC WASHINGTON VALID O50 A SIG N 2009 _ OR TURE 270 300 270 TERED 80/470 390 ฿16799 1390
- 400 450 "PATRICIA 530 500] 500 500 500 480 450 390 390 340 300 1390 340 [270] 200 390 300 5-130
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5.19.3.1 Content.
5.19.3.1.1 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 4/8th, occasional (OCNL) meaning 4/8th to 6/8th, and frequent (FRQ) meaning more than 6/8th coverage. Isolated CBs can only be depicted when they are embedded (EMBD) in clouds or concealed by haze. Occasional CBs can be depicted with or without embedded (EMBD).
The vertical extent of cumulonimbus layer is specified by top and base heights. Bases that extend below FL250 (the lowest altitude limit of the chart) are encoded XXX.
CBs are depicted by enclosed (red) scalloped lines. The identification and characterization of each CB 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 significant weather charts, the inclusion of CB shall be understood to include all weather phenomena normally associated with cumulonimbus
(i.e., thunderstorm, moderate or severe icing, moderate or severe turbulence, and hail).
5.19.3.1.2 Moderate or Severe Turbulence.
Forecast areas of moderate or severe turbulence (see Figure 5-65, High-Level SIGWX Chart Turbulence—Examples ) associated with windshear zones and/or mountain waves are enclosed by bold yellow dashed lines. Intensities are identified by standard symbols (see Table 5-25).
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.
5-132 Figure 5-65. High-Level SIGWX Chart Turbulence—Examples
5.19.3.1.3 Moderate or Severe Icing.
Moderate and severe icing (outside of thunderstorms) above FL240 is rare and is not generally forecasted on High-Level SIGWX charts.
5.19.3.1.4 Jet Streams.
A jet stream axis with a wind speed of more than 80 kts 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 identifies 20-knot wind speed changes (see Figure 5-66, High-Level SIGWX Chart Jet Stream—E xample).
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 flight level 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 kts or more. Jet depth is defined as the vertical depths to the 80-knot wind field above and below the jet stream axis using flight levels. 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.
5-133 Figure 5-66. High-Level SIGWX Chart Jet Stream—Example
Forecast maximum speeds of 100 kts at FL310 at one location and 120 kts at FL290 at another location. At the latter location, the base of the 80-knot wind field it FL210, and the top of the 80-knot wind field is FL340.
5.19.3.1.5 Tropopause Heights.
Tropopause heights are plotted at selected locations on the chart. They are enclosed by rectangles and plotted in hundreds of feet MSL (see Figure 5-67, High-Level SIGWX Chart Tropopause Height—Examples). Centers of high (H) and low (L) tropopause heights are enclosed by polygons and plotted in hundreds of feet MSL.
Figure 5-67. High-Level SIGWX Chart Tropopause Height—Examples
5.19.3.1.6 Tropical Cyclones.
Tropical cyclones, i.e., with surface wind speed 34 kts or greater, are depicted by the symbol depicted in Figure 5-68, High-Level SIGWX Chart Tropical Cyclone—Examples, with the storm’s name positioned adjacent to the sym bol. Cumulonimbus clouds meeting chart criteria are identified and characterized relative to each storm.
5-134 Figure 5-68. High-Level SIGWX Chart Tropical Cyclone—Examples
5.19.3.1.7 Volcanic Eruption Sites.
Volcanic eruption sites are identified by a trapezoidal symbol depicted in Figure 5-69, High-Level SIGWX Chart Volcanic Eruption Site—Example.
The dot on the base of the trapezoid identifies the location of the volcano. The name of the volcano, its latitude, and its longitude are noted adjacent to the symbol.
Figure 5-69. High-Level SIGWX Chart Volcanic Eruption Site—Example
5.19.3.1.8 W idespread Sandstorms and Duststorms.
Widespread sandstorms and duststorms are labeled with the symbol depicted in Figure 5-70, High-Level SIGWX Chart Widespread Sandstorm and Duststorm—Example. The vertical extent of sand or dust is specified by top and base heights, separated by a horizontal line. Sand or dust that extends below the lower limit of the chart (FL250) is identified with XXX.
Figure 5-70. High-Level SIGWX Chart Widespread Sandstorm and Duststorm—Example
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5.19.3.2 Issuance.
In accordance with the WAFS of the ICAO, High-Level SIGWX forecasts are provided for the en route portion of international flights.
High-Level SIG WX forecasts are issued as a global data set in digital format by two WAFCs, one at NWS’s AWC and the other at the United Kingdom’s Meteorological Office. Each center produces a global data set of significant weather that is then made available (displayed) in chart form for different areas of the globe. These charts are available on https://www.aviationweather.gov.
Corrections are issued for format errors or missing information. These charts are not amended.
Table 5-27. High-Level SIGWX FORECAST Issuance Schedule Issued (UTC) Valid (UTC) 0800 0000 (next day) 1400 0600 (next day) 2000 1200 (next day) 0200 1800
5.19.3.3 Use.
High-Level SIGWX forecasts are provided for the en route portion of international flights. These products are used directly by airline dispatchers for flight planning and weather briefings before departure and by flightcrew members during flight.
5.19.4 Alaska Significant Weather (SIGWX) Charts.
The Alaska SIGWX charts (see Figure 5-71, Alaska Significant Weather Chart— Example) are a series of four forecasts (24-hour, 36- hour, 48-hour, and 60-hour) valid at specified times. These charts provide a graphical overview of the specified forecast weather primarily for lower flight altitudes.
5.19.4.1 Issuance.
The AAWU issues the Alaska SIGWX charts. These charts are issued twice a day at 0530 and 1330 UTC during Alaska Standard Time, and 0430 and 1230 UTC during Alaska Daylight Time. The 1330/1230 UTC-issued 24-hour SIGWX chart may be updated around 2145/2045 UTC valid at 1200 UTC the next day.