Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-11
Figure 8-14. Low-Level SIGWX Chart Freezing Level Forecast Example
Multiple freezing levels occur when the temperature is zero degrees Celsius 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.
Figure 8-15. Low-Level SIGWX Chart Multiple Freezing Levels Example
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-12 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 foot freezing level. Multiple freezing levels are possible beyond the 4,000 feet freezing level (i.e., below 4,000 feet MSL), but the exact cutoff cannot be determined (Figure 8-15).
8.2.2 Issuance Low-Level Significant Weather (SIGWX) Charts are issued four times per day by the Aviation Weather Center (AWC) in Kansas City, Missouri (Table 8-2). Two charts are issued; a 12-hour and a 24-hour prog. Both are available on the AWC web site:
http://aviationweather.gov/products/swl/.
Table 8-2. 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
8.2.3 Use The Low-Level Significant Weather (SIGWX) Charts provide an overview of selected aviation weather hazards up to 24,000 feet MSL (FL240 or 400 millibars) at 12- and 24-hours into the future.
Figure 8-16. 12-Hour Low-Level SIGWX Chart Example
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-13
Figure 8-17. 24-hour Low-Level SIGWX Chart Example
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-14
8.3 Mid-Level Significant Weather (SIGWX) Chart The Mid-Level Significant Weather (SIGWX) Chart (Figure 8-18) provides a forecast of significant en route weather phenomena over a range of flight levels from 10,000 ft MSL to FL450, and associated surface weather features. The chart depicts a “snapshot” of weather expected at the specified valid time.
The Mid-Level Significant Weather (SIGWX) Chart is available on the Aviation Weather Center web site at: http://aviationweather.gov/products/swm/.
Figure 8-18. Mid-Level SIGWX Chart Example
8.3.1 Content The Mid-Level Significant Weather (SIGWX) Chart depicts numerous weather elements that can be hazardous to aviation.
8.3.1.1 Thunderstorms 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 or scattered cumulonimbus not embedded in cloud layers or concealed by haze.
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-15 Each cumulonimbus area is identified with CB and characterized by coverage, bases and tops.
Table 8-3. Mid-Level SIGWX Chart Cumulonimbus Coverage
CODED CHARACTERIZATION MEANING ISOL Isolated Less than 1/8th coverage OCNL Occasional 1/8th to 4/8ths coverage FRQ Frequent More than 4/8ths coverage EMBD Embedded CBs concealed by other cloud layers, haze, dust, etc.
Coverage, Table 8-3, is identified as isolated (ISOL) meaning less than 1/8th, occasional (OCNL) meaning 1/8th to 4/8ths, and frequent (FRQ) meaning more than 4/8ths coverage.
Isolated and occasional CBs are further characterized as embedded (EMBD). The chart does not display isolated or scattered cumulonimbus clouds unless they are embedded in other clouds, haze, or dust.
The vertical extent of cumulonimbus layer is specified by top and base heights. Bases that extend below 10,000 feet (the lowest altitude limit of the chart) are encoded XXX.
Cumulonimbus clouds (CBs) are depicted by enclosed (red) scalloped lines (Figure 8-19).
The identification and characterization of each cumulonimbus 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 cumulonimbus area.
On significant weather (SIGWX) charts, the inclusion of CB or the thunderstorm symbol (Figure 8-3) should be understood to include all weather phenomena normally associated with cumulonimbus or thunderstorm, namely, moderate or severe icing, moderate or severe turbulence, and hail.
Figure 8-19. Mid-Level SIGWX Chart Thunderstorm Examples
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-16
8.3.1.2 Surface Frontal Positions and Movements Surface fronts are depicted using the standard symbols found on the Surface Analysis Chart.
(Figure 8-2). An arrow identifies the direction of frontal movement with the speed indicated in knots plotted near the arrow head (Figure 8-20).
Figure 8-20. Mid-Level SIGWX Chart Surface Frontal Position and Movement Example
8.3.1.3 Jet Streams A jet stream axis with a wind speed of more than 80 knots is identified by a bold green line (Figure 8-21). An arrowhead is used to indicate wind direction. Double-hatched, light green lines positioned along a jet stream axis identify 20 knot wind speed changes.
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 “FL” in hundreds of feet MSL is placed adjacent to each wind symbol to identify the altitude of the jet stream axis.
Jet stream vertical depth (jet depth) forecasts are included when the maximum speed is 120 knots 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.
Figure 8-21. Mid-Level SIGWX Chart Jet stream Example.
Forecast maximum speeds of 100 knots at FL310 at one location and 120 knots at FL290 at another location.
At the latter location, the base of the 80 knot wind field is FL210, and the top of the 80 knot wind field is FL340.
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-17
8.3.1.4 Tropopause Heights Tropopause heights are plotted at selected locations on the chart (Figure 8-22). They are enclosed by rectangles and plotted in hundreds of feet MSL. Centers of high (H) and low (L) tropopause heights are enclosed by polygons and plotted in hundreds of feet MSL.
Figure 8-22. Mid-Level SIGWX Chart Tropopause Height Examples
8.3.1.5 Tropical Cyclones Tropical cyclones are depicted by the appropriate symbol (Figure 8-23) with the storm’s name positioned adjacent to the symbol. Cumulonimbus clouds meeting chart criteria are identified and characterized relative to each storm.
Figure 8-23. Mid-Level SIGWX Chart Tropical Cyclone Examples
8.3.1.6 Moderate or Severe Icing Areas of moderate or severe icing are depicted by enclosed (red) scalloped lines (Figure 8-24).
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 (Appendix J). The vertical extent of the icing layer is specified by top and base heights. Bases which extend below the layer of the chart are identified with XXX.
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-18
Figure 8-24. Mid-Level SIGWX Chart Icing Examples
8.3.1.7 Moderate or Severe Turbulence (in cloud or in clear air) Forecast areas of moderate or severe turbulence associated with wind shear zones and/or mountain waves are enclosed by bold yellow dashed lines (Figure 8-25). Intensities are identified by standard symbols (Appendix J).
The vertical extent of a turbulence layer is specified by top and base heights, separated by a horizontal line. A turbulence base which extends below the layer of the chart is identified with XXX.
Thunderstorm turbulence is not identified.
Figure 8-25. Mid-Level SIGWX Chart Turbulence Examples
Areas of moderate or severe turbulence are also depicted by enclosed (red) scalloped lines (Figure 8-24). 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 associated area.
Standard turbulence symbols are used (Appendix J). The vertical extent of the turbulence layer is specified by top and base heights. Bases which extend below the layer of the chart are identified with XXX.
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-19
8.3.1.8 Cloud Coverage (non-cumulonimbus) Clouds are enclosed within (red) scalloped lines (Figure 8-26). Cloud coverage (noncumulonimbus) appears within or adjacent to the outlined area. If the cloud coverage is adjacent to an outlined area, an arrow points to the appropriate area.
The cloud coverage symbols are listed in Table 8-4. See Table 8-3 for cumulonimbus cloud coverage.
Table 8-4. Mid-Level SIGWX Chart Cloud Coverage (Non-cumulonimbus)
CODED MEANING COVERAGE SKC Sky Clear 0/8ths FEW Few clouds 1/8th to 2/8ths SCT Scattered 3/8ths to 4/8ths BKN Broken 5/8ths to 7/8ths OVC Overcast 8/8ths
8.3.1.9 Cloud Type Table 8-5 shows the contractions used to identify cloud type.
Table 8-5. Mid-Level SIGWX Chart Cloud Types
CODED MEANING CI Cirrus CC Cirrocumulus CS Cirrostratus AC Altocumulus AS Altostratus NS Nimbostratus SC Stratocumulus ST Stratus CU Cumulus CB Cumulonimbus
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-20
Figure 8-26. Mid-Level SIGWX Chart - Example of Moderate or Severe Icing, Moderate or Severe Turbulence (in cloud or in clear air), Clouds, and Cloud Types
8.3.1.10 Volcanic Eruptions Volcanic eruption sites are identified by a trapezoidal symbol (Figure 8-27). The dot on the base of the trapezoid identifies the location of the volcano. The name of the volcano, as well as the latitude and longitude are noted adjacent to the symbol.
Figure 8-27. Mid-Level SIGWX Chart Volcanic Eruption Example
8.3.1.11 Release of Radioactive Materials Radioactive materials in the atmosphere are depicted by the standard symbol shown in Figure
8-28. Information on the chart regarding the radioactive material includes the latitude/longitude of the accident site, the date and time of the accident, and a reference to check NOTAMs for further information.
Figure 8-28. Mid-Level SIGWX Chart Release of Radioactive Materials Example
8.3.2 Issuance The Aviation Weather Center (AWC) in Kansas City has the responsibility, as part of the World Area Forecast Center (WAFC), Washington, to provide global weather forecasts of significant weather phenomena. The AWC issues a 24-hour Mid-Level Significant Weather chart, four times daily, for the North Atlantic Ocean Region (NAT) (Table 8-6). The Mid Level Significant (WIGWX) Chart is found online at: http://aviationweather.gov/products/swm/
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-21
Table 8-6. Mid-Level SIGWX Chart Issuance Schedule North Atlantic Ocean Region (NAT) Valid Times (UTC) Issued 0800 Issued 1400 Issued 2000 Issued 0200
0000 0600 1200 1800
8.3.3 Use The Mid-Level Significant Weather (SIGWX) Chart is used to determine an overview of selected flying weather conditions between 10,000 feet MSL and FL450. It can be used by airline dispatchers for flight planning and weather briefings before departure and by flight crew members during flight.
Aviation Weather Services, Advisory Circular 00-45F, Revision 1 AC 00-45F (July 2008) Section 8: Forecast Charts Page 8-22
Figure 8-29. Mid-Level SIGWX Chart Example