11/14/16 Aviation Weather Services AC 00-45H 5-104 Figure 5-46. 300 MB Constant Pressure Forecast—Example
Contours of the height of the 300 MB surface are presented as solid lines. Windbarbs are used to show the direction and speed of the wind. Shading is done for wind speeds greater than 70 kts or greater and generally represents the jet stream. In this example, the jet stream extends from Alaska to Southern California to New England.
5.15.1.1 Issuance.
Constant pressure level forecasts are produced several times a day depending on the model. NCEP’s Global Forecast System (GFS) model and North American Model (NAM) model produce forecasts four times per day, with initial times of 00, 06, 12, and 18 UTC. Other higher resolution models such as the High Resolution Rapid Refresh (HRRR) produce forecasts at hourly intervals.
5.15.1.2 Content.
Constant pressure level forecasts vary in content depending on the selected model and product. Most provide a wind forecast that may be combined with either temperature, relative humidity, or certain derived parameters
(e.g., vorticity).
Many constant pressure levels are available for display depending on the model. For example, the NCEP’s Model Analyses and Guidance Web site (see Figure 5-45) provides displays of NCEP’s GFS model constant pressure levels contained in Table 5-20, Select Constant Pressure Levels from the GFS Model. It should be noted that the levels provided on the Web site are
11/14/16 Aviation Weather Services AC 00-45H 5-105 only a subset of the levels available from the model that are routinely made available to NWS meteorologists and others (e.g., 400 millibars (mb), 600 mb).
Table 5-20. Select Constant Pressure Levels From the GFS Model Constant Pressure Level Approximate Altitude (MSL) 925 MB 2,500 850 MB 5,000 700 MB 10,000 500 MB 18,000 300 MB 30,000 250 MB 34,000 200 MB 39,000
5.15.1.3 Use.
Constant pressure level forecasts are used to provide an overview of weather patterns at specified times and pressure altitudes and are the source for wind and temperature aloft forecasts.
Pressure patterns cause and characterize much of the weather. Typically, lows and troughs are associated with clouds and precipitation while highs and ridges are associated with fair weather, except in winter when valley fog may occur. The location and strength of the jet stream can be viewed at 300 MB, 250 MB, and 200 MB levels.
5.16 Short-Range Surface Prognostic (Prog) Charts.
The NWS’s WPC provides Short-Range Surface Prognostic (Prog) Charts (see Figure 5-47, NDFD Surface Prog Forecast—Example) of surface pressure systems, fronts, and precipitation for a 2 ½-day period. The forecast area covers the CONUS, the coastal waters. The forecasted conditions are divided into five forecast periods: 12, 18, 24, 48, and 60 hours. Each chart depicts a “snapshot” of weather elements expected at the specified valid time.
The Short-Range Surface Prognostic (Prog) Charts combine WPC forecasts of fronts, isobars, and high/low pressure systems with the NWS’s National Digital Forecast Database (NDFD) digital forecasts from NWS WFO.
The Surface Prognostic (Prog) Charts are available at http://www.aviationweather.gov.
11/14/16 Aviation Weather Services AC 00-45H 5-106 Figure 5-47. NDFD Surface Prog Forecast—Example
5.16.1 Content.
5.16.1.1 Precipitation.
The Short-Range Surface Prog Forecast provides precipitation forecasts in the following depiction:
• NDFD Rain (Chance)–(light green)–There is a 25 to less than 55 percent probability of measurable rain (≥0.01”) at the valid time.
• NDFD Rain (Likely)–(dark green)–There is a greater than or equal to 55 percent probability for measurable rain (≥0.01”) at the valid time.
• NDFD Snow (Chance)–(light blue)–There is a 25 to less than 55 percent probability of measurable snowfall (≥0.01” liquid equivalent) at the valid time.
• NDFD Snow (likely)–(dark blue)–There is a greater than or equal to 55 percent probability of measurable snow (≥0.01” liquid equivalent) at the valid time.
• NDFD Mix (Chance)–(light purple)–There is a 25 to less than 55 percent probability of measurable mixed precipitation (≥0.01” liquid equivalent) at
11/14/16 Aviation Weather Services AC 00-45H 5-107 the valid time. “Mixed” can refer to precipitation where a combination of rain and snow, rain and sleet, or snow and sleet are forecast.
• NDFD Mix (Likely)–(dark purple)–There is a greater than or equal to 55 percent probability of measurable mixed precipitation (≥0.01” liquid equivalent) at the valid time. “Mixed” can refer to precipitation where a combination of rain and snow, rain and sleet, or snow and sleet are forecast.
• NDFD Ice (Chance)–(light brown)–There is a 25 to less than 55 percent probability of measurable freezing rain (≥0.01”) at the valid time.
• NDFD Ice (Likely)–(brown)–There is a greater than or equal to 55 percent probability of measurable freezing rain (≥0.01”) at the valid time.
• NDFD T-Storm (Chance)–(red hatching)–There is a 25 to less than 55 percent probability of thunderstorms at the valid time. Areas are displayed with diagonal red hatching enclosed in red border.
• NDFD T-Storm (Likely and/or Severe)–(dark red)–There is a greater than or equal to 55 percent probability of thunderstorms and/or the potential exists for some storms to reach severe levels at the valid time.
5.16.1.2 Symbols.
Figure 5-48. Surface Prog Forecast Symbols
5.16.1.3 Pressure Systems.
Pressure systems are depicted by pressure centers, troughs, isobars, drylines, tropical waves, tropical storms, and hurricanes using standard symbols (see Figure 5-48, Surface Prog Forecast Symbols). Isobars are denoted by solid thin black lines and labeled with the appropriate pressure in millibars.
The central pressure is plotted near the respective pressure center.
5.16.1.4 Fronts.
Fronts are depicted using the standard symbols in Figure 5-48.
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5.16.1.5 Squall Lines.
Squall lines are denoted using the standard symbol in Figure 5-48.
5.16.2 Issuance.
The Short-Range Surface Prognostic (Prog) Forecasts are issued by the WPC in College Park, MD. Table 5-21, Short-Range Surface Prog Forecast Schedule, provides the product schedule. The 12- and 24-Hour Surface Prognostic (Prog). They are available on the WPC Web site.
Table 5-21. Short-Range Surface Prog Forecast Schedule Product Issuance Times (UTC) 6 and 12 hour forecasts 0200 and 1400 18 and 24 hour forecasts 0430 and 1630 30, 36 and 48 hour forecasts 0730 and 1930 60 hour forecast 0900 and 2100
5.16.3 Use.
Short-Range Surface Prognostic (Prog) Forecasts can be used to obtain an overview of the progression of surface weather features during the next 2 ½-days hours. The progression of weather is the change in position, size, and intensity of weather with time.
5.17 Significant Weather (SIGWX) Forecast.
Note: Significant Weather Forecasts may be depicted in monochrome or color display.
The colors used for symbols and as well as the color and style of lines is not standard.
The colors of jet streams, turbulence, cloud cover, and other elements may vary depending on the Web site and service provider. The examples shown in this paragraph are from the NWS AWC’s and AAWU’s Web pages. Users of Significant Weather Forecasts should always refer to any legend or Help Page or user information on the Web site for details on the content and display of the weather information.
5.17.1 Low-Level Significant Weather (SIGWX) Charts.
The Low-Level Significant Weather (SIGWX) Charts (see Figure 5-49, 12-Hour Low-Level SIGWX Chart—Example) provide a forecast of aviation weather hazards primarily intended to be used as guidance products for pre-flight briefings. The forecast domain covers the CONUS and the coastal waters for altitudes Flight Level 240 and below. Each depicts a “snapshot” of weather expected at the specified valid time.
11/14/16 Aviation Weather Services AC 00-45H 5-109 Figure 5-49. 12-Hour Low-Level SIGWX Chart—Example
5.17.1.1 Content.
Low-Level SIGWX Charts depict weather flying categories, turbulence, and freezing levels (see Figure 5-50, Low-Level SIGWX Chart Symbols). In flight icing is not depicted on the Low-Level SIGWX Chart.
Depending on the Web site or service provider, the Low-Level SIGWX Charts may be combined with Surface Prognostic (PROG) Charts to create a four-panel presentation. For example, the left two panels represents the 12-hour forecast interval and the right two panels the 24-hour forecast interval. The upper two panels depict the SIGWX Charts and the lower two panels the Surface Prog.
11/14/16 Aviation Weather Services AC 00-45H Figure 5-50. Low-Level SIGWX Chart Symbols Flight planning only. See TAFs for specific terminal forecast celling less than Luuu tt andror visibility less than 3 miles ceiling 1000-3000 ft inclusive andior visibility 3-5 miles incl moderate or greater turbulence freezing level above mean sea level freezing level at surface TStorms imply possible svr or greater turb, svr icing and LLWS.
MODERATE TURBULENCE IOP (Hundreds of feet MSL) SEVERE TURBULENCE 240/100 Base (Hundreds of feet MSL)
EXTREME TURBULENCE Note: The colors used in the Low-Level Significant Weather Charts may vary depending on the Website or service provider.
5.17.1.1.1 Flying Categories.
IFR areas are outlined with a solid red line, marginal VFR (MVFR) areas are outlined with a scalloped blue line, and VFR areas are not depicted (see Figure 5-51, Low-Level SIGWX Chart Flying Categories-Example).
Figure 5-51. Low-Level SIGWX Chart Flying Categories-Example ef 5-110
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5.17.1.1.2 Turbulence.
Areas of moderate or greater turbulence are enclosed by bold, dashed, brown lines (see Figure 5-52, Low-Level SIGWX Chart Turbulence Forecast— Example). Turbulence intensities are identified by standard symbols (see Figure 5-50). 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 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 5-52. Low-Level SIGWX Chart Turbulence Forecast—Example
5.17.1.1.3 Freezing Levels.
The freezing level at the surface is depicted by a blue, saw-toothed symbol (see Figure 5-53, Low-Level SIGWX Chart Freezing Level Forecast— Example). 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-foot intervals (see Figure 5-53). 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 5-53). The lines are discontinued where they intersect the surface.
11/14/16 Aviation Weather Services AC 00-45H 5-112 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 5-53. Low-Level SIGWX Chart Freezing Level Forecast—Example
Multiple 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.
11/14/16 Aviation Weather Services AC 00-45H 5-113 Figure 5-54. 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 belowfreezing 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 ft freezing level (i.e., below 4,000 ft MSL), but the exact cutoff cannot be determined (see Figure 5-54, Low-Level SIGWX Chart Multiple Freezing Levels—Example).
5.17.1.2 Issuance.
Low-Level SIGWX Charts are issued four times per day by the NWS AWC (see Table 5-22, Low-Level SIGWX Chart Issuance Schedule). Two charts are issued; a 12-hour and a 24-hour prog. Both are available on the http://www.aviationweather.gov.
Table 5-22. 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
5.17.1.3 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.
11/14/16 Aviation Weather Services AC 00-45H 5-114
5.17.2 Mid-Level Significant Weather (SIGWX) Chart.
The Mid-Level Significant Weather (SIGWX) Chart (see Figure 5-55, 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 Mid-Level SIGWX Chart is available on http://www.aviationweather.gov.
Figure 5-55. Mid-Level SIGWX Chart—Example
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5.17.2.1 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 paragraph 5.17.3) except for the addition of Non-Convective clouds with moderate or severe icing and/or moderate or severe turbulence. See paragraph 5.17.3.1 for details on these other weather elements.
5.17.2.1.1 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 5-55). 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 5-23, 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 5-23. Icing and Turbulence Intensity Symbols Intensity Icing Symbol Turbulence Symbol Moderate
Severe
5.17.2.2 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 5-24, Mid-Level SIGWX Chart Issuance Schedule).