5-136 Figure 5-71. Alaska Significant Weather Chart—Example
5.19.4.2 C ontent.
5.19.4.2.1 Surface Pressure Systems and Fronts.
Pressure systems and fronts are depicted using standard 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.
5.19.4.2.2 Areas of IFR and MVFR Weather Conditions.
Areas of forecast IFR and MVFR conditions are shown in red and blue hatching, respectively.
5.19.4.2.3 Freezing Levels.
Forecast freezing levels are depicted for the surface (dashed red line) and at 2,000-foot intervals (dashed green lines).
Note: Areas of in-flight icing forecasts are not included in the Alaska SIGWX forecasts.
5.19.4.2.4 Low-Level Turbulence.
Areas of forecast moderate or greater Non-Convective low-level turbulence are depicted with black dots. Turbulence altitudes are not included but can be
5-137 considered as turbulence that is near the surface as a result of wind interactions with the terrain. In most cases, it would be within 6,000 ft above the terrain.
5.19.4.2.5 Thunderstorms.
Areas of forecast thunderstorms are depicted with red dots. Thunderstorm areal coverage, cloud bases, and tops are not included.
5.19.4.2.6 Use.
These charts provide a graphical overview of the specified forecast weather primarily for lower flight altitudes.
5.20 World Area Forecast System (WAFS).
ICAO’s WAFS supplies aviation users with global aeronautical meteorological en route forecasts suitable for use in flight-planning systems and flight documentation.
Two WAFCs, WAFC Washington and WAFC London, have the responsibility to issue the WAFS forecasts. WAFC Washington is operated by the NWS’s NCO in College Park, MD, and the AWC in Kansas City, MO. WAFC London is operated by the United Kingdom’s Meteorological Office in Exeter, United Kingdom.
5.20.1 WAFS Forecasts.
Both WAFC Washington and WAFC London issue the following WAFS forecasts in accordance with ICAO Annex 3, Meteorological Service for International Air Navigation.
• Global forecasts of:
o Upper wind and temperature (i.e., wind and temperature aloft, which is also issued in chart form for select areas);
o Upper-air humidity;
o Geopotential altitude of flight levels;
o Flight level and temperature of tropopause (i.e., tropopause forecast);
o Direction, speed, and flight level of maximum wind;
o Cumulonimbus clouds;
o Icing; and o Turbulence.
• Global foreca sts of Significant Weather (SIGWX), i.e., High-Level Significant Weather (SIGWX) forecasts (see paragraph 5.19.3).
• Select regional areas of Medium-Level Significant Weather (SIGWX) forecasts (see paragraph 5.19.2).
Note: See paragraphs 5.19.2 and 5.19.2.1 for details on the WAFS High-Level and Medium-Level SIG WX forecasts.
5-138
5.20.1.1 Issuance.
The WAFS forecasts of upper wind, temperature, and humidity; direction, speed, and flight level of maximum wind; flight level and temperature of tropopause; areas of cumulonimbus clouds; icing; turbulence; and geopotential altitude of flight levels are issued four times a day by both WAFC Washington and WAFC London.
These forecasts are produced from weather computer models and are not modified by WAFC forecasters. WAFC Washington’s is from the GFS model.
These forecasts are issued in grid-point format, i.e., WMO GRIB2 format.
The grid-point horizontal resolution for the WAFS forecast is 1.25 degree latitude by 1.25 degree longitude. This is equivalent to a 75 x 75 NM grid box at the equator with the grid boxes becoming progressively smaller toward the poles.
These forecasts are valid for fixed valid times at 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, and 36 hours after the time (0000, 0600, 1200, and 1800 UTC) on which the forecasts were based.
5.20.1.2 WAFS Wind and Temperature.
Wind and temperature forecasts are issued for flight levels (FL) 050 (850 MB), 100 (700 MB), 140 (600 MB), 180 (500 MB), 240 (400 MB), 270 (350 MB), 300 (300 MB), 320 (275 MB), 340 (250 MB), 360 (225 MB), 390 (200 MB), 410 (175 MB), 450 (150 MB) and 530 (100 MB).
Note: ICAO uses flight levels below 18,000 MSL for global weather products.
WAFC wind and temperature forecasts use a plotting model where the air temperature (degrees Celsius) is the center of the data point and the wind direction and speed follows the standard model (see Figure 5-72, WAFS Wind and Temperature 6-Hour Forecast at FL390—Example) with the exception that wind speed for points in the Southern Hemisphere is flipped. Note the data points do not correspond to any airports or reference points with names or identifiers.
WAFS global wind and temperature forecasts are provided in grid point format (e.g., computer format) for use in flight-planning systems. Chart format is also provided on the AWC’s Web page (under “Flight Folder”).
Figure 5-72. WAFS Wind and Temperature 6-Hour Forecast at FL390—Example PWNB20 WORLD AREA FORECAST CENTER WASHINGTON A 694 -70 FL 390 WINDS/TEMPERATURES OOUTC MAR MAR 31 312 2015
HIND SPEEDUNLESS POSITIVES UNSIGNED 145914601 60N 60N 60N + 584-544-531 541-97458 4 95 99 49-50 GOW 40W gs 13810561 5 50N W62 4L60 9 59 85 57-60 61 61-60 474998535861164 65 62 40N se s" 51 38 60 50 9o 6,26 565556 15961 62 64 ''5911-604-60 61 30N 3ON 59 60 616015 458 157 * 55 11 52 2ON 55 5611-58 51 52483 55 85 55 A 20N ' 83"59 848-85-89183 51 51 1-51-52 54 kU54|56 A:-55 4!
10N 10N ' 2'52 52'52 8353-8352 A251 4 52 4l1:52 All 52 A53 A53 AlL53 4 4524 524 524 834831 82182152,5 535355353 52-32/83 53m 531154+54r 545-55755 54 11 51177-527-53 5555 205 99 99 387-82-86 56 55 1 50525453 54177-5455 30S 555-567758 11521 53977753 1 " 52 mistr 52 17-55 07 57 1 58 40S 52 5 83 51 99 49 rr 52 r551/58 771555-568 59-61 2 1 4987-4787-461 477 50S "33160~60W 150 190132 204 95 801-42 1757 5-139
5-140
5.20.1.3 Humidity, Maximum Wind, Tropopause Forecasts.
No specific charts are issued for global upper-air humidity, maximum wind, height of tropopause, and altitude of flight levels. These products are provided in grid point format (e.g., computer format) for use in flight-planning systems.
Data from these forecasts are used by the WAFC forecasters to produce the High-Level and Medium-Level SIGWX forecasts, which contain tropopause and jet stream forecasts.
Humidity data is produced for FLs 50 (850 MB), 100 (700 MB), 140 (600 MB), and 180 (500 MB).
5.20.1.4 WAFS Tur bulence, Icing, and Cumulonimbus Cloud Forecasts.
WAFS global turbulence, icing, and cumulonimbus cloud forecasts are provided in grid point format (e.g., computer format) for use in flight-planning systems, but the AWC does make these available on their website in Web display format and not chart format. The Web display allows the user to select various products and flight levels and view the forecasts as single time steps or in a movie loop sequence. More detailed information is provided on https://www.aviationweather.gov under WAFS Forecasts.
The WAFS global turbulence, icing, and cumulonimbus cloud forecasts are actually a blend of the WAFC Washington global turbulence, icing, and cumulonimbus cloud forecasts and the WAFC London global turbulence, icing, and cumulonimbus cloud forecasts. In other words, each WAFC produces their own global turbulence, icing, and cumulonimbus cloud forecasts using their own global computer models (WAFC Washington uses NCEP’s GFS model). The two WAFC’s forecasts, for turbulence, icing, and cumulonimbus cloud only, are then merged together to eliminate any differences between the two sets of forecasts.
WAFC s provide written guidance material on these WAFS forecasts, available on ICAO’s Meteorology Web page. Consult the Guidance on the Harmonized WAFS Grids for Cumulonimbus Cloud, Icing and Turbulence Forecasts - 11 September 2012 at http://www.icao.int/safety/meteorology/WA FSOPSG/Pages/GuidanceMaterial.aspx.
5.20.1.4.1 WAFS Turbulence.
Two kinds of turbulence forecasts are provided: Clear Air Turbulence (CAT) and in-cloud turbulence.
Clear Air Turbulence.
As of 2015, WAFS CAT forecasts are derived from an algorithm that is based on the Ellrod Index. The Ellrod Index results from an objective technique for forecasting CAT. The index is calculated based on the product of horizontal deformation and vertical windshear derived from numerical model forecast winds aloft. Future upgrades to this product are planned, which may include
5-141 replacing the Ellrod Index with global version of the GTG forecast (see paragraph 5.21.2).
The theoretical limit to the data range is zero to 99, but over 98 percent of the values will be below 11, and they will rarely exceed 40. The numbers are not a probability but are instead a potential of encountering turbulence of any severity. The WAFCs suggests that a value of 6 should be considered as a threshold for moderate or greater turbulence. AWC’s WAFS website provides a display (see Figure 5-73, WAFS CAT Forecast—Example) of WAFS forecasts, which includes CAT forecasts with values from 5 to 35.
CAT forecasts are produced for six vertical l ayers, each having a depth of 50 MB (see Table 5-28, WAFS Clear Air Turbulence Forecasts). Approximate equivalent flight levels for each of the layers are shown in the right column of the table. AWC’s WAFS website provides a display of WAFS forecasts, which includes CAT forecasts.
Table 5-28. WAFS Clear Air Turbulence Forecasts Layer centred at (MB) Layers from (MB) Approximate Flight level 150 125-175 410-480 200 175-225 360-410 250 225-275 320-360 300 275-325 280-320 350 325-375 250-280 400 375-425 220-250
Figure 5-73. WAFS CAT Forecast-Example World Area Forecast System Data INFO Unofficial Display of WAFS Global Grids 6hr Forecast valid 12UTC Mon Mar 16 201!
Looping: << Loop Layers:
Overlays: • Layer-Top CAT FL300/300mb Opacity: 100% 10 15 20 25 30 • 35 FIR OLINES • Layer-Mid CAT FL300/300mb Opacity: 100% $ 5 10 15 20 25 30 35
• Layer-Low CAT FL300/300mb • Opacity: 100% • 5 10 15 20 25 30 35 WAFS CAT 6-hour forecast for a layer centered at FL300 (300MB), i.e., from FL280 to FL320, for values from 5 (yellow shade) to 35 (red shade).
In-Cloud Turbulence.
The in-cloud turbulence algorithms are based on the model indicating the presence of a cloud and the change in potential energy with height, which is a measure of instability. The range of values in the data is from 0 to 1 and is a potential for encountering in-cloud turbulence. AWC's WAFS website provides a display of WAFS forecasts, which includes in-cloud turbulence.
In-cloud turbulence values from .005 to .035 are displayed (see Figure 5-74, WAFS Turbulence Forecast-Example).
In-cloud turbulence forecasts are produced for five vertical layers, each having a depth of 100 MB. Approximate equivalent flight levels for each of the layers are shown in the right column of Table 5-29, WAFS In-Cloud Turbulence Forecasts.
5-142
5-143 Table 5-29. WAFS In-Cloud Turbulence Forecasts Layers centred at (MB) Layers from (MB) Approximate Flight level 300 250-350 270-340 400 350-450 210-270 500 450-550 160-210 600 550-650 120-160 700 650-750 080-120 Figure 5-74. WAFS Turbulence Forecast—Example
WAFS In-Cloud Turbulence 6-hour forecast (purple shade) for a layer centered at FL240 (400MB), i.e., from FL210 to FL270.
5.20.1.4.2 Icing.
The icing forecasts are derived from algorithms that are based on a combination of cloud condensate (ice and water), temperature, relative humidity, and vertical motion parameters that predict the presence of supercooled liquid droplets. The output is in a value range from 0 to 1 and is a potential for the presence of icing. The higher the value, the greater the risk of encountering icing. Future upgrades are planned to this model to change the output to indicate the severity of icing.
WAFS global icing forecasts are produced for six vertical layers, each having a depth of 100 MB (see Table 5-30, WAFS Global Icing Forecasts).
Approximate equivalent flight levels for each of the layers are given in the column of the Table 5-30. AWC's WAFS website provides a display of icing forecasts using a threshold of 0.7 or greater (see Figure 5-75, WAFS Icing Forecast-Example).
Table 5-30. WAFS Global Icing Forecasts Layer centered at (MB) Layers from (MB) Approximate flight levels 300 250-350 270-340 400 350-450 210-270 500 450-550 160-210 600 550-650 120-160 700 650-750 080-120 800 750-850 050-080 Figure 5-75. WAFS Icing Forecast-Example World Area Forecast System Data INFO Unofficial Display of WAFS Global Grids 6hr Forecast valid 12UTC Mon Mar 16 2015 _ooping: << Loop Overlays: • Layer-Top Icing FL140/600mb • Opacity: 100% D б.7 0.8 0.9 1.0 ) FIR outlines • Layer-Mid Icing FL140/600mb • Opacity: 100% € 6.7 0.8 0.9 1.0
• Layer-Low Icing FL140/600mb © Opacity: 100% € 0.7 0.8 0.9 1.0 A 6-hour forecast of icing (blue shade) for a layer centered at FL140 (600MB), i.e., from FL120 to FL160, using a threshold of 0.7 or greater.
5-144
5-145
5.20.1.4.3 Cumulonimbus Cloud (CB) Forecasts.
CB cloud forecasts are based on an algorithm gives information relating to base, top, and horizontal extent (coverage) of any expected CB clouds. The algorithm is based on Convective rainfall rates.
The horizontal extent component is expressed as a value between 0 and 1, representing the fraction of sky covered by CB cloud within a grid box.
A value of 0.5 implies 50 percent coverage of CB cloud in that grid box (a grid box is 1.25 degrees latitude by 1.25 degrees longitude in size).
Where CB clouds are forecast to exist, a base and a top of the CB cloud is provided, represented by a height that can be converted into a flight level.
CB base and top forecasts are provided in meters above MSL. AWC’s WAFS website provides a display of CB cloud forecasts with the CB tops shown in flight levels (see Figure 5-76, WAFS CB Cloud Forecast—Example).
Figure 5-76. WAFS CB Cloud Forecast—Example
A 6-hour forecast of CB clouds (grey shade) where coverage is more than 0.3, i.e., 30 percent.
5-146
5.20.1.5 Use.
WAFS forecasts are for use in flight planning and flight documentation for international air navigation.
5.21 Additional Products for Icing and Turbulence.
SIGMETs and AIRMETs are produced by NWS forecasters and are the advisory products for turbulence and icing information for aviation users. The NWS also produces icing and turbulence products that are derived from NWP model data with no forecaster modifications. This paragraph provides information on the following icing and turbulence products:
• Current Icing Product (CIP);
• Forecast Icing Product (FIP); and
• Graphical Turbulence Guidance (GTG).
By design, the CIP, FIP, and GTG do not use the intensity level severe. Pilots should always refer to SIGMETs for information on severe icing and severe turbulence.
CIP and FIP products contain a heavy intensity level. Heavy icing has been defined, in part, as the rate of ice accumulation that requires maximum use of ice protection systems to minimize ice accretions on the airframe. Immediate exit from the conditions should be considered. Heavy icing is not an intensity level used in PIREPs.
These icing and turbulence products will continue to evolve over the coming years with increased model resolutions, additional horizontal layers, and improvements to the algorithms and/or data sets used to produce the product. Along with these improvements may come a slight change in references to the product update version (e.g., GTG to GTG2, GTG2 to GTG3). Users can find additional information on these products and any changes on the AWC’s icing and turbulence Web pages.
5.21.1 Current and Forecast Icing Products (CIP/FIP).
The CIP combines sensor and NWP model data to provide an hourly three-dimensional diagnosis of the icing environment. This information is displayed on a suite of graphics available for the CONUS, much of Canada and Mexico, and their respective coastal waters. The FIP provides the same suite of products describing the icing environment in the future and is solely NWP model based. The CIP/FIP product suites are automatically produced with no human modifications. Information on the graphics is determined from NWP model output; observational data, including WSR-88D radar; satellite, PIREPs, and surface weather reports; and lightning network data.
Note: CIP is sometimes referred to as an analysis, but it is actually a 0-hour forecast.
5.21.1.1 Issuance.
The CIP and FIP product suites are issued hourly about 15 minutes after the hour by the AWC. The products are available on their website. Gridded
5-147 versions of these products are also delivered for users who wish to integrate CIP and FIP products with flight-planning systems.
5.21.1.2 Content.
The CIP/FI P product suite as appears on https://www.aviationweather.gov consists of five graphics, including:
• Icing Probability;
• Icing Severity;
• Icing Severity—Probability > 25 percent;
• Icing Severity—Probability > 50 percent; and
• Icing Severity plus Supercooled Large Drop (SLD).
The CIP/FIP products are generated for individual altitudes from 1,000 ft MSL to FL 300 at intervals of 1,000 ft. FIP products are available at 1, 2-, 3-, 6-, 9-, 12-, and 18-hour forecast lead times.
The CIP/FIP products can be viewed at single altitudes and FLs or as a composite of all altitudes from 1,000 ft MSL to FL300, which is referred to as the maximum or max. Single altitudes are referenced to MSL from 1,000 to 17,000 ft and FLs above 17,000 ft. The AWC’s website allows for access to every other altitude (1,000 FT, 3,000 FT, 5,000 FT, etc.). However, vertical cross sections and presentation of all altitudes can be viewed using the Flight Path Tool (see paragraph 6.2) on https://www.av iationweather.gov.
Icing PIREPs are plotted on a single altitude CIP graphic if the PIREP is within 1,000 ft of the selected altitude and has been observed within 75 minutes of the chart’s valid time. Icing PIREPs for all altitudes
(i.e., 1,000 ft MSL to FL300) are displayed, except negative reports are omitted to reduce clutter. The PIREP legend is located on the bottom of each graphic.
5.21.1.2.1 Icing Probability.
The icing probability product displays the probability of icing at any level of intensity. Probabilities range from 0 percent (no icing expected) to 85 percent or greater (nearly certain icing). The product is available on https://www.aviationweather.gov in single altitudes, e.g., 3,000 ft MSL (see Figure 5-77, CIP/FIP Icing Probability (3,000 Feet MSL)—Example), or a composite of all altitudes from 1,000 ft MSL to FL300, i.e., max level (see Figure 5-78, CIP/FIP Icing Probability—Max Example).