Chapter 27, Forecasts 27-45
Forecast maximum speeds of 100 kt at FL310 at one location
and 120 kt at FL290 at another location. At the latter location,
the base of the 80-kt wind field is FL210, and the top of the
80-kt wind field is FL340.
Figure 27-19. High-Level SIGWX Chart Jet Stream—Example
27.9.3.2.5 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 27-20). Centers of high ( H) and low ( L) tropopause heights
are enclosed by polygons and plotted in hundreds of feet MSL.
Figure 27-20. High-Level SIGWX Chart Tropopause Height—Examples
27.9.3.2.6 Tropical Cyclones
Tropical cyclones (i.e., with surface wind speed 34 kt or greater) are depicted by the symbol in Figure 27-21
with the storm’s name positioned adjacent to the symbol. Cumulonimbus clouds meeting chart criteria are
identified and characterized relative to each storm.
Chapter 27, Forecasts 27-46
Figure 27-21. High-Level SIGWX Chart Tropical Cyclone—Examples
27.9.3.2.7 Volcanic Eruption Sites
Volcanic eruption sites are identified by a trapezoidal symbol depicted in Figure 27-22. 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 27-22. High-Level SIGWX Chart Volcanic Eruption Site—Example
27.9.4 Alaska Significant Weather (SIGWX) Charts
The Alaska SIGWX Charts (see Figure 27-23) 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.
27.9.4.1 Issuance
The AAWU issues the Alaska SIGWX Charts (see Figure 27-23). 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 aro und 2145/2045 UTC valid at
1200 UTC the next day.
Chapter 27, Forecasts 27-47
Figure 27-23. Alaska SIGWX Chart—Example
27.9.4.1.1 Content
27.9.4.1.1.1 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.
27.9.4.1.1.2 Areas of IFR and MVFR Weather Conditions
Areas of forecast IFR and MVFR conditions are shown in red and blue hatching, respectively.
27.9.4.1.1.3 Freezing Levels
Forecast freezing levels are depicted for the surface (dashed red line) and at 2,000-ft intervals (dashed
green lines).
Note: Areas of in-flight icing forecasts are not included in the Alaska SIGWX forecasts.
27.9.4.1.1.4 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 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.
Chapter 27, Forecasts 27-48
27.9.4.1.1.5 Thunderstorms
Areas of forecast thunderstorms are depicted with red dots. Thunderstorm areal coverage, cloud bases, and
tops are not included.
27.10 Short-Range Surface Prognostic (Prog) Charts
The NWS WPC provides Short-Range Surface Prog Charts (see Figure 27-24) of surface pressure systems,
fronts, and precipitation for a multiday period. The forecast area covers the CONUS and coastal waters.
The forecasted conditions are divided into selected forecast valid time periods. Each chart depicts a
“snapshot” of weather elements expected at the specified valid time.
The Short-Range Surface Prog Charts combine WPC forecasts of fronts, isobars, and high/low pressure
systems with the NWS’ National Digital Forecast Database (NDFD) digital forecasts from the NWS WFO.
The Short-Range Surface Prog Forecasts are issued by the WPC in College Park, MD.
Figure 27-24. NDFD Short-Range Surface Prog Forecast—Example
Chapter 27, Forecasts 27-49
27.10.1 Content
27.10.1.1 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 in) 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 in) at the valid time.
• NDFD Snow (Chance—light blue): There is a 25 to less than 55 percent probability of measurable
snowfall (≥0.01 in 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 in 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 in liquid equivalent) at the valid time. “Mix” 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 in liquid equivalent) at the valid time. “Mix” 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 in) 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 in) 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 a
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.
27.10.1.2 Symbols
Figure 27-25 shows the Surface Prog Forecast symbols.
Chapter 27, Forecasts 27-50
Figure 27-25. Surface Prog Forecast Symbols
27.10.1.3 Pressure Systems
Pressure systems are depicted by pressure centers, troughs, isobars, drylines, tropical waves, tropical
storms, and hurricanes using standard symbols (see Figure 27-25). 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.
27.10.1.4 Fronts
Fronts are depicted using the standard symbols in Figure 27-25.
27.10.1.5 Squall Lines
Squall lines are denoted using the standard symbol in Figure 27-25.
27.11 Upper Air Forecasts
NWP models, run on supercomputers, generate surface and upper air forecasts, known as “Model
Guidance,” to meteorologists. The NWS NCO runs several models daily and produces hundreds of surface
and upper-air guidance products, valid from model run time (i.e., 00 -hour) out to several days or weeks
(e.g., 340 hours after model run time) depending on the model . Their “Model Analyses and Guidance ”
website (see Figure 27-26) contains a User’s Guide as well as a Product Description Document that provides
details on the various products.
Chapter 27, Forecasts 27-51
A User’s Guide, located below the image (lower left), provides descriptions, details, and examples of the
various products.
Figure 27-26. NWS NCO Model Analyses and Guidance Website
27.11.1 Constant Pressure Level Forecasts
Constant pressure level forecasts (see Figure 27-27) are just one of the many products produced by NWP
models. Constant pressure level forecasts are the computer model’s depiction of select weather (e.g., wind)
at a specified constant pressure level (e.g., 300 mb), along with the altitudes (in meters) of the specified
constant pressure level. When considered together, constant pressure level forecasts describe the 3D aspect
of pressure systems. Each product provides a plan-projection view of a specified pressure altitude at a given
forecast time.
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 str ength of the jet stream can be viewed at 300 mb, 250 mb,
and 200 mb levels.
Chapter 27, Forecasts 27-52
Contours of the height of the 300 mb surface are presented as solid lines. Wind barbs are used to show the
direction and speed of the wind. Shading is done for wind speeds greater than 70 kt and generally
represents the jet stream.
Figure 27-27. 300 mb Constant Pressure Forecast—Example
27.11.1.1 Issuance
Constant pressure level forecasts are produced several times a day depending on the model. The NCEP’s
GFS model and North American Model (NAM) 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.
27.11.1.2 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 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” website (see Figure 27-26) provides displays of the NCEP’s GFS model
constant pressure levels contained in Table 27-14. It should be noted that the levels provided on the website
are only a subset of the levels available from the model that are routinely made available to NWS
meteorologists and others (e.g., 400 mb, 600 mb).