A Closer Look at Global Icing, Turbulence, and Surface Analysis Map Layers

Available with ForeFlight Mobile 9.4, five new weather layers bring global Icing, Turbulence and surface pressure data to the map, with detailed Surface Analysis available to much of North America. These weather products significantly enhance local and global flight planning and your weather decision-making abilities. The new layers are available for customers with Pro Plus, Performance Plus, Business Performance, and MFB Performance subscription plans, on mobile and on the web. In addition, the recently introduced time slider provides frame-by-frame control over animation of the new weather layers. Let’s dive into the details!

Layer Animation Time Slider

The animation Time Slider tool provides better control over weather layer animation, as well as more clarity on the age of the weather product you are viewing. The slider automatically displays when you select any Radar, Satellite, Icing, or Turbulence layer.

Weather Layer Time Slider

In addition to the familiar play/pause button, the Time Slider allows frame-by-frame control over the animation. The slider’s position on the time scale indicates the valid time for the currently displayed weather graphic. Tapping to the right of the slider head advances the layer by one time step, while tapping to the left of the slider head retreats the layer by one time step. Alternatively, tap-hold and move the slider head left and right to manually control animation speed and enable back and forth (rocking) animation, a useful capability when analyzing local storm cell changes on radar.

The time scale changes as appropriate for each weather product displayed. Forecast-based weather layers, such as the Icing and Turbulence layers, use a white vertical bar to indicate the present time, splitting the time slider into two parts: forecasts that are valid for the past (gray line) and for the future (white line).

Products that indicate past information only, such as the radar and satellite layers, are presented with a gray time scale with the product age, relative to the current time, displayed with each frame. The absolute time is displayed in a callout with each frame as you manually scrub left or right, as well as to the left of the time scale.

Providing full manual control over the weather layer animation and easily reading relative age and absolute time for each weather product frame provides better insight to how the weather is trending.

Surface Analysis

The Surface Analysis layer adds a global surface pressure overlay, displaying isobar lines and associated pressure values in millibars. For much of North America, the isobar lines are complemented with depictions of surface fronts, troughs, and high/low pressure center markers. Besides displaying the current surface analysis, the forecasted surface analysis can be viewed for up to two days into the future using the Time Slider.

The Surface Analysis product and its forecast is a collaboration between multiple weather centers and is primarily based on the National Weather Service Global Forecast System (GFS) and the North American Mesoscale (NAM) models, with additional guidance from the European ECMWF and the United Kingdom’s UKMET models. Surface features (fronts, troughs, pressure centers) are analyzed manually by a NWS meteorologist, and are therefore only available for the North America region.

Surface Analysis features follow the standard depiction convention as outlined in the table below:

View global isobars and more detailed weather features for the U.S.

Icing

Icing severity data for the United States has been available in the ForeFlight Mobile Imagery view since 2015. Now, icing severity is available as a dynamic weather layer in the Maps view in ForeFlight Mobile and on the web. And not just for the US, but for the entire world.

You will now see two icing products in the Maps view layer selector: Icing (US) and Icing (Global). Both serve the same purpose, but are based on different weather models.

For the US coverage, the Forecast Icing Product (FIP) is a Numerical Weather Prediction model that employs a 20km grid in the horizontal and a 1000 ft grid in the vertical (from 3000’ to FL450) to calculate icing severity and the potential for SLD (supercooled large droplets). The FIP model is run hourly and forecasts are available out to 18 hours.

For a fast analysis of conditions at each altitude, you can quickly scrub between altitudes using the Altitude Slider in the lower right corner of the Maps view. The Altitude Slider is also available on the Turbulence layer.

Altitude Slider

Altitude Slider

Please note that SLD threat is currently only available to customers flying with the SiriusXM SXAR1 aviation receiver and who are subscribed to the SiriusXM Pilot for ForeFlight plan. SLD threats are depicted with red squares.

The Global Icing product is based on the Global Forecast System (GFS) weather model with a coarser horizontal and vertical grid and is run four times per day (every 6 hours). Global Icing forecasts in ForeFlight are available out to 24 hours.

Because of the coarser grid, as well as the less-frequent model runs of the GFS, it’s worth comparing the two icing depictions side-by-side. The difference in horizontal resolution and model update frequency between the US icing layer (left) and the global icing layer (right) is evident. When operating within the limits of the US icing model coverage area, the US icing layer should be referenced instead of the global layer to take advantage of its frequent updates and finer horizontal and vertical grid resolution.

Regardless of whether you have US or global icing selected, each layer uses the same color scale to depict conditions of no icing, light, moderate and heavy icing, as shown in the legend below:

Icing intensity is based on how long it would take for ice to build up on an airfoil

It’s important to note that the icing severity is roughly based on the accretion rate of ice on an airplane. The severity levels are defined by how long it would take for ¼ inch (65mm) of ice to build up on an airfoil. Time ranges are given for each level because the build-up rate depends on variables like airfoil properties, airspeed, and atmospheric conditions.

Since the icing forecasts are produced with no human modifications, they are intended for flight planning purposes only and should always be used in combination with AIRMETs, SIGMETs and PIREPs.

Turbulence

Similar to the Icing map layer, there are two new Turbulence map layers, one for the United States and the other for Global coverage.

The US turbulence product is based on the Graphical Turbulence Guidance (GTG). The GTG and the associated Eddy Dissipation Rate (EDR) data scale used by GTG were both covered in detail in a previous ForeFlight article. It’s worth re-reading that post to brush up on the turbulence product, especially how the relationship between EDR and in-flight turbulence intensity changes based on aircraft weight. The information in that article applies equally to the US and global turbulence layers.

The US turbulence layer data are still based on GTG-3 with an available lead time of up to 18 hours. This layer displays the maximum EDR of clear air turbulence (CAT) and mountain wave turbulence (MWT). It is not intended to predict convection and thunderstorm turbulence sources, but may provide some guidance if the storm event is widespread. Furthermore, the graphics represent a snapshot at that time and not a forecast for a time range. Finally, it is important to realize that turbulence is a dynamic event and rapidly changing conditions may not be accurately reflected.

The global turbulence layer displays EDR derived from the GFS forecast model using a proprietary algorithm. This layer differs from the US GTG-3 derived layer in that the global turbulence data only forecasts CAT and not MWT. The forecast lead time can be up to 24 hours. The same reduced horizontal and vertical resolutions discussed previously in the global icing section apply to the global turbulence layer. Also, since the global turbulence layer uses GFS model data, the same reduced model run rate limitation applies (every 6 hours for GFS versus hourly for GTG-3), resulting in the US turbulence layer being updated more frequently. For all of these reasons, it’s important to use the US turbulence layer when operating within its coverage area.

Since the layer’s EDR scale is aircraft dependent, it is important to review the scale applicable to your aircraft category (the article linked above contains scales for light, medium, and heavy aircraft classes). The following color scheme is used in the turbulence map layer:

These new weather layers are an exciting and useful feature addition for our US and worldwide customers. Keep in mind that there is no human involvement in creating the turbulence and icing products and the information should be supplemented as much as possible with SIGMETs, AIRMETs, and PIREPs to understand the full weather picture.