Weather products in the Flight Path Tool include grids of various weather parameters as well as NWS textual weather observations and forecasts. Some gridded products are three-dimensional including temperature, relative humidity, winds, icing, and turbulence. Other gridded products are two-dimensional and may represent a “composite” of a three-dimensional weather phenomenon (for instance the convection product) or a surface weather variable such as horizontal visibility. A description of each gridded product is given below. The tool displays relevant NWS textual weather observations and forecasts needed for aviation. These data are either points of observed or forecast weather, often at airports, or regions of hazardous weather represented by two-dimensional polygons.
Temperature, relative humidity, and wind speed are available in four-dimensional space and time. These data are taken directly from the RUC numerical model run at NCEP. They update as an analysis every hour and as a 3, 6, 9, and 12-hour forecast every three hours. The RUC data provides the best possible spatial and temporal resolution. Currently the tool only accesses these fields from RUC data, which covers all of the contiguous United States and parts of Mexico/Canada. In the future, with sufficient funding, the development team desires to include global data with forecasts for multiple days into the future.
The turbulence product combines various weather data sources and numerous turbulence diagnostic metrics into a current analysis and future prediction of upper-level clear-air turbulence. For this reason, the data do not extend below 10000 feet MSL. Future upgrades to the product will address the lower levels and other sources of turbulence including convection and mountain waves. The turbulence data are known as GTG, which was developed by the FAA and NCAR.
The icing severity product combines a multitude of weather observations (temperature, humidity, satellite, observed surface weather and pilot reports, radar data, etc.) to diagnose areas of expected trace, light, moderate, and heavy icing. Separate overall icing probability and SLD potential from 5 to 85% are provided as well. These products originate from the Current Icing Product developed by the FAA and NCAR.
The next two weather products depict the latest radar data with each product updating approximately every 5 minutes. There are roughly 150 individual NEXRAD WSR-88D radars in the United States collecting data every 5 to 10 minutes. These are collected centrally and stitched together by various entities. Two radar depictions are provided because each source is currently considered experimental and each is derived from variations in the original source data. One is labeled base reflectivity and will often show ground clutter as well as weather features. This product primarily captures the radar echo nearest the surface. No attempt has been made to remove any non-weather features. The alternative product is the composite reflectivity that attempts to remove non weather features from the data. Both radar products show data with the best possible resolution of approximately 1 square kilometer grid boxes. Due to limitations of radar technology, blockage by mountains, and the spacing of radars, there are weather events when radar data does not detect or show a complete weather picture. The most commonly seen example is very shallow clouds with light precipitation like freezing drizzle or snow. An excellent web site with more information concerning radar technology and limitations is found at Radar FAQ.
Figure 1: Radar Base (top) and Composite (bottom) Reflectivity
Following the radar products are three satellite products, satellite visible, satellite infrared, and satellite water vapor. Visible refers to what typical cameras and the human eye would see. The infrared channel is especially useful at night and can be used together with temperature data to determine the height of the top of clouds. The water vapor channel indicates the moisture amounts in the upper atmosphere and does not directly show clouds. Regions that appear in the color scale stated as dry (orange/red colors) are very likely to be clear skies above FL180 whereas the green/blue/magenta colors are extremely likely to be cloud filled above FL180. In particular, the blue and green colors are very likely to indicate clouds above FL300. Intermediate values shown in gray shades could be either cloudy or clear above FL180. Even the driest zones shown in the darkest shade of red may have clouds below 10000 feet MSL.
The remaining trio of weather products, ceiling, visibility, and flight category originate from the CVA product developed by the FAA and NCAR. The ceiling and visibility are used together to classify the flight category as VFR or IFR. This set of rules is commonly used throughout the aviation industry. These three CVA products derive from all available observations including surface weather reports and satellite data and update approximately every 5 minutes. Due to limitations of the observations, the grid cells are approximately 5 kilometers apart at best. In data sparse regions, the best possible estimate of ceiling and visibility is assumed from the nearest surrounding data and may not represent the actual conditions at a specific point.
Figure 2: Flight Category