Last Revised: February 2016

Map Information

This nowCOAST™ time-offsets map service provides maps depicting the NWS surface wind speed forecasts from the National Digital Forecast Database (NDFD) at 6-hr increments out to 3 days (NDFD has forecasts out to 7 days which are available via the nowCOAST™ time enabled map service for NDFD elements). Each forecast is valid for the specified forecast projection hour with respect to the latest update cycle time. The forecast is valid at 10 m (33 feet) above ground level. The wind speeds are in units of knots (1 knot = 1.15 miles per hour). The wind speed forecast is indicated on the map by different colors for 5-knot increments up to 60 knots (69 mph) and then at 10-knot increments up to 100 knots (115 mph). The forecasts are updated in the nowCOAST™ map service four times per day. For more detailed information about layer update frequency and timing, please reference the nowCOAST™ Dataset Update Schedule.

Background Information

The wind speed is defined as the sustained wind speed for the indicated hour. The term sustained usually refers to a wind speed determined by averaging observed values over a two-minute period.

The NDFD is a seamless composite or mosaic of gridded forecasts from individual NWS Weather Forecast Offices (WFOs) from around the U.S. as well as the NCEP Ocean Prediction Center and National Hurricane Center/TAFB for certain marine weather variables. NDFD has a spatial resolution of 2.5 km (1.6 miles). The time resolution of forecast projections varies by variable (element) based on user needs, forecast skill, and forecaster workload. Each WFO prepares gridded NDFD forecasts for their specific geographic area of responsibility. When these locally generated forecasts are merged into a national mosaic, occasionally areas of discontinuity will be evident. Staff at NWS forecast offices attempt to resolve discontinuities along the boundaries of the forecasts by coordinating with forecasters at surrounding WFOs and using workstation forecast tools that identify and resolve some of these differences. The NWS is making progress in this area, and recognizes that this is a significant issue in which improvements are still needed. The NDFD was developed by NWS Meteorological Development Laboratory.

Time Information

This nowCOAST™ map service is not time-enabled, although it does contain time-varying data. Instead of supporting the time dimension through use of a time parameter in each map request, each individual map layer contains data valid for a different "time offset", or forecast projection, from the dataset's reference time.

Due to software limitations, the full temporal resolution (i.e. maximum forecast horizon and/or all forecast projections) of the data is not provided by this service. Instead, a corresponding time-enabled service containing the full temporal resolution is available for this dataset, and users are highly encouraged to use that service instead, if possible.

This time-offsets map service is provided as a convenience for users who are not yet capable of interacting directly with the time dimension, especially users of legacy nowCOAST™ version 4 map services who wish to access the same data using the new nowCOAST™ version 5 map services. However, this service may be terminated with little advance notice at a later date.

In addition to ArcGIS Server REST access, time-enabled OGC WMS 1.3.0 access is is also provided by this service.

In order to determine the latest time information about the data included in this map service, users have two options:

1. Issue an Identify (ArcGIS REST) or GetFeatureInfo (WMS) request against the proper layer corresponding with the target dataset. For raster data, this would be the "Image Footprints with Time Attributes" layer in the same group as the target "Image" layer being displayed. For vector (point, line, or polygon) data, the target layer can be queried directly. In either case, the attributes returned for the matching raster(s) or vector feature(s) will include the following:
   • validtime: Valid timestamp.
   • starttime: Display start time.
   • endtime: Display end time.
   • reftime: Reference time (sometimes reffered to as issuance time, cycle time, or initialization time).
   • projmins: Number of minutes from reference time to valid time.
   • desigreftime: Designated reference time; used as a common reference time for all items when individual reference times do not match.
   • desigprojmins: Number of minutes from designated reference time to valid time.
2. Query the nowCOAST™ LayerInfo web service, which has been created to provide additional information about each data layer in a service, including a list of all available "time stops" (i.e. "valid times"), individual timestamps, or the valid time of a layer's latest available data (i.e. "Product Time"). For more information about the LayerInfo web service, including examples of various types of requests, refer to the nowCOAST™ LayerInfo Help Documentation

References

• NWS, 2007: National Digital Forecast Database (NDFD) Experimental Gridded Data, Product Description Document, NWS, Silver Spring, MD (Available at http://products.weather.gov/PDD/NDFDGrids.pdf).
• NWS, 2013: Experimental Gridded Marine Offshore and High Seas Forecasts in the National Digital Forecast Database (NDFD) Product Description Document, NWS, Silver Spring, MD.
• NWS, 2013: National Digital Forecast Database Element Definition, NWS, Silver Spring, MD (Available at http://www.nws.noaa.gov/ndfd/definitions.htm).
• NWS, 2014: NDFD Spatial Reference System. NOAA/NWS Meteorological Development Laboratory. (Available at http://graphical.weather.gov/docs/ndfdSRS.htm)