Map Information

This nowCOAST™ time-offsets map service provides maps of the latest nowcasts and forecast guidance of water temperature, salinity, water currents, and combined (tides + wind driven) water level from the NOS Northern Gulf of Mexico Operational Forecast System (NGOFS) at 6-hr increments out to 48 hours. Each forecast is valid for the specified forecast projection hour with respect to the latest forecast cycle time (e.g., 0, 6, 12 or 18 UTC). The surface water currents velocity maps display the direction using white or black streaklets depending on the color background. Four color backgrounds are available: sea surface water temperature, surface salinity, speed of currents, and water level. For water temperature, a color scale from dark blue to medium orange is used to display temperatures from about 28 degrees F to 88+ degrees F at 2 degree increments. For salinity, a color scale from black to dark red is used to display salinity values from 0 to 40 ppt at 4 ppt increments. Colors were chosen to highlight approximate thresholds for freshwater, brackish water, and saline waters. For speed, a color scale from black to gray is used to depict speeds from 0 to 4 knots at 0.5 knot increments, except for 0.25 knot intervals from 0 to 0.5 knots. For water level, a color scale from pink to red is used to depict water levels from -14 ft to +12 ft at mainly 0.5 ft increments. The water levels are referenced to the OFS model sea level and not to MLLW, MSL, or NAVD88. The maps of NGOFS forecast guidance 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

NGOFS is based on the Finite Volume Coastal Ocean Model (FVCOM), a numerical ocean circulation or hydrodynamic prediction model. The OFS grid domain covers Northern Gulf of Mexico from the U.S.-Mexico border to Mobile Bay. NGOFS is a 3-D forecast modeling system with a spatial horizontal resolution ranging from approximately 10 km on the open ocean boundary to 600 m near the coast and 200-300 m along navigational channels within bays. NGOFS has 40 vertical levels. It has four forecast cycles per day (0300, 0900, 1500, and 2100 UTC) which generate forecast guidance out to 48 hours. The forecast cycle uses 3-hourly forecast guidance of surface winds radiation fluxes from NCEP North American Mesoscale (NAM) model (4 km resolution) and derived heat fluxes from NAM model meteorological variables for the FVCOM surface meteorological forcing. For forcing at the lateral ocean boundary, the OFS uses water temperature and salinity forecast guidance from NCEP Real-Time Ocean Forecast System (GRTOFS) and sub-tidal water level forecast guidance from GRTOFS along with tidal harmonics derived from the regional tide model via the ADCIRC ec2011 database. For river boundary conditions, it uses near-real-time discharges from USGS river gages and near-real-time water temperature observations from USGS gages and/or NOS gages. Each forecast cycle is preceded with a 6-hr long nowcast cycle. The final nowcast serves as the initial conditions for each forecast cycle. The nowcoast cycle is driven by very short-range forecasts of surface wind, radiation fluxes, and derived heat fluxes from the NAM model, persisted river observations, and GRTOFS subtidal water levels plus tidal harmonics from the tide model. NGOFS was developed and evaluated by the NOAA/NOS Coast Survey Development Laboratory Coastal Marine Modeling Branch in collaboration with the NOS Center for Operational Oceanographic Products and Services (CO-OPS) and the University of Massachusetts-Dartmouth. NGOFS became operational in March 2012. NGOFS is run on the NOAA Weather and Climate Operational Supercomputer System (WCOSS) operated by NCEP Central Operations. For more information, please see http://tidesandcurrents.noaa.gov/ofs/ngofs/ngofs_info.html.

The maps are generated using a visualization technique developed by the Data Visualization Research Lab at The University of New Hampshire's Center for Coastal and Ocean Mapping (http://www.ccom.unh.edu/vislab/). The method combines two techniques. First, equally spaced streamlines are computed in the flow field using Jobard and Lefer's (1977) algorithm. Second, a series of "streaklets" are rendered head to tail along each streamline to show the direction of flow. Each of these varies along its length in size, color and transparency using a method developed by Fowler and Ware (1989), and later refined by Mr. Pete Mitchell and Dr. Colin Ware (Mitchell, 2007).

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

• Fowler, D. and C. Ware, 1989: Strokes for Representing Vector Field Maps. Proceedings: Graphics Interface '98 249-253.
• Jobard, B and W. Lefer, 1977: Creating evenly spaced streamlines of arbitrary density. Proceedings: Eurographics workshop on Visualization in Scientific Computing. 43-55.
• Mitchell, P.W., 2007: The Perceptual optimization of 2D Flow Visualizations Using Human in the Loop Local Hill Climbing. University of New Hampshire Masters Thesis. Department of Computer Science.
• NOS, 2011: Implementation of National Ocean Service new Oceanographic Forecast Modeling System for Northern Gulf of Mexico, Technical Implementation Notice 12-02, NOS, Washington, DC (Available at http://www.nws.noaa.gov/om/notification/tin12-02nos-ngofs.htm).
• Wei., E., Z. Yang, Y. Chen, J. Kelley, and Z. Zhang, 2013: The Northern Gulf of Mexico Operational Forecast System (NGOFS): Model Development and Skill Assessment. NOAA Technical Report NOS CS3, NOAA/NOS/CSDL, Silver Spring, MD, 202 pp.