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This nowCOAST™ time-enabled 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 Chesapeake Bay Operational Forecast System (CBOFS) at hourly increments out to 48 hours. The surface water currents velocity maps displays 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 CBOFS 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.
CBOFS is based on the Regional Ocean Modeling System (ROMS), a terrain following numerical ocean or hydrodynamic prediction model. The OFS grid domain covers Chesapeake Bay and immediate continental shelf. CBOFS is a 3-D forecast modeling system with a spatial horizontal resolution ranges from approximately 30 m to 4.9 km and 20 terrain or sigma vertical levels. It has four forecast cycles per day (0000, 0600, 1200, and 1800 UTC) which generate forecast guidance out to 48 hours. The forecast cycle uses 3-hourly forecast guidance of surface winds and radiation fluxes from NCEP North American Mesoscale (NAM) model (4 km resolution) and derived heat fluxes from NAM model meteorological variables for the ROMS 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 a regional tide model. For its 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 the GRTOFS subtidal water levels plus tidal harmonics from tide model. CBOFS was developed and evaluated by NOAA/NOS Coast Survey Development Laboratory Coastal Marine Modeling Branch in collaboration with the NOS Center for Operational Oceanographic Products and Services (CO-OPS). CBOFS 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/cbofs/cbofs_info.html.
The maps are generated using a visualization technique developed by the Data Visualization Research Lab at The University of New Hampshire 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).
This map service is time-enabled, meaning that each individual layer contains time-varying data and can be utilized by clients capable of making map requests that include a time component.
In addition to ArcGIS Server REST access, time-enabled OGC WMS 1.3.0 access is also provided by this service.
This particular service can be queried with or without the use of a time component. If the time parameter is specified in a request, the data or imagery most relevant to the provided time value, if any, will be returned. If the time parameter is not specified in a request, the latest data or imagery valid for the present system time will be returned to the client. If the time parameter is not specified and no data or imagery is available for the present time, no data will be returned.
This service is configured with time coverage support, meaning that the service will always return the most relevant available data, if any, to the specified time value. For example, if the service contains data valid today at 12:00 and 12:10 UTC, but a map request specifies a time value of today at 12:07 UTC, the data valid at 12:10 UTC will be returned to the user. This behavior allows more flexibility for users, especially when displaying multiple time-enabled layers together despite slight differences in temporal resolution or update frequency.
When interacting with this time-enabled service, only a single instantaneous time value should be specified in each request. If instead a time range is specified in a request (i.e. separate start time and end time values are given), the data returned may be different than what was intended.
Care must be taken to ensure the time value specified in each request falls within the current time coverage of the service. Because this service is frequently updated as new data becomes available, the user must periodically determine the service's time extent. However, due to software limitations, the time extent of the service and map layers as advertised by ArcGIS Server does not always provide the most up-to-date start and end times of available data. Instead, users have three options for determining the latest time extent of the service: