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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 Chesapeake Bay Operational Forecast System (CBOFS) 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 direction of the water currents are depicted by white streaklets. The surface water currents velocity maps displays the direction using white or black streaklets depending on the color background. The magnitude of the current is indicated by the length and width of the streaklet. 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 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: