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nowcoast/guidance_model_greatlakes_leofs_offsets (MapServer)

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Service Description:
Last Revised: May 2016

Map Information

This nowCOAST™ time-offsets map displays the latest nowcasts and forecast guidance of water temperature, water currents, and water level guidance from the NOS Lake Erie Operational Forecast System (LEOFS) at 6-hr increments out to 60 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 magnitude of the current is indicated by the length and width of the streaklet as well as by the color-coded speed background. The water levels are referenced to Low Water Datum of IGLD85. The maps of LEOFS 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

LEOFS is a 3-D lake forecast modeling system providing updated nowcasts and short-range forecasts guidance of water temperatures, currents, and water levels for Lake Erie. LEOFS is based on the Finite Volume Community Ocean Model (FVCOM), a numerical ocean circulation or hydrodynamic prediction model. The model has a horizontal resolution ranging from approximately 3.5 km offshore to 400 m near the shore and has 20 terrain-following or sigma levels. LEOFS has four forecast cycles per day (0000, 0600, 1200, and 1800 UTC) which generate forecast guidance out to 120 hours, however nowCOAST presently only displays the output out to 60 hours. The forecast cycle uses meteorological forecasts from the NWS National Digital Forecast Database (NDFD) which has a horizontal resolution of 2.5 km. Each forecast cycle is preceded with a nowcast cycle, which uses the 2-hr forecast guidance from the NWS High Resolution Rapid Refresh (HRRR) numerical weather prediction system. The HRRR system has a horizontal resolution of 3 km. Lateral boundary conditions for water levels are provided by near-real-time water level observations from NOS gages at Gibralter, MI (near Detroit River) and Buffalo, NY (near the Niagara River). An offset for each open boundary is applied to the observed water level to account for the river channel representation and/or distance between gage and boundary location on the grid. The lateral boundary conditions for water temperatures are provided by a USGS ADCP located in the Detroit River near Fort Wayne, MI. The boundary conditions for the forecast cycle are persisted from the nowcast cycle. The latest nowcast serves as the initial conditions for each forecast cycle. LEOFS was developed and evaluated by NOAA/GLERL and the NOAA/NOS/CSDL Coastal Marine Modeling Branch. It was implemented operationally on the NOAA Weather and Climate Operational Supercomputer System (WCOSS) by the NOS Center for Operational Oceanographic Products and Services (CO-OPS) in collaboration with NWS/NCEP Central Operations. For more information, please see

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 ( 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

Map Name: NOAA NOS Lake Erie Operational Forecast System (LEOFS) Forecast Guidance (Time Offsets)


All Layers and Tables

Dynamic Legend

Dynamic All Layers

Layers: Description:

Copyright Text: NOAA/NOS/OCS nowCOAST, NOS/CSDL/MMAP, NOS/CO-OPS, OAR/GLERL, Ohio State University and NWS/NCEP/NCO

Spatial Reference: 102100  (3857)

Single Fused Map Cache: false

Initial Extent: Full Extent: Time Info: Units: esriMeters


Document Info: Supports Dynamic Layers: true

MaxRecordCount: 1000

MaxImageHeight: 4096

MaxImageWidth: 4096

Supported Query Formats: JSON, AMF, geoJSON

Min Scale: 0

Max Scale: 0

Supports Datum Transformation: true

Child Resources:   Info   Dynamic Layer

Supported Operations:   Export Map   Identify   Find   Return Updates