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This nowCOAST™ time-offsets map service provides maps depicting the NWS Surface Relative Humidity forecasts from the National Digital Forecast Database (NDFD) out to 3 days (NDFD has forecasts out to 7 days which are available via the time-enabled map service for NDFD elements). Each forecast is valid for the specified forecast projection hour with respect to the latest update time. The forecast is valid at 2m (6.6 ft.) above ground level. The increment and color scale used to display the NDFD RH forecasts were developed to highlight NWS local fire weather watch/red flag warning RH criteria at the low end (e.g. 15, 25, & 35% thresholds) and important high end RH thresholds for other users (e.g. agricultural producers) such as 95%. 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.
The relative humidity is a ratio, expressed as a percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. Relative humidity is derived from the associated forecasts of air temperature and dew point temperature for the same hour.
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. 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 dentify 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.
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: