Nomads Weather Model Informations

General

NOMADS is the National Weather Service data dissemination server/service. WindNinja can now download weather forecasts using this service, which may prove to be more reliable than past services.

Available Models:

NOMADS Dependencies

Due to a bug in the vsizip package in GDAL, version 1.11.1 or newer is required. See http://trac.osgeo.org/gdal/ticket/5530.

NOMADS Configuration

Compile-time options

• NOMADS_USE_IP Use the IP address of the NOMADS server instead of the host name. This was to debug a DNS bug in GDAL, not recommended.

• NOMADS_USE_VSI_READ Use the GDAL VSIL API to download files. This allows for larger files to be downloaded by chunking them into NOMADS_VSI_BLOCK_SIZE chunks for download. Otherwise, the entire file is downloaded at once into memory and then written to disk. Most files are fine either way. If very large weather model files are to be used, set this option to ON.

• NOMADS_ENABLE_ASYNC Allow multiple connections for download. This can provide substantial speed up. The default implementation attempts to download 4 files at a time. This can be modified using NOMADS_THREAD_COUNT at runtime. If this option is disabled, only one file is fetched at a time.

• NOMADS_ENABLE_3D Attempt to use three-dimensional initialization data. Alpha at best. Not recommended.

• NOMADS_RTMA Enable the CONUS RTMA model. The RTMA is a “past cast”, so the duration always defaults to 0. The most recent forecast hour is always used.

• NOMADS_EXPER_FORECASTS Enable some forecasts that may not work under the current implementation. NARR and NEST almost fit, but don’t with the current organization. Primarily a debugging tool. Not recommended.

GFS horizontal grid resolution

GFS has three horizontal grid resolutions, 0.25 (default), 0.5 and 1.0 degrees.

cmake -DNOMADS_GFS_0P5DEG ...

or

cmake -DNOMADS_GFS_1P0DEG ...

will override the default and may be useful in some situations, but not recommended.

Runtime Options

• NOMADS_THREAD_COUNT number of threads to use to download files from nomads(default: 4). You can get good speed up from a ratio of 1:1 threads per file to 1:4.

• NOMADS_MAX_FCST_REWIND number of forecast runs to go back in time to try and get a full forecast(default: 2).

• NOMADS_VSI_BLOCK_SIZE If NOMADS_USE_VSI_READ is set to ON in cmake, this is the chunk size that is read in each download(default: 512).

• NOMADS_HTTP_TIMEOUT The timeout in seconds for a request (default 20)

Known Issues

GFS cloud cover data

The new (2014-01-14) GFS model omits t000 cloud cover data as it is a time-averaged variable. Since no time has passed, it is NULL. The work around is to grab the next time step and use it’s cloud cover data.

Further Information

Model descriptions and other information can be found at http://nomads.ncep.noaa.gov/

Frequent users would benefit by subscribing to NCEP’s list-serv pertaining to the NOMADS server. Changes and downtime are reported there:

https://lstsrv.ncep.noaa.gov/mailman/listinfo/ncep.list.nomads-ftpprd