NOAAWinds

NOAA GFS weather forecasts

Almost all games and weather routers get their weather information from NOAA's Global Forecast System. The GFS makes the forecasts available at this location:
http://nomads.ncep.noaa.gov/cgi-bin/filter_gfs.pl

Each 6-hourly GFS cycle predicts wind at 1° latitude/longitude resolution, at 3 hour intervals up to 192h, then at 12 hour intervals up to 384h. This data is generated by a weather simulation program, using laws of physics and inputs from instruments. Server responses are in grib 2 format. Variable "wind at 10m" is used. Winds are given by its two U and V components, in latitude/longitude directions.

A new folder is created on the NOAA server for each 6-hourly cycle. The folder is named to identify the GFS cycle using the format gfs.YYYYDDMMHH. For example the folder for a 12:00 UTC cycle might be called gfs.2014301012. For convenience this is usually referred to as a t12z cycle.

Grib files are added gradually to current folder as the GFS run proceeds. Grib files are named to identify how many hours in advance the grib applies, using the format gfs.tHHz.pgrbfhh.grib2. For example the +36 hour grib in a t12z GFS cycle is named gfs.t12z.pgrbf36.grib2. For convenience this grib is usually referred to as the f36 grib.

The first grib in each folder (the f00 grib) is already history, and is provided as an interpolation point (in time).

The publication schedule is:

  Cycle  Start   Finish   
  t00z   03:30   04:30 UTC
  t06z   09:30   10:30 UTC
  t12z   15:30   16:30 UTC
  t18z   21:30   22:30 UTC

How games use NOAA GFS gribs

Games typically use gribs from only two of the four daily cycles.

Game winds for 06Z are extracted from folder YYYYDDMM00. Time 00 comes from file gfs.t00z.pgrbf06.grib2, time 12 from gfs.t00z.pgrbf18.grib2 and so on. Winds for 18Z are extracted from folder YYYYDDMM12. Time 0 comes from file gfs.t12z.pgrbf06.grib2, time 12 from gfs.t12z.pgrbf18.grib2.

Game wind downloads

VRTool game wind forecasts are gathered from different files from GFS forecasts. Current wind (forecast 00) is the same as the game's ( file 06 from NOAA folders 00Z and 12Z ). Future winds (12,24,36 etc) are taken from the latest files available at the time of the download.

Note that VRTool future winds (12,24,etc) are different from game's most of the time, because they are taken from newer predictions (probably better).

How VRTool downloads NOAA winds

If you are using weather routing or the track designer, you should update NOAA winds right after the following times: 4Z, 6Z, 10Z, 16Z, 18Z, 22Z (Z=GMT) and recalculate your route. Winds for 4Z and 16Z are particularly important, because the 12h forecast is an accurate preview of winds that will be used in the game two hours later. So you get 14 hours of 100% sure winds (instead of the 12 in the game app)

Tip: The 2 hour advance in relation to game wind shift can be even larger. NOAA file publication is gradual. Files start to be released 30 minutes before 4Z and 16Z, and the folder is completed (up to 384h) about one hour later. So one could advance the computer clock some 30 minutes and get a preview of the first few forecasts even earlier.

Getting long-range forecasts

The NOAA GFS produces long-range Gribs out to 16 days in the future. VRTool can import these, but only during the following hours:

• 1130-1600 UTC
• 2330-0400 UTC

The reason for this is as follows: Each 6-hourly GFS run predicts wind at 3-hour forecast intervals up to 192h and then at 12-hour forecast intervals up to 384h. So each run produces a sequence of Grib files with the following time-stamps:

 f00, f03, f06, f09 ...... f189, f192, f204, f216 ...... f360, f372, f384
|<-------3-hour intervals--------->|<---------12-hour intervals--------->|

When VRTool is set to import weather at 12-hour resolution it automatically chooses the appropriate Gribs from the current GFS run. If you look carefully at the table in the preceding section, in the third column you can see that the time-stamps that VRTool selects change depending on which GFS run is being used.

For example between 0400Z and 1000Z (the yellow part of the column) VRTool uses the t00z GFS run, and chooses the f06, f18, f30, f42 ..... gribs from that sequence. Beyond f192 VRTool will not find matching Gribs unless it is selecting time-stamps that are multiples of 12.

This means that forecasts out to f384 can only be imported only when the t00z or t12z GFS run is current (during the green and blue parts of the table), and only after the GFS run is fully completed.