Grib news Vol-2 (28 Mar 2003)
This is an irregular newsletter on the topic of grib weather data from Saildocs. You can subscribe and a copy will be sent whenever there is an update- see below for info. Previous copies are available via Saildocs by requesting "gribnews-1" for the first volume (Dec-2002). The most-recent version will always be available as "gribnews".
The big news in this issue is that a fancy new download scheme means that data is available earlier, and downloads can be run more often. Grib data will now be updated with each model run, every 6 hours. This will also create some confusion, so please read the next few paragraphs carefully.
Saildocs grib subscriptions were previously sent at 12z by default, as the data was often not available until after 11z. Currently NOAA is making the grib data available by 0400z, our download is usually complete by 0530, and the default for subscriptions has been changed to 0600z. Existing subscriptions have been updated to 06z also. This is the same 00z model-run, just available earlier.
Similarly, the 06z model-run is available by 12z, the 12z run by 18z, and the 18z run by 00z. This is where the confusion starts...
A grib-request specifies forecast-times, for example 24/48/72 hours after the 00z model-run (i.e. the synopsis-time of the data). For requests (or subscriptions) processed between 06 and 12z, the available data will be the 00z model-run and everything is as expected. But for requests processed between 12z and 18z, the available data is the 06z model run. If the request for 24/48/72 hours were taken literally, the data would be valid at 06z each day, not 00z. This is confusing and makes comparisons of subsequent files difficult. So we've arbitrarily stipulated that the forecast-times are based on a 00z valid-time and adjusted accordingly. If other words, if you request a 24/48/72 forecast then it will be valid at 00z each day, no matter when the request is sent. If you request it at 18z, then it will be based on the 12z run and the forecast-times that you see in the returned file will be 12, 36, and 60 hours, so that the actual valid-times will still be 00z each day. So the later your request (or subscription time), the fresher the data. And if you want to see the forecast at 12z, then request 12, 36, 60 hours which will always return forecasts valid at 12z each day. There are a lot of options, just keep an eye on the file-size (shown in Airmail's request-window).
That certainly doesn't resolve the confusion, but at least the results are predictable and don't depend on when the request reaches the Saildocs server.
Currently, forecast data is available from 00 to 96 hours at 12-hour increments (00z or 12z), and from 120-240 hours at 24-hour increments (00z only). This is a limitation of the download size, and as we get more experience with the new download system we should be able to add more valid-times and perhaps more data-fields. Data beyond 180 hours is only available from NOAA at a lower resolution of 2.5 x 2.5 deg, and only available at 12-hour increments. That means that for the 06z and 18z model runs, Saildocs can only supply data out to 180 hours, 7.5 days. And anyone who is concerned with that does not really understand the limitations of computer models.
A word about the computer models: The grib data which Saildocs makes available comes from the U.S. NCEP (National Center for Environmental Prediction), part of NOAA. We use the "GFS" model, formerly known as the "AVN" (aviation) model and descended from the "MRF" (medium-range forecast) model. This is the primary US computer model, arguably as good as any of the world models but the best feature is that it is available without charge.
In any event the forecast data can be expected to be fairly accurate out to 48-72 hours, useful to 96, and beyond that is probably better suited to speculation and "what if" games.
Old news:
For a copy of Vol-1 (info about Airmail's grib-request window and the grib viewer), send a (blank) email to gribnews-1@saildocs.com.
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Good sailing, and thanks for your support.
Jim Corenman
saildocs@siriuscyber.net