Thursday, June 3, 2021

HRRR 48-hr forecasts and sub-hourly forecasts

For some time now, the HRRR model forecast has had options to its base offering  of updates every hour, with forecasts extending out to 18hr. One extension is a longer term forecast updated every 6 hrs, extending out to 48hr, but not many popular sources had the extended data available.  Now we have several sources, so this detail might benefit from a highlight, not to mention that in some cases the presentation might be confusing.


LuckGrib for (Mac or iOS) pioneered the access to this data, and still has the clearest interface to the options, plus LuckGrib also offers access to the unique HRRR sub-hourly wind forecasts (every 15 minutes for 18 hr), which are not available elsewhere to my knowledge—other than a direct download from NOAA, which takes special procedures. The LuckGrib solution is to offer three separate model choices:

(1) HRRR hourly forecasts, out to 18h, updated hourly, delayed 1h 30m.

(2) HRRR 15 min forecasts, out to 18h, updated hourly, delayed 1h 28m (wind and gust only)

(3) HRRR hourly forecasts, out to 48h, updated every 6 hr at the synoptic times, delayed 2h 01m.

These are the most timely forecasts we have, but there is still a latency to account for the model run time plus the 10 or 15 min it takes LuckGrib or other third parties to process the data once ready.  

This means that if you request (1) at 1500, the first forecast you get would be valid at 13z and the last would be 18h later at 07z the next day. Noting the delays to the minute allows you to get new data about half an hour earlier, but you would still be comparing present observations to a forecast that is 1.5 hr old.

The same is true with the sub-hour forecasts (2), but at the earliest you will always get at least 6 forecasts that cover past times. Your logbook records of these winds and then a good check on the actual forecasts going forward in time.

For inland routing or planning a day sail or a race, option (3) is enticing, but the timing might be more crucial if you want the freshest forecast.  Requesting (3) at 1410, would bring 48 hourly forecasts starting at 12z today and ending at 12z in two days. You would have only lost 2 hr of currency, which is unavoidable.  On the other hand, if you requested (3) at 1330 or even 1400 exactly, the first synoptic time earlier than the 2h 01m latency would be 0600, so you would be working with a forecast that is 8 hr old. 

In short, when using the extended forecast HRRR data we must be aware of the latency. Note that the fact that this runs at only the synoptic times means it will make an interesting (timely) comparison with the  3-km NAM forecasts as well as the NBM CONUS data.


Saildocs does this a different way. They have only one HRRR model request, but it can be used for the 18h data or the 48h data.  The result you get back depends on the extent of the forecasts you ask for.  If you ask for less than 18h of data,  ie every hour (or every 3h) out to 18h then this is interpreted as the 18h run computed every hour with a latency of 1.5h.  If you ask for any forecast beyond 18h, then that will trigger the 48-hr forecast  run only at the synoptic times, now with a latency of 2.0 hr. 

In short, it is about the same as LuckGrib on the extended forecast, but Saildocs does not have the sub-hourly forecasts.


Expedition uses Saildocs for access to the HRRR, and to accommodate the 48h option, it shows extended hours for choosing the number of forecasts, and hence the trigger to the type of data. It works as explained above, but users need to know the conventions and latencies.

Other Viewers and Apps

XyGrib does not access HRRR. OpenCPN, qtVlm, and others that use Saildocs show the HRRR interface but are not updated yet, and still assume the maximum hours for HRRR is 18. These programs often generate an email for the user, that we then send to to get the file, which we then import as an external grib file.  To use this same email for the 48h forecast, just change the end forecast date from 18 to 48 or 30, etc.  Keep in mind this is high res data, so the files can get huge fast. We have to make judicious choices of region and parameters or hit server limits on file size.

Some mobile apps allow the choice of HRRR, but default to the 48hr data for all selections, so users must be aware that there is maybe newer data from that model.


Numerical weather models including regional models for inland sailing are discussed in our textbook Modern Marine Weather.

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