GRIB Outputs and the Race to Mackinac

Milan Curcic
Published in
4 min readJul 10, 2019
Photo by Jonathan Smith on Unsplash

Every year, Chicago Yacht Club organizes the Race to Mackinac — a 333-mile long yacht race from Chicago in Lake Michigan to Mackinac Island in northern Lake Huron. This year, Cloudrun has partnered with Chris Bedford from Sailing Weather Service to support the race by providing high-resolution weather forecasts for the official weather briefing, as well as for direct use on board the sailboats for weather routing. Chris is a world-renowned meteorologist with expertise in sailing weather, and has coached many world champions over the last 30+ years. He will provide the official weather briefing on the evening of July 12 and the morning of July 13, shortly before the start of the race. In compliance with the race regulations, Cloudrun’s forecast data will be freely available for download to all competitors.

Introducing GRIB output in Cloudrun

For Cloudrun’s data to be useful to sailboat racers, forecasts now include GRIB outputs. GRIB (short for GRIdded Binary) is a self-described, compressed, binary data format used by operational weather prediction centers around the world, and also by sailors as most sailboat routing software reads weather data in this format. The output in GRIB format is currently available in Cloudrun’s Forecast Wizard mode. It includes hourly fields of u- and v-components of wind at 10-m height, air temperature at 2-m height, sea-level pressure, and precipitation.

How does it work?

To start using GRIB formatted outputs now, simply use the Cloudrun Forecast Wizard as normal. Select a region on the map, choose your forecast dates, and click Start Forecast. The Forecast Wizard will now create GRIB files (alongside the original WRF files) on the fly as the model is running, and you can download the latest GRIB by clicking on the Download latest GRIB button in the Forecast Dashboard.

You can now download all forecast times up to the current time as a single GRIB file.

The downloaded GRIB file is an aggregate of all forecast times (hourly) from the initial forecast time until the latest computed. If you download it more than once while the model is running, make sure you overwrite the previous file or that you always use the latest file.

Once the forecast is complete, you can download the complete GRIB at any time by clicking on the Download Complete GRIB button in the Forecast Dashboard.

You can download the complete GRIB file from the forecast dashboard once the forecast is complete.

How do you read GRIB data?

Cloudrun’s GRIB output files are compatible with Expedition Marine, which is by far the most widely used weather routing and navigation software for sailboats. Expedition Marine is continuously developed by Nick White, a veteran Volvo Ocean Race navigator and Whitbread winner.

Expedition Marine + Cloudrun GRIB files in action: 10-m winds in northern Lake Michigan, computed at approximately 2.7 km horizontal resolution.

Of course, you’re not limited to Expedition Marine for reading Cloudrun’s GRIB data. You can use any other software that has a GRIB v2 decoder, for example CDO (Climate Data Operators), NCEP’s wgrib2, or the Python package cfgrib.

The weather outlook for the race

For a preliminary look into the race weather, check out the video below, showing 10-m wind prediction for July 10–15. This forecast was produced completely in the cloud, with just a few clicks in Cloudrun’s Forecast Wizard.

10-m wind forecast for Lake Michigan and northern Lake Huron at 2.7 km resolution, valid for July 10–15, 2019. Created on Cloudrun with just a few clicks.

The above wind forecast is an unofficial and informative outlook. The official race weather forecast will be delivered by Chris Bedford from Sailing Weather Service on July 12 and 13, before the race start.

How are Cloudrun’s forecasts different from existing GRIB data?

Recreational and professional sailors typically rely on publicly available GRIB data that can be downloaded from operational weather prediction center data servers. However, the global weather data are quite coarse — for example, the global GFS files from NCEP are currently distributed at 0.25 degree horizontal resolution. That’s one value of wind, temperature, or precipitation every 15 miles or so. There are higher resolution weather models that are publicly available, such as the North American Mesoscale (NAM) model or the High-Resolution Rapid Refresh (HRRR) model. However, they are only available over the continental United States, and for shorter forecast lead times. Existing publicly available GRIB data are thus limited at either the level of detail, or the forecast horizon.

Cloudrun allows you to run your own, custom, ultra-high resolution weather model from a web browser, without the need for expensive hardware or weather modeling expertise. Our model uses the latest data and is optimized for high performance and parallel computing in the cloud. You can download your weather forecast data as the model is running. A day of forecast typically completes in an hour, so you can get a high-definition weather forecast for the next several days in an afternoon.

Selecting your custom forecast region in the Cloudrun Forecast Wizard

Are you interested in using Cloudrun for your next sailing race or expedition? Give it a try! Send us an email if you have any questions or if you’d just like to tell us more about your weather forecasting needs. We’d love to hear from you!