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Author Topic:   Using GRIB Weather Data with PolarView NS
jimh posted 07-23-2013 10:56 AM ET (US)   Profile for jimh   Send Email to jimh  
Previously I have written about PolarView NS, the multi-platform, modestly priced computer application for marine mapping and route planning. PolarView NS also contains a weather data download and display function. PolarView NS allows the user to define a region of interest on a chart, and to then download GRIB weather data for that region. I have found this feature to be extremely useful in trip planning. Each GRIB download contains forecasted weather for seven days into the future. I typically download the GRIB data for the area in which I will be cruising just before we depart the dock. During the cruise, if an internet access is available, I download newer data whenever possible. Our recent cruise in Northern Lake Michigan provides a good example of the usefulness of having GRIB weather data onboard.

On Thursday, July 18, we arrived in the port of Charlevoix, Michigan, on the Eastern shore of Lake Michigan. Our arrival was in rather turbulent weather; we made the harbor entrance just as a squall hit, bringing rain so heavy we could barely see and winds so strong the lake surface was turned to windblown foam. That cell quickly passed over, heading on to the Straits of Mackinaw, where it blew over a semi-trailer truck on the Mackinaw Bridge.

For the rest of that Thursday afternoon and into the night, the wind blew from the Southwest, increasing to 25-knots, and blowing steadily all night. We awoke on Friday morning to a mixture of sunny skies and storm cells blowing over, and the wind still strong from the Southwest. Two other boats in the marina left to test conditions, trying to head South, right into the wind and the seas it had created in the past 24-hours. Both returned with reports of waves of six to eight feet, and extremely difficult conditions to make headway.

This was not good news for us, as we wanted to head Southwest about 25-miles to the port of Northport. The option to stay in Charlevoix was not attractive. By chance, this Friday evening was the Venetian Festival, and every slip in every marina in the vicinity was booked with reservations. Even nearby Irish Boat Works, a nice facility at whose docks we have often stayed, were completely full. They were holding their annual customer appreciation dinner that evening, and there was literally no dock space available. Even if we could find an overnight berth, the weather forecast did not look very good for Saturday, as more high winds were forecast.

Here is a screen capture of the GRIB weather prediction for early Friday morning, about the time the two boats (I mentioned above) tried to leave Charlevoix. (The wind flags point at the direction of the wind. A long flag bar is 10-knots; a short flag bar is 5-knots.)

GRIB forecast 1200 GMT
GRIB data for 1200 GMT, or 0800 local. Winds are 25-knots from the Southwest.

This same pattern was seen for most of the afternoon.

GRIB data 1500 GMT
GRIB data for 1500 GMT, or 1100 local. Winds continue at 25-knots from the Southwest.

GRIB data 1800 GMT
GRIB data for 1800 GMT, or 1400 local. Winds show a decrease to 15-knots but hold from the Southwest.

By noon we had left our marina slip, and taken a short cruise in Lake Charlevoix. We anchored in the lee of a long point, had lunch, and went swimming. But even inland and in the lee of the shore, the wind was very strong, gusting to 25-knots and causing the boat to sail on its anchor. About 2:30 p.m. we came back to the marina. Our slip was still empty, as the new boat with reservations for tonight had not arrived yet. We tied back up, informed the harbormaster we were using the slip but could leave whenever he needed it, and pondered how we were going to get to Northport.

I downloaded the latest GRIB model, and studied the forecast. I saw a brief window of opportunity. The forecast called for the wind to drop to 10-knots and shift to the West, beginning around 2100 GMT or 1700 local. The next three-hour forecast for 0000 GMT or 2000 local showed the wind velocity coming up again. There appeared to be a three hour window with winds down to 10-knots and a shift to the West.

This looked like our best opportunity to get out of Charlevoix. Luckily, the incoming boat heading for our slip did not arrive, and we were able to stay at the marina until about quarter to five. We next prepared the boat for some rough seas, ate a small snack, and made sure we had plenty of water available. We did not know exactly what we were going to find on the big lake, and we anticipated that the 25-mile passage might take several hours if we had to go slowly upwind. Here is the GRIB model for 5 p.m.:

GRIB data 2100 GMT
GRIB data for 2100 GMT, or 1700 local. Winds are down to 10-knots and have shifted to the West

As we motored out to the Lake in the break wall channel, we could see swells running up the inlet from the lake, a sign of wave motion coming into the channel. But, as we approached the lake, we were relieved to find the seas were nothing like the six to eight footers of earlier in the day. The harbor mouth at Charlevoix is somewhat sheltered by a point of land to the South, and we knew that conditions would be worse once we cleared that lee.

When we reached the open lake, we found the seas to very manageable, consisting of mostly two- to three-foot waves, with the occasionally large wave mixed in. The waves were just on the Starboard bow. With a classic Boston Whaler boat, you can run into direct head seas or ones just off the bow without too much problem. We found a comfortable speed in the conditions, and headed out. Our course would take us about 12-miles upwind, heading for the tip of the Leelanau Peninsula.

An hour later we were at the Grand Traverse Light, and in the lee of that mighty peninsula. We had made the 12-miles in about one hour, an average speed of 12-MPH. Here the seas decreased to a modest one-foot or less, and we were able to increase speed to about 28-MPH, and run down the remaining 12-miles to Northport in short order.

Without the information from the GRIB model, I don't think I would have been inclined to leave Charlevoix. Seeing the drop in wind speed to 10-knots on the GRIB forecast gave us the encouragement we needed to get out of the shelter of the harbor and take to the open lake. I have found the GRIB data to very useful. Rather than the summary data of a forecast, it shows you detailed wind data in three-hour time and 30-mile location increments.

jimh posted 07-23-2013 11:14 AM ET (US)     Profile for jimh  Send Email to jimh     
Our 5 p.m. departure was also timed to avoid a thunderstorm cell. Being able to access the internet, I was watching weather RADAR on-line at WeatherUnderground. (I find their Wundermap presentation to be very good.) As it happened that afternoon, a large storm cell was making its way across the lake to the South of Charlevoix. A 5 p.m. departure allowed us to miss the rain and wind in that storm cell, letting it pass to our South.

The RADAR also showed a second storm cell, but farther to the West. It appeared to me that there would be a gap of at least one hour between the two cells, which would give at least an hour window of no rain.

The GRIB model showed that later in the evening the wind would shift to the Northwest or perhaps even to North, but it would also increase in strength, back to 15 to 20-knots. While the wind direction would be more favorable for our run to the Southwest, the shift would come too late. I did not want to be out on the open water of Lake Michigan in darkness. It is too hard to see big waves in the dark. Thus, a 5 p.m. departure seemed much better. We would have at least four hours of daylight to make the crossing.


jimh posted 07-23-2013 01:28 PM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE: I was just checking the hyperlink (above) for the Wundermap. I happened to notice the conditions today in Northern Lake Michigan: wind NNW at 30-knots. That is not a fun day to be out in your small boat. Also, the Wundermap page is a large amount of data to download, so if your internet connection is slow, it can take a long time to create the page. A faster, simpler presentation can be found at
jimh posted 07-24-2013 10:06 AM ET (US)     Profile for jimh  Send Email to jimh     
I just discovered that the Wundermap stores historical RADAR data, so I was able to set it to show conditions at 2 p.m. on Thursday, July 18, 2013, about the time I took the picture of the storm cell (seen above). Here is the RADAR for that time:

RADAR shows small but intense storm cell approaching Charlevoix, 2 p.m. July 18, 2013.

jimh posted 07-24-2013 01:24 PM ET (US)     Profile for jimh  Send Email to jimh     
Compare the RADAR image (above) with how the storm cell looked to us off the stern of our boat as we ran for cover in Charlevoix:

I don't think the weather RADAR echoes portrayed this storm with the proper intensity.

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