Thursday, November 17, 2011

Why wind forecasts drop

In recent weeks, the wind forecasts on iWindsurf.com have often shown this annoying pattern:
  • the wind predictions 5 or 6 days out look great, and remain great for a few days
  • about 2 days before, the forecast suddenly drops and looks a lot less impressive
For example, the forecast on Monday may predict 30 mph winds for the weekend. This stays more or less the same until Wednesday - but on Thursday, the forecast drops to the low 20s. This happened a lot in the last few weeks. Almost every time when we went sailing, though, the actual wind was much stronger than the most recent forecast, and closer to the stronger forecast from a few days before. 

What is going on? Well, let's have a look at the iWindsurf forecast tables for Kalmus from this morning. The default tables show the "Quick Look":
Until yesterday, the forecast for Saturday called for wind in the upper 20s to low 30s; now, it shows mostly low 20s. Sunday is still looking a lot better in the table above, but dropped a few miles later during the day.

A look at the different computer models sheds some light on what's happening. Here are the tables from the GFS model:
The tables from the NAM model don't look quite as good:
Both are run by NOAA. The NAM model has a finer geographical resolution (12 km vs. 35 km for GFS), but gives predictions only for 84 hours, compared to 7 days for the GFS model. The "Quick Look" table on iWindsurf will use the NAM model if it has data for the entire day, and the GFS model otherwise. So in the example above, it shows NAM data for Saturday, and GFS data for Sunday.

This time of the year, it seems that the GFS model always predicts stronger winds  than the NAM model, at least for the Cape Cod area. So when the iWindsurf "Quick Look" tables switch from the GFS model to the NAM model, it seems that the wind forecast has suddenly dropped. In reality, however, the wind prediction from the GFS model may not have changed all all, or even gone up!

I do not know why the GFS model seems to make better wind predictions than the NAM model at this time of the year, but I'm glad it does. As I am writing this, the latest NAM model runs predict 23-25 mph for Saturday (that's 5 Beaufort for our European readers), while the GFS predicts 27-29 mph (6 Beaufort). My bet is that we will see 30+ mph again!

So, if you are making plans for windsurfing on Cape Cod for the weekend, check the GFS tables! Just keep in mind that when it gets too warm, decoupling at onshore beaches may cause winds to be a lot lower than predicted. That will hopefully not be an issue on Saturday, but it could be a problem in Kalmus on Sunday.

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