How accurate are the GRIB forecasts?
Forecasts are based on data facts, but they are not statements of fact. They show the trend of weather development. As a general rule you can say, the higher the resolution, the more accurate the forecast. The forecast based on a Global Forecast System model (GFS), for example, displays per default a resolution of 0.5 degree in WeatherTrack which translates into 30 nm. Consequently such a global model does not take into account local effects like thermic winds influenced by heating of the land, sea and land breezes or topographic aspects. Whereas 100 nm offshore a forecast might be pretty accurate, it may not be true for the coast area. Weather phenomena like a shower and its associated gusts or squalls are not predictable even hours before they occur.
Some rules of thumb:
Where do I find the forecast interval(Days and hours)?
Why tend GRIB forecasts to underestimate wind speed?
GRIB files are computer models only with no human forecaster interpretation. In most cases, the wind speeds will be underestimated and it’s important to modify the forecast for local conditions.
WeatherTrack displays wind speed at 10 m MSL. The higher the wind comes from the more wind speed you get. For a sailboat with a mast taller than 10 m you should add 20% to the wind speed. And for the maximum gust windspeed you should add another 20%.
As a general rule: Add 1 Beaufort for planning; 10% for the Tropics, in temperate latitudes and 25% in the Southern Ocean (Frank Singleton's experience).
"In addition these models cannot provide adequate prediction for tropical systems, frontal activity or convergence zones. For example, while global models can provide useful data on the likely track of hurricanes, they grossly underestimate the strength of hurricanes because of their small size compared to the model grid. For hurricane/cyclone forecasts, carefully monitor the appropriate warning messages and do not rely on grib data from any source. That all said, grib data can provide useful guidance not available elsewhere. Understand the limitations and use the data carefully. Grib data should be considered supplemental to other forecasts, and not be relied upon in lieu of professionally-generated charts or forecasts."
Where do the forecast data come from?
Please see the left column.
Why does the GRIB forecast area move?
It is very likely that you have enabled the function "GRIB Optim". After entering course and speed this function moves the forecast area according to the speed and course you have entered to optimize the file size of the GRIB forecast. This can be very helpful when offshore and you do not receive GRIB files via internet (but for example via SSB).
This is how you deactivate this function.
After selecting the area of interest on the blue-yellow map, please tap on the GRIB parameters (wheel, below in the task bar), scroll down until you see "GRIB Optim". Set speed and course to 0.