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Forecasts

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:

  • By night winds often decrease and increase by day.
  • Wind will blow along steep coastline and around headlands rather than over them.
  • Pressure over land increases by night time cooling giving offshore winds.
  • Pressure over land decreases by day time heating forming sea breezes.
  • Pressure falling slowly (0.5 - 3 hPa in 3h): A low is weak, dying or moving slowly. You might get some rain but typically no high winds.
  • Pressure falling moderately (3-6 hPa in 3h): rapid movement or deepening low. Moderate winds and rain in warm front. The low is passing you fast so day after tomorrow will typically be fine.
  • Pressure falling fast (6-12 hPa in 3h): high probability of storm.
  • Where do I find the forecast interval(Days and hours)?

    Please

  • Go to GRIB params (wheel symbol while the blue-yellow map is showing)
  • Choose the weather model (mostly it is GFS)
  • Scroll down to forecast interval (days and hours)
  • Enter the number you wish to see by
  • tapping the right field (for instance 7 days)
  • make sure you see the check sign on the right (of the days line)
  • NOW TAP SAVE.
  • 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).

    Saildocs Warnings:
    "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.

    WeatherTrack- ★★★★★
    UK
    Richard Hammons on Wednesday, October 30 2013 version 1.5.3
    A superb app with great flexibility and reliable. Very detailed worth the money.

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    USA
    by Mahinagirl1234 on Monday, December 31 2012 version 1.4.7
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    Weather Centre ★★★★★
    United Kingdom
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    Great ★★★★★
    Pakistan
    by Sdrroooo on Friday, August 02 2013 v 1.5.1
    A wonderful application. Being professional airline pilot, I find it very helpful and accurate.

    Weathertrack ★★★★★
    United Kingdom
    by Dine December 19 2012 version 1.4.8
    What an excellent App, I use this professionally to cross check weather patterns at work... it is very comprehensive and accurate.... well done developers...!!

    Captain ★★★★★
    Netherlands
    by Zeevaart on Monday, December 31 2012 version 1.4.8
    As a nautical officer I am using this program all the time ..very Handy and very usefull wheater forecasting program.

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    Canada
    by A Very Nice Guy on Tuesday, December 11 2012 version 1.4.8
    The most complete product I have found so fare.
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    Italy
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    United Kingdom
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    Netherlands
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    Netherlands
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    Italy
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    United Kingdom
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    Best weather forecast ★★★★★
    Germany
    by PrinzzIII on Tuesday, July 30 2013 v 1.5.1
    To look at the weather map tells more than 1000 words. GRIB data in two hours steps, wind, precipitation, air pressure. No one needs more to make a travel plan

    Perfect ★★★★
    Spain
    by dani72 on Tuesday, October 30 2012 version 1.4.7
    Does what it says
    (Original: Perfecta - Cumple con lo que dice))

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