I'm getting very inaccurate weather forecast for my location when it's set to get automatically

Which weather station are you using?

You check that by viewing an outdoor activity in Garmin Connect Web. Click the weather "bubble" in the map and look for the source. It is a code that you need to google to find out which station it is.

Google: [your code] weather

Here is from my morning run.

The source here is ESGP.

I google: esgp weather

It is Gothenburg City Airport and that airport is located 10 km from my location and no height difference or mountains that can make the weather different between the locations.

I see what you are saying, in my case the nearest airport is 5 hours away but the landscape is totally different I guess that's why the weather is completely wrong. Is there a way to change this in the settings? I hope so 

e7andy said:

You check that by viewing an outdoor activity in Garmin Connect Web.

This is actually not always true. The map in Garmin Connect uses the Google API for the weather, while the weather shown on the Weather widget of the watch may be using a different weather provider, and also different location.

You can verify rather easily that the activity map uses the starting coordinates of the activity, and its timestamp for loading the weather data. If you edit the starting coordinates, or the timestamp, the map will show different data, whenever you do it (even retroactively).

The Weather widget on the phone gets the data feed from the phone app once in 30 minutes (if BT and mobile data available), and there it either searches the closest weather station according to the phone location, or from a fixed location - it depends on the GC app Settings » User Settings » Weather Location

In other words - if you are not happy about the weather data on the Weather widget, you may set a fixed location (true or slightly away) and see whether you get better data.

Some more details here: Questions About the Weather Widget on a Garmin Device | Garmin Customer Support 

You can change the location that is used. Instead of the phones location you can use a fixed location. You can try to move the location to a point that are closer to a weather station that got a more correct weather for you.

Scroll down in this page until you find "Can I Change the Source of the Weather Widget Data to Display Weather From a Different Location?"

Questions About the Weather Widget on a Garmin Device

Another solution is to use a 3rd party weather widget or watch face from Connect IQ.