Storm alert still not fixed

It's because your change in altitude is calculated from the change in the barometric pressure. GPS (and other GNSS) are not accurate nor reliable enough in the altitude direction to be used for altitude measurement. (For example, when you use GPS to calibrate your altitude, what actually happens is that the watch get a GPS lock for your lat/long coordinates and then consults its internal elevation map for estimated altitude.)

So the watch has to constantly estimate (guess) which portion of the air pressure change is due to you ascending/descending and which is because the air pressure actually changes without you getting up or down. It seems to me that mostly the watch does this by attributing rapid changes to you moving, but in the mountains, I can see where the watch could underestimate your descent, triggering the storm alert.

Thank you for your response and explanation. I do agree to the problem statement, but not to the conclusion.

I frequently pass by geolocation markers (i. e. stones with lat/long/alt engraved) and mountain peaks of known altitude and I use them to calibrate my altimeter. When doing so, I am presented with 3 options: manually providing the known altitude, GPS or DEM. I am hoping that "GPS" actually means "All available GNSS on your watch". 

I have observed that "GPS" is usually within 5 m of the true altitude, while DEM is sometimes off up to 20 m. I agree that accurately fusing multiple GNSS, DEM, acceleration and barometric pressure sensor data into a correct altitude while accounting for all sorts of noise and drifts is not a highschool summer project, which is why we are not buying watches from highschoolers but from Garmin. 

Lastly, for the feature in question using this threshold implementation, a highly accurate altitude measurement is not even needed, but you will have to make sure that you do not mistake a full 1300 m descent for a pressure change due to weather, which is what happens. For that matter, GNSS and a fair DEM would be sufficiently suited, unless you are permanently in a narrow canyon or close to steep walls. 

Bitti Are you sure the watch uses map data to determine gps altitude? I’m not.

For starters. My previous watch, a forerunner 935 didn’t have maps and even without connection to my phone was able to calibrate the altitude based on gps.

Gps is capable of determining altitude. Not very accurate, but within acceptable limits.

But altitude by barometer isn’t accurate either. Due to weather changes the altitude can be reported differently. Your watch hasn’t got a clue about weather changes. (Hence the need to calibrate)

I used to do hanggliding and at the end of the day, gps altitude is a bit more reliable than barometric altitude (although visually beats both) (my vario didn’t had maps, so altitude was solely from gps data)

The only thing a barometer can do better than gps altitude is quickly detect even small changes in altitude (or actually airpressure) great when flying in thermals. Not so needed for other situations.

But I do realize some people obsessively chase accuracy.

I was referring to the altimeter auto-calibration that the watch performs automatically. Of course, if you calibrate manually, you have the option to choose which the calibration is based on.

This page is for Fenix 7, but should be valid for 965 as well: https://support.garmin.com/en-US/?faq=pCL0xvX9lj8hdtZcNf1ZAA

As you can see, using DEM is prioritized over using GPS (GNSS) altitude.