Simply Weather is a Garmin widget for short-term weather forecasting. It is based only on the current air pressure, its trend and the wind direction. It does not need an internet connection. I’ve seen it claimed that a forecast made around 09:00 is greater than 90% accurate.
The app is a software implementation of the Zambretti Forecaster (https://en.wikipedia.org/wiki/Zambretti_Forecaster) produced at about the time of the 1st World War. There have been several software versions of the forecaster, this one is adapted from Beteljuice’s Javascript code (https://www.beteljuice.co.uk/zambretti/forecast.html).
The widget is available from the Garmin Connect IQ store: https://apps.garmin.com/apps/44dcc5e4-8a09-4509-9618-490a88a13856
Icon design is by Freepik from flaticon (https://www.flaticon.com/authors/freepik) licensed by CC BY 3.0 (https://creativecommons.org/licenses/by/3.0).