To convert inches of mercury to millibars, multiply the inches value by 33.8637526
To convert millibars to inches of mercury, multiply the millibar value by 0.0295301.

29.5 inch = 998.9 mbar

And your forecast can be improved still further by combining your record of changing barometric pressure with the changing direction of the winds.  As we have already learned, air circulates in a clockwise fashion around high pressure systems and counterclockwise around low pressure systems.  So if you see a trend toward rising pressure and a northwesterly wind, you might expect generally fair weather to move in, as opposed to a falling barometer and an east or northeast wind which could eventually lead to clouds and precipitation.  


As per:
https://forum.arduino.cc/index.php?topic=512459.0
"The BME280 measures absolute air pressure. It is factory calibrated"
Absolute to Relative Barometric Pressure
Relative air pressure is known as corrected barometric pressure. It is a measurement of how much pressure an air column would exert at sea level. To determine corrected barometric pressure, an absolute air pressure measurement is taken, along with an altitude measurement. The relative air pressure of that column is the amount of air pressure it would exert at sea level, if it remained at a constant pressure all the way down. It is sometimes called relative pressure because it reports pressures relative to sea level.
https://sciencing.com/absolute-vs-relative-barometric-pressure-13425786.html






"The BME280 measures absolute air pressure. It is factory calibrated", but you can do better by comparing the measurements with those obtained from a professional quality barometer, and calculating an improved scale and offset factor to apply to the BME280 data.

As per:
https://forum.arduino.cc/index.php?topic=512459.0
"The BME280 measures absolute air pressure. It is factory calibrated"

https://sciencing.com/absolute-vs-relative-barometric-pressure-13425786.html

Absolute barometric pressure is the actual atmospheric air pressure at a particular location that profoundly depends upon the location altitude. Relative or sea level pressure is the corrected barometric pressure calculated for the sea or zero level, and usually used to refer atmospheric conditions. The importance of the relative pressure (P0) is that it allows calculating the absolute pressure (P) at any elevation (h) using the barometric formula: P=P0*exp(-Mgh/RT), where M molar mass of air, g standard gravity, T temperature and R universal gas constant. The relative barometric pressure is the pressure reported by weather stations.
https://sciencing.com/relative-barometric-pressure-5075062.html