Final answer:
TEMPO 2820/2823 -RA indicates a temporary forecast change where rain is expected from the 28th at 20:00 UTC to the 28th at 23:00 UTC, lasting for three hours.
Step-by-step explanation:
The notation TEMPO 2820/2823 -RA in a weather forecast refers to a temporary weather phenomenon expected to occur. In this case, TEMPO is an abbreviation used in aviation meteorology to denote a temporary change in weather conditions. The numbers following TEMPO represent the time period: starting from the 28th day of the month at 20:00 UTC and lasting until the 28th day of the month at 23:00 UTC. The -RA indicates that rain is the expected weather condition during this time period. Thus, this specific weather phenomenon, the rain, could last for a period of three hours.