Average weather in Warsaw in September
Poland is a central-eastern European country with a continental climate and four seasons: winter, autumn, spring and summer. The ASAIHL Conference will take place at the beginning of autumn in Poland. Visitors to Warsaw can generally expect moderate temperatures in September, ranging from around 19°C (65°F) daytime temperature to the minimum nighttime temperature around 9°C (48°F) and a moderate rainfall. The wind is moderate as well.
How to prepare for the weather in Warsaw in September?
It is advised to take an umbrella, a light to medium jacket and clothes for the moderate weather when visiting Warsw in September. It might be also likely that exceptionally the temperature will be above 19°C (65°F) or below 9°C (48°F), therefore a piece of clothing for unusual weather condition might also be necessary.
See more at https://weather.com/weather/tenday/l/6b3cb483cd1de233d0d12d00212418758b2e7da9bd069dab10adc70098dd89df