TAF EHAM 281500Z 2816/2901 14010KT 6000 RA SCT025 BKN040 BECMG 2816/2818 12015G25KT SCT008 BKN013 TEMPO 2818/2823 3000 BKN005 OVC010 BECMG 2823/2901 25020KT 8000 NSW BKN020=

Period of validity is stated as 16:00 UTC day 28 until 01:00 UTC day 29 (2816/2901).
Initial weather conditions are forecasted to be:

• 14010KT = wind 140º at 10 KT
• 6000 = visibility 6000 m
• RA = moderate rain
• SCT025 = scattered clouds at 2500 ft
• BKN040 = broken layer at 4000 ft

A change of permanent character is expected to occur on day 28 at any time between 16:00 and 18:00 (BECMG 2816/2818). After the change occurs, the following conditions are to be expected:

• 12015G25KT = wind 120º at 15 kt gusting to a maximum of 25 kt
• SCT008 = scattered clouds at 800 ft
• BKN013 = broken layer at 1300 ft

Next changes expected are of temporary character (TEMPO 2818/2823) => which means that the listed conditions can occur on day 28 at any time between 18:00 and 23:00 UTC, but are not of permanent character and may occur momentarily, for a max of 1h at each time. Thereafter, the conditions will revert back to how they were previously.

• 3000 = visibility 3000 m
• BKN005 = broken layer at 500 ft
• OVC010 = overcast layer at 1000 ft

Another change of permanent character is expected to occur on day 28 at any time between 23:00 and 01:00 on day 29 (BECMG 2823/2901). After the change occurs, the following conditions are to be expected:

• 25020KT = wind 250º at 20 KT
• 8000 = visibility 8000 m
• NSW = no significant weather
• BKN020 = broken layer at 2000 ft

=> Concerning lowest cloud base at 21 UTC, we might be affected by the first forecast of permanent character (2816/2818) - BKN013 - and also by the second forecast change of temporary character (TEMPO 2818/2823) – BKN005. Because the question asks for the lowest cloud base and not the most likely, we have to choose the lowest of these two => 500 ft.