Sea level pressure, in millibars, but with only the last two digits transmitted. Since sea level pressure typically varies from 960 mb to 1050 mb, the missing numbers will be either a 9 or a 10, and it's always possible to figure out which. These pressure forecasts are particularly useful for pinning down the passage of a front, with the lowest pressure occurring at about the time of frontal passage. In the example below, there is a frontal passage between 12 and 18 hours (00Z to 06Z October 31), indicated by the pressure (falling, then rising), the winds, and the temperatures.
TTPTTR1R2R3 VVVLI PSDDFF HHT1T3T5
BOS//605860 00719 131410 56050504
06000818448 04209 091622 56080605
12008935114 00802 002126 59121210
18000773014 -1811 022828 55120804
24000743520 -0913 092924 49090400
30000643339 -2216 102818 48080300
36000624749 03318 112519 47080300
42000615956 -0318 112621 42050299