How would SLP997 be calculated on a METAR? I understand that SLP117 say would be 1011.7 hPa, but I am sure that SLP997 is not 1099.7 as that would be impossibly high.
Refer to FAA document, AC 00-45H Aviation Weather Services
188.8.131.52.25 Sea-Level Pressure.
At designated stations, the sea-level pressure is coded in the following format: the identifier SLP, immediately followed by the sea-level pressure in 1 millibars (mb) . The hundreds and thousands units are not coded and must be inferred. For example, a sea-level pressure of 998.2 mb is coded SLP982. A sea-level pressure of 1,013.2 mb would be coded SLP132. For a METAR, if sea-level pressure is not available, it is coded SLPNO.