Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Confusion; Troubleshooting
Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Flight Crew Requested ATC Assistance / Clarification
Chart Or Publication; Weather
While on initial descent into SFO; I obtained a METAR and D-ATIS for the airport prior to my briefing. On this particular day; the wind was from the west and quite gusty between 35 to 40 knots. I wanted to ensure the crosswind component was within my limitations as a First Officer. At the time; I noticed a discrepancy between the METAR wind direction and that of the ATIS. METARs provide wind direction in reference to true north whereas ATIS and D-ATIS provide in reference to magnetic north. At SFO; the magnetic variation is 14E; meaning we would subject that amount the direction in reference to true north to get the wind direction in magnetic north. For simplicity; I will round to 20 degrees. At the time; the wind on the METAR was 280 degrees but that on the ATIS was 300 degrees. Both reports had the same reported time and date and the rest of the information including peak wind was exactly the same on both reports. There seems to be a large discrepancy between the two since if the METAR says the wind is 280; then in magnetic that is 260 and not the 300 that was advertised by the ATIS. This 40 degree difference is quite large. I was curious and began to observe the ATIS and METARs over the next couple days and found that SFO is consistently reporting; on the ATIS; a value that is 20 degrees greater than the METAR. Possibly explanations for this could be that the wind information is coming from different sources; but it seems quite peculiar that the wind direction on the ATIS is consistently 20 degrees greater than the METAR.  Any pilot would use the METAR; subtract 10 or 20 degrees; and get a wind direction of 270 or 280 degrees magnetic. The ATIS is reporting it as 310 degrees. This discrepancy can be large enough to cause confusion with crosswind landing limitations for the aircraft and pilot. I have noticed this issue prior to this flight; but did not think much of it until I noticed it consistently wrong over the past few days. If you view the METAR and ATIS for OAK and SJC; the ATIS for both reports a wind direction that is 10 degrees less than that of the METAR. This is as expected. SFO; however; reports 20 degrees greater. I addressed SFO Tower about this directly; to which they responded that they do not control this data as it comes from the 'weather people downstairs.' I am just curious if this is intentional or if this is; in fact; an error. Perhaps SFO Tower quality assurance can take a look?
Air Carrier pilot reported they believe the SFO METAR reports consistently issue incorrect wind directions.