I noticed on my local airport's METAR that there was distant lightning to the east. I am near Kansas City. When I looked at radar, the only possible system that could produce lightning was in the St. Louis area. Does the METAR really pick up distances like that (250+ miles)? What is the limit on the lightning detector? Perhaps, was there something closer that produced lightning but wouldn't have shown up on radar?
According to NOAA's Federal Meteorological Handbook No. 1, 2019 edition:
- Weather phenomena beyond 10 statute miles of the point(s) of observation shall be coded as distant (DSNT) [12.7c]
- Distance remarks shall be statute miles except for automated lightning remarks which are in nautical miles [12.7d]
- When lightning is detected by an automated system [12.7.1j2]:
- a) Within 5 nautical miles of the Airport Location Point (ALP), it will be reported as TS in the body of the report with no remark;
- b) Between 5 and 10 nautical miles of the ALP, it will be reported as VCTS in the body of the report with no remark;
- c) Beyond 10 but less than 30 nautical miles of the ALP, it will be reported in remarks only as LTG DSNT followed by the direction from the ALP.
This implies that LTG DSNT should not appear on the METAR if the lightning is farther than 30 nautical miles from the airport. FAA JO 7900.5 reiterates this at 13.28b.
Lightning information is collected and disseminated through the Automated Lightning Detection And Reporting System (ALDARS). This thread on a forum for Weather Observers makes for some interesting reading, for example this quote:
Talked to a friend who is NWS forecaster- they know ALDARS is shit, detects crap 200 miles away as VCTS at times- something I can also verify.
So if there was truly no convective activity within 30 miles of the airport, it may have been an issue with ALDARS.