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Old 8th Aug 2018, 18:09
  #5 (permalink)  
good egg
Join Date: Feb 2006
Location: Rapunzel's tower
Posts: 456
In Japan they do something similar...

”In automated METAR/SPECI, thunderstorms (TS) and convective clouds of operational significance (CB and TCU) are identified using data from ground weather radar and JMA’s Lightning Detection Network System (LIDEN) via the calculation process outlined below.

Algorithm for identification of thunderstorms (TS), cumulonimbus clouds (CB) and towering cumulus clouds (TCU)
CB/TCU information*1 is produced every 10 minutes based on the Radar Lightning Analysis Index*2 and other data.
TS, CB and TCU are identified every 5 minutes along with their location and movement direction (based on difference from the previous location) using CB/TCU information and cloud-to-ground lightning data from LIDEN.
If composite weather radar data cannot be obtained due to system failure, the term TSCBNO (“thunderstorm and significant convective cloud information not available”) is used. In the event of LIDEN failure, the term TSNO (“thunderstorm information not available”) is used in the RMK section of automated METAR/SPECI.
*1 CB/TCU information: Automatically produced data showing estimated CB and TCU cells based on extraction of convective cloud cells from Radar Lightning Analysis Index data and subsequent threshold-based classification.
*2 Radar Lightning Analysis Index: An index expressing lightning potential (i.e., the status of convective clouds). Values are calculated every 10 minutes using composite ground weather radar data on radar echo intensity, radar echo top height, vertically integrated liquid (VIL) and other variables.”

So I suspect the something similar is used in UK. I.e. whilst the weather station is not detecting any cloud (below 5000ft) there are other sources which are using data and extrapolation to determine that CB/TCU/TS are likely/possible.

Seems like a worthwhile addition to an AUTO METAR (which, of course, is verified by a human observer...).
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