Updated: May 1, 2019
One of my pet peeves has always been the reporting of hail in a routine surface observation (METAR) or special observation (SPECI). If you look at the Federal Meteorological Handbook it provides two codes for hail, GS and GR. GS was reserved for small hail and snow pellets. Following a safety risk assessment, the FAA decided the best course of action was to change the coding of the GS (small hail and snow pellets) and GR (large hail to greater than or equal to 1/4 inch) to the following:
GR refers to all hail. All reports of hail must include hailstone size diameter in the Remarks (RMK) section of the METAR/SPECI in increments of 1/4 inch. If no hail size is reported it will be assumed to be small hail. Small hail will result in the issuance of a SPECI. GS is used only when snow pellets are observed.
This change will be effective starting September 13, 2018. Per Federal Aviation Administration (FAA) Notice 8900.275 from winter 2014-2015, small hail (GS) was determined to be meteorologically equivalent to moderate ice pellets (PL) for anti-/de-icing purposes. This change meant new de-icing tables for the airlines and necessitated a change to the METAR code in the U.S.
What will be interesting is to see how many of the METAR decoders in the various apps/websites are changed to support this. I suspect you'll see GS decoded as "small hail or snow pellets" for decades to come due to the lack of expertise in weather in those organizations.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS research meteorologist