I thought I would let you all know that I figured out what that code means in the METAR.

The code deals with Ice Accretion:

Hourly Ice Accretion Amount:
Ice accretion for the current hour encoded in hundredths of an inch (0.01 in.) has the following format:
• I1nnn Whe “I” - is the icing indicator for the group “1" - is the reported time period (one hour) “nnn” - is the thickness accumulated to the nearest one-hundredth of an inch (0.01 in.). A trace amount is encoded as “000.”

Three (3-) Hourly Ice Accretion Amount:
• The accretion of ice over the past three hour time period in one-hundredths of an inch (0.01 in.) would have the format: I3nnn Whe “I” - is the icing indicator for the group “3" - is the reported time period (three hours) “nnn” - is the thickness accumulated to the nearest one-hundredth of an inch (0.01 in.). A trace amount is encoded as “000.” The remark may be encoded at the intermediate synoptic times (0300, 0900, 1500, or 2100 UTC). When conditions warrant, the “I3nnn” remark will be encoded immediately following the hourly ice accretion amount (I1nnn).

Six (6-) Hourly Ice Accretion Amount:
• The accretion of ice over the past six hour time period in one-hundredths of an inch (0.01 in.) will have the format: I6nnn Whe “I” - is the icing indicator for the group “6" - is the reported time period (six hours) “nnn” - is the thickness accumulated to the nearest one-hundredth of an inch (0.01 in.). A trace amount is encoded as “000.” The remark may be encoded at the mandatory synoptic times (0600, 1200, 1800, or 0000 UTC). When conditions warrant, the “I6nnn” remark will be encoded immediately following the hourly ice accretion amount (I1nnn).

This could be very helpful to us but the NWS has not installed this update to all stations yet.