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Here is an actual METAR example:

KOMA 190352Z 36017G26KT 3SM -SN OVC012 M08/M11 A3014
RMK AO2 PK WND 36027/0302 FZDZE37SNB27 SLP220 P0000 I1001 T10831111 $

What does the I1001 mean? I have also seen I1000, I1002, I1003, and I1004.

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  • $\begingroup$ Are you sure it's a capital i and not the digit 1? $\endgroup$ – DeepSpace Jan 19 at 19:46
  • $\begingroup$ Yes, it is capital "i". I copy and pasted the METAR. If you goggle I1001 you might get some METARs with it. google.com/search?q=metar+i1001 $\endgroup$ – rvernica Jan 19 at 23:48
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It indicates ice accretion (buildup). This seems to be very obscure, I finally found it in some NOAA release notes for ASOS software v3.07:

  1. (S01126) ADD ICE ACCRETION REMARK TO METAR/SPECI REPORTS

Discussion: The amount of ice accreting on a flat surface (planar icing) as estimated by the data from the freezing rain sensor, will be included in the remarks section of the METAR / SPECI reports. The remark will have the format of “Ihnnn,” where “h” is the hourly time period, i.e., 1, 3 or 6 hours. The value “nnn” is the ice thickness accumulated to the nearest 0.01 inch.

Please Note: In situations when FZRASN is manually entered in the Present Weather field, an inappropriate trace accretion amount (I1000, I3000, I6000) might be entered in the remarks section of the METAR / SPECI reports. This is a known issue and will be addressed at a later time.

Based on that, "I1001" means ice buildup of 0.01 inches in the last 1 hour.

And thanks to Jimmy for finding a more official source: FCM-H1-2017, Federal Meteorological Handbook No. 1 - Surface Weather Observations and Reports (p. 12-18) from OFCM, the Office of the Federal Coordinator for Meteorology.

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  • $\begingroup$ That is correct, but a better source would be the NOAA instruction manual for putting together METARs and other observations. FCM-H1-2017, Para 12.7.2.i. ofcm.gov/publications/fmh/FMH1/FMH1_2017.pdf Ice Accretion was added to the handbook about a year ago. $\endgroup$ – Jimmy Jan 23 at 5:43
  • $\begingroup$ @Jimmy Great find! I've added that to my answer $\endgroup$ – Pondlife Jan 25 at 2:44

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