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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rvernica
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Are you sure it's a capital i and not the digit 1? – DeepSpace Jan 19 '19 at 19:46
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Yes, it is capital "i". I copy and pasted the METAR. If you goggle I1001 you might get some METARs with it. https://www.google.com/search?q=metar+i1001 – rvernica Jan 19 '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:
- (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.
Ryan Mortensen
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Pondlife
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1That 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. https://www.ofcm.gov/publications/fmh/FMH1/FMH1_2017.pdf Ice Accretion was added to the handbook about a year ago. – Jimmy Jan 23 '19 at 05:43
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