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Blowing snow for automated observations

Updated: Aug 18, 2023

In a bona fide blizzard you would certainly expect to see a fair amount of blowing snow, right? Well, take a look at the conditions for these three airports at the height of the nor'easter that recently pummeled the Northeast with a record snowfall.

KEWB 270539Z AUTO 03030G46KT 1/4SM R05/1600V2200FT +SN FZFG SQ VV004 M02/M02 A2958 RMK AO2 PK WND 03046/0538 P0000 T10171022

KTAN 270552Z AUTO 01018G32KT 1/4SM +SN FZFG VV003 M02/M03 A2962 RMK AO2 PK WND 04040/0536 SLP031

KFMH 270535Z AUTO 03032G52KT 1/4SM SN OVC002 M01/M02 A2955 RMK A02

In the belly of this storm, these airports observed strong, gusty winds, moderate to heavy snow and even a squall, but no hint of blowing snow (coded as BLSN) in any of these automated reports. This seems rather odd, but an Automated Surface Observing System (ASOS) is not designed to report blowing snow in the middle of a blizzard. You read that right; it's designed to report blowing snow only after the winter storm has departed as seen here for the observation at Worcester, Massachusetts the following morning. Say what?

KORH 280947Z AUTO 31023G29KT 2 1/2SM BLSN CLR M14/M18 A2972 RMK AO2 PK WND 29028/0913 VIS 3/4V5 T11391178

Before an ASOS can report blowing snow (even when blowing snow conditions are detected by the precipitation sensors), all of the following conditions must also be met -

1. Visibility must be less than 7 statute miles 2. Ambient temperature must be less than or equal to 14 degrees Fahrenheit 3. Sky cover must be less than overcast or the ceiling height must be greater than 10,000 feet 4. Wind speed must be greater than 22 knots.

From the surface observations during the blizzard, only two of the four conditions were met, namely, wind speed and visibility. As a result, BLSN wasn't reported in the automated observation. Keep in mind, at airports or stations with a human observer, the observation may be augmented to include BLSN like it was here for Providence, Rhode Island during the height of the storm.

KPVD 270551Z 01025G34KT 1/2SM R05/3000VP6000FT SN BLSN FZFG VV012 M07/M09 A2965 RMK AO2 PK WND 01038/0538 SLP039

Most pilots are weatherwise, but some are otherwise™

Scott Dennstaedt

Weather Systems Engineer CFI & former NWS meteorologist

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