Blizza Blizza

Every time a major hurricane — or even something so mundane as a severe thunderstorm — hits an area served by the major commercial airlines, folks at work start watching the Automated Surface Observation System (ASOS) reports from the airport. We do this partially because we get bored easily and partially because it’s interesting by itself to see extremes of weather reflected in the reports, which look something like this:

KMSY 010100Z 04005KT 9SM -RA FEW030 BKN055 OVC090 23/22 A2978 RMK AO2 TSE0058 OCNL LTGIC DSNT SW NW CB DSNT SW NW P0000

Translation: New Orleans International (KMSY) observation on 01 [September] at 0100 Zulu (GMT; New Orleans is five hours behind GMT, so this would have been 8 PM local time on 31 August). Winds from 040 degrees at 05 knots, visibility 9 statute miles, light rain (the - in front of RA indicates light, and RA indicates rain). Few clouds at 3000’ above the field, broken layer at 5500’ above the field, overcast layer at 9000’ above the field. Temperature of 23¬?C, dewpoint of 22¬?C, barometric pressure 29.78” Hg.

Remarks: AO2 indicates an automated observation station with a precipitation discriminator (it can tell the difference between, say, rain and snow, or snow and hail, etc.). TSE0058 indicates a thunderstorm ended at 0058Z, or two minutes prior to this report being generated. OCNL LTGIC DSNT SW NW means occasional lightning in clouds, distant to the southwest and northwest. CB DSNT SW NW indicates cumulonimbus clouds (i.e., thunderclouds) distant to the southwest and northwest.

It gets really easy to read these once you’ve read a couple thousand of them, trust me.

Anyway, now we get to the CSI part. Here are some of the ASOS reports from KMSY as Hurricane Gustav came ashore 70 miles to the southwest of New Orleans, presented in chronological order:

KMSY 010153Z AUTO 01010KT 10SM FEW060 25/23 A2978 RMK AO2 TSE0058RAE16B33E45 SLP086 LAST AUGMENTED OBSERVATION UNTIL FURTHER NOTICE P0000 T02500228 TSNO

The “TSNO” at the end means thunderstorm information not available. In other words, the sensor crapped out. This gets interesting later on…

KMSY 010853Z AUTO 04026G41KT 9SM FEW022 BKN029 OVC034 26/23 A2950 RMK AO2 PK WND 03041/0846 RAB09E18 SLP992 P0000 60000 T02610233 58058 TSNO
KMSY 010953Z AUTO 04025G39KT 3SM RA BR SCT017 BKN023 OVC029 26/24 A2947 RMK AO2 PK WND 04042/0919 RAB10 SLP981 P0006 T02560239 TSNO
KMSY 011053Z AUTO 04032G46KT 3SM RA BR BKN020 OVC027 26/24 A2940 RMK AO2 PK WND 04048/1042 SLP959 P0014 T02560239 TSNO
KMSY 011105Z AUTO 05036G49KT 2SM RA BR BKN020 OVC026 26/24 A2939 RMK AO2 PK WND 05049/1103 P0005 TSNO
KMSY 011114Z AUTO 04039G55KT 1 1/4SM +RA BR SCT018 OVC025 26/24 A2938 RMK AO2 PK WND 04055/1108 P0014 TSNO

Note the steadily increasing wind speed (especially the gusts) and steadily falling barometric pressure and visibility (due to increasingly heavy rain), which is to be expected as a hurricane approaches. The last three observations above were all taken within a 21-minute period, from 5:53 AM to 6:14 AM local time. Then, 16 minutes later, a new observation:

KMSY 011130Z AUTO 2 1/2SM -RA BR FEW015 BKN022 OVC027 26/24 A2936 RMK AO2 PRESFR P0026 TSNO $

Wait, where’d the wind information go? Ah, there’s a hint at the end. The “$” denotes “sensor requires maintenance”. It gets better:

KMSY 011139Z AUTO 1 1/4SM BR 26/24 A2936 RMK AO2 RAEMM P0026 PWINO TSNO $

Nine minutes after the first observation with missing wind data, we now have a “PWINO” remark and a “RAEMM” remark. PWINO means the peak-wind sensor has failed, and I think (although I’m not sure) RAEMM means that the time at which the rain ended (the “RAE” part) is also missing. Why? Because the precipitation discriminator — possibly the entire precipitation sensor — has also failed. And Gustav is just getting started:

KMSY 011205Z AUTO 1/2SM FG A2934 RMK AO2 PWINO FZRANO TSNO RVRNO PNO $

By 7:05 AM local time, the ASOS had lost its wind sensor, freezing rain (FZRA) sensor, thunderstorm sensor, runway visual range (RVR, a measure of visibility) sensor, and its precipitation sensor (“PNO” is precipitation information not available). It doesn’t show in the “NO” codes, but you’ll also note that the ASOS is no longer reporting cloud cover or temperature either. The only thing it’s reporting now is rough visibility (RVR equipment is far more sensitive than the sensor that determines “1/2-mile in fog” as seen above) and the barometric pressure (which, incidentally, got all the way down to 29.20” Hg before it started coming back up).