More Re The Cold Flagstaff Sounding On December 31st

Here is the Flagstaff, Arizona, sounding for 00Z 31 December 2010 (again). David Blanchard has commented below and provided the dates of the ten coldest 500 mb observations for both Winslow and Flagstaff. The longer period of record at Winslow leads to most of the coldest observations occurring there.
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PRES HGHT TEMP DWPT WIND HOUR DATE STID
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500 5432 -35.6 -41.2 024/31 12 02-MAR-1971 KINW

500 5395 -35.8 -45.2 305/23 12 08-DEC-1978 KINW

500 5422 -35.8 -46.2 240/25 00 27-DEC-1988 KINW

500 5362 -35.9 -43.1 275/04 12 11-FEB-1965 KINW

500 5360 -36.0 -39.6 262/31 12 04-MAR-1976 KINW

500 5400 -36.1 -40.3 285/19 00 27-DEC-2008 KFGZ

500 5314 -36.4 -45.7 250/31 12 07-DEC-1978 KINW

500 5250 -37.5 -43.5 285/33 00 31-DEC-2010 KFGZ

500 5364 -37.8 -46.8 298/44 12 03-MAR-1966 KINW

500 5282 -39.4 -43.3 266/19 12 15-FEB-1990 KINW
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However, the sounding several days ago turns out to be the third coldest at 500 mb in the observational record for northern Arizona. It is also of note that the December 31st sounding appears to have the lowest observed 500 mb height in the period of records. Quite an impressive system to end 2010.