|Tuesday Afternoon to Wednesday Morning: Change You Can Believe In!|
Let's look at this storm piece by piece.
We'll concentrate here on the University of Utah because we have a long record of 5-minute temperatures allowing for comparison with earlier storms. At the University of Utah, yesterday's maximum temperature was 71ºF, achieved just before frontal passage, which occurred between 2:45 and 2:50 PM MDT. The temperature changes accompanying the frontal passage include 12.4F in 5 min, 17.8F in 10 min, 21.1F in 1 h, and 29F in 2 h. The total temperature change from 71F at 2:30 to 32F at 8:05 was 39F.
The mother of all Salt Lake City frontal passages occurred during the 2002 Tax Day Storm. This was a beast of an event and perhaps the closest analog to yesterday in terms of the strength of the front. During the Tax Day Storm, the maximum temperature was 73ºF, also achieved just before frontal passage, which occurred between 3:20 and 3:25 PM MDT. The temperature changes accompanying the frontal passage include 6.1F in 5 min, 11.2F in 10 min, 28.0F in 1 h, and 31.4F in 2h. The total temperature change from 71.5F at 3:20 to 32F at 11:30 was 39.5 F.
By and large, these two events are very similar in terms of the temperature change. Yesterday's temperature fall appears more rapid than that in the Tax Day Storm, but that might simply be an artifact of the 5-minute sampling and sensor lag. The front could pass at any point in that 5-minute period and it is possible to miss the rapid nature of the temperature fall. During the Tax Day Storm, for example, one of our students measured a 12.6F temperature fall in 10 seconds with a hand-held weather station. We conclude that both events featured a remarkable temperature drop. However, the total fall during yesterday's storm occurred in a shorter period, enabling a more dramatic transition from extreme dust to snowfall in just 3.5 hours.
The winds during yesterday's storm were impressive in both strength and coverage. Going into the storm, I suspected we would see some valley gusts to around 60, but the event was even stronger with widespread gusts > 60 mph and some > 80 mph. Peak gusts at valley and canyon locations include:
Parleys Canyon (5126 ft): 92 (during fropa)
Eureka (6584 ft): 87 (prefrontal)
Saratoga Springs: 87 (postfrontal)
English Village (Dugway): 82 (during fropa)
I-80 MP1: 81 (post frontal)
GSL Marina: 81 (likely during fropa, but no temp obs and wind direction squirrelly)
Lehi: 81 (postfrontal)
Baccus: 75 (postfrontal)
BCC S-Curve: 73 (prefrontal)
Wendover Port: 72 (postfrontal)
Highland: 71 (postfrontal)
Spaghetti Bowl: 66 (postfrontal)
Salt Lake International Airport: 64 (postfrontal)
Although the southerly pre-frontal flow was quite strong and produced an impressive dust storm, the strongest winds at most locations occurred shortly after the frontal passage. A notable exception is the S-Curve site in Big Cottonwood. That's deep within the lower canyon and it is quite remarkable that it got a gust to 73.
Modis imagery from yesterday shows significant dust being transported to the Wasatch Front from several sites in southwestern Utah. For the Salt Lake Valley, key sources include the Sevier Lake Bed, Sevier Desert to the east, and the Escalante Desert west of Cedar City. Further to the west, one can see dust behind the cold front over southern Nevada.
PM2.5 levels reached hazardous levels during and around the time of frontal passage in the afternoon. At Hawthorne Elementary, the hourly average PM2.5 reached 279.8 ug/m3. I have never seen anything that high in the Salt Lake Valley during our dreaded inversion events.
Dust events that reduce visibility at the Salt Lake Airport to 10 km or less occur, on average, 4.3 times per year, although there are large variations from year to year (Steenburgh et al. 2012). From 1930-2010, 2.6% of these events featured visibilities of 1/2-1 km and 2.0% feature visibilities < 1/2 km. Yesterday we were in the former category at the airport. I don't have access to a long record of hourly PM2.5, but Hahnenberger and Nicoll (2012) note that during the dust event of 19 April 2008, PM2.5 levels at Hawthorne Elementary reached 191 ug/m3. For hourly PM2.5, yesterday was probably amongst the nastiest events. For daily PM2.5, Hahnenberger and Nicoll (2012) report a maximum of 49.9 at Hawthorne (and 55.7 at Lindon) on March 30, 2010. We didn't reach those yesterday due to the short-lived nature of the event.
Snowfall yesterday evening was associated with the frontal passage. Overnight, snowfall was generated by several mechanisms, with some contributions from lake-effect processes. Our Ute Weather Center reports that we are closing in on 7 inches on campus. The Alta Collins stake hit 18 inches as of 8 am. Not much to say here except HOORAY! For those of you up there, get some faceshots for me. Ski touring? Keep in mind that the Utah Avalanche Center has issued an avalanche warning. Keep powder fever in check.