Hell of a storm yesterday in Little Cottonwood Canyon with copious amounts of graupel. Observations from the Alta-Collins site showed a storm total of 19" of snow and 2.70" of water which fell over a period of 18 hours. Water equivalent rates remained above 0.2" per hour from 9 AM to 3 PM, above 0.37" per hour from 11 AM to 2 PM, and peaked at 0.41" from 12 to 1 PM. The frontal structure during this event was complex, but I'd say the heaviest precipitation period was prior to and possibly during the frontal passage.
Watching the radar, it appeared that the Alpine Ridge between Little Cottwonwood Canyon was the locus for heavy precipitation development and that is confirmed by the MRMS Radar only precipitation estimate below which showed an elongated area of more than 1.25" of water and a maximum of up to 1.50" from just west of the American Fork Twin above Snowbird to Sunset Peak just east of Alta.
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| Source: https://mrms.nssl.noaa.gov/qvs/product_viewer/ |
We have been looking at extreme events like this in Utah and around the world in collaboration with several scientists including Baker Perry, a geographer and mountaineer who has installed high-altitude weather stations on Mt. Everest and in the tropical Andes. In the case of the latter, they have observed hourly maximum water equivalent of snowfall rates of 20.3 mm (0.80") at Laguna Sibinacocha, Peru (4,895 m), 16.6 mm (0.65") at Chacaltaya, Bolivia (5,160 m), and 14.5 mm (0.57") at Quelccaya Ice Cap, Peru (5,650 m). We've been investigating how extreme these are.
Peter Veals at the U combed through the Alta-Collins data we have stored in MesoWest (special thanks to the Alta Ski Patrol) and it appears that the "mother of all Alta graupel fests" occurred on 5 January 2008 when an incredible 3.99" of water equivalent fell in 21 hours. Hourly water equivalent rates were at or above 0.35" per hour for 5 consecutive hours, totaling 2.18" during the heart of the storm. Hourly water equivalent rates reached 0.54", which is the record at Alta-Collins.
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| Source: MesoWest/Alta Ski Patrol |
Radar imagery at the time of maximum water equivalent snowfall rate was very similar to yesterday with a reflectivity maximum parked right over the Alpine Ridge, again ahead of the approaching cold front which in the image below is producing precipitation to the northwest over the Great Salt Lake.
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| Source: NCEI |
So, yesterday's graupel fest was impressive, but there have been bigger. At issue is what are the processes that contribute to such extremes? It does appear that the interaction of the SW-WSW flow with the Alpine Ridge is probably critical as that ridge appears to serve as a locus for localized storm and graupel development in both cases, but why doesn't this happen more frequently? I don't have answers to these questions, but it would be fun to investigate.
I was happily post-holing up the Jacob's Ladder trail on Wednesday morning toward Enniss Peak before I received a call about a new employment opportunity and turned around. I wanted to see what the extreme warm temperatures had done to the south facing snow pack. Pretty sure I could have made it most of the way toward the 9K foot summit without even the need for microspikes.
ReplyDeleteProbably a little different now!
I have been down near the town of Alpine at least once during one of these graupel events. During strong southwest flow, if you have lots of moisture in the 600-750 mb range and an unstable or neutral lapse rate, the lift over the terrain is incredible. The cloud base really looks like a convectively driven storm and may have similar rates of parcel ascent, with that terrain catch area acting like a giant funnel. You can go from only cloudy skies to heavy graupel or snow in just a couple miles there.
ReplyDeleteVery interesting post. Thank you. Just before reading this was checking my home weather station data to see how last Wednesday’s wind on the benches compared to the max at my house. It’s very fun as a hobby with simple data, can imagine that as a professional it’s fascinating. Anyone that’s into weather and has a few hundred bucks to spare (and has a place they’re going to live in for a bit) should give it a go. Also good material to annoy the neighbors with.
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