Friday's snowstorm in Fairbanks was unusual from at least a couple of standpoints. First, the total of 10.5 inches is a hefty fall in Fairbanks; more winters than not never see a snowfall this heavy in the span or one or two days, and sometimes it doesn't happen for several years. For instance, from winter 1997-98 through 2007-08, it happened only once (January 2000).
Still more unusual was the low temperature during such a heavy snowstorm. Nearly all of the snow fell with a temperature below 0°F, and the high temperature of 13°F didn't occur until late Friday when the snow was ending and a westerly breeze picked up and mixed out the low-level inversion. Here's the balloon sounding from Fairbanks airport at 3pm on Friday, when the substantial snow was just ending: notice the sharp low-level inversion, with surface temperatures around -20°C (-4°F).
Typically a heavy snowstorm is much warmer, because the strong flow required to transport and dump all that moisture also mixes the warmer air aloft down to the surface.
Remarkably, the heaviest rate of snowfall occurred in Fairbanks on Friday morning when the temperature was -11°F. Looking back at the full history of observations from Fairbanks, only one year in the past saw moderate or heavy snow reported at a colder temperature: it occurred twice in 1955 (-14°F in January and -16°F in December). It is simply very rare to have heavy snowfall rates at these temperatures in Fairbanks.
Looking at calendar days with more than 10 inches of snow in Fairbanks (of which there are only 20 in the last 95 years!), Friday was the coldest such day on record. The previous coldest high temperature was 15°F on February 11, 1966, and December 29, 2016. Friday's daily mean temperature of -1°F was also the lowest daily mean temperature for such a snowy day, the previous record being +1°F.
Why so cold? It seems the explanation lies with the fast-moving nature of the disturbance and the fact that it occurred without any warm, southerly component to the mid-atmosphere flow. There was no fundamental change in the cold pattern, with a strong ridge hanging out over the southern Bering Sea throughout the entire event, and the mid-atmosphere flow merely went from northerly to westerly and then back to northerly. Here are 500mb maps from early Thursday, Friday, and Saturday mornings:
The surprise to me is just how much snow fell with such a fast-moving westerly disturbance; I would have expected something closer to 3 or 4 inches. But this was no quick burst of snow from a fast-moving front; visibility was 1 mile or less for more than 8 hours. An unusual event, indeed.
Well told Richard! I was out with my Honda blower for 3 hours cleaning up the yard. The snow was quite light and fluffy....I suspect in town the SWE was lower than normal events like this. Then the remainder fell out as temps cooled as ice crystals. Now it's -36F and clear. Probably like this until February when La Nina goes to sleep.
ReplyDeleteThe 10.5 inches of snow contained .38 inches of water, according to the NWS. The previous record snow for Dec. 12, 9.5 inches in 2012, contained .72 inches of water.
ReplyDeleteSo each inch of snow that fell on Dec. 12 this year contained just 49 percent of the water held by an inch on Dec. 12, 2012.
The result: fluff.
Hey thanks for the SWE info....didn't shear my blower pin that's for sure.
DeleteI just want to say how much I enjoy reading this site. You could write about the color of dirt and it would be a good read!
DeleteNice write-up. Good extra info on the water equivalent, such a better measure of snow. Still a big snow for such a coldish atmosphere and W-NW flow aloft. And I can't remember such a low tropopause, or maybe I just haven't been looking enough.
ReplyDeleteI was dismayed to be out of state during this event, but this post captures succinctly just what occurred. Absolutely fascinating stuff, and I am so happy we have a local who can delve into the deeper facts of why certain wx events are occurring here. Thank you, as always.
ReplyDelete