First a note on the late winter cold snap that is unfolding at the moment. Fairbanks dropped to -46°F yesterday morning and -48°F this morning, a remarkable intensity of cold for this late in the winter. This morning's low temperature is in fact the coldest observed this late in winter since 1956, and few events can rival it. Here are the most notable cold readings at similar dates in the 1930-present Fairbanks history, with the 1932 and 1964 events surely being the most anomalous:
Feb 20-28, 1932 -50°F or colder for 9 consecutive nights
Feb 23, 1933 -49°F
Feb 26, 1954 -50°F
March 1, 1956 -49°F; -45°F on the 8th
March 15, 1964 -46°F
Two more nights of intense cold are possible before the current chill eases.
Looking back at the past couple of weeks, there were two episodes of sustained blowing snow, which is very uncommon in the sheltered Fairbanks bowl. Last Thursday the airport reported blowing snow for 15 straight hours, which appears to be the second longest uninterrupted duration of blowing snow observations since hourly observations began in 1948. The record is 17 straight hours exactly 15 years earlier, in late February 2011.
Then on Tuesday another 8 hours of blowing snow was reported as the cold air moved in after the snowfall. That takes this winter's total to 25 hours, including a brief episode in January, and the historical data shows this is close to a record - see below.
The pronounced drop-off in blowing snow from the mid-1970s through 2005 is very striking, and at first glance it's tempting to ascribe this to changes in observational practice. However, the PDO may again be a plausible explanation, as the PDO flipped positive in 1976. There does seem to be a correspondence between heightened frequency of blowing snow and the most negative PDO periods of 1950-1956, 1970-1975, 2008-2013, and 2021-present.
It's also interesting to observe that blowing snow is easily most common (least uncommon) in the month of February, both in terms of total number of hours and number of unique days and years when it occurs - see below.
I would welcome suggestions for why February is particularly favored for blowing snow, because the wind is not significantly higher than earlier winter months; the frequency of strong breezes (e.g. 15 knots) is barely any higher than December.
Without being an expert on snow pack properties and the physics of blowing snow, I'd suggest that the return of significant solar warming in February may be a key factor, allowing the snowpack to warm from its midwinter lows. As for March, the stronger winds might be expected to produce still more blowing snow, but by that time the sun is often warm enough to melt the snowpack surface, forming a hard crust, and the frequency of blowing snow drops back again. March also tends to be the driest of the winter months, so there's less new snow to blow around.
NOAA defines blowing snow as "wind-driven snow that reduces surface visibility to less than 7 miles. It can consist of either falling snow or snow that has already accumulated but is picked up and blown by strong winds."
ReplyDeleteHas this definition always been used so we can compare over the years?
Also, if heavy snowfall can be interpreted as blowing snow, how do we differentiate over ground snow picked up by wind?
The latest snow storms had very dry snow compared to other snow events. Obviously, drier snow is more easily blown. So what was the water content of the snow of the various blowing snow events throughout the record?
Hi Eric, blowing snow also involves the requirement that snow is lifted from the ground to above 6 feet. If less, it is "drifting snow". New falling snow does not affect the blowing snow classification, because it's not lifted from the ground.
DeleteI don't know how the definition has changed over the decades, and certainly that could affect the historical frequency.
Interesting findings, Richard.
DeleteI notice that on the 24th they were reporting blowing snow with 10 miles visibility for a few hours, ie:
PAFA 242153Z 25015G22KT 10SM BLSN SCT023 BKN035 M17/M22 A2977 RMK AO2 PK WND 26027/2125 SLP093 P0001 T11671217
This may not be significant to the topic but along with other changes (automated station, perhaps changes in human augmentation) it does complicate the picture.
Hi Jim, I noticed they also had BLSN with relatively light winds, as low as 8kt (no gust reported) on the 20th. The ASOS manual actually specifies that ASOS will only report blowing snow when wind speed is greater than 22kt, so human augmentation is clearly a major factor here. But I'm not sure why they wouldn't follow the usual visibility criterion.
DeleteRegarding the historical continuity, I am sure PAFA observations have been augmented for at least a majority of the ASOS era (being a top-tier service level airport), and prior to ~1998 it was all manual of course. But certainly there are questions around how the criteria have changed and whether the observers have been consistent/objective.
In my experience the blowing snow is driven by an atmospheric occlusion...a warmer front followed by a colder front that typically sweeps SW to NE across Interior Alaska. Some call it a triple point, but I'm not a meteorologist so all I see online are the prognostics. And we experience the local effects in Fairbanks. Due to SW hills there is some orographic enhancement due to downslope compression of winds experienced previously in Nenana and others to our SW.
ReplyDeleteThanks, Gary
DeleteBasically it clouds up from the S or SW, and it may warm some. Then it snows for whatever interval. West of Fairbanks at Lake Minchumina, Ruby, Tanana, then Nenana westerly winds commence to blow and surface temperatures drop quickly. The snow clouds thin and start moving NE. The wind affects the surface and it blows eventually in Fairbanks. I assume how long the frontogenesis (?) lingers or how widespread it is determines the duration of the blowing snow event.
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