Statewide Cold

The widespread cold of the last week in Alaska has been really exceptional for this late in the winter.  Only a quick glance at UAF's statewide temperature index is needed to confirm this - see the chart below.  The index has been nearly pegged at the bottom of the scale since last Wednesday.

Recall that the index scale is designed to indicate the departure from normal in terms of frequency, so an index value near -10 corresponds to near-record cold for the time of year (compared to the 1991-2020 climate).

The calculation behind the UAF index is actually the average standardized temperature anomaly across 25 observing sites, taking into account the reduced climatological variance at more maritime locations and the greater variance in the interior.  Last Friday (Feb 27), the average anomaly was 3.1 standard deviations below normal, which is the lowest daily value since the remarkable cold snap of April 2021.  Prior to that, the only dates in the past 20 years with such anomalous statewide cold were in April and May 2013 - the year of the record late breakup at Nenana.

If we only look at December through February, it's interesting to note that only ONE other event since 1991 produced a daily statewide anomaly below -3 SD, and that was early February 1999.  The cold back in December and early January this winter didn't reach anywhere near the same intensity, peaking at only 2.3 SD below normal on January 3 (the persistence of the cold was a more unusual aspect on a statewide basis).

Therefore in the context of normal Alaska climate at the end of February, the last week has been very extreme, a rare cold outlier.

Fairbanks exemplifies this: the low temperature of -49°F on Sunday was the coldest in March since 1911, and it tied the monthly record for the 1930-present era.  It's also the first time since 1932 that 5 consecutive nights have dropped below -45°F after mid-February.  In fact, until this winter such a string of cold nights hadn't happened at all since the New Year's cold snap of 1999-2000.

More on this winter's achievements in Fairbanks in a subsequent post.

Serious Cold

I'm on the road and away from my usual data resources at present, but a quick post is in order to highlight the remarkable National Weather Service forecast for the next few days in Fairbanks:

This morning's NWS forecast for the international airport calls for daily high temperatures below -40° from today through Sunday, i.e. 5 days in row.  This hasn't happened since the intense New Year's cold snap at the turn of the century (Dec 30, 1999 through Jan 3, 2000).

The anomaly is even more striking in comparison to the last 15 years: there have only been 2 individual days with a high temperature of -40° or below at the airport since January 2009.  Early January 2009 saw 4 straight days with a high of -40° or -41°F.

The Fairbanks record for consecutive days below -40°F is 9 days in December 1961.

And here's the forecast for Fort Yukon: chilling out below -50°F for the next several days.

Looking farther ahead, there's a distinct glimmer of hope for significant relief in the second week of January:

More Context for Cold

Another round of downslope/chinook warming brought another widespread thaw to the Tanana River valley and Fairbanks-land yesterday.  Temperatures above 40°F were widespread, including 45°F at Fairbanks airport, the highest February temperature in 20 years.  The year-to-date average temperature in Fairbanks is now only 2.2°F below normal.

Looking back again at the late January and early February cold snap, I was curious to see how the lower atmosphere (not surface) temperatures compared to past cold spells on various time scales.  However, with NWS weather balloons no longer going up during severe cold, there's some missing data for the recent event; so I extracted data from the ERA5 reanalysis.  I looked at Fairbanks 850mb temperatures and 1000-500mb thickness, with the latter being an excellent measure of the average temperature of the lower half of the atmosphere.

Here's the annual (winter) minimum for 3-day average 1000-500mb thickness:

On this time scale, the recent event was one of the coldest since 2000, but it was nowhere near as cold as the more extreme events of earlier decades.  The complete absence of seriously cold events after 2000 is quite remarkable, and suggests that the cold tail of the distribution has been dramatically curtailed since then.

Below is another perspective, showing annual minimum values of 3-day 850mb temperature (y-axis) versus 1000-500mb thickness (x-axis).  The values are quite highly correlated, of course, but there's some variability in the extent to which cold is concentrated at lower atmosphere levels like 850mb.  On a 3-day basis, the recent event was relatively less unusual in terms of 850mb temperature, with several other events being colder in recent years.

The 7-day and 14-day charts below show quite similar findings on these time scales, with the recent event being a bit more anomalous overall on a 14-day basis, as I noted before; but it was still not in the least unusual compared to earlier decades.

The two "granddaddy" cold spells of 1989 and 1999 stand out clearly here; those two events were much more extreme than anything in recent years, and also worse than anything that happened in the otherwise colder 1950s, 60s, and 70s.

Finally, it's interesting to observe that two quite extreme cold spells occurred in the last 20 years when we look at a 30-day average: late February and early/mid March of 2007, and January of 2012.

I hadn't previously realized how anomalous that 2007 event was, but in fact ERA5 shows the event having the lowest 30-day average temperature since 1950, by both metrics (thickness and 850mb temp above Fairbanks).  Given that it was so late in the winter, the surface temperatures were nowhere near as low as they would have been a few weeks earlier; but nevertheless March 2007 was the second coldest on record in Fairbanks, and 27 of 31 days had a low of -10°F or lower (the highest such number for the month).

Here's a look at the 500mb height pattern for the 30-day period: what a classic!

The 500mb height anomaly:

And the 850mb temperature anomaly, according to the older (but still basically reliable) NCEP reanalysis:

Cold in Context

Now that the cold snap is in the past, there will be lots of opportunity for post-mortem analysis when I have a chance (and when I ditch the nasty bug I'm suffering from).  For now, I just wanted to follow-up with a final number on the weekly temperature index that I used last week to show the lack of Fairbanks-Bettles severe cold in recent decades:

https://ak-wx.blogspot.com/2024/01/cold-snaps-in-el-nino.html

Here's the updated chart:

The recent steep uptrend was indeed broken, but on a 7-day time scale the coldest week was not at all unusual by historical standards.

However, the cold spell was more notable in the end for its longevity than its severity at any one time, and so there's a more significant cold anomaly on a 14-day time scale:

Here we see a most remarkable absence of 14-day cold in recent years - almost as if the climate never recovered from the record-warm winter of 2015-16 (during the last very strong El Niño!).  The lack of variability since 2016-17 is quite amazing.

But the cold snap just ended is a significant change.  Only 15 winters since 1950 had a colder 14-day period for these two climate observing sites.

Here's the 500mb height anomaly for the 13 days that had average temperature below -30°F for Fairbanks and Bettles combined.

This too is interesting, because the mid-atmosphere anomaly is right over the northern interior; and while we might expect that to be true, in fact significant cold in Fairbanks is more often associated with a trough over western Canada.  This too will be a topic for follow-up.