Record Chukotka Warmth

Unseasonably cold conditions are hanging on in Fairbanks-land, with temperatures around 10-20°F below normal for the time of year, but some really exceptional warmth occurred yesterday in Russia's Chukotka district, on the other side of the Bering Strait.  Temperatures rose above freezing in a zone stretching from the northern Sea of Okhotsk and the Kamchatka Peninsula up to the Arctic Ocean, and the town of Ostrovnoe at 68°N saw a high of 42°F.

The location of Ostrovnoe (see the map below) means that it is usually a very cold location in winter; it's in northeastern Siberia, above the Arctic Circle, 100 miles south of the Arctic Ocean, and in a river valley.  The normal high temperature in January is well below -20°F and so it's considerably colder than any inhabited place in Alaska.  According to my calculations, yesterday's high temperature was 64°F above the 1981-2010 normal, and the low temperature of 33°F was a remarkable 69°F above normal.  The GHCN daily data from 1936-present (which is a bit patchy in some years) indicates that this is the first time Ostrovnoe has seen a daily minimum above freezing in January or February.

To put these daily anomalies in context, the largest positive daily temperature anomalies on record in Fairbanks are just over 50°F (for example, a high of +52°F on January 16, 2009, compared to normal of about 0°F).

Here's a chart of daily mean temperatures in Ostrovnoe since November 1; the daily mean temperature was 67°F above the 1981-2010 smoothed normal.  It should be noted that this is actually not unprecedented: December 21, 1955, was 68°F above the 1981-2010 normal (and presumably even farther above the then-colder normal).

The profound temperature gradient across the Bering Sea is being caused by an intense ridge of high pressure centered right over the date line - see this morning's 500mb height analysis below.  Powerful southerly flow to the west of the ridge is pulling warm air up into eastern Siberia and the high Arctic, but northerly flow is keeping Alaska rather cold.

The map below shows the average 500mb height anomaly in January and February, for 10 winters with strong La Niña conditions; a Bering Sea ridge is one of the main elements of the mid-high latitude atmospheric response to La Niña.  Although the current La Niña episode is not particularly strong, the current flow anomaly clearly fits the pattern rather well.

Deep Cold and Persistent Snow

[Update 5am Jan 26:  Snow ended before 2pm yesterday in Fairbanks; one more hour would have tied the record for most hours of snow in a 7-day period.  As for consecutive hours with snow, this ended at 69 hours - nowhere near a record, although 45 hours of snow with temperatures below -20°F does tie a record from 1999 (thanks to Brian Brettschneider for that one).]

Very cold air associated with the upper-level low over the western interior has spread east in the past 36 hours, and as a result temperatures have dropped markedly in the Fairbanks area despite plentiful cloud cover and continuous light snow.  Normally the higher elevations in the hills provide some refuge from the valley-level cold, but this morning's sounding shows no low-level temperature inversion; however, there's a very deep inversion extending up to 700mb, and this is more characteristic of a true Arctic air mass.

Temperatures at the standard levels of 925mb (~2000' above ground) and 850mb (~4000') are very low this morning; the 850mb temperature of -30.3°C is the lowest since January 2013, and the 925 mb temperature of -32.1°C is the coldest since late January 2012.

Here are the minimum temperatures (°F) at the surface from the last 24 hours; colder conditions occurred just to the west of Fairbanks where the sky cleared out for a time yesterday.

Also of note is the persistence of the light snowfall in Fairbanks in recent days, caused by the lingering zone of ascent aloft to the east of the upper-level low.  As of the latest hour (6am AKST), snow has been falling continuously for 61 hours.  This is nowhere close to the record from late December 1984, when snow was reported every hour for 5 and a half days (133 hours).

However, if we look at the number of hours of snow in the past 7 days, we are getting close to a record: starting last Friday, there have only been 13 observations at the top of the hour that did not report snow falling - it has been snowing for more than 90% of the time in the past 6+ days.  Based on data since 1950, the lowest number of no-snow hours in a period of 7 calendar days (midnight to midnight) is 22 (in January 1996), so this record will be broken if Fairbanks gets another 9 hours of snow before midnight tonight.

For reference, here's this morning's 500mb analysis.  It's interesting to note the very cold air aloft to the east of the low; the temperature of -41°C at 500mb above Fairbanks is accompanied by wind from the south-southeast.

Wintry Chill

The coldest conditions of the winter so far have developed in western and northern Alaska over the past few days, and today there is quite an extensive area across the northwest with temperatures in the -30s and below.  So far it's a fairly run-of-the-mill cold snap for this time of year - after all the normal seasonal cycle of temperatures reached its low point just a few days ago in the interior - but it's very cold nonetheless.

Here are the lowest temperatures that I've seen so far:

-57°F  Clear Creek HADS site, 40 miles NE of Huslia
-54°F  Norutak Lake RAWS, near the upper Kobuk River at the southern edge of the Brooks Range
-52°F  Huslia airport
-51°F  Hozatka Lake SCAN site, between Galena and Huslia

This morning's FAA webcam photo from Huslia shows the calm and clear conditions that we expect at these temperatures.

Looking back at the winter thus far, there were 3 earlier spells when the temperature dropped below -40° somewhere in Alaska, but up until now the coldest conditions have been restricted mainly to the southeast interior.  The chart below shows the daily minimum temperature in Alaska since October 1, and the red markers denote days when the minimum occurred (or was tied) at the settlement of Chicken.  On another 8 days the minimum was nearby on the Fortymile River, and other southeastern sites show up for a number of other days - so from early November through mid-January the daily statewide minimum was nearly always in the southeast interior.  In the current cold spell, however, clouds have blanketed most of the eastern part of the state, keeping temperatures higher (including in Fairbanks).

The maps below show this morning's 500mb and surface analyses, courtesy of Environment Canada.  Note the strong cold low aloft with a strong contrast between dry northerly flow (thus clear and cold) to the west and moist southerly flow (thus cloudy and milder) to the east.

Freezing Rain

Freezing rain created a fair bit of disruption in the Fairbanks area this week, with an initial round of light rain for a couple of hours on Monday morning followed by much more persistent and significant rainfall on Tuesday evening and overnight into Wednesday.  A very substantial snowfall also occurred, and the liquid-equivalent precipitation total was well over half an inch; this was the wettest January storm in 18 years.  There was also enough wind to produce Fairbanks' first ASOS report of "blowing snow" this winter - not a common occurrence in the sheltered Fairbanks bowl.

Here's the surface analysis from early Wednesday morning, courtesy of Environment Canada; a trough axis and low pressure center are indicated very close to Fairbanks.  The Fairbanks sounding from the same time is also shown below - note the strong southwest flow throughout the troposphere and the high precipitable water (total moisture content) of 1.22 cm.

In view of this rather significant freezing rain event, it's interesting to look back at the long-term history of similar conditions in Fairbanks winters.  The chart below shows the percentage of hourly observations in Fairbanks with either drizzle or rain reported at the top of the hour, for winters back to 1950-51  (thanks to Rick Thoman for the idea for this chart).

There has certainly been an uptick of winter rain and drizzle events in recent years, but the 1960s saw a couple of remarkable winters with a lot of freezing precipitation.  In particular, January 1963 stands out as highly anomalous; here's how it unfolded:

- freezing drizzle for 19 hours beginning at 9am on the 14th
- 6 hours of freezing rain on the evening of the 15th
- 4 more hours of freezing rain on the evening of the 16th
- freezing rain throughout the first half of the 19th, ending as plain rain in the afternoon when temperatures rose above freezing
- the afternoon sounding on the 19th recorded the highest precipitable water on record for the month of January in Fairbanks (1.66 cm)
- more plain rain on the morning of the 20th
- intermittent freezing drizzle and a bit of freezing rain on the 25th and 26th
- a few more hours of freezing drizzle late on the 29th and overnight Jan 30-31

Road conditions must have been somewhere between treacherous and impossible by the end of that month.  A quick look at the 500mb height map for January 1963 (below) shows an interesting dipole pattern, with much above-normal heights near and south of Alaska and also near Iceland.  The Arctic Oscillation was strongly negative, indicating a weak circumpolar flow that allowed cold air to spill south and, conversely, warm air to surge north.  Consequently it was a very cold month in the lower 48, and Europe was extremely cold; England had its coldest calendar month of the entire 20th century.  In some ways the topsy turvy pattern resembles recent conditions: both winters (1963 and 2018) produced headline-making cold at lower latitudes but exceptional warmth in Alaska and the Arctic.

Warmth on the Kuskokwim

Western Alaska's exceptionally warm weather is in the news again today, as open water on the Kuskokwim River has forced a change of plans for the Kuskokwim 300 sled dog race that begins this Friday:

https://www.adn.com/outdoors-adventure/2018/01/16/unprecedented-open-water-causes-route-change-for-kuskokwim-300-sled-dog-race/

I mentioned the warmth in Bethel a few days ago, but it really is remarkable: since October 1, the total number of freezing degree days is easily the lowest in the modern record (1930-present).  As a reminder, freezing degree days are a simple measure of accumulated "freeze units"; we just sum up all positive daily differences between 32°F and the daily mean temperature.  So far this winter the freezing thermal units have amounted to less than 40% of the 1981-2010 normal for the same period.

The previous lowest FDD total for October 1 - January 15 was in winter 2000-2001, which turned out to be Bethel's warmest winter on record overall (November - March).  It was an analogous winter in some respects, with a weak La Niña and without a negative PDO phase to match.  The absence of a negative PDO this winter is the most obvious reason for the lack of colder weather that some of us expected to accompany the La Niña episode.

As another illustration of just how warm it's been in Bethel, consider that more days so far this winter (since November 1) have had a high temperature above freezing (37 days) than below freezing (32 days).  This too is unprecedented for 1930-present.

Chena Basin Snowpack

The Fairbanks area has seen no significant precipitation in over 3 weeks now, but a look at monitoring data from SNOTEL sites in the nearby hills reveals that the snowpack is healthy; indeed the current water content of the snowpack ranges from 124% to 184% of normal, where "normal" is the 1981-2010 median at each location.  Somewhat remarkably, most of the sites already have about as much snow on the ground as they normally do at the end of the snow season; and the Munson Ridge SNOTEL, the highest site at 3100' elevation, is already reporting over 8" of water in the snowpack.  Here's a chart of normal snowpack trajectories that I showed last spring.

Down at valley level in Fairbanks the snowpack is close to normal for the time of year, but there's no question that precipitation has been above normal so far this winter.  Higher than normal precipitation has been observed widely across interior, northern, and western Alaska; here are the liquid-equivalent precipitation totals (percent of normal) from November 1 through January 10:

Fairbanks 134%
Bettles 185%
Nome 145%
Kotzebue 266% (a record 4.39" total, previous record 3.51" in 2010-2011)
McGrath 100%
Bethel 115%
Northway 107%

Looking more closely at the Chena Basin SNOTEL data from recent years, it's interesting to note that this is shaping up to be the 8th consecutive year with generally above-normal snowpack in the latter part of the snow season.  The chart below shows how each of these winters fared, and while some (like last winter) obviously had a slow start, each of the past 7 years was above the 1981-2010 normal by March.  This isn't quite what I expected to find, as several recent winters were exceptionally warm in Fairbanks, and historically most very warm winters are also dry.

The April 1 snowpack data from the past several years show surprisingly little variance compared to earlier decades (see below).  None of the years has been particularly unusual in isolation, but taken together it has been quite a string of relatively snowy winters in the Chena Basin hills; and this year seems to be continuing in the same vein.

Sea Ice Update

At the beginning of last month we noted the remarkable absence of early-winter sea ice in the Bering and Chukchi Seas, and in view of December's warmth it will surprise no one to learn that ice formation has continued to lag far behind normal for the time of year.  In the satellite era (1978-present), the Chukchi Sea ice extent has never before failed to reached 100% by the end of the year, but this year about 5% of the area remained open (less than 15% ice coverage) on December 31, according to NSIDC's regional sea ice index.  The ice extent finally exceeded 99% yesterday.

Here's a plot of the December average ice extent in the Chukchi and Bering Seas.  For the Chukchi, December 2007 ice extent (occurring in the wake of the then-record September melt-out) was nearly as low for the month as a whole, but the Bering Sea has seen nothing like recent conditions in the satellite era.

The warm ocean surface and slow ice growth to the west of Alaska was closely related to the exceptional warmth that prevailed throughout western and northern Alaska, as well as more widely across the Arctic, in December.  The map below shows an estimate of December's mean temperature anomaly from the CFS reanalysis.

The +6-8°C temperature anomaly estimated by the model for the Y-K delta region is supported by observations from Bethel, where the month was over 14°F warmer than normal.  The season-to-date accumulation of freezing thermal units (freezing degree days) is easily the lowest on record in Bethel, and consequently river ice conditions are very poor for the time of year across the region.

New Year's Eve in Fairbanks

How cold will it be when the ball drops on New Year's Eve in New York City is, for better or worse, in the news, so I (Rick T.) thought it might be interesting to look at New Year's Eve temperatures in Fairbanks.

With a little work I was able to pull together the temperature near midnight each December 31st since 1941 (thanks to Brian B. for a last minute data access rescue). These are the temperatures from the the Weather Bureau Cushman Street office for 1941, Ladd Field 1942, Weeks Field 1943-50 and Fairbanks International since New Years Eve 1951. I've used the observation closest to midnight Alaska Standard Time, which has typically been between 1150 and 1155pm, though a couple years in the World War Two era the closet observation was about 1130pm. I've used the standard time zone that was in effect at the time, so there is the one hour change in 1983. Hopefully then, this reflects the temperature (at the official observing site) at the time when Fairbanksians were toasting in the New Year. So here's what we get:

The first thing I notice is the 81F range of temperatures: the warmest New Year's Eve temperature was +28F in 2014, the coldest -53F in 1968, and the standard deviation is a whopping 20F. The second thing that stands out is that there seems to be a lot of New Year's Eve with temperatures near to slightly above 0F. This may be something of a surprise, given that the average temperature (in the 1981-2010 normals) for Dec 31 is -7F. In fact, the distribution of New Year's Eve temperatures (below) is typical of mid-winter temperatures in Interior Alaska, with a marked "left skew". That is, a long tail to the cold side, and a short tail to the warm side. This makes physical sense: in the absence of solar heating,  there are a lot of ways for an inland valley at 65N to be cold but very few ways to be really mild. I will not get into the details here, but my working hypothesis is that this kind of distribution, with evidently two peaks, centered, in this histogram at -20F and +10F, and the long cold tail is the outcome of two distinct temperature regimes: clouds vs. no-significant clouds.  

Lastly, just glancing at the first plot above, the recent mild New Year's Eve temperatures not withstanding, there does not appear to a strong trend in the temperatures, though there are notably fewer years at 20 below or lower since the early 1980s. In fact, there is something of trend toward warmer New Year's Eve temperatures. The linear regression (green line below), which is effectively modeling the mean value and is sensitive to outliers, does not meet the threshold for significance (i.e. we can not say with reasonably high confidence the apparent increase is not due to simple random variation). However, if we are concerned with the median New Year's Eve temperature (not sensitive to outliers), then the trend of 21F warmer now than in the early 1940s does meet a statistical significance threshold. 
So what will the upcoming New Year's Eve bring? Well, the airmass is likely to be pretty mild, and there is reasonable agreement that there will be a broken to overcast cloud cover, so I'll go with a comparatively mild ringing-in of 2018.  Happy New Year to you and yours, and thanks for reading Deep Cold. --Rick T.

Satellite Temperatures

Seasonably cold weather has returned at last over interior Alaska, with Fairbanks seeing generally sub-zero temperatures in the past week and a chilly high temperature so far today of only -19°F.  The cooperative observer at Chicken reported temperatures below -40° the past three nights.

The map below shows a satellite-derived estimate of temperatures at 4am this morning over the southeastern interior, extending from Fairbanks-land (near the top left) to the Canadian border and western Yukon Territory on the right.  The cold conditions of the upper Tanana River valley and the steep valleys of the Fortymile Country and western Yukon are clearly evident, with estimated temperatures around -35°F to -45°F.  Ground truth observations provide confirmation: -38°F in Northway this morning, and below -40° in Dawson, but barely below 0°F in Eagle with a breeze blowing.

In recent months I've been working on a project to decode all of the wintertime historical land surface temperature data for Alaska from the MODIS instruments on the Aqua and Terra polar-orbiting satellites.  The map above is derived from the realtime MODIS feed, but the entire archive is available from NASA (back to 2000 for Terra and 2002 for Aqua).  The full archive is of course a lot of data and takes a long time for my little computer to decode, so I'm only about half way through; but the preliminary results are already quite interesting.

As an example of the kind of analysis that is possible with the data archive, the maps below show the number of days with estimated surface temperatures of -50°F or lower for several winters (November through March).  The numbers are conservative estimates, because I've used only "high quality" pixels and there may well have been times when ice fog, cloud cover, or other problems prevented good estimates; but the temperatures are unlikely to be too low and so the counts of days are probably not too high.

Click the images to enlarge.  The Old Crow Flats of northern Yukon stand out as remarkably and consistently cold.  The results also support Rick's opinion that there are places upriver from Umiat that are quite a bit colder than that notoriously cold place.

Nov 2002 - March 2003:

Nov 2003 - March 2004:

Nov 2004 - March 2005:

Nov 2005 - March 2006:

Nov 2006 - March 2007:

Alaska Range Snowfall

I don't often comment on the weather and climate of southern Alaska, but this week a very interesting paper was published looking at long-term snowfall accumulations high on Mt Hunter in the Alaska Range.  Here's a summary from the Weather Company

https://weather.com/news/climate/news/2017-12-19-warming-ocean-water-double-snowfall-alaska-study

and here's the study itself

https://www.nature.com/articles/s41598-017-18022-5

The paper begins by reminding us of the dramatic increase in winter precipitation that has occurred in recent decades at coastal sites in southern Alaska, for example (and most strikingly) a more than 50% increase in rain and snow at Kodiak.  The chart below shows the December-February precipitation totals since 1950 at Kodiak; note that the increase essentially occurred as a step change in 1976 when the PDO suddenly flipped from negative to positive.  The median December-February precipitation changed from 13.9" (1950-1975) to 23.6" (1976-2016), an increase of 69%, but there has been no significant trend since 1976.

The precipitation estimates from Mt Hunter, as published in the new study, show a very dramatic increase over the past 1000+ years, and they also indicate that snowfall has continued to increase in the past few decades.  Interestingly the 1976 Pacific regime shift is not particularly clear in the Mt Hunter data (see below); a more pronounced increase occurred after 1985, and the highest estimated totals occurred in the last decade of the data (which ends with the winter of 2010-2011).

The absence of a 1976 step change at Mt Hunter raises a question as to how closely the precipitation amounts are connected to the North Pacific circulation pattern.  The authors of the study argue that the long-term strengthening of the Aleutian Low is the main driver for the long-term increase in Mt Hunter snowfall, so this is an important point.

The chart below shows two measures of North Pacific atmospheric variability that are referenced in the paper: the Pacific-North American (PNA) index and a North Pacific (NP) pressure index; both of these are closely related to the strength of the Aleutian Low.  The usual definition of the NP index means that it's more negative when the Aleutian Low is stronger, but I've defined it with an inverse sign to line up with the PNA.  The 1976 regime change is evident for both indices, but as with the Kodiak precipitation data there has been no significant increase since then - in fact the PNA index has zero trend since 1976 and the NP index has a negative trend.

A scatter plot of the PNA index versus Mt Hunter precipitation (see below) reveals a fairly robust connection with - as it turns out - the same correlation for 1950-1975 and 1976-2010.  The correlations are slightly lower for the NP index (+0.39 in both periods).

It's clear from these results that the Mt Hunter data does reflect an influence of the Aleutian Low, but it's equally clear that we can't explain the most recent (post-1985) increase in snowfall by invoking continued strengthening of the Aleutian Low.  The published study alludes to this point and suggests that the explanation for the most recent changes may be a sustained increasing trend in ocean temperatures far away in the western tropical Pacific Ocean; these tropical changes may have caused shifts in the atmospheric flow near Alaska that are not reflected by the PNA/NP indices and/or the sea-level coastal precipitation measurements.  The authors describe this hypothesis as "a heightened sensitivity to tropical SST teleconnections at higher elevation in Alaska".

While I don't have any particular comments on the tropical-connection hypothesis, I thought it would be interesting to look "closer to home" for potential explanations for the most recent increase in Mt Hunter snowfall.  To begin, I calculated the portion of the Mt Hunter precipitation that cannot be explained by a linear regression with the PNA index - see below.  The most obvious aspect of the chart is the preponderance of high precipitation totals, and the absence of small totals, since 1988-89.

It's not immediately obvious what major climate regime shift could have caused a change like this in 1988; as we noted before, the PDO shifted in 1976, and the Atlantic Ocean shifted into a much warmer state (positive AMO phase) in 1995; but I'm not aware of any notable shifts in or around 1988.

To dig in a little deeper, I calculated the correlation between September-April sea-level pressure and the Mt Hunter precipitation residual.  The results show an interesting signal related to pressure over the Chukchi Sea - see below.  When low pressure prevails over the Chukchi Sea, then Mt Hunter snowfall tends to be higher than the PNA would suggest, whereas high pressure to the north and northwest of Alaska tends to occur with lower Mt Hunter snowfall amounts.

Looking at an area-average of sea-level pressure over a box from 65-80°N and 160-180°W, we see an apparent shift to lower values beginning in the late 1980s (see below), so this lines up rather well with the unexplained recent increase in Mt Hunter snowfall.  A scatter plot of the annual pressure and precipitation residual values (second chart below) reveals a 1976-2010 correlation of -0.48, which is nearly as good as the underlying PNA correlation that we started with.

Of course correlation does not prove causation, but there does seem to be some evidence here that lower pressure to the north and northwest of Alaska may have contributed to the continued rise in Mt Hunter snowfall in recent decades.  How would this work from a physical standpoint?  Lower pressure to the north would seem to favor episodes of strong westerly flow across all of Alaska, and while southerly flow is most favorable for heavy snow in the Alaska Range, it's conceivable that westerly wind events could also transport ample moisture into the high terrain.  Further investigation is clearly required to see if there is really a plausible physical connection to storminess in the Chukchi Sea.

In summary, the remarkable hydrological changes that have occurred recently on Mt Hunter and presumably elsewhere in the Alaska Range appear to have some connection to Arctic weather patterns as well as to the North Pacific circulation.  This raises the interesting possibility that the recent reduction in Arctic sea ice may also be implicated, perhaps by altering the winter flow patterns around Alaska in a systematic way.

Yet More Warmth

Blog updates this month are starting to sound like a broken record, but that's also an apt description of the extraordinary weather pattern that continues to pump warm air northward across Alaska.  The warmth has been very anomalous, but the persistence of the pattern may be the biggest story, as Fairbanks has now seen 17 consecutive days with a daily high temperature of 20°F or higher and a low temperature above 0°F.  The 1981-2010 normals for the time of year are around +5°F and -12°F for the high and low respectively.

Looking back at the Fairbanks history from 1930-present, the December-February record for consecutive days above 0°F is 18 days, so it looks like this may be broken soon.  It's a pretty amazing achievement when you consider that any given day is about 80% likely to see 0°F or lower in December in Fairbanks (all else being equal, i.e. based on the 1981-present climatology).

As I mentioned the other day, the 3 long-term climate sites of Fairbanks, McGrath, and Bettles have never recorded such a lengthy period in winter with a combined average temperature of more than 1 standard deviation above normal.  Of the 3 locations, Bettles has been the warmest relative to normal, and they haven't even seen a sub-zero temperature so far this month (see figures below).  No other year in their climate record comes close to the warmth for the first 18 days of December, although of course Bettles climate data does not include December 1934 - and that remarkable month was marginally warmer than this year in Fairbanks through the 18th (but then turned colder in the last 10 days of the month).

Warm Air Aloft

In pondering the nature of winter warm-ups over Alaska, I find it interesting to consider how much of the atmospheric column rises above freezing, even while surface temperatures usually remain below freezing, at least for most interior locations.  In the most recent case, for example, the Thursday afternoon sounding from Fairbanks reported that nearly 20% of the atmosphere (by mass) was above freezing above the Golden Heart City.  The deep layer of above-freezing air is evident in the diagram below where the red line (the temperature trace) extends to the right of the diagonal blue 0°C line from the surface up to about 800 mb.

If we look at historical Fairbanks sounding data back to 1958, we find that the deepest layer of above-freezing air observed in December was in 1985, when the freezing level was way up around 8000 feet above ground (above 750 mb) on the 11th of the month.  The most extreme case to occur at any time in winter was the amazing warm spell of late January 2014, as documented here.  In that case over 30% of the atmosphere by mass was above freezing, and that's higher than normal for the height of summer.

The chart below shows a box-and-whisker depiction of the history of monthly means of the above-freezing fraction.  The numbers are calculated by adding up the pressure depth of each above-freezing layer in each sounding, after interpolating the freezing level(s) between reported heights.  Note that the record values labeled on the chart are for the monthly means, not for individual events.  It's interesting to note the sustained extreme warmth observed in some recent years in spring and summer, but not so much in winter.

A quick look at trends by month reveals that April is easily the month with the strongest warming trend based on this metric; the trend is actually quite remarkable (see below).  The mean value for June-August also shows a statistically significant upward trend, but I'll refrain from making a similar statement about the winter trend as a highly non-Gaussian distribution makes the statistics a bit more tricky.

Exceptional Warmth

Unusual warmth is prevailing across Alaska with remarkable persistence so far this month, and yesterday was the warmest day yet for the interior; McGrath reached 39°F, Fairbanks made it to 41°F, and Bettles (at nearly 67°N latitude) hit 38°F.  The Bettles high temperature tied the record for the month of December, with data back to 1951.

Looking at the joint history of climate data from these three interior sites (1951-present), there is only a handful of days when all 3 locations reached 38°F or higher on the same day in the depths of winter (December through February).  It has happened only 6 other times, with the most recent occurrence being nearly 8 years ago (Jan 17, 2009).  January 2009 was also the last time that the daily mean temperature was at least 2 standard deviations above normal for all 3 of these sites on the same day in winter (November-March).

With respect to persistence, we are currently at 11 straight days with a combined mean daily temperature (for the 3 sites) of more than 1 standard deviation above normal.  The record for November-March is 13 days, so by this metric we're closing in on a record warm spell for the interior.

The chart below shows the kind of warmth that's been observed by the soundings from Fairbanks; for the first half of this week, temperatures were around 40°F or higher at about 600' above the surface.

Here's the 500mb height analysis from 3pm yesterday.  If it looks oddly similar to the chart from last week... that's because it is.

Warm Days in Winter

Warm chinook flow from the south lifted temperatures above the freezing point on Thursday night in Fairbanks for the first time in more than a month.  Thursday's high temperature, which occurred at 10pm, was 37°F, but this isn't close to a daily record; the extraordinary chinook of early December 1934 set all the high temperature records in Fairbanks from the 3rd to the 8th of the month (including 58°F on December 5).  Nevertheless, the flow anomaly from this week's event was impressive; the map below shows the 500mb height analysis from 3pm Thursday.

It's obviously unusual for temperatures to rise above freezing in the depths of winter in Fairbanks, but most years do see at least a brief thaw at some point in December through February; only 10 years since 1930 remained below freezing throughout the 3-month period (most recently 1998).

The chart below shows the number of days each winter on which the daily high temperature was above 32°F in Fairbanks, and the red shading indicates how many days the daily mean temperature was also above freezing - the latter being a much rarer event.  The relative abundance of warm days prior to about 1945 may have something to do with the fact that measurements were taken downtown from 1930-1942, rather than at the international airport as they have been since 1951.

The asterisk annotations indicate winters when La Niña conditions were present in the tropical Pacific Ocean, as they are this year.  Despite the fact that La Niña winters tend to be colder on average in Fairbanks, the frequency of warm days is not much reduced, and this is because the temperature variability is heightened during La Niña winters.

The scatter plot below shows how the number of above-freezing days varies with an index of the El Niño/La Niña (ENSO) phenomenon.  A clear peak is evident for near-neutral ENSO conditions, and this is an interesting example of non-linearity in climate behavior; while El Niño winters tend to be warm on average in Fairbanks, the number of very warm days is no higher than normal, because temperature variability is low.  I've discussed this kind of thing before.

The same chart for the PDO rather than ENSO shows a hint of the same behavior, although there is more asymmetry between the positive and negative phases.  There is a notable reduction in warm day frequency when the PDO is significantly negative, but so far this winter the PDO is near-neutral.