End in Sight

A major warm-up is under way at last across the eastern interior, as the flow has finally gone around to the south; Eagle is up to 46°F this evening, and Fairbanks is expected to break the freezing mark by Friday at the latest.  This will prevent a significant record from being broken: Fairbanks has never seen a March without the temperature rising above freezing at some point in the month.  The closest was in 1997, when the month's high temperature was only 34°F.

[Update March 31: the temperature has risen above freezing at Fairbanks airport as of 11:30am AKDT.  This is the 3rd latest appearance of the first above-freezing temperature in March.  In 2006 it occurred at 1pm on March 31, and in 2007 the thaw began at 2pm on March 31.]

The last time that above-freezing conditions were observed either at the surface or in the air column above Fairbanks was on February 26, so the period of having a continuously sub-freezing atmospheric column will end at about 30 days.  The longest such period this winter was 46 days, from mid-November through December 30, and this was considerably longer than any of the past 3 winters.  The chart below shows the length and ending date of each winter's longest stretch of sub-freezing conditions (both surface and aloft) back to 1948-49.  (Click to enlarge the chart.)

It's a funny coincidence that 4 of the last 5 winters have seen the longest sub-freezing spell come to an end very close to the end of the year (December 27-31).  Reader Eric first pointed out the similarity in timing of cold and warm spells between this winter and last (see here), and in fact a distinct warm-up can be identified near the turn of the year in each of the past 5 winters.  We'll call it the New Year's Thaw and I'll look at it more closely in a future post.

Cold and Dry

Wintry cold continues to hold on tenaciously across interior Alaska, at least during the nighttime hours.  Here we are after the spring equinox, and temperatures in the -30s Fahrenheit are still occurring overnight in the usual cold spots.  This morning the Kanuti Lake SCAN site reported -34°F, and the Salcha RAWS hit -31°F.

In Fairbanks-land, Goldstream Creek reported -27°F this morning, and the airport thermometer registered -18°F.  Remarkably, every night but one so far this month has seen a low of -13°F (-25°C) or colder at Fairbanks airport.  The 22 nights at -15°F or colder is tied with 2007 for most on record in the month of March, and the NWS expects another one tonight.

Here's a chart showing the last two weeks of 2m temperature measurements from UAF's Smith Lake site: sub-minus 20 every night, and daily temperature traces indicative of generally clear skies.  So far this month more than two-thirds of the sky condition measurements at Fairbanks airport have indicated either clear skies or "few" clouds (25% coverage or less).

Total snowfall this month in Fairbanks has amounted to only 1.7", but this in itself is not very unusual: the 1981-2010 median is only 3.2" for the month.  April is usually even drier, with a normal liquid-equivalent precipitation of only 0.1".  March and April are the driest months of the year, climatologically speaking, in Fairbanks, and a few days ago Brian showed that this is also the driest time of year across most of interior northwestern North America:

https://www.facebook.com/AlaskaClimateFacts/photos/pcb.825178884300842/825177814300949

Why is late winter generally a sunny and very dry time in interior Alaska?  Part of the reason is that the air is very dry, and that's because evaporation from the earth's surface is much reduced in the Northern Hemisphere at this time of year: ocean surface temperatures are near their coolest, ice cover is near its peak on both oceans and fresh water bodies, and forests are still mostly dormant.  The lifetime of water vapor in the atmosphere is only about 9 days on average, so precipitation must decrease in tandem with evaporation rates on a hemispheric scale.

The graphic below shows an estimate of how the total atmospheric water vapor content (precipitable water) varies through the year in the Northern Hemisphere: the values shown in the chart are the west-east averages for each month and each line of latitude.  It's interesting to see the earlier summer peak of vapor content in the Arctic compared to the tropics, as solar heating drops off more rapidly in late summer at high latitudes.

Here's a similar chart, but for a single line of longitude running approximately through Fairbanks (148°W).  Over Alaska and the oceans to the south, the lowest normal value of precipitable water occurs in March.

A second reason for dry weather in Fairbanks in March and April is that there's simply not much storminess to generate clouds and precipitation.  The absence of fronts and storms is a more localized factor than hemispheric dryness, because other parts of the hemisphere (indeed of the USA) have plenty of storminess and wet weather in spring.

In the vicinity of Alaska, storminess is reduced in the spring because the North Pacific Ocean is cool, leading to reduced north-south temperature gradients and lower potential energy for storms.  Another reason is that high pressure tends to build over the Arctic basin in late winter and spring.  The chart below show the time-latitude cross-section of sea-level pressure along 148°W; notice the prominent high pressure zone around 75-80°N from February through April.  At the latitude of Fairbanks (65°N) the pressure is lowest in October and then increases into late winter; and while there's much more to the weather pattern than just MSLP, this does illustrate the overall tendency towards generally more clear and dry weather as the winter advances.

Arctic to Mid-Latitude Connections

Yesterday the scientific journal Nature published a brief but helpful summary of the state of research concerning the linkage between climate changes in the Arctic and mid-latitudes.  For context, bear in mind that a popular hypothesis in recent years has been the idea that enhanced warming in the Arctic slows the jet stream and therefore creates more blocking and extreme weather at mid-latitudes.  However, the real world may not have such a simple chain of causality; the article emphasizes how much there is yet to learn on this subject.  Here's a key quote:

"...the connection is not one-way from the Arctic to the mid-latitudes but also works in reverse, and observations and climate models give differing estimates of the extent to which mid-latitude climate is influenced by Arctic warming".  Also: "we have a poor grasp of how much of the Arctic amplification is caused by warm and moist transport from the mid-latitudes to the Arctic".

Click here to view the article

Update: see comments below for links to the workshop website, containing much more information.

New Blog

Regular visitors may be interested to learn that our own reader Eric has started his own blog on interior Alaska's weather and climate; I expect I will link to it occasionally or even repost some material.  Take a look here:

http://climateoftheboreal.blogspot.com/

On another note, the webcam from Dawson (Yukon) reveals that the Yukon River has at long last frozen over completely next to the town.  There's been no lack of cold weather this winter, but an unusual ice jam upstream allowed an open lead to remain in the middle of the river throughout the entire winter until now.  It looks like the ice road to West Dawson may finally have opened just in time for the onset of spring.

March 1:

March 7:

March 17:

Winter Hangs On

The sun is shining for long hours in central Alaska now, but temperatures have remained decidedly wintry of late.  This morning the thermometer registered -25°F at Fairbanks airport, which is where it has been on average for the first 18 days of the month (for overnight lows).  The overall mean temperature so far this month is 17°F below normal.

The 500mb height anomaly map for March 1-16 reveals the pattern that we would expect to find in association with unusual cold in much of Alaska: a big ridge over the Bering Sea and relatively low heights over Southeast Alaska.

The sea-level pressure map (below) shows the surface-level ridge extending across the interior into western Canada, providing the clear sky conditions that are conducive to enhanced overnight cooling.  The ridge has also shut off the moist westerly flow and brought an end to the snowy pattern.

It's interesting to examine the subtle contrast between this month's pressure pattern and that of November through February, see below.  For most of the winter the ridge was centered farther to the south, and the Arctic trough extended down to the Chukchi Sea, so the flow into Alaska was both wetter and warmer (relative to normal).

The map below shows the average MSLP pattern during 10 strong La Niña winters; notice the strong similarity to this winter's pattern.  This is somewhat remarkable to me, because this winter's La Niña episode was very weak in intensity - and yet the circulation pattern over the Aleutians strongly resembled a classical La Niña pattern.  This seems unlikely to be a coincidence, and La Niña probably bears some of the blame for the current cold as well.

For completeness, the maps below show the typical El Niño pattern and the MSLP anomaly from last winter (2015-2016).

North Pacific Blob Update

Hot off the press this evening: a new post on the Alaska "Blob Tracker" blog, showing interesting similarities of recent events with those that followed the 1997-1998 El Niño:

https://alaskapacificblob.wordpress.com/2017/03/13/the-blob-in-hibernation/

Cold Nights

Abnormally cold conditions have returned to interior Alaska again, with widespread -40s at night in the past week or so, and a few -50°F readings.  The Salcha RAWS hit -51°F last night and has fallen below -40°F every day this month except one.  Here's a sample of the locations that dropped below -40° last night:

In Fairbanks the minimum temperature of -39°F this morning is the coldest observed so late in the season since 1964, although -37°F was seen on March 15 only 2 years ago.  Assuming today's high temperature is 0°F or less, which appears likely, this will go down as the coldest day of the winter in terms of standard deviations below normal.  But of course the contrast between today and the January 17-19 cold snap is like night and day: the number of hours of sunshine has doubled since then.

Here's an updated chart of daily mean temperature anomalies for Fairbanks; the mean temperature since November 1 is now 1.7°F below the 1981-2010 normal.

Record Cold in Arctic Canada

[Update March 5] The 1000-500mb thickness dropped still further today, reaching 4619m at 1200 GMT over westernmost Victoria Island.  This appears to be the coldest air mass since March 1996 in the general vicinity (90-180°W).

Also of note: yesterday's all-time low temperature at Mould Bay is the first all-time low at a long-term observing site north of 70°N since March 18, 2008, when Danmarkshavn in northeast Greenland set its all-time low of -49°F.  (This is based on NOAA's GHCN data, which is reasonably but not entirely comprehensive.) [End of Update]

In recent months and years we've often discussed the extraordinary and persistent warmth around the Arctic Ocean, so it makes a change to consider an episode of extreme cold for once.  This morning the temperature dropped to -66°F at Mould Bay on Prince Patrick Island, which is north of Banks Island and is the westernmost of Canada's Queen Elizabeth Islands; Mould Bay is at a latitude of 76°N.  I've marked the location on a couple of the maps below.

Remarkably, today's low temperature of -66°F sets a new all-time record low for Mould Bay, which has more-or-less continuous weather records since 1948.  The previous record was -65°F, in 1967 and 1987.  The last time -60°F was observed at Mould Bay was in 2004.  Here's a chart of the minimum temperature each winter since 1948-49; note that some winters are missing (including 2003-2004) because the daily data are incomplete in those years.

It's interesting to note that today's cold at Mould Bay is not far from an all-time record low for all of Canada's high Arctic region (north of 70°N).  According to GHCN data, only two locations, Eureka and Lake Hazen (both on Ellesmere Island) have been colder, at -68°F and -69°F respectively.

The circulation feature responsible for the cold weather today is a vortex of extremely cold air that has been hovering over the Queen Elizabeth Islands for several days.  Back on Monday the coldest air was over Ellesmere Island, but the vortex has drifted southwestward and the cold has intensified considerably in the past few days.  Bear in mind that of course there is still very little solar heating at this time of year north of 75°N - the sun rose in Mould Bay only 3 weeks ago and reached only 8° above the horizon today.

The map below shows the GFS model's estimate of 850mb temperature today at 1800 GMT or about noon local time in Mould Bay, when the surface temperature was reported as -64°F.  A substantial area of sub-minus 40° air at 850mb is evident from Prince Patrick Island southward over half of Banks Island; this is an extremely cold air mass.

Here's a map of 1000-500mb thickness at the same time; the thickness is an excellent measure of the mean temperature in the lower half of the atmosphere.  The gridpoint minimum of 4634m is extremely low by historical standards; according to NCEP reanalysis data, this is the lowest 1000-500mb thickness value anywhere in the Northern Hemisphere since March 2011.  For comparison, and again according to reanalysis data, the all-time lowest 1000-500mb thickness value over Alaska was 4598m on 27 January 1989 (towards the end of that month's deep cold snap).

The map below shows the reanalysis-era minimum in 1000-500mb thickness; we see that most of the Arctic basin has seen thickness values below 4650m at one time or another.  However, the second map below shows that only a few selected areas have seen sub-4650m thickness since January 2000.  In other words, the kind of cold we're seeing today in northwest Canada has become rare in the past couple of decades; this is a really extreme and notable event.  It's remarkable and very interesting to see this event occur at the end of what has otherwise been a winter of record-breaking warmth in the Arctic.

Snow Water Equivalent

Following on from the last post, here's a quick look at the seasonal maximum in snow water equivalent in Fairbanks, as measured at the airport and at the Snotel site (off College Road and not far from the Fairgrounds, according to Rick Thoman).  The automated Snotel instrument monitors snow water content via a pressure pad, whereas the airport measurement is done manually as part of the daily observing routine.  The two time series track together pretty well, although there was a big discrepancy in 2013-14.

The overall downward trend since the early 1990s is rather noticeable, although of course the interannual variance is very large.  So far this winter the SWE has reached 5.2" at the Snotel site; again we see that while this is above normal, it's not highly unusual compared to the long-term climate of Fairbanks.

Snow Piling Up

Fairbanks has seen something of a snow onslaught in the past week, with a remarkable 19" of snow over six days, taking the seasonal snowfall total to 78".  This is the highest in over 20 years.

The entire month of February normally only brings about 6" of snow, as the weather usually becomes increasingly dry in late winter.  On average, the heaviest 1-day snowfall in February is 2.5", but Fairbanks just saw 4 consecutive days with more than 2".  Interestingly this has only happened 8 times before since 1930, and never after mid-January.

Another interesting factoid: the months of December through February brought 15 calendar days with at least 2" of snow in Fairbanks this winter, and that's the highest on record; the previous record was 13 such days in December 1936 through February 1937.

The map below shows reported snow depths today.  A remarkable aspect to me is how high the valley-level snow depths are from the Fairbanks area down to Nenana; the snow depths are no greater at some of the elevated Snotel sites northeast of Fairbanks.  Keystone Ridge is reporting 35" on the ground - barely more than Fairbanks airport (32").

The percent of normal snow water equivalent at the long-term Snotel sites ranges from 112% at Mt Ryan to 198% at Monument Creek.

Here's a chart showing the seasonal snowfall totals and maximum snow depth in Fairbanks for each winter since 1930-31; the long-term median values are indicated with dotted lines.  For as unusual as the snow has seemed this winter, the season as a whole is nowhere near some of the extraordinary winters of the past; but what has been very unusual is the concentration of snowfall in the past three months.

Yukon River at Dawson

Temperatures are near or above freezing across most of the western half of Alaska south of the Brooks Range this evening, as a very warm and moist airmass has been imported by strong southwesterly flow aloft.  In Fairbanks the temperature has been close to the melting point since around noon yesterday, and today's minimum temperature is so far sitting at 29°F; this is close to record territory for the time of year.  The westerly component of the flow means that it's not a chinook event; in contrast, heavy snow is occurring in many areas.

Interestingly Anchorage has a high temperature of 27°F so far today.  It's very rare for the low temperature in Fairbanks to be above the high temperature in Anchorage during winter; it's less rare in June and July, but it hasn't happened in the winter months since 1974.

On another topic, regular readers may recall that a couple of months ago I drew attention to the lack of complete freeze-up on the Yukon River at Dawson.  Remarkably, the same situation is still in evidence, with a persistent lead of open water in the middle of the mighty river.  Here's a webcam image from earlier today.

Can the presence of open water be explained in terms of unusually warm weather?  No; the chart below shows the accumulation of freezing degree days this winter (black line) compared to the last 5 winters.  This winter has been colder, on balance, than any of the last 3 winters, and the river did not fail to freeze over in those years.

The explanation appears to lie in the early formation of an ice jam a short distance upstream, cutting off the formation of ice floes that would normally provide the building blocks for a complete ice cover.  As the images below demonstrate, the open lead has instead been freezing over from the edges, which is a very much slower process.  It will be interesting to see if the lead manages to disappear before the end of winter; my guess is that next week's much colder weather (indeed perhaps extremely cold) may finally do the job.

February 6:

January 16:

December 31:

Daily Minimum Temperatures

Here's a chart showing the lowest temperature reported statewide in Alaska each day so far this winter, along with the same analysis for last winter.  This is constructed with data from NOAA's ACIS tool, but I've excluded Snotel sites because of quality concerns.  Many, though not all, remote automated observing sites are included in the analysis.

The contrast with last winter has become pronounced since the beginning of the year: from January 1 to present, only one day had a higher statewide minimum than the same date in 2016.  The Kanuti Lake SCAN site dropped below -50°F again this morning, so that makes the 4th separate episode with sub-minus 50°F observed in Alaska; and that's a lot more like normal than recent winters.

Update Feb 22: here's the same chart for the two prior winters, 2013-2014 and 2014-2015.  A similar peak occurred at the turn of the year in 2014-2015, so this is the third winter in a row with this feature.  Based on the current forecast, it looks like this will also be the third year in a row with a notable late February warm spell.

Duration of Cold Spells - Part 2

A few weeks ago I showed some evidence that severely cold spells have become shorter in recent decades in Fairbanks - in other words, the coldest extremes of the modern climate (which are warmer than earlier cold extremes) have become less persistent as warmer conditions have tended to return more quickly.

This follow-up provides a quick look at the average 500mb height patterns associated with the most severe cold spells in two contrasting periods, 1931-1960 and 1981-2015.  To examine this I used data from the 20th Century Reanalysis, courtesy of NOAA/ESRL/PSD.  The 20th Century Reanalysis uses historical sea-level pressure and sea surface temperature observations to produce an evolving 3-D estimate of atmospheric conditions back to 1851.  Owing to the strong correlation between sea-level pressure and upper-level heights, the historical 500mb height estimates should be broadly correct back to the early 20th century or before; by 1925 there were already well over 1000 daily pressure observations north of 20°N (although not at all evenly distributed around the Northern Hemisphere).

The two sets of maps below show the mean height anomaly (departure from normal) for 1931-1960 cold spells on the left (as defined in the earlier post) and 1981-2015 on the right.  The first pair of maps is for 10 days prior to the first day exceeding the cold threshold, and then pairs of maps are shown at 5-day intervals until 15 days after the cold spell onset.  Click to enlarge the images.

	1931-1960	1981-2015
-10 Days
-5 Days
0 Days
+5 Days
+10 Days
+15 Days

With respect to the main question of interest here, it's immediately clear why cold spells in the earlier period tended to last longer than more recent cold spells.  In 1931-1960 the average flow pattern remained quite similar from the date of onset through 10 days later, with high pressure over the Bering Sea and low pressure over western Canada; this pattern funnels cold air southward across the interior.  In contrast, the 1981-2015 pattern is completely different by +10 days, with low pressure over the Bering Sea and high pressure over western Canada; this pattern brings warmth to Fairbanks.

It's also interesting to note that the precursor flow patterns were quite similar between the two periods at a lead time of 10 days, with high pressure over and south of the Aleutians and low pressure over the Arctic Ocean.  However, at 5 days in advance, the modern pattern is much more amplified, with a very strong northerly flow anomaly over the Bering Sea (rather than northwesterly in 1931-1960).  It seems as if modern severe cold spells tend to arrive with greater ferocity, but the high-amplitude pattern continues to evolve rapidly so that the cold blast is followed quickly by warmth.  In the "old days" the cold-advection pattern gives the impression of being more stable and long-lived, at least as far as the Alaska-centric flow anomalies are concerned.

A possible next step would be to investigate historical sea surface temperature anomalies to see if we can identify a reason for the preferential patterns shown above.

Extreme Warm-Up

I was going to write a few words on the brief but notable cold spell over the weekend, but the cold has already been eclipsed by an incredibly rapid warm-up over the eastern interior.  The map below show 24-hour high temperatures through 5pm today in the wider Fairbanks area: most places above freezing, and +40's in the hills to the east.

A broader view shows clearly where the southerly chinook flow reached the surface today and, conversely, where it did not; the Yukon Flats have remained cool, and locations west of Fairbanks have been less warm; even Nenana was far enough west to stay 10°F cooler than Fairbanks.  Interestingly the upper Tanana Valley was cooler as well.

The rate of warming at certain spots has been really remarkable: the Salcha RAWS, for example, warmed from -45°F at 9am on Sunday to +45°F at 1pm today.  This is extreme even by interior Alaska standards.

At Fairbanks the low on Sunday morning was -41°F, and today's high so far is +35°F; this is the warmest it has ever been within two calendar days of a -40°F or colder reading.  The warmest it's ever been within a single day of -40°F is +21°F (in 2012), and the warmest within 3 days is +44°F (2009).  So this is one for the history books.

Here's a sequence of 500mb charts showing the flow adjustment that has brought such a dramatic change of conditions.  The maps are from 3am on Sunday, Monday, and today, respectively; notice the strong upper-level low dropping south over western Alaska, allowing the the flow to pivot quickly to the south across the Alaska Range.

The readiness of the low to drop south appears to have been aided by the deep trough that was already in place over the North Pacific at around 160-180°W (evident in the first map above).  This in turn was favored by widespread below-normal sea surface temperatures in a west-east band across the North Pacific, reflecting the negative NPM phase that has become entrenched this winter.  Here's a recent SST analysis:

And here's the typical 500mb height pattern when the NPM is strongly negative in February; notice the similarity of the trough position to where the low ended up this morning.  This is a nice example of a single weather event reflecting the favored mode based on long-term (seasonal-scale) forcing mechanisms.