Tuesday, June 15, 2021

Summer Heat

The heat is on for the eastern half of the interior today, with widespread 80s, including 83°F so far in Fairbanks and 84°F in Eagle.  Many RAWS site are hotter, but that's typical under strong summer sunshine.  More notable is the heat across the eastern North Slope: 72°F at Barter Island, and 80°F at Deadhorse on a southwest wind before the sea breeze kicked in.  Click to enlarge:


The 80°F at Deadhorse is the first time in nearly 3 years that the temperature has exceeded 73°F.  The calendar years of 2019 and 2020 had maximum temperatures of 73°F and 71°F respectively, which is actually the coolest pair of summers (by this metric) since 1987 and 1988.  But all four years from 2015-2018 saw at least one 80°F day in Deadhorse.  Here's a visual representation of all 75+ and 80+ days since 1970 in the combined history from Deadhorse and (prior to 1999) Prudhoe Bay.


It's interesting to note the occurrence of warm days in August - remarkably late in the summer for the Arctic coast - between 1989 and 2007.  Also, the trend towards earlier hot days seems notable: prior to 1990, the earliest 75°F was July 3, and today was the earliest 80°F on record.

In other (but related) news, Fairbanks airport has been reporting distant wildfire smoke this afternoon: presumably the result of the Haystack fire 20 miles north of town.  This is the first smoke report at the airport since June 1st of last summer, and that was the only day last year that reported smoke.

Friday, June 11, 2021

Stubborn Northwest Chill

It's been a cold start to climatological summer in the Bering Strait region, with a remarkably stubborn upper-level low residing over the area for an entire week.  Cut-off low pressure features like this are often persistent, but it's unusual to see almost no movement for such a long time.  Here's a simple animation of 500mb height analyses, courtesy of Environment Canada, at 3am each day from June 1-10.


With cloud and cold air stuck overhead, Nome has seen the coldest first 10 days of June since 2006.  Another statistic: the week ending June 8 was the third coldest week entirely within June since 1977; but 2006 and 2013 both had slightly colder weeks in early June.  Like this year, 2006 and 2013 both had major disruptions to the stratospheric vortex in January; there may or may not be a connection (perhaps more likely not), but I'm always on the lookout for these things.

Some fresh snow has been seen at various locations around the Seward Peninsula: here's a photo from Shishmaref on Tuesday (June 8), courtesy of Alaska DOT&PF on Twitter.



And here's a video of snow in Nome on June 5.



As for the larger context of Nome's temperatures so far this year: it was a chilly end to winter, but spring has been considerably warmer than normal most of the time since mid-April.
 


Saturday, June 5, 2021

Arctic-Wide Temperatures

Several years ago I spent some time looking at Arctic-wide air temperature trends based on measurements at climate observing sites around the basin, and it's probably worth returning to that idea for an update.  See here (and links therein) for previous work.  In my fresh look at this, I found 32 sites - distributed fairly evenly around the Arctic land areas - that have nearly complete data since 1980 and also report reliably on a daily and monthly basis.  Here's a map of the sites:


This set of locations is designed to sample both inland and coastal environments above the Arctic Circle (neglecting interior Greenland); previously I was focused on coastal sites clustered around the central Arctic Ocean.

The chart below presents a multi-decadal view of monthly mean temperatures at these locations; the black line shows the 32-station average of the monthly departure from normal, with "normal" being defined as 1991-2020.  The red and blue lines show the highest and lowest monthly anomalies within the set (with the largest anomalies occurring in winter, when variance is highest).  Click to enlarge:



The overall warming trend amounts to +0.76°C per decade (using linear regression), or a little over +3°C in the 41-year period.  This is much more warming than has occurred across the globe as a whole, illustrating the reality of "Arctic amplification".  I should note, however, that this analysis is based on the unadjusted ("unhomogenized") data from GHCN-M, so it's the "raw" data without corrections for changes in station siting, instrumentation, or observing practices; read more here.

Looking at the last decade or so, it's remarkable to see the jump up that occurred in winter 2015-2016, when the super El Niño occurred; temperatures seem to have remained at a higher baseline since then.  Since 2016, it has become fairly uncommon to have any station with a -5°C anomaly (even relative to the warmer 1991-2020 average), whereas most months in the past few years have had at least one site with a +5°C anomaly.


Interestingly, November of last year was the second warmest month in this period of record, with a +3.7°C average anomaly for the 32 sites.  Only January 2016 was warmer, at +4.3°C.  Again, this is relative to the new 1991-2020 baseline.

However, there was quite a drop-off in Arctic-wide temperatures as last winter progressed, and both February and March were colder than the new normal.  Here's a chart of the daily progression (with a few missing days just before the turn of the year).  For a little over two months in late winter, the Arctic was generally colder than normal, but it warmed up considerably in April, reaching 4.7°C above normal by late April.  The year-to-date temperature anomaly so far (January through May) is +0.47°C.



Now that I've identified this more comprehensive set of stations, I'll be able to slice and dice the historical data in various ways, and keep on top of Arctic temperature variations going forward.  If readers have any suggestions for interesting analysis, feel free to leave a comment.

Wednesday, June 2, 2021

Winter Lives

The calendar may say June, but the weather has been more than a little wintry in the past few days at Howard Pass in the western Brooks Range.  Readers of this blog are well acquainted with the extreme wind chills that occur regularly up there in the winter, but it turns out that the site can put on a good show even in "summer".

As the chart below illustrates, two multi-day episodes of wind chill below +10°F have occurred in the past few days, and early yesterday morning the wind chill dropped below 0°F for a few hours.  This is the first time a sub-zero wind chill has been measured at the site in the summer months; previously the latest was on May 19, 2013, and the earliest was on September 9, 2012.  (However, there are only 5 complete June's in the station's history since 2012, because the anemometer had been disabled by winter winds and was not yet repaired in June 2015, 2018, and 2020).


An interesting difference from the winter wind chill setup is that the cold air blowing through Howard Pass this week is not being drawn from a locally reinforced cold pool on the North Slope.  In the cold season the extreme wind chill occurs when severe cold develops across the interior North Slope under a surface-based inversion, and then a strong pressure gradient forces that cold air up and over the shallow pass.  But in this case it's just an early summer Arctic air mass; there's no serious radiative cooling at the surface in the continuous daylight of Arctic summer.

To document the nature of the air mass, here are the surface observations at 5am yesterday, and the 3am Utqiaġvik sounding (click to enlarge).




Sunday, May 30, 2021

Wild Temperatures

The weather of the past week in mainland Alaska is a tale of a remarkable ridge of high pressure that intensified and migrated northward across the west coast and up into the Arctic.  At its onset the northward surge of warm air brought very warm conditions to southern and western Alaska on Tuesday, including 71°F in Anchorage, 78°F in Talkeetna, 75°F in Bethel (tied for 2nd earliest 75°F on record), and 80°F at Sleetmute on the Kuskokwim River.  The high-quality CRN site at Port Alsworth on Lake Clark reached 77°F, after a morning low temperature of 28°F; that's the largest daily temperature range outside of winter at that site since the sudden heat wave of late May 2013.

The heat extended farther north too: Nome reached 71°F on Tuesday, and as in Bethel, this was tied for second earliest on record for such warmth (72°F occurred in May 22, 2002).

Below are 500mb height charts from 3am on Monday, Tuesday, and Wednesday mornings: the ridge ballooned up into the Arctic by Wednesday, with a remarkable peak geopotential height of 5700m.  This is the highest on record for the month of May over the Pacific side of the Arctic Ocean (based on ERA5 data since 1950).




With a strong anticyclonic circulation taking shape around the ridge as it became cut off from the mid-latitude jet stream, the flow turned to the north over central and eastern Alaska - and the temperature started to drop.  Arctic air began to intrude, and on Friday an upper-level low pressure system dropped in from the northeast, forced down to the south and west by the flow around the ridge.  Here are Thursday, Friday, and Saturday mornings:




And so by Friday morning wintry conditions had returned to northeastern Alaska: Toolik Lake dropped to 17°F with fresh snow.  Freezes occurred widely in the eastern interior; and to cap it all off, snow fell yesterday in parts of the eastern interior and at low elevations in the Alaska Range.  Eagle reported an inch of snow - the latest on record for the town, according to Rick Thoman.  Here are low temperatures ending at 8am Saturday morning: click to enlarge.




The sequence of events is a bit reminiscent of what happened back in April, when a record-breaking cold air mass dropped south in response to a strong ridge to the west (but over the Aleutians in that case).  The ebb and flow of these circulation features has been quite striking for some months now, with notable cold outbreaks causing problems in the middle latitudes (Texas in February, spring freezes in Europe and the eastern US); but I haven't seen any evidence of an increase in volatility or extremes on a hemispheric scale.

Here's a simple animation of the 500mb heights at 12-hourly intervals from last Sunday afternoon through yesterday afternoon.  A fascinating sequence for meteorologists!



Monday, May 24, 2021

Late Snow in Bettles

I was too busy to post on this at the time, but last Wednesday a very late snowfall occurred at Bettles, on the south side of the Brooks Range.  Rain turned to snow in the early morning hours, and even though the temperature remained above freezing, 2.4" accumulated by late morning before melting later in the day.  This is actually the largest snow event on record for so late in the season, with data back to 1951 (and some data back to 1944).  The only other snowfalls of 1.5" or more after May 15 were May 19-20, 1996 (1.6") and May 22, 1956 (2.0").

 

Here's a visual depiction of all 1" or greater calendar-day snowfalls in May through September since 1951, for both Bettles and Fairbanks.  Click to enlarge:

A couple of comments are in order.  First, that August 9 snowfall in Bettles is real: 2.6" fell in the middle of the day on August 9, 1969, and 2" was still on the ground the next morning.  The next day 3" was reported far to the southeast in Northway, so obviously it was an extraordinary cold snap for the time of year.

But the most striking feature of the chart is the increase in mid-September snows at Bettles in recent decades.  The change is remarkable: in the 40 years from 1951-1990, only 2 years saw a 1" snowfall on or before September 20, but in the latest 30-year climate "normal" there are 9 such years.  Digging into the reasons for this would make for an interesting study.


Tuesday, May 18, 2021

Warm Season Precip Normals

I mentioned in the last post that July retains the top spot for monthly mean precipitation in Fairbanks, according to the new 1991-2020 climate normals, and that historically this was not true for most of Fairbanks' modern climate history; August used to be wetter than July.  A July rainfall peak also differs from most of the state, as August is considerably wetter than July in all but the eastern interior and the North Slope.

To look at the shifting climate normal in more detail, here's a chart showing the overlapping 30-year normals since 1931-1960 for the warm season months.  A spate of wet July's from 2001-2010 brought a significant increase to the July average in the 1981-2010 normal, but the other months were largely unchanged until more recent years.  However, the 1991-2020 normal is wetter in each of June through September (but interestingly there's a slight downward trend in May).

 

Here's a time series view of the June-September totals (blue columns) and the fraction that is attributable to July (gray columns).

 

The change in July is largely attributable to a handful of very wet months, but the consistency of the high seasonal totals since 2014 is quite remarkable.  Here are a few blog posts from last summer about this:

https://ak-wx.blogspot.com/2020/07/wet-summers.html

https://ak-wx.blogspot.com/2020/07/rainfall-trends.html

https://ak-wx.blogspot.com/2020/08/back-to-rain.html

Another interesting aspect of the Fairbanks warm season climate history is that the early decades were also relatively wet, but the excess was seen mostly in August back then; August used to be distinctly wetter than July.  But this change isn't just about a shift in rainfall timing from one month to another; it also reflects a major change in the character of summer rain events.

Consider the chart below, showing the total precipitation in categories of daily totals, i.e. how much of the long-term normal is attributable to light versus heavy amounts.  The first two decades in this analysis (1931-1950) saw a higher fraction of precipitation from relatively light daily amounts, but over time there's been an upward trend in the contribution of high daily amounts.


The change in the distribution of daily amounts is connected to the July-August shift, because July rainfall is typically much more convective (brief, heavier events), whereas August rains tend to be stratiform in nature (lighter, more extended rains).  In recent years Fairbanks has seen a decreased frequency of August rains, but very heavy rains in July - while still quite rare - have begun to occur frequently enough to make a significant difference in the long-term rainfall totals.

For more on this topic, see this post from a few years ago:

https://ak-wx.blogspot.com/2017/07/wetter-in-july.html