Radar Trends

Back in early August, when interior Alaska was seeing an unusually late spell of thunderstorm activity, I commented that it would be interesting to work with the Fairbanks radar data to investigate the variability of strong storms:

https://ak-wx.blogspot.com/2023/08/severe-thunderstorm-warnings.html

I ended up digging into this, and I've succeeded in producing an archive of integrated storm activity back to 2014.  The Fairbanks (Pedro Dome) radar has been around for a lot longer than that of course, and there is some historical data available back to 1997, but unfortunately there is just too much missing data in the early years to use it in this analysis.  There's also a complete gap from 2004 through 2013, and regrettably NOAA/NCEI says that data does not exist any more.

But we'll work with what we have.  My approach was to calculate the spatial area for which the radar reflectivity exceeded various thresholds at each radar scan, across the entire available scan domain, as archived in NOAA's Big Data project.  I used the base (lowest scan elevation) reflectivity and extracted the data on a 1km grid.

There is a myriad of ways that the results could be analyzed, but for starters I took the May-September average of each day's maximum area at or above the reflectivity threshold - such as 40 dBZ, see below.  A 40 dBZ reflectivity is typical of moderate convective rainfall in summer; a typical robust thunderstorm core would be more like 50 dBZ.

Of course many days have no thunderstorms and little or no rainfall across the domain, and most of each season's total activity is concentrated in a rather small number of days, but the season-long average of daily peak activity seems to provide a suitable integrated measure.  Note that I excluded days with less than 100 reflectivity scans in 24 hours; about two-thirds of days have more than 200 scans in May-September.

The overall year-to-year variability in 40+ dBZ radar echoes does not seem particularly great, but the monthly breakdown shows that both July and August of the last two summers were considerably more active than the decadal average.

Raising the bar to a 50 dBZ threshold emphasizes the high level of storm activity in the past two summers, especially in July (both years) and August (this year).  August 2023 actually stands out quite dramatically compared to the previous 9 years:

Taking it up still another notch to 60 dBZ, which would be typical of a strong storm producing at least some hail (possibly large), summer 2023 stands out even more compared to prior years, and there's a strong suggestion of an upward trend.  Despite the very short period of record, the linear trend in the first chart below is highly statistically significant.

This data obviously begs for further analysis and investigation, and I'll aim to do that as time permits.  For now, it's just interesting to note that perhaps the unusual spate of severe thunderstorm warnings issued by NWS Fairbanks this summer wasn't as out-of-line as I thought at the time.

Kaktovik Wind

Blizzard season is off to an early start in Alaska's capital for that kind of thing: Kaktovik, on Alaska's eastern Arctic coast.  Snow has been flying since Thursday morning, and winds increased to 50 mph yesterday evening (with gusts to 60 mph).  Thanks to reader Mike for bringing this to my attention.

October is rather early for this kind of weather in Kaktovik, and it's been more than 10 years since such high winds were observed in October.  In winter proper it's more common, with more than 5% of days seeing sustained winds over 45 mph in November through February.

Here's the meteorological setup: high pressure over western and central Alaska, and a deep low pressure system over Banks Island (as of 4pm AKST yesterday).  My clumsy annotation shows the direction of the strong pressure gradient, and Barter Island's location is indicated with the red dot: click to enlarge.

Statewide Temperature Index

Last week I mentioned the UAF/ACCAP statewide temperature index (developed by Rick Thoman and Brian Brettschneider), which is a useful tool for quickly visualizing the overall ebb and flow of temperature anomalies in Alaska.  Here's the last year or so (click to enlarge):

Two of the more striking anomalies in recent months were the extraordinary warmth of late summer and the April cold spell.

While I'm a big fan of the index that Rick and Brian came up with, I've been tinkering to see if it can be improved still further.  One of the limitations of the existing index is that it assumes a Normal distribution for daily mean temperatures, but we know that the distribution is significantly skewed at certain locations and times of year.  Here's an old post illustrating this:

https://ak-wx.blogspot.com/2014/03/temperature-skewness-maps.html

To account for skewness, I was able to fit a Skew-Normal distribution to the historical temperatures at each of the 25 ASOS locations used in the ACCAP index.  Using these distributions - which vary through the year - the statewide index can then be recalculated from the newly standardized anomalies at the individual locations.

This modification makes a fairly big difference at certain times of the year.  An extreme example is late March 2019, when extremely warm weather occurred at a time of year when skewness is negative for most of the state.  In other words, negative departures from normal tend to be larger than positive departures, and so a large warm anomaly is actually much more unusual than a Normal distribution would imply.  Here's an illustration of the standardized anomalies on March 22 that year, using a Normal distribution (top) and a Skew-Normal distribution (bottom):

The standardized anomalies are much larger in the Skew-Normal framework, and the statewide anomaly jumps from +2.15 to +2.71.  In terms of degree-of-rarity, the Normal distribution estimates a 98.4% percentile for the day, but the Skew-Normal distribution shows 99.7%.  Both are in the upper tail, but accounting for the negative skewness reveals that the warmth was statistically much more extreme.

It works the other way too: a few months later in 2019, extreme heat occurred in early July, but with positive skewness around Alaska's coastal regions, the large warm anomalies were somewhat less significant than a Normal distribution would indicate.  The statewide standardized anomaly drops from +3.16 to +2.88, which although not a large change implies more than a 50% increase in the climatological probability of such an event (0.08% vs 0.20%).

Of course we could find examples on the cold side too.

In addition to the skewness modification, I also adjusted the statewide index calculation to account for the fact that the (weighted) average of the 25 individual (standardized) anomalies has larger variance in winter than in summer.  This is because there is more spatial correlation of temperatures across the state in winter, so the 25 (standardized) temperature measurements are more prone to move up and down together, and the combined average has a relatively high variance.  In contrast, summer temperature anomalies are more prone to cancel each other out across the state in summer, and the statewide average (of standardized anomalies) varies less.  (This is a completely separate issue from the reduced variance of actual temperatures in summer.)

So rather than using a single number to convert the statewide mean into its own standardized index, I used a seasonally varying number, and this varies by nearly 40% between about January 20 and June 20.  The result is that my modified index shows systematically smaller statewide anomalies in winter than the ACCAP method, but larger anomalies in summer.

Here's a comparison of the past year's statewide standardized anomaly from the two methods.  Notice that the revised method shows smaller values in winter, larger values in summer, and significantly less extreme cold in April (because of the negative skewness).

And here's the actual ACCAP index, which shows the statewide value in terms of percentile/frequency rather than standardized anomaly.

The major difference between the anomaly view and the percentile view is that the percentile view doesn't differentiate the magnitude of extreme events, i.e. once you're in the top or bottom 5-10% of events, they all look quite similar in the percentile view.  In contrast, the anomalies clearly delineate the amplitude of extreme events, but on the other hand they don't provide an intuitive measure of frequency: most people don't have a sense of how rare a 2 or 3 standard deviation event is.  Both perspectives are useful.

My next step with this work is to set up an automated process to create daily graphics and perhaps a simple webpage for monitoring purposes.

Early Snowpack

The onset of winter sometimes seems to occur almost instantaneously in Alaska.  The sudden change of season happens most obviously when temperatures drop more rapidly than usual around the climatological transition into the long freeze of winter, leading to a nearly binary switch from above-freezing to persistently below-freezing.  This year is an example: here's a chart of daily mean temperatures in Fairbanks since August 1.  The arrival of cold with the first snow on October 3 marked the sudden change.

Assuming the five inches of snow now on the ground in Fairbanks doesn't melt out, October 6 this year will mark the earliest onset of the sustained winter snowpack since 2014, and within the top 10 earliest since 1930.  Read some comments from 2014 here:

https://ak-wx.blogspot.com/2014/10/early-snowpack.html

There's no significant trend in snowpack onset dates for Fairbanks, and interestingly the 10 earliest dates have occurred at quite regular intervals over the decades.  The latest onset dates show more clustering: a few in the 1930s, and another group in recent years.

Looking farther north in the interior, Bettles also shows no trend (with data since 1951).  The roughly 180 miles makes a difference of about 9 days in the median date (October 10 in Bettles, October 19 in Fairbanks).

However, it is interesting to see that Bettles has not seen a September onset for 15 years now, whereas it occurred 8 times between 1950 and 2010.  Only once (1992) has Fairbanks seen a September snowpack start.  Conversely, Bettles hasn't seen bare ground at the end of October since 1974, whereas it's not too unusual in Fairbanks (and seemed to become commonplace in the last decade or so).

In other interior weather news, Tanana reached a chilly 0°F yesterday morning.  This is the earliest 0°F reading in Tanana since 1996, and it's good for 10th earliest (tied) in the history since 1901 (with some missing data).

Overall the state is seeing the coldest start to October since 2014, judging from the ACCAP standardized statewide index.  Except for 2021, every year from 2015-2022 was quite a bit warmer than normal for the first two weeks of October, so it's a refreshing change.

Signs of Freeze-Up

A dry and chilly north wind has kept wet bulb temperatures at or below freezing today across much of western and northern Alaska, and that means fresh water ice formation is likely beginning on many water bodies.  One of the first places where it's easy to spot from afar is the Koyuk River where it empties into Norton Bay.  Here's this afternoon's webcam view courtesy of the FAA:

The ice collected on both sides of the river over the course of the day today; here's a simple animation.

Looking back at this blog's archives, I noted the ice formation at Koyuk on the following dates in previous years:

October 24, 2013

https://ak-wx.blogspot.com/2013/10/start-of-freeze-up.html

October 9-11, 2014

https://ak-wx.blogspot.com/2014/10/freeze-up-progress.html

October 18-20, 2017

https://ak-wx.blogspot.com/2017/10/winter-descends.html

https://ak-wx.blogspot.com/2017/10/freeze-up-progress.html

October 14, 2019

https://ak-wx.blogspot.com/2019/10/upper-tanana-snowstorm.html

October 14, 2022

https://ak-wx.blogspot.com/2022/10/changing-fast.html

In other news, it was 0°F at Anaktuvuk Pass this morning, and nearly as cold at some of the interior RAWS sites: +1°F at Norutak Lake and +2°F at the Beaver RAWS in the Yukon Flats.

And yesterday the rarely-operational Galbraith Lake thermometer reported -8°F on the north side of the Brooks Range.  This appears to have been the first sub-zero temperature of the season observed in Alaska.

First Snow in Fairbanks

The first accumulating snow fell in Fairbanks today - right on time according to the calendar.  The median date since 1930 is October 2nd.  But these were also the first flakes to be reported in the air at the airport, and that typically happens a few days earlier: September 26 on average.

Its getting white outside our office this morning. ❄️A weather front moving east across the Interior will bring areas of light snow ranging from one half to 2 inches before the snow ends tonight. How much snow have you seen so far? Have a nice day out there. #akwx pic.twitter.com/3AezK9w2JX

— NWS Fairbanks (@NWSFairbanks) October 3, 2023

This morning's Fairbanks sounding showed a temperature profile entirely below freezing, and it's also the first time that has happened so far this season.  This is also close to normal.  The odds of any particular 3am sounding being entirely below freezing in Fairbanks rises approximately as follows:

10%   September 20

25%   October 1

50%   October 10

75%   October 27

85-90%   November 15

It's more uncommon to see a sub-freezing profile in the 3pm sounding at the beginning of October, because there's still a modicum of solar heating to produce a modest diurnal temperature cycle.  Today's 3pm sounding reported exactly 0°C a small distance above the ground.  But the historical frequency of a fully sub-freezing afternoon profile rises quickly as that solar heating gets snuffed out:

10%   October 3

25%   October 9

50%   October 18

75%   November 1

85-90%   November 15

Appealing views from the Nenana webcam today:

Brink of Winter

Autumn is quickly drawing to a close for interior Alaska, with a severe freeze in the usual cold spots this morning, and significant snow predicted for later this weekend in and around the eastern Alaska Range.

This morning's chill was notable in the hills above Fairbanks, with widespread lows below 20°F.  Down at valley level, the Goldstream Creek observer measured 18°F, and the Salcha RAWS reached 10°F, the third morning at 11°F or below in recent days.  It's not particularly unusual, but the rate of the temperature decline never ceases to amaze (me) at this time of year.  [Update Saturday morning: down to 7°F this morning at Salcha.)

Looking at statewide data, it's typical to see a temperature below 10°F somewhere in the state before September ends, and sub-zero is not rare.  Two years ago the early cold was more severe.  Usually the coldest reports at this time of year come from locations in and around the Brooks Range, such as Ivotuk, Toolik, or Chandalar Lake.

Despite the chill, there's less cold air around in the Northern Hemisphere than in previous years.  Here's a chart I made today to illustrate the daily evolution of the areal extent of air below -5°C at the 850mb level (roughly 4500' above sea level):

A threshold of -5°C at 850 mb has been suggested as a good way to delineate the lower tropospheric cold pool that expands each winter and virtually disappears in summer.  The figure above shows that the cold pool, so defined, typically extends across 25-30% of the hemisphere in January.  This year the cold pool's growth is lagging the 1991-2020 normal significantly, and in fact this month's cold pool area is the lowest on record so far (for the month to date).

Here's a map to illustrate the cold pool's extent in the most recent day of data from ERA5: 

Based on data since 1950, the year with the most expansive cold pool in September was - surprise, surprise - 1992, when Alaska saw its record-setting early cold snap.  Here's what the 850 mb temperature map looked like on the same date in 1992:

Clearly it wasn't just Alaska that was affected by unusual early cold in the high latitudes that year.

Finally, it's worth noting that despite the sharp freezes, there is still a lot of color on the trees in and around Fairbanks.  I find this quite remarkable at the end of September.  Nenana and Cleary Summit webcam views from the last day or so:

Here's an animation of the foliage changes at Nenana:

Getting Frosty

Widespread overnight freezing temperatures arrived across Fairbanks and much of the interior late last week.  This is a "top 10 latest" first freeze in the official Fairbanks climate record; the 1991-2020 median date for first 32°F or lower is September 11.

Fairbanks climate data since 1930 shows the expansion of the growing season at both ends:

The trend is somewhat greater for the first freeze of autumn (1.8 days later per decade) than for the last freeze of spring (1.2 days earlier per decade).  Combined, the growing season (based on this 32°F metric) has lengthened by 3.0 days per decade, or nearly a full month since 1930.  The last time the growing season was 100 days or less was in 1996, but such a short growing season used to be fairly common.

Localized urban warming does not explain the trend at the official climate site (the airport since 1951, Fort Wainwright in the 1940s, downtown previously), as the data from the university farm paint a similar picture - see below.  The farm is at the far western end of campus, in a relatively open and undeveloped location, and has always been far more prone to early and late freezes.

Based on the 32°F threshold, the trends from 1930-present are actually greater at the farm, with the growing season lengthening by 4.2 days per decade.  The official climate site has never reported a freeze in July, but it occurred in some of the earlier years at the farm.

For the full history since 1905 at the farm, the trends are 1.5 days per decade later for first freeze, and 1.6 days per decade earlier for last freeze.  Note that I used July 15 as the cut-off between "late" versus "early" freeze.

The annual total of growing degree days (accumulated "heat units" above 50°F) has risen tremendously in Fairbanks, with the linear trend amounting to 5% per decade, or about a 60% increase from the 1930 baseline (change from ~750 to ~1200).  The chart below shows that a very noticeable jump occurred in the late 1960s, and it's also striking to see that sub-1000 GDD seasons have all but disappeared in recent years.

Subsea Cable Fixed

Good news this week for many residents of northern and western Alaska: the Arctic undersea fiber optic cable that was damaged back in June has been repaired, allowing internet and phone service to get back to normal.  This is a story I've been following, because the cable was apparently severed by ice scouring on the sea bed about 35 miles north of Oliktok Point, just up the coast from Prudhoe Bay.  Read more here:

https://alaskapublic.org/2023/09/19/northern-alaska-cable-break-repaired-after-14-weeks-of-internet-outages/

https://alaskapublic.org/2023/06/12/cut-cable-causes-weeks-long-north-slope-northwest-alaska-internet-and-cellphone-outages/

The aspect I find interesting here is that the break occurred in water "about 90 feet deep", and it seems remarkable to me that Arctic sea ice could ever reach the ocean floor at that depth.  Frankly I was skeptical to begin with, thinking that either the depth was misquoted, or that the cable was damaged for some other reason.  But Quintillion's webpage confirms "ROV inspection shows 20 meters of accessible cable on the seabed floor. The target is 50-60 meters of accessible cable in order to reach the surface with workable slack for splicing operations. The water depth in the area is ~25 Meters."

The second article linked above cites Quintillion's president with the following explanation:

“It’s in an area where the land-fast ice and the sea ice kind of meet,” he said. “And so ice scour – which is essentially moving ice hitting the land ice and then (sinking) kind of like a tectonic plate – I suppose that it scarred the seabed floor, and then went down below the depth where we were buried.

(end quote)

It seems that a large wind stress event would be needed to drive sea ice down to such a great depth, so it's worth looking at the weather data.  As it happens, there is some evidence of high winds on June 11, the date the damage occurred.  The following chart shows hourly 10m wind speed from the ERA5 reanalysis at 71°N, 150°W, which is very close to the stated location of the event:

According to the ERA5 data, the wind speed peaked on the afternoon and early evening of June 10, from a direction just slightly north of easterly.  The peak sustained wind speed is not particularly high - less than 30mph - but the ERA5 wind speeds can't necessarily be taken at face value.  The Deadhorse ASOS reported sustained winds of 30-35mph, with higher gusts, at the same time, so the winds may have been somewhat higher over the ice (and there was still 100% ice cover at that time).

To put the peak ERA5 wind in context, I pulled out ERA5's maximum hourly wind speed for the month of June in past years at the same location.  Since 2000, nearly half of all years have seen a June wind speed higher than this year's event, so it really doesn't stand out as an unusual event.

Here's the surface analysis chart for the same time, 4pm AKDT on June 10, courtesy of Environment Canada; click to enlarge.  There's a modest pressure gradient across the near-coastal waters north of Alaska, consistent with brisk easterly winds, but again nothing especially unusual.

It seems that further analysis of the event would require information about ice thickness and motion in the area, and perhaps satellite imagery would reveal the ice subduction process described by Quintillion.

Any sea ice experts care to weigh in?

August Climate Data

Looking back at climate data for August, it was a very warm month indeed - the 3rd warmest on record for the state overall.  Only in 1977 and 2004 was the month of August warmer for Alaska as a whole.  It was also the warmest month of the 2023 summer relative to normal; there were only a few days in August that were marginally cooler than normal overall.

Regionally, the warmth was most unusual across the eastern interior, and southwestern Alaska was relatively cooler.  A major trough-ridge contrast explains the pattern; both the trough over the Aleutians and the ridge over western Canada (and SE Alaska) were quite strong for the time of year, and the circulation anomaly pumped a great deal of warm air northward into Alaska.

South-central Alaska saw yet another wet and cloudy month, and the Southeast had more rain than in the very dry July, but it was still drier than normal.  ERA5 and NCEI disagree on whether the Southeast Interior saw moisture relief, but Rick Thoman notes that rains were highly variable in that area; some places remained very dry.

The trend towards warmer conditions as summer progressed probably had at least some connection to the developing El Niño, because a ridge over southern and eastern Alaska, and northwestern Canada, is a typical feature when El Niño is occurring in August.  The map below shows the typical August 500mb height patterns during El Niño.

However, other aspects of the circulation pattern from eastern Asia to Canada were highly atypical for El Niño.  Notice the tendency for lower heights/pressure near the Sea of Japan: that's the opposite of what happened in August this year (see the first map in this post).  Also, most of Canada tends to be cooler than normal in August during El Niño (first map below), but in fact it was very warm across Canada's northern tier, and the record fire season just kept on going.  The heat over Japan and northern Canada was relentless this summer, and that certainly can't be pinned on El Niño.

A better explanation for the overall North Pacific - North America pattern is the ongoing negative PDO phase.  It's very unusual indeed for a negative PDO to persist as El Niño emerges, but this year the PDO has remained entrenched in the negative phase; and the warmth to the east of Japan has a lot to do with that (i.e. the warm SSTs there correspond to a negative PDO).

Here's the spatial (air) temperature pattern for August when the PDO is negative.  It doesn't line up with the warmth in the eastern Pacific this year, but otherwise it's not a bad match from Japan to northern Canada.  Texas tends to be hot too - and they had a really awful summer.

Rounding out the Alaska climate maps for August, it was cloudier than normal once again for western Alaska and the northern Gulf coast.  It was also relatively windy from the Bering Sea to the western and northern interior.

It's also interesting to note the very high humidity (dewpoint) for much of the state.  This goes a long way to explaining the extremely high overnight temperatures and therefore also the remarkably delayed autumn foliage in the Fairbanks area.

For future reference, I'll include maps below for climatological summer, June through August.  It was the second warmest summer on record for the North Slope, and third warmest for the eastern interior climate divisions, according to the NCEI data.  2004 still holds the summer temperature record by some margin for the state as a whole and for the west coast, interior, and North Slope, as well as the northeast Gulf and central and southern panhandle regions.  2019 was the warmest summer for south-central and southwestern Alaska.

Rainfall, wind, and cloudiness were all exceptionally high for the northern Gulf Coast and southwestern mainland, but Southeast had really quite a dry and sunny summer.

And finally, it's just interesting to note again the dramatic shift that took place in mid-July as the weather suddenly and belatedly turned very warm.