Bristol Bay Snow

In my last post I mentioned how cold the eastern Bering Sea waters are compared to normal, and as if to illustrate the point, snow was in the air at times today along the northern coast of Bristol Bay.  Dillingham saw mixed rain and snow for a couple of hours, nearby Manokotak had a period of snow, and out to the west snow has been falling all day on the elevated headland of Cape Newenham.

The observations from Cape Newenhem airport (elevation 541') suggest that snow was accumulating this morning, with temperatures below freezing and visibility below 1 mile for a couple of hours.

Looking back at the surprisingly complete history of hourly weather data from Cape Newenham, it looks like this is the most significant snow after May 20 since 1973.

Snow Analysis

Fairbanks reached 50°F yesterday for the first time this season - a couple of weeks later than normal - and thaw season is now fully under way, with no sign of an imminent return to unusual cold.

Winter's last gasp earlier this week had a sharp bite to it in the interior and north, with overnight temperatures dropping well below zero at many locations.  Here are a few notable readings from Wednesday morning:

-24°F at the Birch Creek HADS

-16°F at the Livengood RAWS

-17°F in Bettles and Coldfoot

-27°F at the Dahl Creek HADS

-20°F at the CRN site to the east of Kaltag

It's pretty late for this kind of cold.  In Bettles, for instance, only one year (since 1951) has recorded a temperature below -15°F after April 20, and that was only marginally later in the month (April 22-25, 1966).

Accumulating snowfall is likely finished in Fairbanks, so the tentative total for the season fell just short of 100".  It was the snowiest winter since the triple-crown big snow winters of 1990-91 through 1992-93.

In view of the Fairbanks snow onslaught in February, and similar heavy snows in Anchorage and Juneau, I thought it would be worth looking at the contribution of the snowiest days and months to seasonal snow totals at all three sites.  For example, the top 10 snowiest days of each winter generally contribute about half of a winter's total snow accumulation in Fairbanks; but has this changed over time?  Fairbanks typically "nickels and dimes" its way to a substantial snowpack, but is this more true of recent years or of an earlier climate?

The answer is there hasn't been any significant change over the decades.  Click on the following chart to enlarge; it shows the seasonal snow totals in light blue, the contribution of each winter's 10 snowiest days in dark blue, and the ratio of the two totals in red.

There are slight increasing trends in snow totals in Fairbanks, and there's a hint of a decreasing trend in the "top-10-days to seasonal total" ratio, implying that heavy snow days used to contribute a bit more of the total snowfall - but the trends are nowhere near statistical significance.

Another perspective on the question is to look at the snowiest 30-day period each winter and calculate its contribution to the seasonal snow total.  Again, there's a slight decrease in the snowfall fraction attributable to the snowiest month, but it's not a significant change.

Anchorage also exhibits slight - but not significant - upward trends in snowfall, both in terms of seasonal totals and each component of the segmentation.  The snow fraction derived from top-10 days and the snowiest month haven't changed significantly over the 72 years of data in Anchorage.

A more striking result is found in Juneau, where both fractions have increased very significantly in the 82-year climate history.  The snow totals themselves haven't changed significantly, but there's clearly a substantial increase in the concentration of snowfall in the heaviest episodes, starting in 1976-77.

It seems to me that the change in Juneau must reflect the increase in winter average temperatures that also developed in the late 1970s, when the PDO phase turned positive.  In Juneau's mild climate, warmer winters have more days with rain rather than snow, and therefore the snow events make up a larger fraction of the winter's overall snowfall.  (To understand this, consider the extreme of a much warmer climate where snow is rare; 100% of the snow will occur in the top-10 snowiest days each winter.)

The surprise, perhaps, is that Juneau hasn't seen a more significant decline in total snowfall; there is a negative slope, but it's not close to statistical significance.  However, it seems possible that the inter-annual variance is increasing; many recent years have had low snow, but there have been very snowy winters too, culminating in this winter's all-time record.

Winter Storm Warnings

Winter storm warnings have been hoisted from the Yukon Delta and eastern Norton Sound to the western interior, as a decaying Bering Sea low joins forces with an Arctic trough to bring significant snowfall.  Here are snow forecast graphics from the NWS (click to enlarge):

Interestingly, however, the model forecasts have adjusted warmer for next week, with cold disappearing quickly by mid-week; warmth will quickly surge up from the southwest, replacing the cold air mass centered over northwestern Alaska.  Here are the latest 850mb temperature forecasts for late Tuesday, with cold poised to depart:

If warmer weather can stick around for the rest of the month, as seems possible, then thawing will start to make some headway.  Of course we don't want it to get too warm too quickly.

Here's a historical perspective on Fairbanks temperatures in previous years when La Niña transitioned rapidly into El Niño between winter and summer, as it is doing this year.

Interestingly, this set of "analog" years suggests that the odds most favor below-normal temperatures for 30-day periods ending late April through late May - or right around breakup season.  The median breakup date at Nenana for these 10 years is May 6, i.e. a few days later than normal.  However, not all ENSO transition years are cold around this time; two of the last three (1997 and 2009) were not, and Nenana breakup was on April 30 and May 1 in those years respectively.

Cold and Snow Correlation

Last week I alluded to the linkage between the Pacific Decadal Oscillation and seasonal snowfall in Anchorage, with a negative PDO phase often producing increased snow relative to normal.  Of course the PDO also influences seasonal temperature variations - a negative PDO typically brings colder conditions to Alaska, especially in the south.

These twin correlations imply that seasonal mean temperature and snowfall are also correlated to each other in Anchorage: increased snowfall tends to accompany cold.  Here's a chart illustrating this relationship in the last half-century.

I thought it would be interesting to look at the snow/temperature relationship elsewhere around the state.  I started by examining ERA5 reanalysis data, but it's not particularly helpful: it actually suggests a positive correlation in the Anchorage area, presumably because the model's resolution is inadequate (i.e. the results reflect the influence of conditions at higher elevation).

However, one region where the model likely gives a good picture of the low-elevation snow/temperature relationship is in Arctic Alaska.  In the far north, warmer conditions are favorable for increased winter snow, simply because of increased availability of moisture.  This is supported by data from Utqiaġvik up until measurements ceased in 2019: warmer winters tend to be more snowy.

However, historical data from Kotzebue and Nome do not have a positive correlation, despite what the model says.

How about the interior?  The relationships are weak, with little correlation, although there's a suggestion that the warmest winters tend not to be very snowy in McGrath and Fairbanks (especially the latter).  Particularly for Fairbanks, this presumably reflects the drier weather that occurs when the pattern favors warm downsloping/chinook winds from the south and southeast.

Data from Bethel suggests a hint of a negative correlation like Anchorage: more often than not, colder-than-average winters are also snowy.

Turning to Southeast Alaska, Juneau shows a robust negative correlation, as we would expect for a much warmer climate where unusually warm winters tend to be more rainy than snowy.

These charts probably give a good sense of how seasonal snowfall is likely to trend over time if Alaska's climate continues to warm in future decades.  Broadly speaking, we would expect winter snow accumulation to diminish in the low-elevation south and to increase in the Arctic, while the interior may remain relatively unchanged.  But of course large annual and decadal variability will continue, and there may be long-term circulation changes (e.g. related to the PDO) that have significant impacts on long-term trends.

I'll be happy to add charts for other locations upon request (if the data is adequate).

More Anchorage Snow

Anchorage has seen another big snow event this week, occurring three weeks after the record (for January) snowstorm earlier in the month.  The new snow accumulation of 11 inches takes the January total to just over 40 inches, which is a record for the monthly total.  It's also the fifth snowiest calendar month in the modern Anchorage climate history (1953-present).

Here's a visualization of calendar month snow totals in Anchorage, with the top 10 events for the main snow months marked in red.  February has two huge outliers - 1996 and 1955 - but the most favored month for big totals is clearly December.

The climatological drop-off in January is actually rather significant; December reaches 20" over a third of the time (26 of 73 years), while January sees 20" in less than 15% of years (10 of 73, including 2026).  So this month's big snow total is a notable outlier for the time of year.

Another interesting tidbit is the preponderance of high monthly totals in recent years: 2023 saw the snowiest November, 2022 and 2023 had the 2nd and 3rd snowiest Decembers, and now we have the snowiest January.  February 2023 gets an honorable mention with the 4th snowiest February (32").  These are good times for snow lovers in Anchorage.

The statistics are at least as striking when we look at liquid equivalent precipitation in Anchorage, i.e. it's been wet in recent winters.

2022 - 2nd wettest February

2022 - wettest December

2023 - wettest November

2025 - 3rd wettest January

2026 - wettest January

What changed in 2022?  The PDO (Pacific Decadal Oscillation), for one thing:

See this 2022 post for some comments on the correlation between the PDO and Anchorage snowfall:

https://ak-wx.blogspot.com/2022/12/anchorage-snow.html

Late Snow in Bettles - Again

I mentioned a few days ago that the contrast between fire activity in Alaska versus Canada so far this season is very reminiscent of 2023.  Here's another facet of the similarity: Bettles briefly saw accumulating snow yesterday, just like in early June 2023:

https://ak-wx.blogspot.com/2023/06/record-late-snow-in-bettles.html

Unfortunately the climate observer in Bettles didn't (yet) report a snow depth for yesterday, so we can't say whether the maximum snow depth was measurable - but if it was, it would be the latest measurable snow on record.  Judging from webcam imagery (see below), the situation yesterday looked similar to the 2023 event; but in 2023 the light snowfall persisted (at above-freezing temperatures) for longer, and it redeveloped again the following day.

[Update June 8: the accumulation report came in at 0.4", so that confirms it's the latest measurable snowfall on record in Bettles.

End of update]

Yesterday:

2023:

Yesterday:

2023:

The NWS in Fairbanks issued a Winter Storm Warning for central Brooks Range locations above Bettles, and the warning extends through tomorrow afternoon.  It's the first WSW issued by NWS Fairbanks in June since 2018.

Anaktuvuk Pass looks to have quite a bit of fresh snow:

Late Winter Chill

Clear skies and a cold air mass have produced very chilly overnight temperatures in the past several days across interior and northern Alaska.  Some of the coldest readings have come in from the remote (but high quality) CRN sites, including the Selawik NWR (-38°F this morning), the Koyukuk NWR (-35°F), and the Toolik Lake site (-31°F).  It was -36°F this morning at the Beaver RAWS on the Yukon Flats, and Anaktuvuk Pass dropped to -33°F at the other end of the elevation spectrum.

Fairbanks airport dropped to -7°F, but this isn't at all noteworthy from a historical standpoint; it is typical to see -20°F at some point in the second half of March.

Here are this morning's lows across the central and northern interior (click to enlarge):

Interestingly, the change of weather pattern has brought significant snow to Anchorage for the first time since autumn.  Anchorage saw its least snowy winter (December-February) on record, with a measly 4.6" of snow in those three months, but now they've managed to accumulate 14.6" in the past week.  Adding this month's total to October's heavy snow gives a total of 36.1", well over 3 times the amount that fell in November through February.

The following chart illustrates the contribution of November-February to the total seasonal snowfall in the modern Anchorage climate history.  Typically about 30% of the seasonal snowfall occurs outside of Nov-Feb, and this ratio has been as low as 11% (1986-87) and as high as 62% (2001-02) in past years.  This year the ratio is 78% and the snow season isn't over yet.  Note that there's zero correlation between the two sub-totals over the years.

It's a topic for another day, but there is actually a statistically significant downward trend in the contribution of autumn and spring snow to the seasonal total in Anchorage.  This is what we would expect if the climate warms, because the autumn and spring temperatures are more marginal for snow.  So this winter very much counters the trend, because of the extremely dry winter (from the standpoint of snow - there was rain instead).  Last winter was the polar opposite, with only 12% of snowfall outside of Nov-Feb.

Fairbanks sees only slightly more of its seasonal total snowfall in months other than November-February; the average is about 33%.  The ratio has ranged from only 7% (2016-17) to 63% (1947-48).  Unlike in Anchorage, there is a slight positive correlation between the two sub-totals; but there is no significant trend in the contribution of autumn and spring snowfall in Fairbanks.

Flood Follow-Up

A few different items of comment today.  First, a significant snowfall is under way across the south-central region today, signaling the start of winter proper for many valley locations in that area.  If Anchorage's snow cover hangs around in the coming weeks, it will mark an earlier than normal onset of permanent snow pack; the normal date is about November 9th.  Of course Anchorage occasionally lacks snow cover even in the depths of winter, but it's unusual, occurring most recently in 2015-16 and for a few days in late December 2019.

As for Fairbanks, the normal date for establishing permanent snow cover is October 18, and it was right on time this year.  However, the most recently reported snow depth of 10" is the highest for the date since 1992, and more fell today.  (But 11" was on the ground in September 2015.)

Looking back at last week's historic west coast storm, it's worth highlighting blog comments by Rick and Gary.  Rick Thoman confirmed that long-time Kotzebue residents reported the highest water in living memory, which is why it was such a devastating event for some in the community:

https://www.adn.com/alaska-news/rural-alaska/2024/10/26/after-flood-kotzebue-residents-take-stock-of-damaged-homes-and-property-and-take-care-of-one-another/

Although the meteorology was different, we might say this was Kotzebue's version of ex-Typhoon Merbok, which caused such havoc for coastal locations farther south about two years ago:

https://ak-wx.blogspot.com/2022/09/more-on-ex-merbok.html

The article highlighted by Gary indicates that flooding was not quite as bad this time around from Nome down to Unalakleet:

http://www.nomenugget.net/news/high-winds-cause-erosion-and-flooding-region

Rick also commented that storms in November 1973 and 1974 may be the most analogous events in the modern climate record.  Certainly the storm of November 10-11, 1973, shows a rather similar sequence of events: low pressure over the Chukchi Sea created strong winds out of the south early on the 10th, and then the winds went around to the west and blew fiercely with a strong north-south pressure gradient, pushing water into Kotzebue Sound.  Here are MSLP maps:

Compare to the recent event:

Interestingly, the wind speed itself was not particularly unusual with last week's storm, and there have been many past events with equally strong, or stronger, winds from a westerly direction in Kotzebue.  I think therefore it was the very low pressure, the large size of the storm, and the long duration of southerly fetch that allowed such a large surge to develop before the winds turned westerly and pushed the water onto the coast.

Bering Sea Cold

Alaska's west coast and southwestern peninsula have been remarkably chilly in the past couple of days - the coldest in many years for this date on the calendar.

But before I get into details on that, it's worth noting that the Fairbanks NWS office has issued 4 severe thunderstorm warnings (so far) today: one early this afternoon to the east of Fairbanks, and then 3 more for severe storms west of Nenana and up towards Manley Hot Springs.  Historically, many years pass with less than 4 severe thunderstorm warnings issued for the whole summer in the Fairbanks office.  Here's a photo taken around 4:20pm from UAF, courtesy of the NWS via X/Twitter:

A very strong and cold upper-level trough to the west and south provides part of the explanation for the stormy weather: there is unusually strong convective instability caused by the cold air aloft. 

The trough has also produced extremely chilly conditions for the Bering Sea and southwestern Alaska yesterday and today.  Bethel's high temperature was only 40°F yesterday, and snow was observed briefly in the morning.  Bethel hasn't seen a daily high temperature that low in June since 1972.  Similarly, yesterday's high in Cold Bay was only 39°F - again with some snow - the coldest this late in the spring since 1985.

Out in the Bering Sea, St Paul Island reported several lengthy periods of light snow earlier today, with visibility down to 1.5 miles and the temperature hovering at or just below freezing.  The FAA webcam showed some accumulation, as illustrated below:

Remarkably, this morning's balloon sounding from St Paul Island reported a temperature of -9.7°C at 850mb, which is the lowest this late in the season since 1985.  This is about the same as the normal 850mb temperature in January.

The unseasonable cold is related to a suddenly very intense negative phase of the PDO.  Check out the SST anomaly pattern across the North Pacific: extreme warmth extends eastward from Japan, but the Bering Sea has become much colder than normal.

The daily PDO index managed to reach -3.0 for the first time since September 2012, and it's only the second time it's happened in the history of daily data since 1982.

What's driving the anomaly?  Well, the negative PDO regime has been more or less locked in for several years now; but the current amplification is happening as the perennial North Pacific ridge has joined forces with Arctic high pressure via a ridge over eastern Russia, allowing the Bering Sea trough to deepen and intensify.  As for the Arctic ridging, it's a "blocking pattern" that's conducive to extremes at lower latitudes - for example, flooding in central Europe a couple of days ago, and snow in upland Scotland yesterday.

Cold and Snow

Here's a bit of follow-up on the unusually late cold and snow.  According to Rick Thoman (and relayed via Brian Brettschneider on Twitter), Monday's half-inch of snow in Fairbanks was the first time since 1978 that measurable snow occurred after green-up.

The late snow in 1978 was much later - May 27 - but it was also more marginal, with mixed rain and snow reported, and officially-measured accumulation of only 0.1".  A more legitimate late snowfall occurred in 1966 in Fairbanks, with 1.6" measured on May 15-16; that was 8 days after breakup at Nenana, and almost certainly after green-up too (although the NWS green-up history only goes back to 1974).

Not to be left out, Anchorage also saw accumulating snow on Wednesday night.  This is a top-10 latest date for snow, and it's actually the latest on record with measurable snow on the ground at the daily observation (data back to 1953).  The previous record was May 6, 1955.  Interestingly last year there was snow on the ground on May 4.

The cold hasn't stopped breakup from proceeding, however.  The ice went out on the Yukon River at Tanana on Wednesday, and the Kuskokwim tripod at Bethel stopped the clock on Wednesday morning, despite a temperature of 25°F with snow falling.  Unfortunately significant flooding is now occurring in Bethel and elsewhere on the lower river, as the ice is jammed up downstream.

https://www.kyuk.org/public-safety/2024-05-09/as-lower-kuskokwim-river-breaks-up-bethel-sees-highest-river-gauge-level-in-almost-20-years

On the other side of the Arctic, very unusual cold is also affecting western Russia; it was snowing in Moscow yesterday for their big parade.  Severe freezes have also occurred in the Baltic states in recent days - a rare event for this late in the season.  As I commented in my last post, I think the "perturbed" flow pattern can be traced back to the lingering circumpolar disruption caused by the March stratospheric warming event.

Late Snowfall

Green-up was declared in Fairbanks on Saturday, but that distinctive first shade of green on the hillsides was obscured by white today, as light but steady snow moved into the area mid-morning.  With temperatures dropping slightly below freezing for a while, there was some minor accumulation on grassy and elevated surfaces; the NWS posted this photo:

Accumulating snow in early May in Fairbanks isn't too unusual.  There have been three May snows of more than half an inch since 2013; but of course it quickly becomes a more uncommon event as the month advances.  May 1992 was the exceptional outlier, with 9.4" on the 12th and another several inches in the subsequent days.  1992 also produced an absurdly cold and snowy September in Fairbanks, and it seems rather likely that this "coincidence" was somehow linked to the Pinatubo eruption (1991) and temporary global cooling.

But back to more recent events: read about the 2022 and 2013 May snows here:

https://ak-wx.blogspot.com/2022/05/spring-snow.html

https://ak-wx.blogspot.com/2013/05/big-snowfall-and-precip-differences.html

In the northern interior, Bettles saw a daily high temperature below freezing yesterday, marking the coldest day so late in the season since the (much colder) conditions of 2013.  Bettles also saw a bit of snow, but that's less uncommon: accumulating snow occurs in May in more than half of years up there.

And in Nome it's a very chilly day for the time of year: the temperature hasn't yet risen above 20°F today.  Again the last time that happened in May was in 2013.

The immediate cause of the cold weather is a trough that dropped down from the Canadian Arctic Ocean over the weekend and "joined forces" with another trough moving across the Aleutians into the Gulf of Alaska.  Here's the 500mb analysis from 4am this morning, courtesy of Environment Canada:

Arguably the chain of events was set in motion by a strong ridge stretching across the central Arctic Ocean late last week; and that in turn reflects the lingering influence of a "sudden stratospheric warming" event way back in early March.  The map below shows last Thursday's 500mb height anomaly, with generally above-normal heights from the Arctic Ocean to the northern North Atlantic, i.e. a negative Arctic Oscillation.  This is a typical outcome in the wake of a wintertime stratospheric disruption, although the timing and persistence of the anomaly varies widely from case to case.  Meteorologists watch for these events closely, because they tend to presage mid-latitude cold outbreaks and generally volatile weather owing to the Arctic "blocking" patterns.