Runoff and Temperature

My post about the Fairbanks spring flood of 1948 elicited a comment from reader Andy, who suggested that cold in the second half of April might be a more significant indicator of breakup flooding than unusual cold earlier or later in the spring.  It certainly played a role in the 1948 flood: April 16-30 was by far the coldest in Fairbanks history.  More recently, 2013 saw a very cold April, and breakup was record late with a near-record peak discharge on the Tanana River.

I decided to take a look at this more closely using discharge data from the Chena River in Fairbanks.  Peak discharge isn't the same thing as breakup flooding, because the worst floods are caused by ice jams, but discharge gets at the runoff/meltwater part of the problem.

Here's a chart of the peak daily mean discharge in Fairbanks between April 1 and June 15:

Importantly, the Chena River Flood Control Project was completed in 1979, so the peak flows in Fairbanks have been constrained since then.

The flow typically peaks in May, but there is great variability in the date of peak discharge.  There's a long-term trend towards earlier peak flow, including since 1980; it has become unusual to see peak discharge after May 15 in recent years, but it used to be common.

This year we're seeing a secondary peak as a heavy snowpack is still melting out on the higher hills:

I chose June 15 as the cutoff date for "spring" discharge because that's when the flow is typically minimized prior to its gradual increase through summer; the weather gets wetter in interior Alaska as summer advances.

With all of this as background, I looked at the correlation between the peak discharge and the average temperature in Fairbanks for 15-day intervals throughout the spring.  Here's the result:

The negative correlation indicates that colder conditions correspond to higher peak discharge, which is what we expect: snowmelt is delayed and then occurs more rapidly once the inevitable warmth arrives.  In contrast, unusual warmth allows meltout to get under way early, increasing the window of time for all that meltwater to traverse the drainage basin.

Interestingly, there's no obvious date window that is particularly linked to peak flows, and in fact the greatest correlations are found quite early: late March into early April.  However, the correlations are quite small, so temperature in these 15-day windows does not explain much of the variance in peak discharge.

The potential relationship of temperature with discharge is somewhat confounded by the influence of snowpack depth: greater snowpack implies higher discharge, and spring snowpack is also correlated with temperature.  Not surprisingly, there is quite a good relationship between peak snowpack water content and peak discharge; the following chart uses snow water equivalent data from five SNOTEL sites in the hills above Fairbanks.

Using this relationship, we can remove the linear effect of the snowpack on the peak discharge, leaving us with the "residual" discharge that can't be explained by snow water content.  If we then compare the residual discharge with temperature, the correlations are a bit different:

This result suggests that temperatures in early spring (March 15-April 15) are actually more significant for determining peak discharge than temperatures in late April and early May.  I find this a bit hard to believe, because late March temperatures still average well below freezing in the Fairbanks area, so even warm years wouldn't be expected to see significant meltout that early in the season.  But perhaps there are still other correlations involved; warmth in late winter tends to occur with dry, sunny, and perhaps windy weather, and it's certainly true that humidity, sunshine, and wind can affect snowpack.

As for the larger correlation that shows up in early-mid May, this is right around the average time of peak flows, but at that time I would expect cold weather to actually reduce flows as the latter part of meltout slows down.  Again, perhaps the relationship - which is not particularly strong - is confounded by other factors.

I would be interested to hear any suggestions on how to refine the analysis in order to gain more insight.

Flood of 1948

On this date 77 years ago, Fairbanks was experiencing the worst flood in the short history of the city up to that time.  It would go on to be exceeded by the August 1967 flood, but May 1948 remains the second worst flood, and the worst springtime flooding event.

The event was not a breakup flood, although breakup was very late that year, occurring on May 10 on the Chena River in Fairbanks, and May 13 on the Tanana River at Nenana.  Rather, the flooding arose about a week later from tremendous snowmelt runoff in the hills.

The stage was set a month or more earlier, as very heavy precipitation occurred from late March into April.  Valley-level temperatures were marginal for snow during the sustained precipitation of April 7-14, but it would have been cold enough in the hills for snow to continue accumulating.  Here's the data from the Fairbanks climate record:

Notice the cold that developed in mid-April, preserving the snowpack at a time of year when it would normally be starting to diminish, depending on elevation.  Overnight temperatures were still falling below 20°F in early May - hence the late breakup - but then spring finally emerged on May 9, and there was no looking back.

It's difficult to say how much snow may have actually been on the hills above Fairbanks by the time the big thaw arrived.  The preceding winter was quite warm until February, and not particularly wet, so it seems unlikely there was a big snowpack going into March; but the 3" of new precipitation in Fairbanks in late March and April would have added at least 4-5" of liquid to the elevated snowpack, perhaps much more.  There have been other years since with snowpacks that were greater, but back in 1948 the combination of sudden melt and a lack of modern flood defenses produced a very unfortunate outcome.

Here's a silent clip from the Alaska Film Archives documenting the event.

https://www.youtube.com/watch?v=S-u7m0uL7Kc

Wind Trends

In last week's post, I noted that Alaska's 10-year running mean precipitation is at a record high, reflecting the fact that the state's climate has become generally wetter in recent decades.  It's common knowledge that temperatures have also increased on a timescale of several decades, especially in the Arctic.  But what about wind?  Are there any notable multi-decadal trends in wind speed for Alaska?

The question of climate trends is not as straightforward for wind, because there is no good long-term "ground truth" data for wind speeds; measurement practices have changed too much over the years.  We have to rely on retrospective estimates from a model, which in my case means the ERA5 reanalysis from the European modeling center.  So take the following results with a pinch of salt.

First we can observe that last year was apparently the windiest year in many decades for Alaska as a whole (according to ERA5); and it was particularly windy over western regions:

This wasn't just caused by one windy month; 6 of 12 months saw statewide winds at least 5% above normal, and January and August were particularly windy.  Only one month - September - was notably calm compared to normal.

Does this reflect a long-term trend towards greater winds?  No: ERA5 data indicates no significant trend over Alaska's land area, and if anything it suggests an absence of windy years in the last 20-30 years.  2024 was quite anomalous compared to the previous couple of decades:

Here's a look at the 75-year change in annual mean wind speed, using a linear trend, and including 2024:

The results in the complex terrain of southern Alaska are highly variable and probably not representative of the actual trends at populated valley-level locations.  (The ~30km horizontal resolution of the ERA5 data is far too coarse to handle complex terrain.)  There's a marginal hint of decreasing wind speeds across parts of the interior, but clearly the model shows increased wind speed across the near-coastal waters of the Bering Sea as well as more widely in the Arctic Ocean.

The seasonal breakdown of the trends shows a few interesting results - see below.  First, the largest percent increase in wind for the Bering, Chukchi, and Beaufort Seas is found in winter, although the Arctic waters have also become more windy in summer and autumn, if the model is to be believed.  The winter trend of increasing wind is also evident across the western land area and the northern interior.  However, it seems that the North Slope and the northern interior have become less windy in autumn, and there is a distinct trend toward less windy summers in the southwestern interior and Bristol Bay region.

The rising wind trends over the Arctic and Bering waters may have more than one cause.  First, there seems to be a trend toward greater storminess over the Bering Sea in winter; the two maps below show the multi-decadal difference of 500mb heights (top) and MSLP (bottom) for 1950-1990 versus 1991-2024.  This is consistent with the December-February wind increase extending inland across northwestern Alaska and the northern interior.

But the general warming trend in the Arctic is also a direct cause of rising wind speeds across the Arctic waters, because the surface-based temperature inversion has weakened, allowing more atmospheric momentum to mix down to the surface.  This effect is pronounced in autumn, when sea ice loss has been dramatic (see the Sep-Nov map above).  Here's a paper from last year on this topic:

https://www.nature.com/articles/s43247-024-01428-1

As for the summer wind decrease in southwestern Alaska, this is probably related to increasing high pressure and fair-weather "ridging" over the Bering Sea at that time of year.  In contrast, the Arctic Ocean seems to have become more stormy, especially in the Eastern Hemisphere.

So far this year, we've seen a very windy January - one of the windiest on record statewide (2024 was the windiest) - but February and March were somewhat less windy than normal.  It will be interesting to see if we have another windy summer, following last summer's record high wind statewide.  I see some reasons to believe the high-latitude summer patterns could indeed be similar to last summer - but more on that another time.

April Climate Data

Last month was the second wettest April in recorded climate history for Alaska as a whole, according to climate data released today by NOAA/NCEI.  The data spans from 1925 to the present.  The only wetter April was 1977, and the margin of difference is very small (3.11" vs 3.09"), so it's essentially a tie.

This marks the third month that has been either wettest or second wettest in the last year, with January and July both equaling or exceeding the previous record for those months.  Interestingly, there have also been very dry months in the past year (notably June, November, and February), so the 12-month running mean statewide precipitation is only slightly above the 30-year normal; but the 10-year running mean is at a record high.

Here's the regional distribution of the April precipitation percentile.  The northern Gulf coast was nearly record wet for April, and the Bristol Bay, South-Central, and Northeast Interior divisions also saw very anomalous precipitation.

ERA5 data paints very much the same picture.

The weather pattern responsible for the wet weather involved a monthly-mean trough from the Bering Strait region to northwestern Canada; here's the 500mb (mid-atmosphere) pressure anomaly for the month.

This is quite similar to the wet pattern in January, particularly in terms of the anomalous ridge axis from the Sea of Okhotsk to the Gulf of Alaska.  Here's the 500mb map from January:

The main difference between the two months is that January saw a much stronger ridge near Southeast Alaska, leading to a more southerly flow direction and therefore more anomalous warmth: January was in the top 10 for warmth statewide, but April lacked unusual warmth in the west and north.

April mean wind speeds were mostly above normal in southern and eastern Alaska, but the west coast was relatively calm, especially around the Seward Peninsula.

With April still being cold enough in northern Alaska that precipitation often falls as snow, the relatively wet weather allowed the northern interior snowpack to become more anomalous compared to a month earlier.  Indeed, ERA5 data suggests the snowpack is one of the greatest in recent decades for parts of the north-central interior.

NRCS SNOTEL data supports this, with Bettles and Coldfoot snow water equivalent currently at 91% and 93% respectively compared to the historical distribution.  1993 and 2019 had greater early May snowpacks at these two sites.

Finally, Bering Sea ice extent was near the multi-decadal normal for April, as the 2018-2019 low continues to look more like a short-term "blip" - although we're nowhere near the high ice coverage of the 2008-2013 period.

Interior Contrast

This is the time of year when there are big climate contrasts across the Alaska interior, depending on where snow is still lying.  Fairbanks versus Bettles serves as a good example: the snowpack melted out (officially) last Wednesday in Fairbanks, and the temperature hasn't been below freezing since then.  The Tanana River ice at nearby Nenana went out yesterday.  In contrast, there are still 24 inches of snow on the ground in Bettles, and only a handful of days have reached 40°F so far.  Of course it's much more difficult to warm up with deep snow on the ground, so locations with earlier meltout get a big boost to temperature.

Using climate data from 1991-2020, April is typically a whopping 9°F warmer in Fairbanks than in Bettles, a bigger difference than in any other month.

Looking at the overlapping climate history for the two sites, the median snow meltout date is April 25 in Fairbanks (so this year was only a couple of days early), but it's May 12 in Bettles - a difference of 17 days.  Obviously part of this is caused by Bettles being farther north and therefore colder to begin with.  Bettles also gets a lot more snow - nearly 50% more total precipitation in winter than Fairbanks - so the snowpack tends to be deeper.

I think another factor is that the Tanana River valley sees an additional boost of spring warming from its location downwind of the Alaska Range, i.e. downslope warming.  The chart below shows the monthly average wind speed and direction in the middle atmosphere above Fairbanks (calculated using vector components).  Notice that the average wind direction becomes progressively more southerly in April, May, and June, before returning to southwesterly in July.

The winds at lower levels tend to be oriented even more from the south for most of the year.  For instance, at 700mb - approximately the height of the Alaska Range - the April wind above Fairbanks is typically from 210° on the compass (see below).  At 850mb (about 4000 feet above ground), the average wind is almost out of due south in April, although with great variability of course.  Episodes of southerly air flow are therefore very common, and the more abundant sunshine of spring tends to produce vertical mixing that brings warmth to the surface very effectively in the southeastern interior.  (Contrast this with winter, when a surface-based temperature inversion is typical.)

The northern interior, on the other hand, sees a much smaller warming influence from Alaska Range chinook flows; and with a slightly higher latitude and locally greater snowpack on the southern slopes of the Brooks Range, the positive feedback of snowmelt and reduced albedo is considerably delayed.

Here's a webcam view from Bettles today:

And just for fun, Arctic Village and Chandalar Shelf: glorious views.

Trends in Meltout Date

The snow is going quickly at valley-level around Fairbanks, more quickly than expected, as there were several very warm days in the past week.  Thursday's high temperature of 58°F was nearly a record high for the date, and the daily mean temperature of 46.5°F was very nearly the earliest on record for such warmth.  Today's official measurement of snow depth for Fairbanks is 10 inches, down from 21 inches a week ago.

It's interesting to observe that there's no significant long-term trend in the date of meltout in Fairbanks.  Meltout is defined here as the first date with zero snow or a trace of snow on the ground, where a "trace" means less than 50% area snow cover OR more than 50% but too little to measure (less than 0.1 inches).

The absence of long-term trend is more than a little surprising in view of the fact that average temperatures have risen substantially at this time of year:

There may be several possible reasons for this discrepancy, but digging into them is a topic for another day.  Two obvious possibilities are (a) snowpack water content has increased, offsetting the increased warmth; and/or (b) changes in measurement location and/or method have influenced the meltout dates.  The measurement location certainly has changed a few times, most recently just a few years ago when (I believe) the location changed from the airport to the university's West Ridge campus.

But on to the main topic of today's post.  I was interested in a spatial view of meltout trends across Alaska, so I used ERA5 reanalysis data to take a stab at this.  First I examined whether ERA5 is able to capture year-to-year variability in meltout for two locations with reliable long-term snow depth data: Fairbanks and Bettles.  The results are quite encouraging, with correlations of 0.86 (Fairbanks) and 0.89 (Bettles) since 1950.

Note that I used slightly different definitions of ERA5 meltout for the two locations, based on trial and error.  For Fairbanks I found that the correlation of ERA5 and observed dates is best when "ERA5 meltout" is defined as the date when ERA5 snow water equivalent (SWE) drops below 0.5cm (liquid equivalent), whereas for Bettles a threshold of 1.0cm works slightly better.  Encouragingly, not only are the year-to-year correlations optimized at these thresholds, but the average dates line up too, i.e. the ERA5 dates are not systematically earlier or later than the observations.  This in itself is quite surprising; I frankly did not expect the model to do this well.

(Note that using zero SWE for ERA5 meltout is not reasonable, because the ERA5 grid cells are almost 20 miles wide and include higher elevations where snow lingers much longer.)

It's interesting to see that, unlike the official Fairbanks observations, ERA5 shows a substantial trend towards earlier meltout in the Fairbanks area.  The ERA5 result is perhaps more like what we would expect in response to the temperature trend; so this does make me wonder about the representativeness of the historical snow cover record from Fairbanks.  In Bettles the ERA5 trend is less than at Fairbanks, and it's also closer to the "ground truth" trend.

Finally, having established that 0.5cm SWE is a reasonable threshold for ERA5 meltout, here's a map of the ERA5 trend across the state.

A 75-year trend of 1-2 days/decade is widespread across central, western, and northern regions, corresponding to about 1-2 weeks of change since 1950.  However, more rapid trends are evident in southern areas.  Note that I have only calculated the trend in locations where the ERA5 snowpack reached at least 0.5cm in every year, and I also excluded locations where meltout did not occur by July 1 in any year; so the analysis is only for areas with a completely reliable late winter snowpack that then always melts out before summer.

Here are maps for North America and for the Northern Hemisphere, using the same 0.5cm SWE threshold for meltout (and I don't know how well that works in other regions).  Click to enlarge.  There are a couple of small zones with slightly positive trends: in interior northern Canada and in far northern Finland.  But overall the picture is one of dramatically earlier meltout, especially in the more southern latitudes.

Breakup Flooding Outlook

Despite the wintry chill in the air this morning (-6°F in Tanana, -16°F near Huslia), the sun is winning the seasonal battle, and major river breakup is only a few weeks away.  The Alaska-Pacific River Forecast Center in Anchorage produces a breakup outlook once a week, and yesterday's update discusses the wide variation in snowpack and therefore flood potential across the state.

https://www.weather.gov/aprfc/breakupProducts

Snowpack is greater than normal from the central interior up to the Brooks Range, and so it's no surprise to see above-average expected meltwater runoff for these areas, and a "moderate" flood potential for several communities:

In the breakup outlook's table of flood potential, the Chena River basin is listed as "above normal", and that's consistent with the hefty snowpack in the hills above Fairbanks.  The NRCS April 1 snow survey report describes it thus:

The report lists Cleary Summit (2250' on the Steese Highway) as having a snow depth of 48", with water content of 11" (177% of normal).  This is one of the most impressive snowpack numbers in the Alaskan interior, and is quite remarkable for the elevation.

Hopefully the process of meltout and breakup will be gradual and not all at once.  The short-term forecast looks encouraging: near normal temperatures in the next 10 days, allowing some melting to get under way.

Here's a link to get updates for this graphic (enter the ICAO code for any airport location, e.g. PAFA for Fairbanks, PAEG for Eagle, etc):

https://www.weather.gov/aprfc/breakupAirTemps

And for bonus content, here's an animation of the Yukon River view at Dawson today, courtesy of dawson.meteomac.com.  It looks like people are still crossing the ice bridge, although it officially closed on Monday:

https://www.yukon-news.com/news/dawson-city-ice-bridge-closes-for-season-7931513