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

March Climate Data

March was another warmer-than-average month for most of Alaska, although not excessively so; it was the 15th warmest March in the NOAA/NCEI history since 1925.  The only region that wasn't warmer than the 30-year normal was the west, and the Bering Strait region was actually significantly colder than normal.  Here are my usual "percentile rank" maps, showing how the month compared to the same month in the past 30 years. 

The monthly average mid-atmosphere circulation pattern isn't what we would usually think of as producing a warm month for the state as a whole: a ridge to the north and a trough over the Gulf of Alaska tends to be a colder pattern in the cold season.

However, there were tremendous changes in the flow orientation through the month.  The first 10 days were very warm owing to a Bering/Aleutian trough (a typical warm pattern), but this reversed to an Aleutian ridge by late in the month.  Here are 500mb height anomaly maps for one-third portions of the month:

It seems that the northern Bering Sea and Bering Strait region managed to remain north of a frontal zone for much of the month, and so Arctic air dominated that region and kept temperatures relatively low.  The persistent frontal zone can be seen on a map of solar radiation, which was below normal (i.e. above-normal cloudiness) from the central Bering Sea to Bristol Bay:

March precipitation was below normal for more of Alaska than it was above normal, and according to NCEI both the Northeast Interior and Northeast Gulf divisions were significantly drier than normal for the second month in a row.

Given that March is typically a dry month, the precipitation anomalies made relatively little difference to the snowpack, which remains quite similar to a month earlier - although the positive anomalies have generally diminished in the western and northern interior.

Snowpack remains seriously lacking in the southwest, parts of South-Central, and the Seward Peninsula, according to ERA data:

Winds were lighter than normal for large parts of the state in March, which makes sense in view of the overall circulation anomaly (ridge to the north, trough to the south).

The extended winter period of November through March ended up as the sixth warmest on record - not as anomalous as December through February (third warmest).  November was the coolest month relative to normal (although still not cooler than normal), and January was by far the warmest and wettest.

The North Slope climate division had its third warmest November-March, trailing only 2017-18 and 2018-19, and the Northwest Gulf was fourth warmest on record.  In the case of southern Alaska, it's worth considering again how remarkable it is that this kind of warmth can prevail with a significantly negative PDO phase, although admittedly the PDO index did rise to neutral by the end of March.  Here's a chart of the PDO index for the last decade or so (click to enlarge).

Precipitation for the extended winter season was above normal for most of the state except the Panhandle, largely because of January; but as noted above, this generally only produced a good snowpack to the north of the Alaska Range, because of all the warmth (again, especially in January).

More on Persistence

I suspect that not everyone finds this as interesting as I do, but nevertheless here's a follow-up on seasonal temperature persistence in and around Alaska, this time from a map perspective.  Using ERA5 reanalysis data, I calculated the correlation of consecutive monthly temperature anomalies from 1950 through 2020, with the linear trend (specific to each month) removed.  A positive correlation means that the sign of the anomaly (i.e. above or below trend) tends to persist from one month to the next, but a negative correlation indicates that it tends to reverse.

It is usually a safe rule of thumb that weather and climate anomalies tend to be "persistent" even over land - there is a positive autocorrelation - so it's a surprise to see that the overall correlation is slightly negative for a portion of east-central Alaska and an adjacent zone in northwestern Canada.

This is actually the only place on the planet that has a negative month-to-month correlation, according to ERA5 data for this particular historical period.  There are plenty of regions with very low correlations, but this small region just to the north of Eagle is the only place with inverse persistence.  Here's a map for the Northern Hemisphere extratropics.

The tendency for temperature anomalies to reverse sign is mostly found in the winter for interior and eastern Alaska.  Here's the December-January correlation:

The rest of the year is added below.  The maps confirm the observation I made in the previous post: for the state as a whole, persistence is very much heightened in April-May and in July-August.  One might say those pairs of months are temperature twins: they tend to resemble each other in terms of departure from normal.

The other striking point is how much higher persistence is in summer than in winter for the maritime southern regions, and for late summer and early autumn near the Arctic Ocean.  Clearly the warm season temperature anomalies are dominated by slowly-varying ocean temperature regimes in locations close to open water, whereas relatively chaotic atmospheric flow patterns tend to control the month-to-month temperature changes during winter or over ice-covered ocean.