Big El Niño Coming

Most readers are probably well aware that El Niño is on the way for next winter; the forecasts have been well-publicized.  Normally one would be wise to take El Niño forecasts with a grain of salt at this time of year, because there is a notorious "spring barrier" of predictability in Northern Hemisphere spring, but in this case the signals are so strong and coherent that there's little doubt of the outcome.

The most compelling evidence that a major El Niño is about to emerge can be seen in the profile of subsurface temperature anomalies in the equatorial Pacific.  The figure below is a couple of weeks out of date, but it shows a very intense warm anomaly below the surface, and this is migrating eastward and upward.  As the warmth continues to emerge at the surface, East Pacific equatorial surface water temperatures will become much warmer than normal, consistent with a strong El Niño signature.

The warmth produced by the subsurface anomaly will add to the substantial surface warming that has already occurred in the last few months, so it seems inevitable that the equatorial SST anomalies are going to become quite extreme in the next few months.  The canonical Niño3.4 SST index is already near 1°C above normal, although this includes a contribution of about 0.5°C from long-term warming in recent decades:

Here's a chart showing the recent progression of Niño3.4 SST anomaly forecasts from ECMWF's seasonal model.  The red line is the latest forecast; the model has been adjusting warmer with every run in recent months.

If the forecast is correct, the Niño3.4 anomaly may approach +3°C by autumn, with the modern record being +2.7°C in November 2015 (relative to a 1991-2020 baseline).

What does this mean for Alaska?  We'll discuss prospects for next winter another time, but for summer the history of past years with strong El Niño conditions (see below) suggests a heightened probability of above-normal temperatures, driven by relatively high pressure (ridging) from Alaska to the Canadian Arctic.  The top analog years also show dry summer conditions in Southeast Alaska and hints of dryness in southern interior and western Alaska.  Note that this is based simply on the 10 years since 1950 with the most positive Multivariate ENSO Index in June through August.

However, not all El Niños are created equal, and analog years show a lot of variability.  To illustrate this, here are temperature percentile maps for six summers when a robust El Niño developed by autumn following La Niña-like conditions the previous winter.  Where possible, I've used a preceding 30-year baseline for a fair comparison across the decades.

More recently, 2015 produced a very strong El Niño, but that episode didn't emerge from a cool phase in the previous winter.  There was also a robust El Niño in 2023, although its oceanic warm signature was inflated by unusual global warmth.

Not surprisingly, temperatures from the northern Gulf Coast to southwestern Alaska tend to be linked to the PDO phase, which was positive in 1957, 1986, (especially) 1997, and 2015, but negative in 1965, 1972, and 2023.  Elsewhere, variability is large among the analog years, although there isn't much cool to be found in northern and western - or southeastern - Alaska.

Similar maps for precipitation are included below.

It's difficult to pick out any consistent moisture signals on the maps, but interestingly ground-truth data from Fairbanks shows that none of the top 6 El Niño analog years had above-normal rainfall for May through September.  Click the figure below to enlarge (2014 is included only to show the record wettest summer):

Juneau also shows distinct indications of a (relatively) dry summer when El Niño develops in a similar way:

Summer rainfall can go either way in Anchorage, however:

For a more comprehensive look at all the seasonal climate guidance for this summer, I highly recommend tuning in to Rick Thoman's briefing on May 22:

https://uaf-accap.org/event/may2026-climate-outlook/

Arctic Sea Ice Update

Notwithstanding very abundant sea ice in the eastern Bering Sea this winter, Arctic-wide sea ice is now vying with 2019 for record-low extent at this time of year; it has been running near the bottom of the "pack" (pun intended) since November.  The following chart provides a comparison to 2019 (the previous record low at this date), 2012 (the year of the record low in September), and last year.

Deficits relative to the 1981-2010 normal are widespread across the marginal seas of the Arctic:

It's an interesting question as to whether the developing big El Niño episode will have a positive or negative effect on Arctic sea ice this summer.  Several of the strongest El Niño events of recent decades developed in parallel with substantial year-on-year declines in Arctic sea ice extent, so the risk of another major drawdown would seem to be elevated.  Here's a scatterplot of year-on-year change in September ice extent versus the June-August Niño3.4 SST anomaly; the notable recent years are 2023, 2015, 2002, and 1997 (lower right quadrant).

However, it's equally clear that there is actually not much correlation overall between year-on-year ice change and the phase of ENSO during summer.  In particular, pre-1997 El Niño events didn't have a consistent relationship with ice changes, and many stronger La Nina summers show modest ice declines rather than gains.

For a bigger picture, here's the correlation of June-August detrended SSTs with the year-on-year ice change:

The colors indicate the SST anomalies that would normally be associated with an increase of sea ice.  Clearly, the North Atlantic and North Pacific have more correlation than the tropical Pacific, but this doesn't imply causation; the SST patterns revealed here may simply reflect the impact of circulation patterns that affect sea ice extent.  Nevertheless, it's interesting to observe that a negative PDO pattern (like recent years) tends to favor higher sea ice extent, and that's confirmed in a scatterplot - although the correlation is not statistically significant:

The atmospheric anomalies themselves show a greater connection with sea ice change, of course; but those anomalies are relatively unpredictable at seasonal time scales.  A positive Arctic Oscillation, for example, slows the summer ice melt, whereas a negative AO phase tends to bring a reduction in ice extent.

ENSO does tend to be inversely correlated with the AO phase in summer, so it's more likely than not that we'll have a negative AO pattern this summer, i.e. more high pressure over the Arctic (and favoring warm and dry conditions in much of Alaska).  This points toward greater sea ice loss, as in other recent El Niño summers; but time will tell if this year fits the same pattern.

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.

Breakup Delayed

It's a bitter pill to swallow after such a harsh winter, but significantly colder than normal weather is set to return next week for much of Alaska, and there will be snow for many.  Breakup will be pushed back, and unfortunately the risk of a dynamic breakup with flooding problems is now on the rise, so to speak.

Here are ensemble-mean temperature forecasts from the latest ECMWF and GEFS models (temperature departure from normal at 850mb) for early next Wednesday:

The very good agreement at a 6-day lead time suggests that confidence is rather high.

Interior river ice thaw still has quite a way to go.  In Fairbanks, although temperatures have been above freezing by day for the last two weeks, the accumulation of daily "thaw units" has been minimal, and snowpack depth is still reported at 18".

The Tanana River ice at Nenana doesn't go out until at least 75 thawing degree days have accumulated in Fairbanks, which would equate to (for example) a week of high temperatures in the low 50s Fahrenheit and lows around freezing.  (Thawing degree days are calculated by summing the daily differences between daily mean temperatures and 32°F, for positive values only.)  And ordinarily a lot more thawing is needed - unless breakup is very late, in which case the sun's strength can get the job done without the same amount of warmth in the air.  Here's a chart to illustrate (click to enlarge):

Speaking of the sun's strength, data from the CRN site north of Fairbanks shows it has been cloudier than usual in the last 10 days, so that's another strike against breakup progress.  With Nenana ice thickness most recently measured at 51 inches, it's going to be a while.

March Climate Data

Winter 2025-26 is in the books, and it was a memorable one for many in Alaska.  The headline story, of course, was the interior cold - exemplified by the remarkably stubborn valley-level chill in Fairbanks, but experienced also more widely across the central and eastern interior.  The following figure gives a sense of where the cold was really anomalous, as in "colder than any winter in the 1991-2020 climate period":

From this perspective, interior Alaska was the most anomalously cold region on the planet for the 5-month period:

December and March were the really cold months statewide, although January and February were also below normal overall.

With November being milder, the statewide average temperature for November through March was the coldest since 2011-12, but December though March was the coldest since 1971-72.

As in Fairbanks, the greatest superlatives were reserved for March, which was truly extreme.  Six of 13 climate divisions saw their coldest March of record (1925-present), and 3 others were in the top 5 coldest.  This ranks up there with the likes of January 2012 and February 1990 for widespread record-breaking cold.

The reason for the unending supply of bitter cold was a pronounced and highly anomalous ridge over the western Bering Sea, bringing northerly flow to Alaska.  The pattern was very similar for March and for the winter as a whole:

As noted in earlier posts, eastern Bering Sea ice extent reached a record for the satellite era, peaking around March 24:

The dominance of northerly flow produced dry weather for most of the state except the North Slope and Southeast Alaska in March, and the November-March precipitation map also reflects a persistent moisture deficit from western to south-central Alaska:

Snowpack is therefore well below normal in much of south-central Alaska, and especially across the Kenai Peninsula, where the April 1 snow survey reports that many sites are ranking second lowest for snowpack.  However, the eastern interior across to the southern Yukon has well above normal snowpack.

The latest breakup outlook - updated today - highlights an expected return to below-normal temperatures, especially across the eastern interior, and that's not good news.  Unusually thick river ice and unseasonably cool weather point to a delayed breakup that may well be more dynamic (violent) when it finally occurs, especially if the ample snowpack eventually melts out in a hurry.

https://www.weather.gov/media/aprfc/BreakupProducts/ESFAK_ACR_20260410.pdf