Spring Contrasts

Clear skies and calm winds produced a widespread freeze across eastern and northern Alaska this morning, with the temperature dropping into the 20s in most locations.  Fairbanks airport reached 29°F, which is a little unusual in the modern climate for this late in the season; other than last year, it hasn't happened after May 10 since 2013.  The low temperature in Chicken was 19°F, which is the first time below 20°F this late in the season since 2014.

Farther north, accumulating snow fell in Fort Yukon yesterday morning.  The accumulation was probably less than an inch, given that the snowfall rate was light, but temperatures were below freezing, and it was falling for over three hours.  Snowfall measurements haven't been available from Fort Yukon for many years, but historical data shows an inch of snow as late as mid-June in 1971, and as early as late August in 1961.  It's a short summer in the northern interior.

Farther north still, the North Slope is still locked in with wintry weather.  The high temperature in Utqiaġvik yesterday was only 13°F, and that's a little more notable: we have to go back to 2007 to find a colder day this late in the season.  Utqiaġvik temperatures have been oscillating between above and below the 1991-2020 normal so far this year:

The contrast between North Slope winter and interior spring made me wonder when the seasonal contrast between the two regions is greatest.  It is indeed in spring: the following chart shows the difference between normal temperatures in Utqiaġvik versus the average of Bettles, McGrath, and Fairbanks.

The Arctic coast doesn't begin climbing out of winter cold until nearly April, but the interior has gained a lot of warmth by then, and the temperature contrast peaks in late April.  Of course, the interior cools off much more quickly in autumn, and the valley-level interior becomes briefly colder than Utqiaġvik in November.

Looking back at April, it was a cloudy and wet month for most areas except the North Slope and Southeast.  With precipitation well below normal in the usually wet Southeast, the state-average precipitation was not much above normal, but the southwest and southern interior were unusually damp for the time of year.

The broad swath of wet weather was caused by strong westerly flow to the north of a robust Gulf of Alaska ridge, and downstream of the ridge the dry anomaly was inevitable over Southeast Alaska.

Statewide mean temperatures in April were very close to the modern normal, with equally balanced ups and downs during the month.  The Aleutians were significantly warmer than normal, although not to the extent of some recent years (2016, 2020).

Cloud cover was well above normal for most of the state.  The combination of cloudy and damp weather certainly wasn't a bad thing in terms of wildfire risk as we advance quickly toward fire season.

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/