June Chill, May Climate

Yesterday NOAA/NCEI came out with their climate statistics for May, and it was the sixth consecutive month with a statewide mean temperature below the 1991-2020 normal in Alaska (although April was only marginally below).  It was also the third consecutive May with a cool anomaly relative to recent decades:

Significantly above-normal temperatures have been rather scarce in most areas in recent months, and Rick Thoman noted that many interior locations had their lowest May maximum temperature since 2000.  The statewide temperature index barely rose above normal at any time in the month, although the first few days of June turned much warmer.

As if to re-emphasize the cool theme, a very notable cold front swept across the interior and north yesterday, ushering in a very chilly air mass for the time of year.  Here's an enhanced satellite image showing the frontal cloud band at 3:20pm AKDT.  Blue colors across the North Slope and high terrain indicate snow cover, and sea ice is visible in Kotzebue Sound and along the northern Arctic coastline.

The 500mb analysis from about the same time yesterday shows the connection to a very strong mid-atmosphere cyclone over the Arctic Ocean.  Judging from ERA5 data, it appears to be one of the strongest - if not actually the strongest - on record for the month of June in this region.

Temperatures this morning were near or below freezing widely across the interior, with hard freezes across the northwest and Brooks Range.  The NWS (courtesy of Rick Thoman) noted that the wind chill of 13°F in Kotzebue at 5am was the lowest on record for this late in the spring.  Here are a couple of zoomed-in maps of minimum temperatures across the Arctic northeast and interior/Arctic northwest.

Looking back in more detail at the month of May, NOAA's data indicates that the Bristol Bay climate division saw its coldest May since 2001.  However, the coarse climate division perspective completely obscures the fact that it was notably warmer than normal in Alaska's northwest; Kotzebue had its warmest May since 2020.

The mid-atmosphere pressure pattern responsible for this odd outcome is illustrated below.  A robust trough over Alaska brought chilly and wet conditions to most of the state, but the setup was conducive to above-normal sunshine over the northern Bering Sea and southern Chukchi Sea, and it seems that helped boost temperatures in the northwest.  I think there was also an element of chinook/downslope warming over the Seward Peninsula and near Kotzebue Sound, as winds from the northeast passed over the Brooks Range and then warmed on their way back down to sea level.

Winds were stronger than normal across the North Slope and much of the interior and south of Alaska:

Courtesy of the wonderful toolset at the Iowa Environmental Mesonet, here's a comparison of the May 2026 wind rose (top) compared to the long-term normal wind rose for May (bottom) in Utqiaġvik:

While easterly or east-northeasterly winds are always heavily favored in Utqiaġvik at this time of year, the regime was even more persistent, and with stronger winds, than normal this year (compare the frequency of winds in the 15-20 mph category).

For the climatological spring season as a whole (March through May), statewide average temperatures were the lowest since 2013, mostly because of March.  In the south it was the coldest spring since 1972 in the NCEI data from Bristol Bay to the mid-Panhandle, including Anchorage (and since 1974 in Juneau).

Wet and cloudy were also major themes from South-Central to the northern interior; but the bright side, of course, is that an early start to fire season was avoided.

Cool Trends Persist

Cooler than normal temperatures have persisted recently in both southern and northern Alaska, and a quick glance at UAF's statewide temperature index confirms the decidedly chilly nature of this month for the state overall.  Indeed, the month-to-date average of the UAF index suggests this is the coolest May since 2012, and it might be the coolest since 2001 when the final numbers come in.

It's rather notable that Utqiaġvik hasn't yet reached 32°F this year, and if it happens today or tomorrow it will be the latest since 2001 or 2000.  If Utqiaġvik manages to get into June without a 32°F reading, it will be the latest since 1955.

Here's a map view of the temperature anomaly in the past 30 days, as estimated by ERA5 and ECMWF model data:

Note that the anomaly is expressed in terms of standard deviations, which gives a good sense of how unusual the situation has been: for example, 1.5-2 standard deviations cooler than normal in the vicinity of Cook Inlet.  The standard deviation is calculated for the same 30-day window as the anomaly calculation.

Here's the 500mb height anomaly from the same perspective:

Here's the link to view these graphics for yourself: it's a page that I put together recently.

https://s2s.worldclimateservice.com/wcs/era5_monitoring/

Clicking on the 2m temperature map near Utqiaġvik produces a similar chart to the first one above (confirming the veracity of the model data), but the chart also appends the latest (00UTC initialized) ECMWF ensemble forecast.  Cooler than normal conditions appear likely to persist on the North Slope in the next week or two.

Interestingly, the forecast suggests the 30-day temperature anomaly is likely to become rather strongly statistically significant to the north of Alaska in the coming days:

A caveat is in order, however: part of this anomaly could reflect a discrepancy between ECMWF's current forecast model and the much older model version that is used for the ERA5 reanalysis (historical baseline).  Many tweaks and upgrades have been made by ECMWF since the ERA5 version was implemented, and the new model may have cooler 2m temperatures over Arctic sea ice at this time of year.  We'll find out in a few weeks when the ERA5 data includes these dates and we can do the apples-to-apples historical comparison.

Nevertheless, the forecast does show a robust trough over the Arctic waters to the north and northeast of Alaska, so persistently below-normal temperatures are certainly plausible.

Wet and Cool Spring

Spring's progress continues to be rather fitful this year in Alaska.  Much of the state has been unusually wet for the time of year in recent weeks, and temperatures have become decidedly cooler than normal again in much of the south.  A very vigorous Bering Sea trough is to blame, along with substantially cooler than normal water temperatures in the eastern Bering Sea - the latter being a lingering reflection of the expansive winter sea ice.  Here's a recent SST anomaly map:

Interestingly, the PDO index is drifting into more negative territory again, despite the fact that El Niño is developing rapidly in the equatorial Pacific.

The 30-day precipitation anomaly map shows a widespread and significant wet signal across Alaska - see below (NWS data, estimated).  Of course, it's usually a rather dry time of year, so the actual amounts are not enormous in most interior and valley locations; but it's a notable statistical anomaly.  Anchorage, for example, is running at record wet so far for the April-May period, with just over 3 inches of liquid-equivalent precipitation.  The record for the two-month total is 3.29" in 2019.

A temperature chart from Homer illustrates how warmth has been generally lacking in recent weeks.  This morning was especially chilly, with many locations near or below freezing across the Kenai Peninsula and farther afield.

Yesterday morning Fairbanks also dropped to 32°F, the latest freeze at the airport in 18 years.

A notable exception to the cool weather pattern has been in the Arctic northwest; Kotzebue was extremely warm for the time of year last weekend, breaking daily record high temperatures.

Here's the 500mb height (mid-atmosphere pressure) anomaly in the last 14 days, expressed in terms of standard deviations departure from normal:

And the 850mb temperature anomaly:

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.

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.

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.