Cold and Snow

Here's a bit of follow-up on the unusually late cold and snow.  According to Rick Thoman (and relayed via Brian Brettschneider on Twitter), Monday's half-inch of snow in Fairbanks was the first time since 1978 that measurable snow occurred after green-up.

The late snow in 1978 was much later - May 27 - but it was also more marginal, with mixed rain and snow reported, and officially-measured accumulation of only 0.1".  A more legitimate late snowfall occurred in 1966 in Fairbanks, with 1.6" measured on May 15-16; that was 8 days after breakup at Nenana, and almost certainly after green-up too (although the NWS green-up history only goes back to 1974).

Not to be left out, Anchorage also saw accumulating snow on Wednesday night.  This is a top-10 latest date for snow, and it's actually the latest on record with measurable snow on the ground at the daily observation (data back to 1953).  The previous record was May 6, 1955.  Interestingly last year there was snow on the ground on May 4.

The cold hasn't stopped breakup from proceeding, however.  The ice went out on the Yukon River at Tanana on Wednesday, and the Kuskokwim tripod at Bethel stopped the clock on Wednesday morning, despite a temperature of 25°F with snow falling.  Unfortunately significant flooding is now occurring in Bethel and elsewhere on the lower river, as the ice is jammed up downstream.

https://www.kyuk.org/public-safety/2024-05-09/as-lower-kuskokwim-river-breaks-up-bethel-sees-highest-river-gauge-level-in-almost-20-years

On the other side of the Arctic, very unusual cold is also affecting western Russia; it was snowing in Moscow yesterday for their big parade.  Severe freezes have also occurred in the Baltic states in recent days - a rare event for this late in the season.  As I commented in my last post, I think the "perturbed" flow pattern can be traced back to the lingering circumpolar disruption caused by the March stratospheric warming event.

Late Snowfall

Green-up was declared in Fairbanks on Saturday, but that distinctive first shade of green on the hillsides was obscured by white today, as light but steady snow moved into the area mid-morning.  With temperatures dropping slightly below freezing for a while, there was some minor accumulation on grassy and elevated surfaces; the NWS posted this photo:

Accumulating snow in early May in Fairbanks isn't too unusual.  There have been three May snows of more than half an inch since 2013; but of course it quickly becomes a more uncommon event as the month advances.  May 1992 was the exceptional outlier, with 9.4" on the 12th and another several inches in the subsequent days.  1992 also produced an absurdly cold and snowy September in Fairbanks, and it seems rather likely that this "coincidence" was somehow linked to the Pinatubo eruption (1991) and temporary global cooling.

But back to more recent events: read about the 2022 and 2013 May snows here:

https://ak-wx.blogspot.com/2022/05/spring-snow.html

https://ak-wx.blogspot.com/2013/05/big-snowfall-and-precip-differences.html

In the northern interior, Bettles saw a daily high temperature below freezing yesterday, marking the coldest day so late in the season since the (much colder) conditions of 2013.  Bettles also saw a bit of snow, but that's less uncommon: accumulating snow occurs in May in more than half of years up there.

And in Nome it's a very chilly day for the time of year: the temperature hasn't yet risen above 20°F today.  Again the last time that happened in May was in 2013.

The immediate cause of the cold weather is a trough that dropped down from the Canadian Arctic Ocean over the weekend and "joined forces" with another trough moving across the Aleutians into the Gulf of Alaska.  Here's the 500mb analysis from 4am this morning, courtesy of Environment Canada:

Arguably the chain of events was set in motion by a strong ridge stretching across the central Arctic Ocean late last week; and that in turn reflects the lingering influence of a "sudden stratospheric warming" event way back in early March.  The map below shows last Thursday's 500mb height anomaly, with generally above-normal heights from the Arctic Ocean to the northern North Atlantic, i.e. a negative Arctic Oscillation.  This is a typical outcome in the wake of a wintertime stratospheric disruption, although the timing and persistence of the anomaly varies widely from case to case.  Meteorologists watch for these events closely, because they tend to presage mid-latitude cold outbreaks and generally volatile weather owing to the Arctic "blocking" patterns.

April Snow and Temperature

This year in Fairbanks, the date of snowpack meltout - the first day with less than 50% snow cover, or less than an inch depth reported - was April 23.  This is just a day or two ahead of the long-term normal, despite April being distinctly warmer than normal: the month was nearly 3°F warmer than the 1991-2020 average, and fell just inside the warmest 33% of the historical distribution.

However, the snow depth at the beginning of April wasn't greater than normal - in fact it was slightly less (15" versus a 1991-2020 median of 20") - so it's not immediately clear why meltout didn't occur sooner.  But of course there's a certain amount of randomness in the meltout dates at a single location, and the measuring site hasn't remained the same over the years.

One other note on the meltout date: two of the earliest meltout dates in Fairbanks history were in 2016 and 1998 (April 9 and 10 respectively), and those two years were coming out of intense El Niño episodes.  Given that we also had a very strong El Niño this winter - almost on a par with those earlier events - it's again interesting that meltout wasn't sooner.

Let's consider briefly the relationship between April snow cover and April temperatures.  Of course this is a chicken-and-egg problem: temperature affects snow cover, but snow cover also influences temperature.  The basic relationship is obvious, with a tendency for less snow on the ground in warmer Aprils.

Notice that there's a better correlation for daily minimum temperatures than for daily highs.  This makes sense, as nighttime temperatures are prone to dropping more sharply over snow-covered ground than over bare ground.

The major outlier with record April snow depth was 1991 (42" remaining on the 15th!), but the monthly mean temperature was only slightly cooler than this year.  The incredible snow pack that month lingered in spite of the monthly temperature, not because of it.

If we examine the 850mb temperature in relation to April snow depth, the correlation is weaker: see below.  This chart more cleanly illustrates the influence of air mass temperature on snow depth, rather than the other way around, because we wouldn't expect the snow cover to greatly affect the air temperature several thousand feet above the ground in the "free" atmosphere.

In an attempt to extract the reverse causation - the influence of snow depth on surface temperature - the following chart looks at the temperature difference from the surface to 850mb.

It's a weak relationship overall, but better for daily low temperatures than for high temperatures.  Note that 1991 is less of an outlier here.  In April 1991, particularly at night, the snow pack seems to have held back the surface temperatures compared to what the 850mb air mass temperatures would otherwise suggest.

I also started looking at daily temperatures in an effort to see the seasonality (calendar timing) of snow's influence on temperature in Fairbanks, but I'll discuss those results in another post.

Winter Lives

Breakup and snow melt are well advanced for parts of the Alaskan interior, but winter isn't quite ready to relinquish its grip in the far north.  Temperatures dropped below 0°F this morning across much of the western and interior North Slope, and a strong northerly wind event developed at Howard Pass, with brisk wind chills below -30°F.

This morning's low temperatures:

And conditions at Howard Pass in the western Brooks Range:

Compared to winter cold events, this is nothing to write home about, but nevertheless it's been more than a decade since Howard Pass had wind chills this low so late in the season.  However, back in 2013, the first day of May brought temperatures of -15°F with 40+mph winds.  It's unusual but certainly not unprecedented.

The surface analysis from this morning shows high pressure (although not especially intense) over the Arctic Ocean, with a north-south pressure gradient across the Brooks Range.  The red dot marks the approximate location of Howard Pass.

In contrast, the NWS breakup map shows lots of open water on the Tanana and upper Yukon Rivers.  The ice went out at Nenana on Saturday, and it happened early in the morning, which is very unusual.  The breakup date of April 27 was close to the average of recent decades, but also earlier than the last 3 years.  Rick Thoman has more comments on his blog:

https://alaskaclimate.substack.com/p/2024-tanana-river-at-nenana-break

Warming Up Fast

A very strong upper-level ridge over Southeast Alaska and northwestern Canada has produced a big warm-up for eastern and southern Alaska in the last week, with temperatures reaching record levels for the time of year in some spots.  Yakutat reached 64°F on Friday, which would be the warmest on record this early in the season if it were not for a similar warm-up in 2021.  Numerous locations set daily record highs on Friday and/or Saturday, from Ketchikan (65°F) in the far south to Bettles in the northern interior (56°F).

Here's the 500mb analysis from 4pm AKDT on Thursday, courtesy of Environment Canada:

The most unusual warmth I spotted was at Cordova, where the daily minimum temperature of 47°F on Thursday was the highest overnight minimum this early in the season by a full 3 weeks.  Bettles also saw a record warm night for this early, with a low temperature of 39°F on Thursday despite having 29" of snow on the ground at that time.

As of the last daily report, Fairbanks snow depth is down to 3", so the official meltout date may well be today.  The ice at Nenana is looking very rotten, and the accumulation of thawing heat units is now in the historical range for breakup.  However, with unusually thick ice this year - measured at 50" a couple of weeks ago - perhaps it will be a few more days before the tripod goes out.

With meltout occurring earlier this year than the last two seasons (both in early May), green-up and fire season in the eastern interior will probably come to life a bit earlier as well.  Here's an article discussing this, with a few comments from Rick Thoman:

https://www.webcenterfairbanks.com/2024/04/17/fires-floods-risk-with-early-may-green-up/

It makes sense that there would be a connection between early snowpack loss and increased early-season fire activity, so I had a quick look to see if this shows up in the data.  The chart below shows June 1 fire acreage statewide versus the snowpack meltout date in Fairbanks:

There does seem to be something of a relationship, although it's marginal and the sample size is small (I only have daily fire data back to 1995).  If we step forward to July 1, the relationship seems to disappear:

The highest July 1 acreage on the chart was in 2022, when the Yukon-Kuskokwim region saw unprecedented early fire activity owing to a very dry spring; there's a lot more to the puzzle than just the timing of meltout.

https://ak-wx.blogspot.com/2022/06/early-fire-and-may-climate-data.html

Breakup Outlook

Breakup concerns will be front and center in many minds over the next few weeks.  The National Weather Service is assessing a somewhat elevated risk of flooding in some locations along the Yukon and Kuskokwim Rivers, as illustrated by the following map:

Generally above-normal snowpack is the main concern, along with some risk of below-normal temperatures in the latter part of April, raising the possibility of a more dynamic breakup with ice jams.  However, the temperature outlook has warmed in the last few days for central and eastern Alaska, so it may be western regions that are most at risk.

Here's the breakup outlook:

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

Looking at the Tanana breakup at Nenana, there's still some way to go before the tripod is at risk; it's warm today (nearly 50°F), but the accumulation of thawing degree days hasn't made much progress yet, as is typical for the date.  As of yesterday, Fairbanks had reached 20 TDDs (accumulation of daily mean temperatures above freezing), and at least 75-100 are needed for breakup - and typically more like 100-150.

It's interesting to see some multidecadal variability in the amount of thawing needed for breakup at Nenana.  This could be attributable to variations in ice thickness and snowpack, both of which affect the breakup date.

Despite this variability, however, simply using the accumulation of TDDs allows the breakup date to be predicted with a correlation of 0.9; it's mostly about temperature.  Here's what the (in-sample) predictions look like using each year's date when TDDs reached the historical median for breakup:

Notice that very late breakups occur earlier than predicted, because the sun is an increasingly important factor as May advances.

Obviously the TDD calculation uses 32°F as the threshold for accumulating heat units, and I got to wondering if a different threshold would provide better predictions.  The answer is no: the in-sample correlation drops off for thresholds above or below 32°F.  It turns out that ice really does melt at 32°F.

Here's this evening's view of the Nenana tripod: the ice is looking dark, so the process is certainly under way.

March Climate Data

I'm not quite back to a normal posting schedule, but here's a quick look back at the March climate anomalies across Alaska.  It was another wetter-than-normal month for the state as a whole, the 5th in a row according to NOAA/NCEI, and indeed only October was drier than normal in the last year.  It was also the 6th consecutive March with above-normal statewide precipitation; the last significantly dry March was way back in 2017.

As in February, the wet weather was focused in western Alaska owing to a Bering Sea trough:

The past three months have certainly been very wet for the Y-K Delta region.  On the flip side, notice how persistent the dryness has been in the southern Panhandle:

 

According to ERA5 data, March was also an exceptionally cloudy month, except in the Panhandle.

Monthly mean temperatures were significantly above normal in the southeastern interior and near the coastline from Anchorage eastward, although not approaching record levels.

ERA5 snowpack data shows a significant excess for most of western and northern Alaska, as well as south-central, but low snow in the southern interior and the Panhandle.

The April 1 NRCS snow survey has some very interesting comments:

"The most exceptional April 1 snowpack in Alaska exists around Valdez. This is a place known for massive snowfall and this year’s snowfall has not disappointed. NRCS Snow Survey uses software to quality control station data based on previous values recorded at a site. The first time our quality control software had to be adjusted for the Upper Tsaina SNOTEL, near Thompson Pass, was in November, when a massive snowstorm eclipsed the amount of snow that had ever been recorded at this site for the date and flagged the data as erroneous. The same thing had to be done in December, and then again in January before we finally set the snow depth higher than the value the station is capable of measuring, which it is currently at. This station was installed in 2002 and has been reading its period-of-record maximum value for most of the year. Right down the road, the Lowe River Snow Course has a much more robust history, and on April 1 it was measured as the highest value in fifty-three years of observation.

Exceptional snowpack continues north from Thompson Pass into the Copper River Basin. Several April 1 measurements in the Copper River lowlands were made as the second highest on record. This basin snowpack has been hearty all year but is not as outstanding as it was last year at this time, when most of the measurements were period-of-record maximums. The other record snow measurements in this report were taken from our partners in Canada, where there are two April 1 records in the upper Porcupine.

The snowpack around Anchorage has been making headlines this year. This highlights a difference in how snow measurements are taken. The Anchorage National Weather Service office at Sand Lake uses a snow board and records the amount of snowfall that falls on the board several times a day throughout the winter. The amount of snow that has been measured is currently the third highest on record and will crown 2024 as the snowiest if a few more inches of snow falls this spring. Snow Survey measures snowpack as an quantity of snow water equivalent (SWE) a site has at a given date. The Kincaid snow course is very close to the Sand Lake office and recorded its sixth highest April 1 reading in its much less robust thirty-three-year period-ofrecord. This is most likely a function of melting during periods of above Normal temperature in February and March.

There are a few places in Alaska with below Normal snowpack on April 1. The measurements taken on islands in Southeast Alaska are below Normal. Several stations are also reporting below Normal snowpack in the interior, where slightly below Normal snowfall combined with warmer than Normal temperatures. In Northwest Alaska Kelly Station SNOTEL has reported below Normal SWE for all of 2024 and is reporting less than half of Normal snowpack on April 1.

In Western Alaska and on the North Slope above Normal precipitation through the winter months hints at above Normal snowpack. Bethel, Aniak and McGrath have had considerably wetter than Normal monthly precipitation totals in February and March. In an effort to understand snowpack in the Kuskokwim basin, the McGrath SNOTEL was installed in 2019. This station is proximal to the no longer measured McGrath Snow Course. The 7.6 inches of SWE reported at the SNOTEL would be considerably higher than the 5.6 inches that is the period-of-record April 1 median for the snow course. Interestingly the reported SWE is the lowest in the McGrath’s five-year history, a testament to how snowy the last five years have been in this region."

Arctic Ice and Temperatures

Last week I mentioned that Bering Sea ice managed to reach near-normal levels this winter despite relatively warm conditions.  The January-March average ice extent was only slightly below the normal of recent decades, continuing the recovery from the very low ice conditions of (especially) 2017-18 and 2018-19.

The Arctic-wide seasonal sea ice maximum was likewise higher than in most recent years, as documented by Rick Thoman on his blog:

https://alaskaclimate.substack.com/p/march-2024-arctic-sea-ice-update

The relatively abundant ice cover is perhaps consistent with the fact that while tropical and mid-latitude temperatures were extremely elevated in recent months, the Arctic did not see quite the same magnitude of warmth that it did during and after the last big El Niño in 2015-16.  Last time around it seemed that the intense El Niño kicked off a several-year period of really excessive warmth in the Arctic, but so far that hasn't happened with the latest (now dissipating) El Niño.

Here's a simple depiction of mid-latitude versus high-latitude temperature trends in December through February, according to the ERA5 reanalysis:

In the mid-latitude sector from 30-60°N, the winter's average temperature was the second highest on record, behind only 2019-20; but for 60-90°N the winter wasn't quite as warm as the three winters from 2015-16 to 2017-18.

Here's a map comparison of this winter (below, top) with the record warmest winters in the mid-latitude (middle) and high-latitude (bottom) sectors.

Clearly the 2017-18 winter had a very strong Arctic focus of unusual warmth, while it was a colder winter in many mid-latitude locations, at least over the continents.  The Arctic was colder in 2019-20 , but there was extreme warmth across Eurasia as a result of a strongly positive Arctic Oscillation (low pressure over the Arctic, and strong circumpolar flow).

In contrast to both of those years, this winter saw very widespread unusual warmth from the tropics to the Arctic.  Below-normal temperatures at high latitudes were largely confined to certain land areas (Greenland, Scandinavia, parts of Russia, and of course some of Alaska), while air temperatures over the Arctic Ocean were significantly above normal.

Given the very modest reduction in the Arctic's warmth this winter, the recovery in sea ice is interesting and, I think, a little surprising.

Sea Ice Update

I'm in a busy stretch for travel and work at the moment, so posts may be light and sporadic for a few weeks.

But here's a quick update on Bering Sea ice, which appears to have reached its seasonal maximum on March 19, according to NSIDC ice extent data.  The peak of 734,000 km² is about 14% below the 1991-2020 median value for the seasonal peak, although the daily values have been generally quite close to the climatological daily median for the latter part of the winter: compare the black and orange lines in the figure below.

The shortfall of the seasonal peak value reflects the fact that we never saw a significant surge above normal, whereas a typical winter would tend to see such an event as a result of subseasonal variability.

Given that January-February temperatures were above the 1991-2020 normal across most of the Bering Sea (at least according to ERA5), it's actually surprising that the sea ice managed to reach near-normal levels by the end of February.

Here's the NWS ice analysis for March 18, when low-concentration ice just about reached St Paul Island:

Last winter the ice did not reach St Paul Island, but it did in 2022, when ice coverage was the most abundant since 2013.

Snowpack

As the weather suddenly turns distinctly springlike for much of Alaska, let's have a look at snowpack across the state.  According to the March 1 snow survey from NRCS:

"... snowpacks throughout the state are generally robust.  In the Interior, portions of the Copper and Upper Susitna are reporting historic March 1 snowpack.  The stations around Anchorage and Valdez are reporting snowpack way above Normal, in some cases historic.  So is that of the Lower Yukon.  On the other end, Northwest Alaska, the Upper Yukon and the lower elevations of Southeast Alaska are reporting below Normal snowpack on March 1."

Click to enlarge the snow survey map:

Comparing this to ERA5 model estimates, there's agreement that the central and eastern interior has relatively lower snowpack than many surrounding areas.  Note that the "Upper Yukon" region in the NRCS summary is located almost all in Canada, and so the ERA5 agrees with low snowpack there too.

There's obvious disagreement regarding the Arctic northwest near and above Kotzebue, but the NRCS assessment of very low snowpack is based on data from a single SNOTEL site.

Here's a monthly series of ERA5 maps to give an idea of how the snowpack evolved earlier in the season.  It was a slow start in the southwest, but the exceptionally wet and not-too-warm winter produced ample snowfall in the end, especially in February.

North Slope Chill

The first half of March was the coldest since 2012 for the North Slope, providing a notable contrast to the last decade - although it wasn't significantly colder than the long-term mean for the second half of the 20th century.  Utqiaġvik had a mean temperature of -21°F, and Deadhorse came in at -29°F.  Unfortunately the Umiat RAWS hasn't been reporting all winter, but the nearby HADS site reported an average temperature of -32°F for March 1-15, and that's close to the -33°F reported by the RAWS instrument in 2012.

Here's the last month of temperatures from the Umiat HADS site (in Fahrenheit):

The -52°F on March 11 seems pretty cold for the time of year, but it's not especially unusual in the context of earlier years.  The RAWS site saw -52°F a week later in 2008 (March 19), and much colder episodes occurred at the pre-RAWS climate observing site (1976-2001, and also 1945-1954).  Back in 2001, -63°F was observed on March 13, and perhaps the most astonishing of all is -50°F on April 5, 1986 - and that's the all-time state record low for April.

The number of -40° nights at Umiat in March has dropped off sharply since 2012:

As for Deadhorse, the 7 nights with -40° so far this month is a bit more unusual, as not many years have exceeded that number in the combined history from Prudhoe Bay (admittedly slightly warmer) and Deadhorse.

The cause of the cold is an unusually strong trough over the Beaufort Sea and extending down across Alaska.  The following maps show the estimated departure from normal of the 500mb height (top) and 850mb temperature (bottom) for the first half of the month: