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:

Sustained Wind Chill

Long-time blog readers will recall many previous comments about wind chill at Howard Pass in the Brooks Range.  It's a notorious location with the unusual climate characteristic that the windier it gets, the colder it gets, as low-level cold from the North Slope gets funneled through the low pass (only 2062' elevation) from north to south.  We know this because of instruments maintained by the National Park Service.

The past few days have seen a prolonged episode of hefty wind chill at Howard Pass: at or below -60°F since Friday evening, or nearly 120 hours.

Temperatures have hovered in the range -22°F to -32°F, with sustained winds of about 30-45 mph.  Howard Pass has seen much worse in the past (search the blog archives), but this episode is actually the longest on record with wind chill remaining continuously at or below -60°F.  The previous record was 105 hours in February 2013, the first winter we have data for.  Also, late November 2021 saw 10 straight days with an average wind chill below -60°F, but with higher daily maximum values.  [But note that the Howard Pass instrumentation was knocked out by severe conditions in a couple of winters, so there may have been more prolonged episodes that weren't recorded.]

[Update March 14: the wind chill finally rose above -60°F at 1pm today, so the new record is 136 hours.]

As in every other case of severe wind chill at Howard Pass, the wind has been continuously out of the north-northeast, i.e. perpendicular to the mountain range.  The second graphic below shows the location:

Actual temperatures on the North Slope have been very cold: -40s for overnight minimum temperatures in many locations since Sunday, and even below -50°F at the Umiat HADS site (the Umiat RAWS isn't reporting).  The Deadhorse ASOS reported -46°F on the 11th, which ties the coldest so late in the season since 2012 (when it was -49°F on March 16th).

The MSLP analysis from Sunday morning shows a very typical setup for North Slope cold and Howard Pass wind chill: high pressure draped across the Arctic coast, and a significant (but not extreme) north-south pressure gradient across state.  The bunched isobars across the Brooks Range (to the north of Bettles, circled) highlight the potential for strong winds in the mountains.  Click to enlarge: