Thursday, March 28, 2024

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.

Friday, March 22, 2024


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.

Tuesday, March 19, 2024

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:

Wednesday, March 13, 2024

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:

Friday, March 8, 2024

February Climate Data

Climate data for February has arrived, showing that it was a warmer and wetter than normal month overall for Alaska.  Both December and January were slightly colder than the 1991-2020 normal, so February was the only month of climatological winter that was on the warm side; but actually all three months were quite close to normal statewide.

As is often the case, of course, "near normal" masks a lot of spatial and - in this case - temporal variability, as Alaska started and ended February with well below normal temperatures.  Here's the UAF statewide temperature index for December through February:

For February as a whole, colder than normal conditions were confined to some parts of the eastern interior, whereas western Alaska and the North Slope were significantly - but not dramatically - warmer than the baseline of the last 30 years.

The temperature rank map for Dec-Feb shows a moderately significant cold anomaly in the eastern interior, but more significant warmth occurred across  the North Slope and Southeast Alaska.

Fairbanks was colder than 8 of the 10 past winters with a Dec-Feb mean temperature of -5.5°F, but that's only slightly below the 1991-2020 average of -4.4°F.  On the other hand, Utqiaġvik had its 3rd warmest winter on record (only 2016-17 and 2017-18 were warmer).

February precipitation was very high in southwestern Alaska, locally over 300% of normal according to ERA5 data, and this was caused by a persistently strong Bering Sea trough:

December and February weren't particularly wet over the southwestern mainland, but February made for a much wetter than normal Dec-Feb overall.  The northeastern Gulf Coast also had a very wet winter, but that was mostly December.

As for wind, February was a stormy month for southern (but not Southeast) Alaska, with a big north-south pressure gradient across the Aleutians driving strong winds:

The Dec-Feb wind anomaly pattern is remarkably similar to the February pattern: it was an unusually windy winter from the Aleutians to southwestern and south-central Alaska.

All three aspects of the winter's climate outcome (temperature, precipitation, wind) were quite atypical for a strong El Niño winter.  The typical influence of El Niño is to produce unusual warmth in eastern (not western) Alaska, generally below-normal precipitation in most areas, and more often than not reduced winds from the Bering Sea to the northern Gulf.

It seems that the main reason for the difference is that El Niño's typical "Aleutian low" was shifted northward into the Bering Sea, and that's likely because there was a strong mid-latitude ridge associated with very unusual oceanic warmth extending east from Japan.  The persistence of such widespread warmth in the North Pacific was really unusual for El Niño, which historically has been associated with below-normal wintertime SSTs in the central North Pacific.


Wednesday, March 6, 2024

Yukon at Dawson Still Open

This has been a recurring theme in recent years, and a problem for residents of Dawson, YT: the Yukon River is reluctant to freeze over next to town.  As in some other recent winters, it hasn't been possible to build the usual ice bridge across to West Dawson.  Here's today's webcam view, suggesting that a complete freeze-up won't occur at all this winter:

And a video confirming the flow of water in the open channel:

November and December were significantly warmer than normal in Dawson, but January and February were both slightly colder than normal, so it seems unlikely that the lack of ice can be blamed on the weather.

Monday, March 4, 2024

Anchorage Wind

A reader inquired about the persistent north wind event of last week in the Anchorage area, so let's take a look at this.

First, the facts on the ground: there was indeed a very lengthy period of stiff northerly winds, lasting from early morning on Wednesday (Feb 28) through Saturday morning (March 2).  Here's a graphical look at observations from Anchorage airport, with wind speeds in mph:

Much the same sort of thing was observed up in Palmer, and with higher wind speeds, although there was a break on Thursday morning:

Remarkably, the peak wind gusts were 60mph or higher for 4 consecutive days in Palmer.  The maps below (click to enlarge) show the daily peak gusts from Feb 28 (top) to March 2 (bottom).  This was a remarkably persistent event:

Taking a quick look at historical hourly data from Anchorage airport, I see only one other event since 2000 with comparable sustained winds: it looks like a very similar event occurred at the same point on the calendar in 2017 (Feb 28 - March 3).  Here's the average MSLP for the 3 windiest days in both cases:  2017 (top) and 2024 (bottom):

Clearly the driving factor in both cases was the strong pressure gradient between low pressure in the eastern Gulf of Alaska and high pressure to the west and north.

The long duration of the event reflects the unusually persistent MSLP setup, with low pressure slow to depart or weaken in the Gulf of Alaska, and with building high pressure to the north maintaining the gradient as the low pressure eventually decayed.  Here's a sequence of MSLP analyses at 3am AKST for 5 consecutive days: February 27 through March 2.

Farther aloft, the situation was unusual, with a sharp ridge over the Bering Sea progressing slowly eastward behind a broad and deep trough over the Gulf of Alaska.  The 500mb map from Thursday afternoon shows powerful northwesterly flow in the very tight pressure gradient over southwestern Alaska, but yet the 500mb wind was out of the south at Anchorage (highlighted with the red circle below):

To me, this highlights the large degree of spin (technically "vorticity") in the atmosphere aloft, which helps explain the longevity of the low pressure in the northeastern Gulf of Alaska.  This upper trough and associated jet stream, by the way, is what brought extreme mountain snows to California over the weekend.  It was windy down there too: there's a lot of energy in this jet stream, and this is probably attributable to El Niño.

Friday, March 1, 2024

ECMWF AI Forecasts

The weather industry has been abuzz with excitement in the past year about the new AI (Artificial Intelligence) forecast models; I penned a few comments back in November:

The latest news is that ECMWF is now providing the realtime forecast data from its AIFS model, and it's open and free for all to use.  You can see the 4 forecasts per day on Levi Cowan's website:

The skill of the model is comparable to the leading physics-based models, so the new data will provide a useful tool for forecasters.  I'll be keeping an eye on it for Alaska.

We should bear in mind, however, that ECMWF currently runs only a single AIFS forecast each time, rather than an ensemble of forecasts like the ECMWF, NOAA, and Canadian ensemble systems.  Ensemble forecasts provide valuable information on confidence ("how similar are the ensemble members?"), and the ensemble-average forecasts tend to be more stable from run to run.  The AIFS forecasts will have a tendency to jump around from run to run, so take each iteration with a pinch of salt.

For example, here are the 4 latest AIFS forecasts for the morning of March 11, i.e. 10 days ahead, and probably at or beyond the limit of deterministic predictability for the Alaska region.  From oldest to newest forecasts:

The general theme is the same - cold in northern Alaska - but the individual forecasts disagree on the extent of cold farther south.

For comparison, here's the NOAA GEFS ensemble mean for the same time.  In this case the overall agreement is pretty good; and these forecasts (GEFS vs AIFS) are produced by completely different methods.  Impressive technology for sure!