Snow and Wind

The weather of the past couple of weeks has been very unsettled across western and interior Alaska, with multiple stormy episodes, and the contrast to the first 10 days of the month could hardly be greater.  Fairbanks has seen accumulating snow on 12 of the past 14 days, which is the most for this late in the season since 1971; snowfall normally becomes less frequent and lighter as winter advances.

The recent snow has brought the seasonal total to 67.1 inches, or 20" above the long-term median for the date (see below, click to enlarge).  This is marginally more than last winter for the same date, and it's the most since 2000.  A few more inches in the next few days could push February into the top 10 for snowfall, but we probably won't catch last February's total of 23.3" (see here for last year's blog post).

Another symptom of the stormy pattern has been the recurrence of relatively high winds of late in Fairbanks.  Conditions have been notably breezy in 5 separate events of the past 2 weeks, and 5 calendar days have seen peak 2-minute wind speeds above 20 mph (an unusual occurrence in Fairbanks).

The history of 2-minute wind speeds in Fairbanks only goes back to 1996 (the beginning of the ASOS era), but 5 windy days in 2 weeks ties the winter-time record from both March 2003 and December 2013.  In both of those previous cases most of the wind was in one prolonged multi-day event (see here for a blog post on the December 2013 wind); whereas this month we've had repetitive events, and this is really unusual.

The 500mb and 850mb height maps from the past 2 weeks show a pattern that is quite typical for warm and snowy conditions in Fairbanks, with a long fetch of southwesterly flow entering western Alaska.  Westerly flow aloft is easily the most favorable wind direction for heavy precipitation in the cool season, as we saw here.  It's also most common for strong breezes in Fairbanks to come from the west, as they have this month; the last figure below shows that the vast majority of windy days occur with westerly flow in deep winter.  (But note that this is not true in March, when winds are typically stronger and much more often come from the northeast.)

North Pacific Blog Post

I added a new post this evening on the Alaska/North Pacific "Blob Tracker" Blog, discussing the potential for extraordinary warmth this summer in the North Pacific:

https://alaskapacificblob.wordpress.com/2018/02/21/north-pacific-forecast/

Bering Sea Ice Loss

[Update Feb 17: the Bering Sea ice extent fell another 10,000 km² yesterday; I've updated the chart.  Also, I added an animation of February 15 ice extent maps and a comment on St. Lawrence Island.]

In Tuesday's post I highlighted the strong ridge over Alaska that produced remarkable temperature inversions across the interior a week ago.  Another effect of the amplified circulation pattern is that the Bering Sea has been subject to strong and persistent southerly flow, and this has really done a number on the sea ice.

Bering Sea ice was already running at record lows for most of this year so far, and now the ice extent has dropped farther below the long-term normal than at any other time in the modern era.  The chart below illustrates this: the gray shading indicates the historical range of daily ice extent anomalies (departure from a 1979-2016 mean), and the red curve is this year's anomaly; as of yesterday it was more than 400,000 km² below the mean for the first time.

I've annotated four previous years in which sea ice extent also dropped to very low levels in late winter and early spring, and it turns out that each of these was a La Niña winter (like this winter).  I'll do some more analysis later, but it seems clear that La Niña favors the kind of high-amplitude trough-ridge pattern that can bring disruptive southerly flow to the Bering Sea ice.

As an aside, it's interesting to note that the current ice loss episode has produced a decrease of 133,000 km² in ice extent, or over 30%, in just 8 days.  As extreme as this seems, especially for mid-winter, it's not unprecedented; in the early February 1985 episode, the Bering Sea lost 161,000 km² of ice in 10 days - but then gained it back, and more, in the next 10 days.  The difference this time, of course, is that we're starting from a very low point owing to the season-long shortfall in ice.

Here are the daily ice analyses from February 7 (left) and today (right), courtesy of NOAA.

Here's a simple animation of the February 15/16 sea ice extent maps from NSIDC (clipped to a suitable domain).  This gives a bit of context on interannual variability.  The maps for 1985, 1989, 2001, and 2018 are included separately below.

Note that St. Lawrence Island was only partially surrounded by ice in mid-February 2001, and this is similar to the current situation; webcam images from Savoonga and Gambell today both clearly showed open water.

Extreme Inversions

This is a few days late but still worthwhile as an interesting note, I think: late last week some very extreme temperature inversions were observed over Alaska's interior as a strong upper-level ridge moved eastward.  On Friday morning the balloon sounding at McGrath measured a temperature difference of 62°F between the surface (-21°F) and 3500' above ground (+41°F); this is the strongest inversion measured at McGrath since March 2008.

At Fairbanks the inversion strength peaked on Saturday morning, right under the ridge axis, with a temperature difference of 60°F between the surface (-19°F) and 3000' above ground (+41°F).  This is essentially a tie with the strongest inversion ever measured by radiosonde in Fairbanks; the record inversion was just 0.1°C stronger in December 1956.  Here's the so-called skew-T diagram.

Here are the 500mb maps from Friday and Saturday mornings (top and bottom respectively), showing the slow progress of the ridge eastward; the peak inversions were closely aligned with the central axis of the ridge.

Rick Thoman posted a very remarkable map of Saturday morning temperatures around Fairbanks, showing a full 60°F of temperature difference at the surface in a distance of under 10 miles.  Locations in the hills were above freezing while some valley locations were in the -20s.

Looking at historical sounding data from Alaska's upper-air observing sites, the state record for inversion strength is 72°F at McGrath on January 13, 1966 (-41°F to +31°F).  McGrath has seen quite a number of occasions with inversions of more than 65°F, but on a typical winter day the inversion strength is greater in Fairbanks.

The chart below shows the all-time records (blue columns) for 13 sounding sites in Alaska that have been continuously active since around 1950, and the red columns indicate the 1981-2017 median inversion strength for morning (3 am) soundings in winter (December through February).  Only Annette Island has a median of zero, i.e. the average winter 3am sounding does not show an inversion.

How about the depth of inversions?  In general, stronger inversions are deeper, and so Fairbanks, McGrath, and Utqiaġvik (Barrow) are the winners for both strong and deep winter inversions.  The chart below shows the record and median values for inversion depth.

An interesting side note is that upper air observations were made at Northway from 1948 to 1955 and at Barter Island from 1953 through 1988, and both of these sites observed some very strong inversions.  In just a few years at Northway a number of 60+°F inversions were observed, with the record being 68°F of inversion on January 24, 1952.  The record at Barter Island was 64°F on January 25, 1983.

Warmth to Return

The southern and eastern interior of Alaska has seen a decent spell of cold weather in the past couple of weeks, but it's on the way out now, and unseasonable warmth looks likely to return in the near future.  The chart below puts the recent cold in perspective relative to the anomalous warmth earlier in the winter; notice how few days have been more than 1 standard deviation below normal, compared to the many days of 1SD or more above normal.

As chilly as the recent spell may have seemed, the departure from normal for the past 2 weeks is "only" 14°F, which is quite modest for interior Alaska; the 2 weeks ending December 19 were over 23°F above normal.

The impending shift back to warmth is partly related to a very dramatic weather event that is unfolding in the stratosphere: a "sudden stratospheric warming" (SSW).  In these events, which occur a few times a decade, there is a weakening and disruption of the winter-time vortex of westerly winds that usually prevails in the stratosphere above the Arctic.

In some SSW events the vortex is merely weakened and displaced away from the pole, but in the more dramatic cases the vortex splits or breaks down completely and the flow reverses to easterly around the pole for a time.  The upcoming SSW will be of the latter variety; the two maps below, courtesy of tropicaltidbits.com, show the change over the course of a week beginning last Sunday.  The second map shows that the vortex will split into two daughter vortices by this Sunday, with high pressure and anticyclonic flow in the middle.

These events are followed very closely in the long-range forecast community, because the disruption of the vortex usually works its way down to the troposphere and leads to a weakening of the westerly flow closer to the surface in the subsequent weeks.  This in turn often allows blocking high pressure to set up and cold air to spill south into the mid-latitudes, and in particular Europe and western Asia have a strong tendency to be cold in the weeks following a SSW.  Conversely, the blocking pattern tends to favor unusual warmth in southern and interior Alaska as low pressure sets up over the Bering Sea.

Of course the SSW isn't the only driver of the current circulation pattern, but the latest long-range forecasts are certainly quite consistent with the expected SSW impacts.  Here are the latest MSLP and temperature forecasts for the North Pacific sector over the next 6 weeks from NOAA's CFSv2 model; the maps show the ensemble mean forecast anomaly of MSLP (left) and temperature (right) relative to a 2000-2016 normal.  Based on this forecast, it wouldn't be a surprise to see some record breaking warmth in Alaska in the next several weeks.

Week 1
Week 2
Week 3
Week 4
Week 5
Week 6

Yukon River at Dawson

Long-time readers may recall a few posts last winter that mentioned the exceptionally slow freeze-up of the Yukon River in Dawson (Yukon Territory).  It's interesting to note that the same thing has happened this winter - there is still a substantial gap of open water that has prevented the construction of the usual ice road to West Dawson.  This year an effort was launched to accelerate the freeze-up with a "slush cannon", but lack of progress led to the project being called off recently.

Here's how the river looked last Sunday.

Last winter we noted that an unusual ice jam upstream was the primary reason for the open water, rather than exceptionally warm weather.  It appears that the same may be true this year, because again the weather has not been particularly warm overall this winter in Dawson; the chart below shows that total freezing degree days through January 22 (when the ice-making project was canceled) were very close to the normal of the last 20 years.

The repetition of the slow freeze-up this winter suggests that it's not a random occurrence - "something" has changed - but the temperature data rules out a simple explanation based on excessive warmth.  I started to look for river flow data from the autumn to see if the river discharge has been unusually high or low, but I had difficulty acquiring suitable data.

One possibility (and this is mere speculation, being outside my field of knowledge) is that there could have been a subtle change in the river profile/cross-section upstream of Dawson, owing to deposition or erosion, that may have altered the hydrodynamics in a way that now favors the formation of an ice jam in a new location.  If any readers have comments on this idea, or other suggestions on how to identify the underlying cause of the open water (i.e. is it related to weather and climate), then I'd be glad to hear them.