Another Kotzebue Blizzard

It's only been 3 weeks since I wrote about back-to-back blizzards in Kotzebue that led to a local disaster declaration (news link courtesy of reader Gary), but another major storm affected the area last weekend.  Remarkably, hurricane-force wind gusts were reported by the airport ASOS, although the observations were missing for several hours, so perhaps the instruments malfunctioned.

By my count, this makes five real blizzards in Kotzebue this winter: November 7, December 10-11, February 21-23, March 4, and March 25.  Each of these had sustained winds above 40mph.  There was also a storm on January 27-28 that briefly met the blizzard criteria (35mph wind and 1/4 mile visibility).

In light of the recurrent storminess in Kotzebue, reader Andy's recent comment about Nome being "inundated with snow in many recent winters" is very interesting and worth a closer look.  Let's have a look at November-March precipitation for the two sites: the charts below show results both from the standard climate observations and from the nearest grid cell in the ERA5 reanalysis.

 

The large wet bias in the ERA5 data is not unexpected in model data, and it's not generally regarded as a problem - the user just has to implement a bias correction.  Much more importantly, the year-to-year correlation between ERA5 and the ground truth data is fairly good back to about the mid-1970s, giving us some confidence in both sources for the last several decades.  (The ERA5 does not use precipitation observations in creating its global gridded estimates.)

Excluding pre-1980 data, there's little trend in Nome winter precipitation, although the two very wet winters of 2017-18 and 2018-19 certainly count as being "inundated".  At Kotzebue, however, there does seem to be a significant trend towards wetter winters in both the ERA5 and ground-truth data, and the absence of a "dry" winter since 2016-17 is notable.

As for the earlier decades, the situation is more uncertain.  The Nome precipitation data doesn't seem to have enough year-to-year variability in the 50s and 60s.  The correlation with ERA5 is poor in the early decades for both sites, although that by itself doesn't invalidate the climate observations, as the ERA5 uncertainties are much greater for that time.  The pre-1970 numbers from Kotzebue are so low that they seem unreasonable at first glance, but if indeed the winter climate was more truly Arctic back then, then perhaps much lower precipitation is plausible.  Utqiaġvik averages less than 2 inches total precipitation in November through March even after becoming much wetter in recent years.

So much for precipitation; but how about high winds (the "blizzard" aspect of the problem)?

We don't have consistent wind measurements across the decades, so I again turned to ERA5 and extracted the hourly wind speed back to 1950 for both sites.  I determined the peak wind speed for each winter (November-March), calculated the median of those numbers, and then found the number of hours each winter that exceeded that "typical winter maximum".  The ERA5 winds are a bit lower than reported by the ASOS instrument of today, and in reality this "typical winter maximum" corresponds to sustained winds of about 40mph in Nome and 50mph in Kotzebue.  Half of all winters see winds of at least this strength at some point.

 

To bolster confidence that ERA5 winds are realistic, I checked the highest values in the history.  The highest ERA5 hourly wind speed for Nome was at 9pm on December 28, 2021, and indeed that day saw a major wind storm that peaked at 8:30pm.  In Kotzebue, the highest ERA5 wind speed in recent decades was at 11pm on February 18, 2006, and the observed wind speed peaked at the very same hour.

It's interesting that both sites have seen an increased frequency of winters with relatively high winds in the past 20 years; there have been few winters without high wind, especially at Kotzebue.

As for the maximum winds each winter, there's a hint of an upward trend in recent year for Nome, with few recent winters on the low side of the distribution, and the change seems a bit more pronounced at Kotzebue - see below.

Finally, looking at the number of days that met the blizzard criteria in the past 25 years (approximately the ASOS era), there's again perhaps just a hint of an increase in Nome, and Kotzebue has had 9 or more blizzard days in 5 of the last 6 winters.

All in all, it does seem that a trend is emerging at Kotzebue, although as usual the year-to-year variability makes it difficult to be sure, and formal significance testing would likely not give a resounding message.  The trends at Nome seem more tentative and uncertain.  Remind me to take another look at this a few years from now...

Kotzebue Blizzards

Kotzebue has been seeing some rough weather lately, with the second big blizzard of the past two weeks winding down today.  Twitter user Tundrabilly has been documenting the conditions.

3-4-23, noonish, Kotzebue, Alaska, -9 d Fah (-23 C), wind chill -41 d Fah (-41 C), wind speed E 38 mph (61.16 km/h, gusting to 48 mph (77.25 km/h). Didn't see anyone out . Winter's here perty hard. pic.twitter.com/kTburqpgNm

— Tundrabilly ❄☃️🇺🇸 (@tammaq13) March 4, 2023

 

Here's a chart showing the frequency of blizzard conditions as a percent of all hourly ASOS observations in the last 25 years.  February is peak blizzard season, and the climatological frequency doesn't drop off a whole lot in March.  However, the percentage only peaks at about 1%, so a true blizzard is unusual weather even at this time of year.

 

The much lower frequency in January might be at least partly a statistical artifact, but it appears in both the first and second half of this 25-year period, so I'm inclined to think it is a real feature of the climate.  Presumably the jet stream drops far enough south in January that strong storms are less common in Kotzebue; but this is a topic for further investigation.

If we exclude the visibility requirement of the blizzard definition, then the December and November frequencies jump up above the February and March frequencies, respectively.  It seems that low visibility is more readily attainable in late winter, perhaps because there's much more snow lying on the ground throughout the region (available to be blown around), or perhaps because the air tends to be drier in late winter (making snow more powdery).

Weather Contrast

The weather has been bad today in western Alaska, with wind, snow, and rain moving up from the south.  Kotzebue developed a white-out this afternoon with 40-50 mph winds.

2-22-23, 2:27 m, Kotzebue, Alaska, 20 d Fah (-7 C), wind chill 9 d Fah (-18 C), winds E at 37 (59.55 kmh), what we be like pic.twitter.com/0MNph9gjX7

— Tundrabilly ❄☃️🇺🇸 (@tammaq13) February 22, 2023

 

But far to the east on the other side of the ridge axis, it has been a stunning day (after a chilly start) in eastern Alaska and the Yukon, with lots of glorious sunshine.  Here's the webcam view from Dawson:

And in video format:

Here's the 3am 500mb analysis, courtesy of Environment Canada: click to enlarge.  Note that ridge axis right over Alaska.

West Coast Storm

In the past couple of days a low pressure system has tracked up the middle of the Bering Sea, bringing blizzard conditions to the west coast of Alaska.  Kotzebue in the northwest had quite an extended episode, with sustained winds of around 30 knots and low visibility for a full 24 hours.  Here's the surface pressure analysis from 3pm AKST yesterday, showing the low near the Seward Peninsula; the storm was then winding down in Kotzebue.

How does this event compare for lengthy and severity to blizzards of the past?  It's not particularly unusual; a quick search of the hourly data shows at least 7 other events with 24+ hours of sustained 30+ knot winds and sub-1 mile visibility.  In terms of extremes, consider the storm of late February 1951: sustained winds reached hurricane force with a temperature near 0°F, and 30+ knot winds lasted for 46 hours.

Here's a chart of the frequency of windy, low-visibility conditions since 1981.  February is the peak month, but it's not too uncommon even in April.

For a look at the Kotzebue weather scene, check out the following Twitter profile:

https://twitter.com/tammaq13

Update March 31:

Here's a page from Kotzebue's monthly climatological summary in February 1951.  Note the remarkable period of persistently high wind speeds between the 20th and the 25th.

 

 

 

On a side note, the report also includes ground temperature data down to 22 feet.  It would be interesting to "dig up" all this early subsurface temperature data and compare it to modern observations.

Winter Rain History

After a nasty ice storm a couple of weeks ago, Bethel experienced blizzard conditions today, with temperatures below 10°F and 40+mph wind gusts combined with heavy snow this morning.  However, as the warm front approached this afternoon, warm air aloft caused the snow to change over to a brief period of freezing rain again before precipitation ended.

The (nominally) 3pm balloon sounding from Bethel shows the dramatic temperature gradient just above the surface.

At the time the balloon was launched (2pm), the surface air temperature was 16°F, but freezing rain was already occurring as there was enough warm air upstream to melt falling snow; the sounding profile shows the temperature just nudging above freezing at 400m above sea level.  Notice the veering (clockwise directional change) of the wind with height, indicative of the rapid approach of warm air.

As of 6pm this evening, the temperature is up to 36°F, and the wind has gone around to the southeast; it must feel positively tropical compared to the conditions only 8 hours earlier.  Check out the temperature gradient and wind shift across the Y-K delta region in the 6pm surface data (click to enlarge):

Here's the 3am surface analysis, courtesy of Environment Canada; note the strong pressure gradient over southwestern Alaska, with a cold high to the north (-28°F at Galena this morning, -26°F at Kaltag).

In light of the ice storm earlier in the month (see my earlier post), I took a quick look at the historical frequency of liquid precipitation (rain or drizzle) in winter at Bethel and a few other sites.  The chart below shows the annual percentage of winter (November-March) hourly observations that reported rain or drizzle, regardless of temperature, in Bethel.  Note that I've excluded supplementary observations that were not taken at the top of the hour.

I'd be surprised if there are not some systematic biases between the pre-ASOS era (pre-1998) and the modern era, so I think about all we can say is that there's no evidence of an increase in the frequency of winter rain in Bethel.  The frequency has been relatively high in the past decade, but it seems to have been high also in the 1980s and in many previous years.

Here's the same chart for McGrath and Fairbanks, with the same vertical scale to emphasize the drastic difference in climatological frequency of liquid precip in winter.  If the trends are reliable, McGrath seems to have seen a rather steady decline in frequency, whereas Fairbanks has seen an upward trend in the last 20-30 years (but perhaps not exceeding the levels of earlier decades).

For completeness, I include the chart for Anchorage below, which also shows nothing particularly exceptional.  However, the second chart below shows the frequency of liquid precipitation at sub-freezing temperatures, i.e. freezing rain or freezing drizzle, and this does seem to show a decline since the 1980s (the 2013-2014 spike notwithstanding).  It seems possible that the warming trend in the more maritime zone of south-central Alaska has tended to produce more plain rain at the expense of freezing rain in winter.

Another Seward Pen Blizzard

Here's the FAA webcam shot from Nome at 1pm AST Saturday. Nome's been reporting blizzard conditions since before 10am, with winds gusting to nearly 50 mph. Similar at Golovin.




The 9am AST Saturday surface analysis from NWS Anchorage shows a complex low in the western Bering Sea.

North Slope Blizzard

Here is the 10am AST Wednesday Northern Alaska Weather Depiction chart from the Alaska Aviation Weather Unit. Note the 30 to 40 knot sustained winds on the North Slope from Prudhoe Bay westward . Unusually for the North Slope, there is a lot of new snow falling in the strong warm advection over the top of the ridge, though the 40-1 raito snow used at Barrow at 18z is unrealistic.