Tuesday, October 15, 2019

Upper Tanana snowstorm

Following up on Rick's post yesterday, it's interesting to look at the analysis charts from last week's snowstorm in the upper Tanana valley.  Maps shown below are courtesy of Environment Canada's analysis page:


The fundamental cause of the event was a very strong upper-level trough that migrated eastward and slightly southward from the central Bering Sea across southwestern Alaska to the northern Gulf of Alaska.  Below are 500mb maps from 3am AKST Thursday, 3pm Thursday, and 3am Friday; the "x" symbol marks the center of the upper-level low ("L").  Click to enlarge.

The system intensified considerably as it approached the Gulf of Alaska on Thursday, and a strong surface low pressure system quickly developed just to the south of the Alaska coastline on Thursday night, as illustrated by the surface charts from 3pm Thursday, 3am Friday, and 3pm Friday:

The north and northwest side of a strong low pressure system is a "good" place to be for heavy snowfall (in the Northern Hemisphere), assuming there is sufficient cold air in place; think of the coastal winter storms that occasionally affect the northeastern states of the Lower 48.  In this case, remarkably, the storm was able to produce a heavy band of snow on the interior side of the Alaska Range, i.e. in the upper Tanana River valley; this is a climatologically unfavorable area for heavy snow, because of the prevalence of dry downslope flow from the south - but this setup was very different from normal.

Rick highlighted the contrast in snow cover across the southern interior, and I too grabbed yesterday's beautiful satellite image to get a wider view.  The red dot marks the approximate location of Fairbanks (click to enlarge).

Particularly noteworthy to me are the obvious lack of Arctic sea ice - a remarkable sight for mid-October - and the still-unfrozen surface of North Slope lakes such as Teshekpuk Lake.  Here are a couple of yesterday's webcam views of Tesh Lake, showing just a little near-shore ice.  It would be interesting to take a look at observations of this summer's lake water temperature compared to previous years.

However, the situation has changed a bit today.

Freeze-up progress has been more substantial on Toolik Lake - recall it was still mostly open a week ago.

How about the rivers?  Ice was developing quickly in the Koyuk River at Norton Bay yesterday.

Some scattered ice is running in the Tanana River at Nenana today:

But as of yesterday the George Black ferry was still crossing the river at Dawson City in the Yukon:

Monday, October 14, 2019

Snow Cover in the Interior

Rick T. here with with a (mostly) clear sky from Monday afternoon October 14, 2019, showing a fairly well defined line between areas to the south and east of Fairbanks that picked up quite a bit snow during the second week of October and areas to the west, where low elevations are still snow-free. Urban Fairbanks is right on the edge. The snowfall from Delta  Junction southeastward was quite impressive. 15" of snow at Tok was the greatest October snowstorm (over two days) there on record (climate obs since the late 1950s) and second greatest overall. Delta Junction does not often get a foot of snow in one pop, but they did from this event, including severe drifting. 

Monday, October 7, 2019

Snow Arrives

As befits the date on the calendar, snow cover has started expanding across Alaska, with many northern and/or elevated locations now seeing a light covering.  This afternoon's land cover classification image from the Suomi polar orbiter is obscured by cloud cover across much of western Alaska and parts of the North Slope, but nevertheless the image clearly shows some snow on the ground in much of the north (except the Yukon Flats) as well as the Yukon-Tanana uplands.

FAA webcams revealed a pretty scene in Bettles this morning, where the temperature dropped to a chilly but not unusual +7°F.

Up in the northeast at Arctic Village, things are starting to look wintry as well, with a covering of ice on the ponds.

But perhaps surprisingly, Toolik Lake (on the north side of the Brooks Range) is not yet frozen over despite some rather chilly weather in the last couple of weeks.

Moving farther north still, Teshekpuk Lake shows no sign of any ice yet, and the same appears to be true of the smaller lake that's visible on the webcam at Inigok:

And finally, today's climate report from Utqiaġvik stated that there was zero snow on the ground, although the webcam view from earlier today suggests this may not be accurate (or is no longer true):

A couple of weeks ago I mentioned that the first measurable snow of the season has never occurred in Fairbanks earlier than in Utqiaġvik, but this year there has at least been a tie, and Fairbanks may in fact have been earlier (the daily snow numbers are missing from Utqiaġvik for Oct 5th and 6th).

The absence of snow in the north until now is not because it's been dry; Utqiaġvik had nearly an inch of precipitation in the second half of September, but it nearly all fell as rain (there was just a trace of snow).  The average temperature for September was easily the highest on record, exceeding 40°F for the first time on record, and every single day had a daily mean temperature above freezing (also for the first time).  Previously the latest arrival of a sub-freezing daily mean temperature was September 30, 1998, but Utqiaġvik has yet to see such a day this autumn.  And so the warmth in Alaska's far north continues to break records as the trend of recent years persists apparently unabated.

Sunday, September 22, 2019

Cooling Off

Late September is late autumn in interior Alaska, and after an unusually warm first half of the month, the calendar is now exerting its influence.  Some accumulating snow occurred yesterday in the higher hills around Fairbanks, and the international airport finally observed its first freeze of the season this morning.  Only 5 years in Fairbanks history have waited longer for the first freeze, with the record being September 27 (1974).

Of course the usual cold spots have been seeing freezes for weeks (see the chart below for 2m temperatures from Smith Lake on the UAF campus).  Smith Lake dropped to 22°F last night, and so did Tanana on the middle Yukon River as well as Shungnak in the northwestern interior.  Tanana had a freeze more than a month ago, on August 19.

What about snowfall?  Fairbanks often doesn't see its first accumulating snow at valley-level until early or even mid-October, but late September is more common in the northern interior.  Bettles already saw a notable 3.4" of snow a few days ago, and this provides an interesting contrast to the mild and snowless conditions on the other side of the Brooks Range (i.e. the North Slope).  Utqiaġvik (formerly Barrow) has not seen accumulating snow yet, and interestingly there has only been one other year - 2007 - when Bettles beat Utqiaġvik to first measurable snow, and that was only by 3 days.  Fairbanks has never (in the modern climate record) seen measurable snow before Utqiaġvik, but it might not be impossible this year.

Below are charts of the date of first measurable snow for Fairbanks, Bettles, Utqiaġvik, and a couple more sites for good measure.

(Note to regular readers: I'll be traveling for the next couple of weeks, so probably won't be posting anything in that time.)

Tuesday, September 17, 2019

North Pacific Blog Post

After a hiatus of many months, I've posted an update on the North Pacific "Blob Tracker" blog.  There has been considerable interest recently in the return of the so-called "blob" pattern of warm SSTs in the northeastern North Pacific, so it's worth taking a look at how current conditions compare to the earlier version of the anomaly.


Monday, September 9, 2019

More North Slope Warmth

A remarkable spell of early autumn warmth has developed across the North Slope in the past week, with daytime temperatures mostly in the 60s and even up to around 70°F in some spots (e.g. 71°F in Umiat).  As noted by Rick Thoman on his Twitter channel, the 70°F at Deadhorse last Tuesday (September 3rd) was the highest temperature on record for the month of September, with a history back to 1968 if we include the old Prudhoe Bay climate data.  The chart below shows that daily high temperatures have been hovering around record levels since then.  Many years do not exceed 60°F after August in Deadhorse, and in fact this year is only the second year since 2010 to do so.

The data from Utqiaġvik (formerly Barrow) paints a similar picture, and here it's remarkable to note that daily low temperatures have been at or above the 1981-2010 normal for daily mean temperature almost constantly since the start of July.  This is a striking effect of exceptionally warm water temperatures and absence of sea ice in the nearby ocean.

But perhaps just as unusual as the warmth of recent months is the quantity of rain that appears to have fallen on the north side of the Brooks Range.  I don't have a comprehensive assessment of rain amounts, but the NWS has been warning of flooding on North Slope rivers, and the Imnaviat Creek SNOTEL reported nearly 13" of rain for June through August (see below).  The Umiat RAWS instrument, at a much lower elevation, measured 5.9" of rain for the same period, and this is close to a record for the location.

Here's a map of the summer's precipitation in terms of departure from normal, according to the new state-of-the-art ERA5 reanalysis.  It's interesting to see the sharp north-south gradients in precipitation anomaly across the divides of both the Brooks Range and the Alaska Range, and this appears to be realistic even though it's just a model estimate and not obtained directly from ground truth measurements.

And here's the corresponding temperature anomaly from the model.  The warmth looks a bit less amplified across the north than I would have expected, but I haven't yet had a chance to do a detailed comparison at the major observing sites.

Saturday, August 31, 2019

Sea Ice Update

Before I get to the main subject of this post, I would be remiss not to mention a publication released by UAF's International Arctic Research Center last week, titled "Alaska's Changing Environment".  The document compiles a wide range of observations from diverse sectors of Alaska's physical and biological environment, with an emphasis on illustrating the changes that have occurred in recent years and decades.  It's a really nice summary of the many ways that change has been observed across the state, and the graphical content is splendid.  Here's just one example, showing the sudden onset of drought in southeast Alaska in the last year or so, which is particularly dramatic when viewed in contrast to the wet regime that preceded it.

Here's the link to the report, whose principal author is Rick Thoman:


And now on to the topic of sea ice.  In my last update from a month ago, Arctic-wide sea ice extent was running very close to the record low of 2012, but in the past few weeks the rate of decline has slowed and the two curves have diverged; and so it now appears very unlikely that a new record low will be set this year.

A possible cause of the slowdown in ice meltout may be the unusual northerly component to the wind over the Arctic waters north of Alaska and eastern Siberia.  In the past two weeks the ridge of high pressure that has been affecting southern Alaska extended up from the Bering Sea to the East Siberian Sea, and a contrasting zone of low pressure has prevailed near the western edge of Canada's Arctic Archipelago.  Consequently there has been a strong northwesterly flow across the sea ice edge near or just south of 80°N (see the graphics below), and I surmise that this has greatly reduced ice melting and retreat on this side of the Arctic Ocean.  (It is more common for winds to be westerly or southwesterly at this time of year between 75 and 80°N, although of course there's a lot of year-to-year variabiltiy.)

Here are the latest ice extent charts for the Beaufort and Chukchi Seas.  Even though the ice meltout has slowed lately, there is still considerably less ice than last year, and only a few years in the satellite era have had less ice at this date.  Given the very late meltout in the Chukchi Sea last year, it's still entirely possible that significant ice loss still lies ahead in the next few weeks.

The chart below shows the annual minimum extent for both of these basins as a percent of total area; this year will probably come in around 35% for the Beaufort and 5% or less for the Chukchi.  The Chukchi Sea essentially melted out completely (less than 1% ice) every year from 2007-2012, except 2009.  We should bear in mind, though, that ice "extent" includes any sea area with at least 15% ice coverage, so the actual area fraction covered with ice could be close to zero whenever the basin-wide extent drops below 5% or so.  Sea ice area is tracked separately, but extent is the preferred metric at NSIDC for reasons that are explained here:


Saturday, August 24, 2019

Wildfire Latitude

Last weekend, as I was writing about the stark rainfall contrast across Alaska, the effects of the drought in the south-central region were unfortunately illustrated by an outbreak of several rare late summer wildfires, leading to evacuations and significant property damage.  And "drought" it most definitely is: Anchorage has still seen no measurable rain this month and remains on track for the driest summer of record.

Even the national Drought Monitor, a subjective drought assessment issued every week by experts in the field, shows widespread drought, both in south-central Alaska and in the southeast of the state.  In the vicinity of Anchorage the drought is classified as "extreme", the second highest category, and remarkably the analysis estimates that there are twice as many people in "extreme drought" in Alaska as in the rest of the USA put together.  Here's the Drought Monitor website link and the latest analysis for Alaska:


An article in the Anchorage Daily News this week discussed the broader context for the unusual late wildfire activity near Anchorage: check out the article for excellent commentary from Rick Thoman.


In light of the discussion in the article, I thought it would be interesting to look at the historical variations in the latitude of Alaska wildfire; for example, do the historical data show a notable southward shift in fire locations?  Somewhat remarkably, fire acreage and location data is available from the Alaska Fire Service/AICC all the way back to 1939; I have no idea how reliable the numbers are from the earlier decades, but the analysis produces some interesting results.

First, the total annual fire acreage from this source shows the 80-year history of overall fire activity in Alaska.  As many others have noted, the frequency of big fire years has increased since about 1990, although there have always been highly active years every now and then.

Here's a histogram comparison of fire acreage in latitude categories for three different periods in the history.  In this chart we're looking at the fraction of total acreage in each period, so the columns add up to 1.0 for each period (i.e. the columns do not correspond to absolute acreage numbers).  Note that the AFS data provides the location at which each fire started, not (for example) the centroid of the acreage burned.

The overall pattern of latitude dependence has of course remained quite similar over the years, but it is intriguing to see that the peak latitude band changed from 66-67°N in the first 40 years, to 65-66°N in 1979-2008, and then to 64-65°N in the past 10 years.  This does suggest a southward shift, although it's not easy to judge by eye whether the median of the distribution has changed; more on that later.

The next chart shows absolute acreage totals (average per year) for each period, and here the increase in the 64-65°N band stands out more distinctly because of the upward trend in total acreage.

To gain another perspective, I calculated the acreage-weighted mean latitude of fire activity (for all fires of 10 acres or greater) in overlapping 15-year periods; see below.  Here the result is interesting and quite different, because it seems that Alaska wildfire acreage occurred relatively far south - indeed farther south than in recent years - prior to about 1970.  But beginning with the big fire year of 1969, more northern acreage tended to be burned, and that trend continued until 1997, when the trend switched back; in that year, the 2 million acres of fire were focused around 63°N, which is the farthest south of any 1+ million acre fire year.

Finally, here's a scatter plot of acreage versus acreage-weighted latitude by year, with different colors highlighting the two halves of the 80-year history.  Recent decades have seen some fairly big years focused south of 64°N (1997, 2002, 2013), but if the data are to be believed, 1940 and 1957 were very active fire years with a southern focus (around 64°N).  Surprisingly, the overall acreage-weighted mean latitude is actually higher for the last 40 years than the preceding 40 years.

In conclusion, the historical trends in Alaska wildfire latitude are not simple to interpret, and the older history clearly suggests that the baseline of recent years may not be atypical on a longer time scale.  However, a couple of caveats are in order: first, my acreage-weighted analysis is obviously dominated by a relatively small number of big fire years, so the trends may not be statistically robust.  And second, it seems wise to remain cautious about the quality of the data from the early decades; for example, if acreage was systematically underestimated in the northern interior (where perhaps aerial monitoring was less intensive in the early days), then the early fire acreage reports would be biased towards the south.  Perhaps someone among the readership can chime in on the history of fire monitoring in Alaska.