Tuesday, April 24, 2018

When Will The Tripod Go Out?

The spring thaw is getting under way in earnest this week across the interior, so breakup will soon be in full swing.  It's an interesting little exercise to try to pinpoint when the tripod may go out at Nenana, based on the 15-day forecast combined with the historical range of heat units that have been required to produce breakup in the past.  Rick Thoman has produced this kind of analysis for predicting greenup in Fairbanks, so credit goes to him for the idea.

The chart below shows the latest outlook.  Note that we're using temperatures in Fairbanks, not Nenana, because of the long-term history in Fairbanks.  The green line shows the observed accumulation of thawing degree days (excess of daily mean temperatures above 32°F) so far this year in Fairbanks (through today), and the colored fan indicates a realistic probability distribution of future TDDs based on today's ensemble forecast for the next 15 days (courtesy of NOAA's EKDMOS product).  The horizontal lines show several percentiles of the historical TDDs that have been observed up to and including the date of breakup in the past (1930-2017).

According to the analysis, there's little chance that Nenana's tripod will go out before Saturday - even if the weather is warmer than expected, and the river breaks up "easily" (i.e. with less thawing than usual), the earliest we might expect breakup is Saturday, April 28.  It's more likely that the date will be April 29 or later, and it seems quite likely that it won't be until May 1 or later.

However, I would make one cautionary note: with snowpack being well above normal this year, runoff will be greater than normal, and this should bring the date forward a little.  The Tanana river ice is also rather thin at Nenana, but ice thickness is not a strong predictor of breakup date.  With all things considered, I'll go for April 30 - May 2 as the most likely 3-day window of dates.

Saturday, April 21, 2018

Cold Spring Day

Yesterday was a very chilly day for the time of year across interior Alaska, with breezy below-freezing conditions all day in most areas north of the Tanana and lower Yukon rivers.  Temperatures struggled to reach 20°F in the Yukon Flats despite plenty of sunshine, and Fairbanks only made it to 31°F.

The last time the daily high temperature was below freezing this late in the season in Fairbanks was during the record cold April of 2013 - you can read all about that in the blog archives here (Rick Thoman was the author then).  Prior to that it had been more than 10 years since a sub-freezing day in late April (2002).

The winds did not go calm last night, but temperatures dropped off sharply nonetheless, and on the North Slope this morning's temperatures were, well, Arctic.  Umiat made it to an impressive -27°F with still a light breeze this morning.  This is cold even by Umiat standards; in the combined history of data from Umiat sites, only 4 years were this cold so late in the season (but 1984 takes the cake, with -43°F on April 23).

Here are 24-hour low temperatures as of 5pm AKDT today:

To add insult to injury yesterday, a couple of inches of snow fell in parts of Fairbanks in the evening.  The airport's total of 1.8" was the most to fall this late in the spring since - you guessed it - 2013.  A 5-year return period is very consistent with the long term history for this kind of event, although there have been occasions with 6" or more at this late date or even into May.

The scene down on the Tanana River at Nenana was not encouraging for those with money on a very early breakup, and this morning looked less than spring-like at UAF:

The culprit for the renewed bout of winter was a sharp upper-level trough extending southwest from a cold low that dropped south to near Canada's northwest coast a couple of days earlier.

Thursday, April 19, 2018

Sea Ice Update

One of the biggest Alaska-related climate stories of this winter has been the dramatic reduction in Bering Sea ice compared to recent decades (and probably much longer, as noted by this recent article on climate.gov).  The latest data show no late-season recovery, and in fact the ice extent deficit has "stabilized" at more than 500,000 km2 below normal.  As of April 18, the Bering Sea ice extent of just 142,000 km2 is equivalent to the 1981-2010 normal for the beginning of June.

In seeking an explanation for the absence of sea ice, I previously noted the role of reduced northerly flow.  The connection between Bering Sea ice and the local circulation pattern is also reflected in the following map, which shows the correlation between 500mb heights and sea ice extent for December-March.  When there is a strong trough over Alaska, Bering Sea ice expands, but when the winter is dominated by a ridge over Alaska, ice cover is reduced.  Not surprisingly, this winter's upper-level height pattern was very similar to the pattern that produces strong variations in Bering ice extent - see the second map below.

An interesting question that arises is "to what extent can this winter's ice loss be explained by the flow pattern?"  We can address this in a simple manner by estimating a linear relationship between sea ice extent and 500mb heights at the epicenter of (inverse) correlation near 65°N 160°W.  The chart below shows the reasonably good relationship (R=-0.67) and indicates that this winter's 500mb heights were the highest in the modern era of sea ice data.  However, the ice extent was clearly well below the level that would be expected solely from the strength of the ridge.

The chart below shows the same data in time series form.  This winter's 500mb heights were not much higher than in some recent years, but sea ice extent was far lower.

Performing the linear regression allows us to quantify the unusual shortfall of ice relative to what would might be expected from the flow pattern; see below.  Based on 500mb heights alone, the expected value of Bering Sea ice extent for 2017-2018 is 423,000 km2, or 168,000 km2 below the 1981-2010 mean.  The actual deficit this winter was 364,000 km2, or more than twice what can be explained by the circulation pattern alone.

Perhaps the most interesting aspect of the analysis is that there has been an ice deficit relative to the regression model in each of the last four years, and it has grown steadily; here are the ice extent residuals from the last four years:

2014-15   -60,000 km2
2015-16   -107,000 km2
2016-17   -128,000 km2
2017-18   -195,000 km2

These results are consistent with the idea that the sea ice loss is not simply an expression of unusual weather patterns; rather, sea ice deficits have increased owing to persistent regional ocean warming and, presumably, Arctic-wide amplification of the background warming trend.

Saturday, April 14, 2018

April Temperature Variance

Fairbanks saw its first 50°F temperature of the spring on Wednesday, and this was right on schedule according to the long-term history.  Based on data since 1930, the median date is April 11, but the first 50°F has typically arrived a few days earlier in recent decades.

Here's a chart of average daily high temperatures in each April since 1930 (blue markers) along with the date of the first 50+°F temperature (red markers).  Note that I've excluded 3 instances when 50°F occurred prior to March (1943, 1981, and 2009); these were exceptional winter chinook events, and for today's analysis I'm interested in what happens during spring.

The month of April has seen pronounced long-term warming in Fairbanks, with the linear trend line showing more than 5°F of warming over 87 years.  It's no surprise, then, to see that the first 50°F has become slightly earlier - although there's only a weak trend in the date of first 50°F.  The trend in April mean high temperature is highly significant (p<0.01), but the first 50°F trend is not significant at all (p~0.20).

At first glance it's surprising that the first 50°F trend is so much less robust, but this is partly because the variance of 50°F dates is so large.  Over the 87 years, the first 50°F date has advanced about 4-5 days, but the standard deviation is about 3 times as large (14 days).  In contrast, the interannual standard deviation of mean April high temperatures is almost exactly the same as the 5-6°F of warming that has occurred, so the long-term change has produced a very significant shift in the frequency distribution of mean temperature.

If we consider the normal rate of warming at this time of year in Fairbanks, we find that 5-6°F of warming is equivalent to about 8 days of climatological warming.  Naively, then, we might have expected the first 50°F to have advanced about 8 days; but even that would not correspond to a statistically significant trend in the date.  The fact is that the variance of the first 50°F date is so large that sampling variability could plausibly produce trends that are nearly zero or trends that are much greater than expected.  And so we might say that dates of "first warm day" or "first cold day", at least in Fairbanks, are not particularly useful as indicators of long-term change.

Tuesday, April 3, 2018

Another Warm Winter

For the fourth time in the past five years, the November-March mean temperature in Fairbanks was significantly warmer than the 1981-2010 average, i.e. the mean temperature was in the upper tercile of the 1981-2010 distribution.  Each calendar month was also warmer than normal, and in fact this warm "streak" goes back to last June; each of the past 10 calendar months has been at least 1°F warmer than normal.  This kind of persistent warmth is not unprecedented, but it's certainly unusual.

The chart below shows running annual mean temperatures (daily max/min/mean) in Fairbanks since the beginning of the NWS/Weather Bureau era.  The recent warmth has raised the annual mean to nearly 32°F, a threshold that was exceeded for part of 2016.

It's interesting to note that some very cold years in Fairbanks had annual daily maximum temperatures below freezing, but that hasn't happened for many years.  Also of note: the long-term trends in maximum and minimum temperatures are quite similar over the entire period, but minimum temperatures increased more dramatically in the climate shift of 1976-77.  Consequently, the linear trend in minimum temperatures since 1976 is actually negative (-0.15°F/decade), but daily maximum temperatures have continued to increase (+0.20°F/decade).  Of course we have to take these numbers with a grain of salt owing to location and instrumentation changes in the official climate record.

Saturday, March 31, 2018

Warm April More Likely

With just a few days left to submit guesses for the Nenana Ice Classic, breakup prognosticators are pondering the extended-range weather forecast for the next few weeks.  The date of the Tanana River's breakup at Nenana is closely tied to local temperatures in April, so an early breakup will be likely if the unusual warmth of recent weeks and months persists.

Looking quickly at some of the forecast data, CPC's most recent outlook for April shows a slight warm tilt to the temperature probabilities over most of the interior. 

NOAA's CFSv2 model has recently signaled an increasingly pronounced warm anomaly centered over far eastern Russia and the Bering Sea.  The maps below show a progression of April forecasts from oldest at bottom to newest at top.  A few weeks ago it appeared possible that the pattern would favor a cold April in the Tanana valley, but this now seems unlikely.

As an aside, I'm in the middle of a very busy period of travel and work, so blog posts will continue to be thin for a while.

Friday, March 23, 2018

Late Winter Snow Showers

My visit to Fairbanks ended on Monday with a rare March instance of strong westerly breezes, adding further to the tally of "strong" westerly wind days in recent weeks (see here and here).  While it's typical for winds to pick up in late winter in Fairbanks, the seasonal breezes are usually from the northeast; in the past 20 years, there have only been 2 other days in March with similar winds (20-30 mph) from the western half of the compass.

In this week's event, the wind was associated with a strong cold front aloft; the 500mb chart below shows the trough axis near Fairbanks at 4pm AKDT on Monday.  Note the temperature of -39°C reported above Fairbanks; by Tuesday morning the 500mb temperature was down to -43.5°C, which was very nearly the coldest air of the entire winter at this level.

However, even as the cold air moved in aloft, temperatures at the surface were up near the freezing point on Monday afternoon.  The chart below shows the full sounding from 4pm on Monday; the (red) temperature trace shows a strikingly steep vertical temperature gradient, indicating an environment with potential for some convective overturning.

In response to the increasingly unstable environment, heavy snow showers emerged on Monday afternoon and moved through the Fairbanks area.  See below for a sequence of radar images at hourly intervals from 2pm to 7pm AKDT.

Interestingly a similar round of showers occurred on Tuesday afternoon as surface temperatures made their way up to the freezing point again, while very cold temperatures persisted aloft.  Tuesday afternoon's temperature difference (42.6°C) between the surface and 500mb was one of the greatest on record for March in Fairbanks and was actually the largest on record for so early in the year.  Here's a Tuesday afternoon radar image:

Reader Bill suggested that it's not uncommon to see heavy snow squalls like this in late winter in Fairbanks, but without going through a long-term archive of radar data, it's difficult to say what the frequency actually is or whether it might be changing over time.