Saturday, April 22, 2017

Spring Progress

After a cold March (14°F below normal), Fairbanks has seen warmer than normal weather overall this month so far (4°F above normal).  Recent days have been relatively cool, however; the high temperature was only 35°F on Wednesday, compared to a normal of 47°F.

Total thawing degree days (accumulation of mean daily temperatures above freezing, in Fahrenheit) are up to 48 as of yesterday, which is the lowest for the date since 2011 and far behind last year's near-record pace.  However, we're actually only a couple of days behind the normal pace for thawing.  The snowpack is diminishing steadily and river ice is starting to look a bit rotten.






The chart below shows daily statewide maximum and minimum temperatures for the last several months according to data from NOAA's ACIS tool.  I've excluded Snotel stations, and for maximum temperatures I also removed RAWS sites because of their known warm bias in sunny weather.  It's striking to see the dramatically higher variance of statewide daily minima compared to maxima; this is largely because the warmest parts of the state in the cold season have highly maritime climates and therefore low temperature variance in comparison to the much colder continental areas.


Sunday, April 16, 2017

Snowy Weeks in Fairbanks

Rick T. here with a post about snowy weeks in Fairbanks.

Interior Alaska has a long snow season, easily from late September to early May, and occasionally even longer. The average number of days between the first measurable snow in the fall and last measurable snow in the spring is a bit shy of seven months (202 days). But of course, the snowfall is not evenly distributed during that time. On the seasonal timescale, early winter is, on average, much the snowiest time of year, while March and April are the climatological dry season. On shorter time scales, depending on what's happening with the flow aloft, we go through days or weeks that are drier and then snowier. However, over the years it sure seemed to me that a significant fraction of the seasonal snowfall comes in a relatively short window. Is that so, or is it just my back complaining from too much shoveling? To address that question, below is a plot of the maximum 7-day snowfall as a percentage of the winter total, i.e how much of the winter's snowfall fell in the snowiest week (any 7-day period) for the Weather Bureau/NWS era of observations (since 1929-30).

As I expected, a fair chunk of the total snowfall does indeed fall in short stretches. The 88 winter average is that almost 22 percent of the seasonal total falls in some seven day period. Although not statistically meaningful, it is interesting that the two highest percentages have occurred since 2010 and that there have been no winters since the early 1990s with unusually low (compared to the long term average) weekly snow percentages.

A surprise to me was the lack of correlation between the total seasonal snowfall and snowiest week. In the graphic, each year's dot is sized according to the total seasonal snowfall: bigger dots indicate higher seasonal totals. Just eyeballing it you can see the size of the dots varies randomly above and below the average line (correlation only +0.03, effectively zero). I expected that low snowfall winters might tend to have higher percentages fall in a few days, but this analysis says otherwise.

———Update April 17, 2017

Inspired by Richard's question on extremes of timing of the snowiest week, here's a histogram of the date of the the snowiest week by half-months. Note that there is no well defined peak: the late November spike is likely no more than random variability.
And for completeness, here's a histogram of the amount of snow that fell in the snowiest week each winter. Not surprisingly, there is a concentration of values around a foot but with a long tail of higher amounts.

 

Friday, April 14, 2017

Arctic Update

It's been more than two months since my last update on basin-wide Arctic warmth, but not much has changed; even the remarkable cold spell in Arctic Canada in early March didn't put a dent in the overall high-latitude temperature anomaly.  According to the mean temperature from my set of 19 long-term surface observing sites, both February and March were more than 4°C warmer than the 1981-2010 normal.  The coolest of the last 6 months relative to normal was December, at "only" 3.4°C above normal.

This winter's November-March average temperature was the highest since at least 1971 for the 19 stations, as shown in the chart below.  Remarkably, Vize Island (on the northern side of the Kara Sea) was more than 10°C above normal for the 5-month average.  To get a sense of how enormous this anomaly is, consider that 10°C is greater than the difference between the record coldest and record warmest November-March periods in Fairbanks.



If we look at the temperature in terms of standard deviations away from normal, the warmth really hasn't taken a break since the remarkable events of January 2016.  Monthly mean temperatures now seem to be remaining near levels that were very unusual only a few years ago.


Not surprisingly, March average Arctic sea ice extent was the lowest in the satellite era according to the NSIDC, and estimated sea ice volume is also well below previous records.


Interestingly, however, the Danish Meteorological Institute estimates that the Greenland ice sheet has gained considerably more mass than usual so far this cold season.  This was probably caused by the unusual North Atlantic circulation pattern, as low pressure and storminess have been more dominant than usual near southern Greenland.


Monday, April 10, 2017

Chena Basin Snowpack

The April 1 snowpack update from the National Resources Conservation Service (NRCS) shows the cumulative effect of a very snowy winter in the east-central interior, as the snowpack is considerably greater than normal in the vicinity of Fairbanks - see below.  Fairbanks airport has received 83.1" of snow in total, the most since 1992.  However, most of the measuring sites to the south of the Tanana River did not do nearly as well, and it appears that nowhere else in the state saw such a snowy winter in relation to normal.



The chart below shows the 1981-2010 normal snow water equivalent for the SNOTEL sites near Fairbanks, revealing that peak water content is typically reached in mid to late April, depending on elevation.  The higher sites receive more snow in total owing both to orographic forcing and to the longer period with sufficiently low temperatures; so Munson Ridge, at 3100' elevation, tends to build snowpack all the way until the end of April.  In 1982, Munson Ridge was still reporting snow on June 13!


Friday, April 7, 2017

North Slope Winter Warmth

Yesterday NOAA released the U.S. climate division monitoring data for March, and the results for Alaska show a bit of a puzzle.  I was expecting to see significantly below-normal March temperatures for most of the state and a significant warm anomaly for the North Slope district.  However, it turns out that the climate division data show the North Slope division with a -3.0°F anomaly relative to the 1981-2010 normal, and -1.3°F relative to the 1925-2000 normal (as shown in the map below - click to enlarge; note that the normal is 1925-2000, not 1901-2000 as stated in the legend).



In contrast, the graphic below (courtesy of Rick Thoman) shows that Barrow, Deadhorse, and Umiat all saw temperatures in the upper two-thirds of the 1981-2010 distribution; the month of March was significantly warmer than normal at these sites and almost certainly at many other locations north of the Brooks Range.


Here's a time series of the March mean temperature in the North Slope district according to the climate division data:


Oddly the March 2017 temperature is well below the March 2016 temperature, even though Barrow and Umiat were warmer this year than last year in March.  Looking back over the past 5 months (November-March), the same discrepancy is observed: the climate division data indicate that this winter was colder than 2015-2016, but in fact this winter was the warmest on record at Barrow and appears to have been warmer than last winter at Umiat too (although there is a fair amount of missing data).


The two charts below show a closer comparison between the two winters for Barrow, Deadhorse, and Umiat.  The January through March mean temperature was slightly cooler this year, but November and December were much warmer this winter than last winter.  Notice the +54°F daily temperature anomaly at Umiat on January 2 of this year: with a high temperature of 38°F and a low of 29°F, this is the largest departure from normal of any day in Umiat's history.  The 1981-2010 normal for Umiat on January 2 is -20°F, the lowest of any station in Alaska for that date.



What can we conclude from all of this?  First, it's plain to see that climate monitoring is a challenge in the data sparse regions of northern Alaska; different analysis methods can give very different answers even for something as "simple" as year-to-year temperature changes.  It's possible that NOAA's methodology for the climate division calculations allowed this winter's North Slope result to be influenced by colder temperatures south of the Brooks Range and outside the boundary of the district; this would be unfortunate, but these kinds of issues are not uncommon in the world of near-realtime climate monitoring.

An interesting question to consider is, was this the warmest winter on record for the North Slope as a whole?  According to the climate division data it wasn't even close, but my cursory analysis of the numbers from Barrow, Deadhorse and Umiat suggest it may well have been so.  A more thorough investigation, including a careful accounting for missing data, and additional observing sites, would be required to say anything with greater confidence.

Saturday, April 1, 2017

Sub-Seasonal Forecast Skill

I'll be traveling for the next few days, but some readers may be interested in a quick glance at some results I found recently with regard to sub-seasonal temperature forecasts for Fairbanks.  Sub-seasonal forecasts are made about 2 to 6 weeks in advance and therefore deal with prediction time scales in between more traditional medium-range weather forecasts (~5-10 days ahead) and seasonal forecasts (a month or more in advance).

Sub-seasonal forecasting has long been regarded as a very challenging problem, but increasing demand is driving research and investment in this area; the National Academy of Sciences published a report last year on strategies for research into improving sub-seasonal and seasonal forecasts.  I'll be giving a talk in Anchorage on May 2 at the Climate Prediction Applications Science Workshop; hence the new research reported here.

The result that's interesting to me is illustrated in the chart below, which shows a simple measure of performance (R-squared correlation) for forecasts of weekly mean temperature anomaly (departure from normal) in Fairbanks for week 2 (days 8-14 mean), week 3 (days 15-21 mean), and week 4 (days 22-28 mean).  I've taken the ensemble mean of forecasts from the U.S. CFSv2 model and the European (ECMWF) model, and then used a simple mean of the two models.  Generally the ECMWF model is superior to the CFSv2, but the average of the two models is better than either model by itself.


It's interesting and intriguing to see that the forecast performance is considerably better in spring and autumn than in winter; I would not have expected this, because large-scale flow anomalies are largest in winter, and I would expect predictability to be higher as well.  It seems possible that unusual boundary condition forcing (i.e. sea surface temperature and snow/ice cover anomalies - which are a key source of model skill) have greater influence in spring and autumn relative to the magnitude of random/chaotic variations in the flow.  It's also possible that the near-permanent surface-based temperature inversion in winter in Fairbanks makes long-range forecasts particularly difficult in that season.

The best month of the year for sub-seasonal temperature forecasts in Fairbanks is April, and especially at weeks 3 and 4 relative to other times of the year.  I'm quite impressed, actually, that forecasts issued in April capture nearly 50% of the variance at week 3 and almost 30% of the variance at week 4.  The skill is nowhere close to this for most of the rest of the year, although October is also fairly good at week 3.

So what are the current forecasts showing?  As of yesterday morning, when Thursday's extended-range ECMWF forecast came out, the multi-model ensemble is showing warmer than normal conditions persisting into the second week of April, and there is a suggestion of ongoing warmth over southern and western Alaska into weeks 3 and 4, but the signal is not particularly amplified.




A warm outlook is consistent with the CPC's forecast for April - see below - but given the very warm start to the month, it would be surprising to see anything else in the one-month forecast.


Wednesday, March 29, 2017

End in Sight

A major warm-up is under way at last across the eastern interior, as the flow has finally gone around to the south; Eagle is up to 46°F this evening, and Fairbanks is expected to break the freezing mark by Friday at the latest.  This will prevent a significant record from being broken: Fairbanks has never seen a March without the temperature rising above freezing at some point in the month.  The closest was in 1997, when the month's high temperature was only 34°F.

[Update March 31: the temperature has risen above freezing at Fairbanks airport as of 11:30am AKDT.  This is the 3rd latest appearance of the first above-freezing temperature in March.  In 2006 it occurred at 1pm on March 31, and in 2007 the thaw began at 2pm on March 31.]

The last time that above-freezing conditions were observed either at the surface or in the air column above Fairbanks was on February 26, so the period of having a continuously sub-freezing atmospheric column will end at about 30 days.  The longest such period this winter was 46 days, from mid-November through December 30, and this was considerably longer than any of the past 3 winters.  The chart below shows the length and ending date of each winter's longest stretch of sub-freezing conditions (both surface and aloft) back to 1948-49.  (Click to enlarge the chart.)


It's a funny coincidence that 4 of the last 5 winters have seen the longest sub-freezing spell come to an end very close to the end of the year (December 27-31).  Reader Eric first pointed out the similarity in timing of cold and warm spells between this winter and last (see here), and in fact a distinct warm-up can be identified near the turn of the year in each of the past 5 winters.  We'll call it the New Year's Thaw and I'll look at it more closely in a future post.