Saturday, September 30, 2017

Fairbanks First Autumn Snowfall: the Long View


Rick T. here with a quick follow-up on Richard's post on Friday with the first snowfall of the season at Fairbanks Airport. Below is a plot of the date of first snow (any amount) each autumn since 1904, along with the regression plot for the 10ᵗʰ (very early), median and 90ᵗʰ (very late) percentiles. Note that this list aims to give the first snow date exclusive of convective hail. Unfortunately, snow and hail have been lumped together in the US climate record since early 1950s, so a "snow only" time series for Fairbanks requires a little bit of QC.

Prior to 1930, when the Fairbanks observations were done by cooperative observers, it is quite likely that snowfalls like the Friday morning occurrence (mixed rain and snow for a few hours in the middle of the night, no accumulation) would have not been reported, so some of the pre-1930 dates are quite likely later than would now be recorded. Even in the post-1929 Weather Bureau era, around-the-clock observations did not start until World War 2, though there were observations made every six hours, so there was a "middle of the night" presence.  

The regression analysis shows practically no change in the median and very late dates over the past 114 years. I've plotted the regression slopes, but neither the 90th (top red line) or median (green line) pass statistical tests that would allow us to say with high confidence that the slope (i.e. trend) is really different than zero. In light of the probable "late bias" in the pre-1930 data (at least in aggregate) the 90th percentile seem likely to me to have no trend, while the same bias would serve to "push down" the left side of the median date, i.e. increase the century scale slope a little (though this is quantile regression, so it's less sensitive to outliers than ordinary linear regression).

The big change is obviously in very early "first snows" Prior to 1980, a first snow before Labor Day, while not common, occurred 11 times in 75 years. However, in the past 30 years that has occurred just once. The 10ᵗʰ percentile (bottom red line) can be thought of as the date for which there is a ten percent chance in a given year that the first snowfall would occur on or before that date. So prior to WW2, there was about a ten percent chance of first snow September 1ˢᵗ or earlier. Nowadays, we have to get to September 10ᵗʰ or even a bit later to get to the same threshold.

Friday, September 29, 2017

First Flakes

The first snow of the season was reported at Fairbanks airport early this morning, but so far it's only a trace amount and mixed with rain.  According to long-term records, more than 80% of years bring at least a trace of snow to Fairbanks in September, and in recent years it's been more like 90%; the only years in the past two decades without any snow in September were 2007 and 2010.  Of course two years ago 11 inches fell on this date:

Up in the hills, rain turned to snow yesterday afternoon as colder air moved in aloft.  Here are a couple of webcam shots from 3pm and 7pm yesterday at 2200' on the Steese Highway.

Temperatures in Fairbanks have now entered the 6-week period of most rapid decline, with peak cooling of around 6°F per week occurring near October 20.  It's interesting to note, however, that daily maximum temperatures cool more quickly at first, and daily minimum temperatures then catch up; the difference in cooling rates is most pronounced from mid-September to early October.

This nuance in the seasonal transition is related to snow cover, because having snow on the ground makes a bigger difference for minimum temperatures than maximum temperatures; with snow on the ground, the temperature can drop very rapidly at night with clear skies and light winds.  Most years do not have snow on the ground in Fairbanks at the beginning of October, but snow cover becomes 30% likely by October 9 and 50% likely by October 14.  So the establishment of snow cover plays an important role in reinforcing the overall downward temperature trend after about the first week of October.

Saturday, September 23, 2017

Mild North Winds in Barrow

The equinox is now behind us, but only in the past few days have temperatures in Barrow (or Utqiaġvik) begun to make regular excursions below freezing.  Of course this isn't too surprising with a vast expanse of open water to the north; the Arctic sea ice edge to the north of Barrow is about 500 miles away.

We've looked at this type of thing before, but curiosity led me to re-examine the average temperature around the year in Barrow when the wind is blowing from a northerly component; see the chart below.  The purple line at the top shows the warming that occurred between the 1951-1975 and 2001-2016 periods.

It's interesting to observe that for 1951-1975, winds with a northerly component tended to bring sub-freezing temperatures beginning in early September, but that date has been pushed back to the equinox in recent years.  The warming has been more pronounced in October, of course, as the heating effect of open rather than ice-covered ocean becomes greater the colder it gets.  The difference between 2001-2016 and 1951-1975 peaks at about 12°F in late October for winds with a northerly component.

Another way of looking at the change is to consider the latest date in the year when a temperature above freezing was observed in combination with a northerly-component wind.  Prior to 1995, early October was about the latest that this ever happened, but since then it has been rare to see the last such date before the end of September; and there have been several very late outliers, including January 10, 1997.

Below is a pair of contrasting sea ice extent maps to illustrate the change: yesterday versus the same date in 1983, when the Beaufort Sea coast was already locked in with sea ice.  It's no surprise at all that NOAA's Climate Prediction Center is predicting a 75% chance that Barrow's October mean temperature will be significantly higher than the 1981-2010 normal (i.e. in the upper tercile); see the third map below.  This may even be conservative, as only 2 of the last 11 Octobers have failed to be at least this warm.

Wednesday, September 20, 2017

Autumn Cooling

There's a slight chill in the air today across interior Alaska, with afternoon temperatures around Fairbanks only in the mid-40s at valley-level thanks to clouds and some light rain.  Temperatures are in the mid-upper 30s in the higher hills; and a glance at the calendar reveals the reason.  Here are some of the observations as of about 6pm (click to enlarge).

Yesterday was the first day with a high temperature below 50°F in Fairbanks, and this is about a week late compared to normal.  Fairbanks usually sees its first sub-50°F day more than a week earlier than Anchorage, which illustrates the contrast between the rapid cooling of the interior and the slower cooling that occurs closer to the waters of the North Pacific.  Anchorage has never seen a sub-50°F day in August during the modern historical record, but it's not too uncommon in Fairbanks and happened just two years ago.

Here are a couple of webcam views of Fairbanks; autumn colors look to be a little past peak.

At this time of year it's always interesting to be reminded of the rapid drop-off in solar radiation at northern latitudes during autumn.  Thanks to 15 years of high-quality data from the Fairbanks CRN site (actually 11 miles northeast of town), we know what the normal incoming solar radiation curve looks like - see below.  I've added the normal cloud cover from Fairbanks airport to give an idea of relative changes in cloudiness through the year, although cloudiness is presumably a bit greater at the CRN site.

Notice how quickly the solar radiation diminishes in mid to late September; the average amount of incoming solar energy drops by 50% in the last 3 weeks of the month.  Of course, at the beginning of September it's already down by about 50% from the peak in June.

Here's the equivalent chart for Barrow, also from 15 years of CRN data; there's little available solar energy by this date in Barrow, and the very high cloud cover in late summer and autumn only exacerbates the rate of decline.

Just for fun, here's the equivalent solar data from the most southerly CRN site in the continental U.S., at Everglades City in Florida, along with the normal cloud cover from nearby Fort Myers.  By September 20 the normal solar radiation has decreased by only about 20% from the solstice, although the annual peak in south Florida occurs in May prior to the wet season.  The slow decline of the solar input, along with the maritime environment, explains why September is basically still high summer in Florida.

But the most interesting aspect of the comparison may be that Barrow receives more solar energy on average in June than south Florida - even though Barrow is very much more cloudy!  This is the result of 24-hour daylight; the noon sun in Barrow is nothing like the noon sun in Florida, but the hours of sunshine really add up in the far north.

Monday, September 18, 2017

La Niña Emerging

While the attention of many meteorologists (this one included) has been drawn to the remarkable storminess in the tropical Atlantic Ocean lately, significant events have been unfolding at the same time in the tropical Pacific.  Here's a sequence of maps showing the departure from normal of the sea surface temperature (SST) at 4-week intervals; notice the expanding area of below-normal SSTs along the equator in the central and eastern Pacific.

In tandem with the clear trend towards more La Niña-like conditions, the leading seasonal computer models have shifted quite dramatically towards a La Niña outlook for winter, and in response the most recent IRI/CPC forecast showed a sudden change to a much higher probability of La Niña.  Compare the two forecasts below, issued only a few weeks apart.  This is as dramatic a shift as I can recall in what is usually a slowly-evolving assessment of long-range forecast possibilities.

So what does the prospect of La Niña mean for Alaska?  One could be forgiven for having a hazy recollection of the last La Niña episode - the last time a modest La Niña occurred in winter was 2011-2012, and the last strong episode occurred the year before that.

The key feature of a La Niña winter, from the Alaskan viewpoint, is that the normal trough of low pressure over the Bering Sea and Aleutians tends to be relatively weak, and episodes of high pressure in this area are more common than normal.  A ridge over the Bering Sea is a cold set-up for most of the state, so it follows that most of Alaska is usually colder than normal during La Niña, although the signal is strongest in the southern part of the state.

The propensity for high pressure to Alaska's west and southwest reduces the frequency of storm systems moving into southern Alaska, so the amount of snow and rain is typically lower than normal in the south.  However, the west and north of mainland Alaska tend to see above-normal snowfall.

Here are maps of the November-March pressure, temperature, and precipitation patterns that were associated with the 10 strongest La Niña episodes since 1950, based on the Multivariate ENSO Index of Klaus Wolter.

Another aspect of the La Niña influence is that the flow pattern and associated temperatures tend to be more variable than normal over Alaska, so while the mean is below normal, fluctuations from week to week tend to be large.  This means that very cold conditions (much below normal) are considerably more likely than normal, but the chance of occasional very warm conditions is not much reduced except in southern and southeast Alaska.  For more reading on this, search "ENSO variance" on this blog.

Finally, the charts below provide another viewpoint on Fairbanks winter temperatures as they relate to the ENSO phenomenon. For this analysis I've calculated the November-March temperature anomaly relative to two different climatology periods in an attempt to remove the 4.5°F systematic difference between the early (1950-1975) and later (1976-2016) periods.  The vertical colored lines show the ENSO tercile boundaries.

The cold signal associated with La Niña is evident in the lower left, but it's clear that marginal La Niña winters are not always colder than normal, and the overall correlation between the ENSO index and Nov-Mar temperatures is not particularly good.  This partly reflects the fact that the PDO phase is not always aligned with the ENSO phase, and of course the PDO is more closely connected to Alaska winter temperatures than ENSO - see the corresponding chart below for the PDO correlation.

If we re-do the ENSO chart after excluding winters with an out-of-phase PDO, then the correlation improves quite a bit as we would expect, and especially for El Niño winters.

So in conclusion, if La Niña continues to develop into a strong episode this winter, then the usual La Niña patterns will become quite likely in Alaska; but if we see a less robust La Niña, then the outcome is much less certain.  Another way of saying this is that the PDO is more important; but unfortunately it is much more difficult to predict the PDO phase.  The PDO has been bouncing around near neutral recently and shows no indication of becoming significantly negative - and indeed I would say a significantly negative phase is probably unlikely because of the lingering subsurface effects of the strongly positive PDO phase that we've experienced in recent years.

Friday, September 8, 2017

Cooler Arctic Summer

Arctic sea ice extent is now close to its seasonal minimum, but in the past few weeks there has been less ice retreat than in some recent years.  Consequently a new record minimum extent is not likely to occur this year, although the National Snow and Ice Data Center notes that the ice edge in the Beaufort Sea is currently farther north than at any other time in the satellite era.  Here's the latest map of ice extent (defined as areas with greater than 15% ice concentration).

The summer circulation pattern was quite unsettled over the Arctic Ocean, with persistently below-normal sea-level pressure, especially to the north of Siberia.  The Arctic Oscillation was generally positive, corresponding to lower than normal pressure in the high latitudes.  Here are monthly maps of sea-level pressure anomaly from June through August.

Based on the 19 Arctic coastal observing sites that I've used before on this blog (e.g. here), temperatures were quite close to the 1981-2010 normal this summer on average around the Arctic basin, and this is a marked contrast from the persistent and extreme warmth of 2016.  The first chart below shows this year's June-August mean temperature in the context of recent decades, and the second chart shows the rather sudden cooling (relative to normal) that has occurred since late winter.

Given that Arctic sea ice extent is still far below normal, it's very likely that unusual warmth will return around the Arctic basin this autumn as the wide expanse of open water provides a direct heating influence; this post from last year shows the extraordinary warming trend for the month of October.  It will be interesting to see how the winter turns out, but in light of recent years it would be surprising to see anything other than significantly above-normal temperatures again for the seasonal mean over the Arctic basin.

Saturday, September 2, 2017

Brooks Range Chill in Context

Following up briefly on the last post, temperature data from the Atigun Pass SNOTEL provide a bit more context for the rather chilly conditions observed at this high-elevation site in recent days (down to 17°F on Wednesday).  Historical temperature data go back to 1999, although prior to 2006 there is only one data point provided per day - the temperature at midnight.

The chart below shows the accumulation of freezing degree days through August and September for each year since 1999 (calculated with only the midnight temperature throughout, for consistency).  The black line, indicating this year's pace of freeze-up, is a little ahead of the 18-year normal, but only by a few days, and the early chill this year has been much less severe than in 2000 and 2002.

Looking just at August total FDDs, the data from the early years confirm what I suspected - that the apparently cooler August conditions of the past few years are not unusual relative to a longer history.  From this perspective, the relative absence of chill in a number of years from 2004-2012 seems more unusual.

A longer and higher-quality history of temperature data is available from Toolik Lake Research Station, just 40 miles farther north up the haul road - and while this site is not quite in the mountains, the elevation at Toolik is still a respectable 2500'.  The chart below shows August FDDs at Toolik, confirming that recent years have brought freezes closer to the long-term normal, although not as severe as in some of the earlier years.

What could explain the early arrival of autumn chill in the Brooks Range in recent years?  The 500mb height chart from Wednesday provides a clue: notice the strong, cold upper-level trough over the North Slope.  More analysis would be needed to be sure, but it's likely that recent years have seen this kind of feature relatively more often at about the same time on the calendar.

Finally, lest we be tempted to infer that Arctic-wide conditions are cooler now than in recent years, the latest Alaska-centric sea ice analysis shows open water for more than 300 miles north of Barrow.  Arctic sea ice extent won't set a new record low this year, but it is still far below the long-term normal, and estimated ice volume is tracking near record lows.  More on that another day.