Monday, February 23, 2015

Freezing Rain Trends

Fairbanks and many other interior communities were afflicted with freezing rain over the weekend in what seems to have become an annual winter ritual of late.  Last winter I looked at the history of significant (0.1" or more) winter rain and freezing rain events in Fairbanks; we can now add another event to the list, which means that four of the last five winters have seen a significant event.  This is quite a remarkable sequence, but as we noted before there have also been periods in the past when winter rain occurred with uncommon frequency.

Another way of looking at the historical trends is to extract reports of rain or freezing rain from the hourly observations.  This method includes all winter rain events, not just the most significant ones, but it does not account for the quantity of rain that fell; some of the events will have been just a trace.  The series of charts below shows the results when we do this for six locations with a reliable history of hourly observations since 1950.  Note that I looked at just the deep winter (Dec-Feb) period, and I've used the same vertical scale for all the charts to highlight the differences in the typical frequency of winter rain.







It's interesting to observe that in Fairbanks, none of the recent winters comes close to 1962-63 in terms of the number of days with rain.  I confirmed this by looking at the Local Climatological Data publication; remarkably, Fairbanks reported "glaze" in January 1963 on 10 separate days (some of these were freezing drizzle events).  However, the persistent recurrence of rain in recent winters appears to be unprecedented since 1950, with this winter (2014-15) being the 7th in a row with at least one report of rain.  The results also show that Nome and (especially) Kotzebue have seen a higher frequency of winter rain in the past decade than in earlier years.

Another interesting observation is that last winter (2013-14) was the first one with at least 2 rain reports (days) at all 6 locations.  Only 4 previous winters had at least one report at each location.  We can also note that only 5 winters had zero reports of rain at all of the locations - most recently in 2003-4.  In other words, nearly every year brings rain in deep winter to at least one of these locations.

The chart below shows an attempt to combine the 6 station series into an overall mean.  For each winter, I took the percent of normal at each station and then calculated the 6-station mean of these numbers.  This is better than just averaging the 6 series together, because that result would be dominated by the higher values at Nome and McGrath.



The mean chart illustrates the highly anomalous nature of last winter and also shows that the 10-year trailing mean is slightly higher than at any earlier point since 1950, although the winter of 1962-63 produced a similar trailing mean.  The 10-year trailing median provides an alternative measure of the long-term variations, but this is no higher than the value in about 1980.

9 comments:

  1. We should keep in mind that if the temperatures were a few degrees cooler we would have snow. So is the number of rain days a good proxy for number of "extreme" warm days? And how many close rains are in the record (temperatures just below freezing)?

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    1. Eric, I regard rain days as a subset of very warm days, because extreme warmth is also associated with chinook conditions, which are dry. It takes a particular setup to get extreme warmth along with moisture (i.e. ascent, not subsidence) into the interior.

      Interesting question regarding temperatures during rain. Over the whole period (1950-2014) in Fairbanks, 39% of Dec-Feb rain observations are accompanied by sub-freezing temperatures at the surface. However, this changed from 54% in 1950-1981 to 30% in 1982-2014. So plain rain has become more common at the expense of "freezing rain" (of course the plain rain also freezes on the ground). This would be interesting to examine in more detail.

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  2. Fairbanks schools have closed a number of times in the last few years due to ice. I recall seeing a FDNM story about 4 or 5 years ago that stated (at the time) that it was the first school closure in decades. I might be mis-remembering that though.

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    1. Brian, you're spot on as usual:

      http://www.newsminer.com/news/local_news/fairbanks-schools-have-first-full-day-closure-in-more-than/article_e56b42d2-8773-53a5-ac40-1b4fd180b1b9.html

      One might speculate about changing societal attitudes to risk and inclement weather, but nevertheless your observation is significant.

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    2. While there is probably more hesitancy with ice on the road (lawsuits and all) and the weather has had more ice storms, there is also a lot more people living in the hills and outskirts too. The roads are generally fine within Fairbanks proper. It's when you move out into the hills and less-kept roads is where school buses have difficulty. On top of this the hills are warmer and so can be even more slick. Some bus drivers have told me stories about how ridiculous the roads were that they had to drive on. They are extremely underappreciated.

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  3. I wonder how connected our rain events have been in the past with harsh weather in the mid-eastern US?

    It's easy to visualize the current northward looping of warm moist air through Alaska accompanied by SE flow of cold and snow through Canada and into the CONUS. Is that a recent or historic trend?

    Gary

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    1. Here's a recent paper that presents arguments for the atypical weather patterns (the Arctic Amplification Theory) recently experienced :

      http://iopscience.iop.org/1748-9326/10/1/014005/article

      Gary

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    2. Thanks, Gary. I happened to be looking at the same paper earlier today - no doubt we saw it linked in the same place. I want to go through it more carefully, but it's curious to note that their results show the Pacific sector with a DECREASE in "high-amplitude days" during winter in the recent period. Not what I expected to see. I think their winter results are mostly related to changes in the NAO.

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    3. I made it through one read but will return tomorrow. I heard the lead author recently on NPR and had read previous documents via the Internet. The link in Alaska Dispatch was fortuitous and made my morning considering the question above I posed earlier.

      Regardless of the eventual outcome on climate, I feel it's important to become sensitized to analysis and provoking ideas similar to this analysis. Whether the author's "waviness" theory continues to result in a maintenance or even increase in extreme events of weather will be fascinating to observe and reanalyze.

      We in Alaska are quite possibly one of the canaries in the mine.

      Gary

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