Monday, November 25, 2013

Winter Rain Climatology

In view of recent freezing rain in Fairbanks, and also in many other Alaska locations, it's interesting to look at the history of winter rain events in Fairbanks.  Of course, the phenomenon is very unusual because of the difficulty of getting warm enough air into the Alaskan interior in winter, but when it does occur it's very problematic.  As Rick wrote elsewhere, "Freezing Rain in Fairbanks is a serious pain. Because the ground and road surfaces are well below freezing from October through March, any rain, even if it falls when air temperatures are above freezing, can cause severe travel problems. Significant amounts of rain will usually produce significant ice accretion on roads that persists, at least on some lower traffic roads, until late March or early April."

The chart below shows the annual number of significant winter rain or mixed rain/snow events (November through March) since the winter of 1917-18. The cluster of events in recent years is evident (Nov 2010, Jan 2013, Nov 2013) and has resulted in an up-tick in the 15-year trailing average frequency. However, there have also been periods in the past when winter rain occurred with higher than normal frequency, notably in the 1960's, in the 1920's, and in the remarkable winters of 1935-36 and 1936-37, when rain occurred five times. If anything, it appears that winter rain has become somewhat less common over the past century, although I am not sure if the trend is statistically significant.

Winter rain in Fairbanks is most common in November and least common in December and March, as the chart below shows. It is curious that significant winter rain has never been observed between November 25 and December 23 inclusive; it's not clear if this might possibly be a statistical quirk, but the sudden cessation of rain events in late November does look as if it represents a true feature of the Fairbanks climate. From Christmas on, the distribution of rain events seems fairly random.


  1. Here's a non statistical observation. I tend to associate rain events in Fairbanks frequently with the "Pineapple Express", the unique jet stream flow that transports warm moist air south to north from the mid Pacific. Hawaii calling via jet stream looping?

    For we scent starved Interior Alaskan residents, the influx often brings the odors I associate with warmer oceanic environs. Despite the associated hazards it's nice to smell the tropics in mid winter.

    It's not always the case that the source is equatorial. It may be a residual from some Pacific storm or Bering Sea storm to the SW. I suppose the orientation of the flow determines whether we receive the wet, or it gets lifted and dried by the Alaska Range to the south.


    1. Gary, I agree that the flow orientation is important; it takes a special combination of inputs to get rain in winter, and hence the low overall frequency, 27 events in 97 years.

  2. Richard, I agree that it is difficult to identify a trend in the charts you generated but how do the ,agnitude of the events look? Are they getting more severe over time?

    1. Brian,

      That's a difficult question, although perhaps quite important. To address this properly, I suppose one would need to look at the number of hours of rainfall; the total rain amount might not correlate as well to perceived impacts. Without scrutinizing each event carefully, here's a quick list of events that seem to have had rain for more than 12 hours:

      Nov 2010
      Nov 2003
      Feb 2003
      Nov 1979
      Jan 1963
      Jan 1937
      Nov 1935
      Mar 1921

      The most significant events were Nov 1935, Jan 1937, and Nov 2010, each of which had many hours of rain with more than 0.8" in total.