tue, 15-jan-2013, 08:48

Over the past couple days in Fairbanks, there has been a strong flow of warm, moist air from the Pacific which culminated in a record (for January 14th) 0.22 inches of precipitation, most of which fell as rain. Nasty. Similar events happened in 2011 and in November 2010, which everyone will remember for the inch or more of ice that glazed the roads for the rest of the winter that year.

The question people always ask after a series of events like this is whether this is a new weather pattern (let’s hope not!) and whether it may be the result of global climate change (which I probably can’t answer).

To look at this, I examined the historical record for Fairbanks, searching for dates that met the following criteria:

  • At least six inches of snow on the ground
  • During the winter months (October through February)
  • Daily high temperature above freezing
  • Precipitation falling as rain

The last criteria isn’t part of the historical record, but we can guess the amount of rain by comparing the amount of snow (measured each day on a snow board that is cleared after measurement) with the amount of liquid precipitation gathered in a tube and melted, if necessary. In my experience, the ratio of snow to liquid precipitation is almost always less than 10 to 1 (meaning that 10 inches of snow melts down to less than an inch of liquid), so I’m looking for dates where the precipitation amount is greater than 10 times the snowfall for that date. I’m also estimating the amount of rain by subtracting (snow × 10) from the precipitation total.

Here’s the query:

SELECT dte, tmin_f, tmax_f, prcp_in, snow_in, rain_in,
       row_number() OVER (ORDER BY rain_in desc) AS rank
FROM (
    SELECT to_char(dte, 'YYYY-MM') AS dte, round(avg(tmin_f), 1) AS tmin_f,
           round(avg(tmax_f), 1) AS tmax_f, sum(prcp_in) AS prcp_in,
           sum(snow_in) AS snow_in, sum(rain_in) AS rain_in
    FROM (
        SELECT dte, tmin_f, tmax_f, prcp_in, snow_in, snwd_in,
               round(prcp_in - (snow_in / 10.0), 2) AS rain_in
        FROM get_ghcnd('Fairbanks Intl Ap')
        WHERE extract(month from dte) IN (10, 11, 12, 1, 2)
            AND snwd_in > 6
            AND tmax_f > 32
            AND prcp_in * 10 > snow_in
        ORDER BY dte
    ) AS foo
    GROUP BY to_char(dte, 'YYYY-MM')
) AS bar
ORDER BY dte;

And the results, ordered by the year and month of the event. None of the winter rain events stretched across a month boundary, so it was convenient to aggregate them this way (although 1937 is problematic as I mention below).

Winter rains, Fairbanks Airport station
Date Min Temp (°F) Max Temp (°F) Precip (in) Snow (in) “Rain” (in) Rank
1920-02 27.7 38.4 0.26 0.9 0.17 11
1931-01 12.0 33.1 0.13 0.0 0.13 12
1932-02 7.0 33.1 0.77 7.1 0.06 16
1933-11 25.0 41.0 0.11 0.0 0.11 14
1935-11 30.4 37.2 1.51 3.2 1.19 2
1936-11 30.0 37.0 0.44 0.0 0.44 5
1937-01 24.3 36.2 2.83 16.1 1.22 1
1941-02 28.0 42.1 0.02 0.0 0.02 23
1941-11 -2.9 33.1 0.20 0.9 0.11 15
1943-02 30.5 41.0 0.12 0.0 0.12 13
1944-02 21.5 36.5 0.65 2.9 0.36 7
1948-01 7.0 33.1 0.01 0.0 0.01 26
1957-01 30.9 35.1 0.03 0.0 0.03 22
1961-01 17.1 33.1 0.04 0.0 0.04 20
1963-01 22.5 35.1 0.56 0.7 0.49 4
1967-12 20.0 33.1 0.43 0.5 0.38 6
1970-02 10.9 43.0 0.05 0.0 0.05 17
1970-10 28.0 44.1 0.04 0.0 0.04 19
1970-12 5.0 36.0 0.43 2.4 0.19 9
1986-02 10.9 37.9 0.03 0.0 0.03 21
1989-02 24.1 37.0 0.40 3.8 0.02 24
2003-02 27.0 35.0 0.29 0.0 0.29 8
2006-02 17.1 42.1 0.06 0.1 0.05 18
2010-11 26.1 34.3 0.95 0.1 0.94 3
2011-12 26.1 46.9 0.03 0.2 0.01 25
2013-01 24.0 37.0 0.22 0.4 0.18 10

The 2010 event was had the third highest rainfall in the historical record; yesterday’s rain was the tenth highest. The January 1937 event is actually two events, one on the 10th and 11th and one on the 20th and 21st. If we split them up into two events, the 2010 rainfall amount is the second largest amount and the two January 1937 rainfalls come in third and tied for fifth, with November 1935 holding the record.

Grouping the events into decades, we get the following:

Winter rains by decade
Decade Rain events
1920s 1
1930s 6
1940s 5
1950s 1
1960s 3
1970s 3
1980s 2
1990s 0
2000s 2
2010s 3

Here’s a visualization of the same data:

I don’t think there’s evidence that what we’ve seen in the last few years is exceptional in the historical record, but it does seem like the frequency of winter rainfall does come in cycles, with a peak in the 30s and 40s, and something of a decline in the 80s and 90s. That we’ve already had three events in this decade, in just over two years, seems like a bad sign to me. I wonder if there are larger scale climatological phenomena that could help to explain the pattern shown here?

tags: weather  rain  winter  SQL 
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