Last night we got a quarter of an inch of rain at our house, making roads “impassable” according to the Fairbanks Police Department, and turning the dog yard, deck, and driveway into an icy mess. There are videos floating around Facebook showing Fairbanks residents playing hockey in the street in front of their houses, and a reported seven vehicles off the road on Ballaine Hill.
Here’s a video of a group of Goldstream Valley musicians ice skating on Golstream Road: http://youtu.be/_afC7UF0NXk
Let’s check out the weather database and take a look at how often Fairbanks experiences this type of event, and when they usually happen. I’m going to skip the parts of the code showing how we get pivoted daily data from the database, but they’re in this post.
Starting with pivoted data we want to look for dates from November through March with more than a tenth of an inch of precipitation, snowfall less than two tenths of an inch and a daily high temperature above 20°F. Then we group by the winter year and month, and aggregate the rain events into a single event. These occurrences are rare enough that this aggregation shoudln’t combine events from different parts of the month.
Here’s the R code:
winter_rain <- fai_pivot %>% mutate(winter_year=year(dte - days(92)), wdoy=yday(dte + days(61)), month=month(dte), SNOW=ifelse(is.na(SNOW), 0, SNOW), TMAX=TMAX*9/5+32, TAVG=TAVG*9/5+32, TMIN=TMIN*9/5+32, PRCP=PRCP/25.4, SNOW=SNOW/25.4) %>% filter(station_name == 'FAIRBANKS INTL AP', winter_year < 2014, month %in% c(11, 12, 1, 2, 3), TMAX > 20, PRCP > 0.1, SNOW < 0.2) %>% group_by(winter_year, month) %>% summarize(date=min(dte), tmax=mean(TMAX), prcp=sum(PRCP), days=n()) %>% ungroup() %>% mutate(month=month(date)) %>% select(date, month, tmax, prcp, days) %>% arrange(date)
And the results:
|Date||Month||Max temp (°F)||Rain (inches)||Days|
This year’s event doesn’t compare to 2010 when almost and inch of rain fell over the course of three days in November, but it does look like it comes at an unusual part of the year.
Here’s the counts and frequency of winter rainfall events by month:
by_month <- winter_rain %>% group_by(month) %>% summarize(n=n()) %>% mutate(freq=n/sum(n)*100)
There haven’t been any rain events in December, which is a little surprising, but next to that, February rains are the least common.
I looked at this two years ago (Winter freezing rain) using slightly different criteria. At the bottom of that post I looked at the frequency of rain events over time and concluded that they seem to come in cycles, but that the three events in this decade was a bad sign. Now we can add another rain event to the total for the 2010s.
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).
|Date||Min Temp (°F)||Max Temp (°F)||Precip (in)||Snow (in)||“Rain” (in)||Rank|
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:
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?