Since this June-July has been the wettest on record for Fairbanks (by far) I wanted to see if a correlation existed between the average daily PW value and the precipitation totals during this two month period. Using a June-July time period also has the advantage of eliminating snow events and it is also the season of peak thunderstorm activity.
When looking at it on an annual scale (June-July constituting a single year) there is no correlation between average PW over the 61-day period and the total rainfall over the same period (see Figure 1). For Fairbanks, this June-July is already the wettest on record but the average daily PW has been unremarkable.
Figure 1. Average June-July PW (orange) and total June-July precipitation (green) in Fairbanks between 1971 and 2014.
When we group days by the amount of precipitation that fell and calculate an average of those days, a strong correlation exists (see Figure 2). Days with no precipitation or only a Trace have substantially lower PW than those days with over 0.50" of precipitation. The three days this month with over 1.00" of rain in Fairbanks had a PW average of 1.16".
The apparent discrepancy between the two charts is primarily a result of the number of days with precipitation not taken into account in the first graph. If most of the rain falls in 3 or 4 days, the PW from those days is averaged with the PW from the other 57 or 58 days and thus blends in with the other values.
Figure 2. Average June-July PW value grouped by observed precipitation amounts in Fairbanks between 1971 and 2014.
Note: Here is a site ( http://www.crh.noaa.gov/unr/?n=pw ) that has PW climatology for all upper air stations in the U.S.