Fairbanks Precipitation Classes

Readers of the this blog do not need to be told that this has been an extraordinarily wet summer. This is already the second wettest June through August on record in Fairbanks. Are we nearing the wettest summer on record because is has rained often or because when it does rain, it rains a lot. To answer this question, let us look at all rainy days in the Fairbanks historical record. Figure 1 shows a series of lines representing the number of days with A) any precipitation, B) measurable precipitation, C) >= 0.10" precipitation, and D) >= 0.50" precipitation for all years during the June 1 to Aug 5 time period.

On average, there are 24 days during this 66-day window with measurable precipitation. In 2014, there have been 29 days between June 1 and August 5 with measurable precipitation (range: 10 to 53). If you include Trace amounts, the long-term average is 38 days and the 2014 value is 47 (range: 14 to 58). In both instances, 2014 is above the long-term average but quite far from the largest values. However, 2014 does lead in several important categories – days with >= 0.25", >=0.33", >=0.50", and >=1.00".

Figure 1. Number of days with A) any precipitation, B) measurable precipitation, C) over 0.10" precipitation, and D) over 0.50" precipitation for all years during the June 1 to Aug 5 time period.

This leads to the conclusion that when it rains, it pours. Figure 2 shows the June 1 to August 5 total precipitation (green line) and the per rainfall event amount (purple line). As you can see, 2014 has reported approximately 0.37" of rain per day with measurable rainfall. Only one other year (1962) even comes close to this year's per rainy day average. That year had 10 fewer days with measurable precipitation through August 5.

Figure 2. Total precipitation and average precipitation per rainy day for all years during the June 1 to Aug 5 time period.

Nine times since June 1st the Fairbanks daily precipitation has exceeded the 95th percentile for the date. In theory, during any 66-day period, the 95th percentile should be exceeded 3.3 times. Figure 3 shows the Fairbanks precipitation exceedance probabilities with 2014 precipitation events overlaid.

Figure 3. Daily precipitation exceedance probabilities with 2014 precipitation events overlaid. Note: The probability lines only take into account days with 0.01" or greater.