Sunday, September 8, 2013

Precipitation Frequency In Fairbanks

The past couple of weeks in Fairbanks have seemed damp, with measurable rain on nine of the past 14 days. Rainfall has not been excessive, with about three-quarters of an inch of rain during that time (thru 10am AKDT Sunday). How does this stack up against the climatological frequency of rainfall?

To address this, I've calculated the frequency of days with measurable precipitation (≥0.01") by week (starting Jan 01, not smoothed) for the past 84 years:

By this definition, the climatologically wettest time of year is from mid-July through August, with a sharp drop-off in the frequency of measurable precipitation the first week in September. This is followed by a weak secondary maximum in late October. I'm speculating here, but I suspect that the sharp decline of damp days in early September is due to a decline in the climatological frequency of a big North Pacific ridge centered south of south of Alaska (which results in southwest flow aloft that brings moisture around the Alaska Range).