Thursday, May 8, 2014

Fairbanks vs Keystone Ridge Precipitation

Precipitation has been scarce for some time now in Fairbanks-land, but a couple of weeks ago the Keystone Ridge observer reported rain, snow, and snow pellet showers that brought a total of 0.15" of precipitation over two days.  Meanwhile the Fairbanks airport reported only a trace of precipitation; so this led me to wonder how the differences in precipitation between the two locations tend to change through the year.

First we can look at the mean monthly precipitation for the common period of record, 1997-2013; see the chart below.  Not surprisingly, Keystone Ridge picks up more precipitation in every month, with an annual excess of 51% (15.8" vs 10.5") compared to the airport.  However, the magnitude of the monthly differences is much larger in the warm and relatively wet season, both in absolute and percentage terms; almost three-quarters of the excess at Keystone Ridge occurs in May through September.  June and August are the months with the largest absolute difference in mean precipitation amount, but the largest percentage difference is in May, when Keystone Ridge typically receives more than twice as much precipitation as the airport.

The smoothed daily climatology of precipitation amounts is shown in the next chart, below.  An interesting feature here is the August peak in precipitation amounts for Keystone Ridge; the data from the airport do not show the same late-summer peak.  This difference is also suggested by the monthly chart above: precipitation peaks in July at the airport, but August is just as wet on the ridge (although with a pronounced drop-off by late month).

Thirdly, we can consider the mean frequency of measurable precipitation - see the chart below.  The frequency is also higher at Keystone Ridge throughout the year, but the relative differences in summer are not as great as for the total precipitation amounts.

Given these comparisons, we see that precipitation is both heavier and more frequent on Keystone Ridge throughout the year.  But which factor is more important for the annual precipitation totals?  Is the precipitation excess at Keystone Ridge caused mainly by more frequent precipitation events or by heavier precipitation when it falls - and how does this change through the year?  I'll look at this in a subsequent post.


  1. Elevation, seasonally lower temp at an increased altitude (+~1160 ft) w/o an inversion, dew point, and perhaps more direct exposure to weather traversing the Fairbanks area, likely contribute to the differences vs the lower hill-sheltered airport location. This is what happens when you live closer to the clouds.

    Plus, I hear the station owner runs a sprinkler on occasion to grow his garden or make snow.


  2. Plus...

    The Keystone location lies at the head of a NE-SW orientated valley that's subject to orographic lifting if the flow is from the SW...our favorite direction for incoming moisture. The hills to the west of Fairbanks lift the flow and protect us for some time before we get the same fallout.

    In summer, the hills to the north where Keystone is located often experience the initial effects of daily insolation, unstable air, and resultant thunderstorm activity and precip., well before it heads south and hits downtown.


    1. Gary, I agree - orographic lifting boosts precipitation in all seasons, and in the unstable season the higher terrain benefits from elevated heating that sets off convection. It would be interesting to categorize the historical data by flow direction and see how strong the dependency is.

    2. I'm stretching a qualitative hypothesis here.

      In summer thunderstorms form in an arc over the hills W>N>E from Fairbanks. Typically they tend to move to the south to share their weather. There are exceptions of course depending upon the prevailing winds at various levels.

      In fall and winter, we get the major effects of moisture from the SW. The Alaska Range often drys any incoming air mass to the south and southeast unless the fetch and duration are long.

      Weather/moisture that flows to the NE will often be lifted and congest near the Keystone Ridge constriction in that valley extending from Nenana.

      For that reason that area can be difficult to traverse in an aircraft, unlike flight further to the south of the hills along the Tanana River.