Wednesday, November 6, 2013

October anomalies

[Updated to include Anchorage temperature at the forecast office, which is the official climate observing site - thanks Rick]

Below are maps showing the October temperature and precipitation anomalies at some primary observing stations across the state.  The area of each circle indicates how unusual the month's conditions were, as measured by the anomaly in standard deviations for temperature and the percentile rank for precipitation (based on the 1981-2010 normals).

The month's average temperature was more than 1 SD above normal at all of the locations shown except Annette Island; the warmest place relative to normal was Cold Bay, with a remarkable monthly average of 3.5 SD above normal.  The return period for an anomaly this large (either warm or cold) in any calendar month is about 185 years (i.e. if the climate were stationary and if the true distribution of monthly temperatures were Gaussian with mean and variance described by the 1981-2010 climatology, then a calendar month this unusual might be expected to occur once every 185 years on average at a single location).

Needless to say, the month was the warmest October on record at many locations, including (ranked from most significant to least):
  • Cold Bay, 46.0 F vs 44.0 F in 2002
  • Gulkana, 37.8 F vs 35.4 F in 2002
  • McGrath, 38.7 F vs 35.0 F in 2006
  • Anchorage, 43.9 F (airport), 43.0 F (forecast office), vs 42.1 F in 1936
  • Big Delta, 37.2 F vs 35.1 F in 1969
  • Kotzebue, 34.5 F vs 32.6 F in 2006
  • Bettles, 31.3 F vs 29.4 F in 2003

Precipitation was well above normal in most locations and was the highest on record for October in McGrath.  Barrow experienced its 8th consecutive month of above-normal precipitation.  However, below-normal precipitation was observed at Fairbanks and Big Delta, where downsloping chinook winds kept things dry, and Annette Island, which was close to the apex of the big upper-level ridge.  The map below shows the monthly departure from normal of the 500 mb heights:


  1. In the precipitation graph, Glenallen is missing it's colored dot. Must be HAARP :?

    How often do ridges like those in the last graph form? I guess about every 20 years - some more powerful than others. It was still a remarkable October.

    1. Eric,

      Thanks for the comment... the reason for the missing dot (at Gulkana) is that the precipitation (0.71") was very close to the 1981-2010 median (0.77"), so it is close to the 50th percentile and there is nothing to show. The circles get bigger according to the departure from the median. I hope this isn't too confusing.

      I am not sure about the return period of a strong ridge over the northeast Pacific; my sense it would be less unusual in mid-winter, but in October the climatological mean trough axis is along the west coast of Alaska and therefore the trough is normally the dominant influence.

  2. It's not confusing. Your explaination makes sense. Since everyone else had a colored dot I assumed that Gulkana was missing one.

    If we normally don't have ridges over the NE Pacific, what caused such a powerful one to form?

    1. Eric,

      That is a great question and regrettably I don't have an answer. Separating cause and effect from random variability and statistical noise is a tremendous challenge when trying to analyze these things.