Autumn Forecast Verification

Regular readers may recall that way back in August we discussed some long-range analog forecast indicators for September through November, based (only) on the warm, dry summer that occurred in interior Alaska.  It's always fun to go back and check long-range forecasts to see if there was any semblance to the actual outcome; so here we go.

Listed below are the main (though admittedly tentative) conclusions of the analog investigation, along with the corresponding outcome:

• Dry September in Fairbanks: incorrect  1.74" vs 1.08" 1981-2010 median precipitation.  Three-quarters of an inch of rain fell on one day, September 11.
• First 25 °F frost in Fairbanks earlier than normal: correct  Sep 20 vs Sep 26 1930-2012 median
• First 0.5" or greater snowfall in Fairbanks earlier than normal: correct  Sep 18 vs Oct 6 normal
• Wet September in Barrow: correct  1.29" vs 0.59" normal
• Warm October in Barrow, Fairbanks, and Anchorage: all three correct, much warmer than normal
• Warm November in Anchorage: correct but only barely, 21.9 °F vs 21.3 °F normal

The discussion also showed maps of the 500 mb height and sea-level pressure patterns that have historically been observed in September following warm, dry summers in Fairbanks.  Below is the observed September 500 mb height anomaly: this agreed very nicely with the forecast map, with one of the main features being low heights over the Chukchi Sea.

Finally, here's the observed sea-level pressure pattern from September; this doesn't show the same degree of correspondence with the analog forecast map, but nevertheless below-normal pressure occurred as expected over eastern Alaska and western Canada.

All in all, the forecast exercise was fairly successful and illustrates that simple statistical techniques can - if you're lucky - yield limited long-range predictability.  In the next week or so I'll put together another analog forecast based on the very wet, warm year that Barrow has observed.