Last May I made, with colleagues, some estimates for September 2011's extent. As usual, I also included some other forecasts for comparison. The NSIDC figure for August 29th is:
While not always true, it's generally the case that the September average extent is lower than the end of August level. Our end (well, 29th) of August value is about 4.8 million km^2. So we can be fairly confident that the September figure will come in not much higher than 4.8 (5.0 would be high, my intuition says), and probably some tenths below 4.8. But probably not many -- look how little the curve changes across late August through September.
Wang, Wu, Grumbine May -- 5.0
Wu, Grumbine, Wang May (December 2010) -- 4.8
Wu, Grumbine, Wang August (June 2011) -- 4.6
Grumbine, Wu, Wang May -- 4.4
... where I've struck through the estimates that are already (very likely) busted. The 5.0 of our first model estimate is probably too high. The 4.4 of our statistical guess is probably too low. Since I said that I thought this was their probable sense of error, I'm not too mortified. The two estimates which Wu took the lead on making both look plausible. 4.8 might be a touch high if the current figure is this. The 4.6 looks pretty good. The parenthetical dates are when the initial conditions for the model runs came from; we're not entirely up to real time for making our experiments.
The striking thing about the estimates that have already busted is that they're climatology (or someone known for making dramatic statements and doesn't like to see them checked, which is not striking but is worth noting if only because forecasts should be verified). Normally, you expect climatology to be fairly good, not among the first forecasters eliminated. Also, you expect, if climate is stable, that the mean will be a better forecaster than a trend (which results only from noise if climate is stable). Instead, the mean is a horrible predictor. And even the trend is very bad -- even assuming that ice is declining steadily is insufficient. The ice extent is down even more than a linear trend.
Update: For looks at the Arctic, including 3 days of sea ice drift forecast from NOAA/NWS/NCEP, see http://www.arctic.io/observations/520//