Attached are two images from Surfline.com showing the SLP, wind speed, and precipitation forecast for Friday, Nov 6 at 12Z (top image), and Wednesday, Nov 12 at 12Z (bottom image). Notice when looking at the GFS forecast for tomorrow at 12Z, there's a weak area of low pressure almost over Tahiti embedded in the currently weak SPCZ (South Pacific Convergence Zone). The SPCZ can easily be spotted as the the yellow, orange, and red colors showing projected precipitation and the lower SLP in between the two larger areas of high pressure to south on either side of Tahiti. Now notice when looking at the GFS forecast for Wednesday, Nov 12th at 12Z, the low pressure and SPCZ have weakened/faded, as the high pressure system that was located to the southwest of Tahiti (in the top image) has migrated towards the east (normal direction that high and low pressure systems typically move in the S PAC), causing SLP at Tahiti to rise. If this were to occur, we would either see daily SOI values return to weak negative territory or even become slightly positive (this is just according to todays forecast and is obviously subject to change). I have not included Darwin in this post as SLP at Darwin has been and is expected to stay relatively stable (Note that when looking at daily SLP readings from the Long Paddock site, daily SLP readings a Darwin don’t fluctuate nearly as much as they do at Tahiti). Also, I have found that the GFS forecast for SLP at Darwin and Tahiti for12Z (and only 12Z), corresponds well with the daily SLP readings issued by the Long Paddock, which is why I'm using the forecasts for 12Z in this post. Other times such as 06Z, 18Z, etc.. don’t seem to correspond very well at all with daily readings issued by Long Paddock, making it near impossible to predict or even tell what the SOI might do. SLP fluctuates slightly throughout the day due to ever changing weather conditions, and the GFS model seems to obviously factor this in (though I don’t know why the 12Z forecast is the only time that works for forecasting SLP readings in regards to the Long Paddock). If you're able to get a good idea of the possible SLP readings, then you're able to get a good idea of what the possible future daily SOI values might be, which then of course gives you a good idea of what the 30 and 90 day avg might do. I have found this method to work about 65-70% of time for up to 4-5 days out.
Rubikscube,
Very much agreed, -27 is not all that low when looking at the historical data. Many other years, even La Nina years have seen daily values below -27. However, it's all about the long term trend (say over a few months). You could easily see very strong negative daily values for a few days due to a tropical cyclone passing right over Tahiti (as the low pressure from the cyclone would drop the bottom out of daily readings), while at the same time high pressure happens to be passing over Darwin. Such a difference in SLP between the two regions would cause daily SOI values to drop into the -30's or -40's, but only for a short time (a week or less). This is why ASLR, myself, and others have emphasized on the importance of the 30 and 90 day avg as they filter that problem out (daily and weekly spikes due to weather and even the 30 day avg SOI values must remain negative for several months). If one were to look at the table of SOI values I usually provide, one will notice that the 90 day avg is in the largest font size. That's because that's the most important value provided in determining the long term trend. If the 90 day avg is at or below -8.0 for even a month it would be suspicious (it takes a lot to get the 90 day avg below -8, so much so that it hasn’t been since 2010). That suggests that overall, SLP at Darwin is slightly higher than normal and SLP at Tahiti is slightly lower than normal. Daily SOI values do have their importance. If you start seeing negative daily values week after week after week then one starts to wonder. Since the end of June 2014, the 30 day avg has not left negative territory (but has been somewhat weak at times) this suggests ENSO-neutral, bordering on weak El Nino. Something is going on and unlike 2012, it's not fading out, but its pretty weak.