Here is a great description of the SOI from NOAA:
"The Southern Oscillation Index (SOI) is a standardized index based on the observed sea level pressure differences between Tahiti and Darwin, Australia. The SOI is one measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical Pacific (i.e., the state of the Southern Oscillation) during El Niño and La Niña episodes. In general, smoothed time series of the SOI correspond very well with changes in ocean temperatures across the eastern tropical Pacific. The negative phase of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. Prolonged periods of negative (positive) SOI values coincide with abnormally warm (cold) ocean waters across the eastern tropical Pacific typical of El Niño (La Niña) episodes"The Australian Bureau of Meteorology site is another great resource
Over the last 3 days the SOI changes have been significant. A large drop occurred between June 20-22nd. (See this site for the graph).
Looking at the long range composites during similar drops in summer (during an El Nino which we are still in albeit weak) shows this 500mB pattern between July 7th and July 16th.
Warmer shades indicate high pressure. Notice the lack of sustainable high pressure in the middle of the US and central Canada. It tries to develop but flattens out.
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This is not the only driver of this pattern to be sure. But this DOES point to several things:
> The dominant middle US storm track shows up again (shades of blue and purple which indicate low pressure) with frequent storm systems and frequent rainfall.
> Big difference is that these storm systems will tend to feature less large scale rain. Storms will be more localized. Daytime warmth/higher humidity will fuel storms more than in May and June
> The normal high temperatures across much of the central US/Ohio Valley in July is lower 80s NOT the 70s like it was in late May/early June.
> Lack of long stretches of heat (90s) in the first half of July (central US east into New England) and possibly longer. Periods of below normal temperatures
> Through mid July Near normal rainfall in Ohio back into Pennsylvania--slightly above normal in spots--corn belt (Nebraska east into the Great Lakes)
