By Jeff Ray

The Climate Prediction Center has officially put us all on a “La Nina Watch”. The powerful El Nino that provided us our warm winter (4th warmest on record, coldest temperatures at DFW all winter: 27-degrees) is cooling down from its winter peak. Not only is it cooling down it appears it might actually flip its opposite cool sister, La Nina.

The El Nino/La Nino cycle has many different effects to our weather here in north Texas depending on the season, timing and strength. It is not always a clear signal either, other global weather patterns can counteract or even exaggerate its influence. None the less, some trends can be discerned if you can find enough sample years.

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Lets take the current case: a very strong El Nino dissipating rapidly. What kind of May or Summer can we expect to have?

I’m using a list of years from the researchers at AccuWeather who did a similar study for the Pennsylvania area. I then matched up the DFW average temperatures and precipitation for the months from May thru August as well as the Summer of that year. The table looked like this (not meant to be read really, difficult to get Excel data into WordPress):

Capture

Again, don’t try to read this it’s way to blurry. Let me tell you what it suggests: on years where there is a strong El Nino dissipating you typically get a cool and dry May followed by a cool and wet summer. The big (and it is big) exception is 1998 which was VERY hot (3rd warmest summer ever) and VERY dry (9th driest summer on record).

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I’ve looked at ’98 before as an analogy year; the El Nino to La Nina change over is very similar (if indeed we switch over to a La Nina). However there is one big difference and that is the PDO. The Pacific Decadal Oscillation is centered in the Pacific basin. Use the link to get the full explination but generally this is a much slower moving pattern of water temperatures in the Pacific that also has a “warm” or “cool” phase. It has recently changed over to a “warm phase”.

They don’t currently even try to forecast the PDO but you can look at its past cycles and gleam an influence it has on the El Nino/La Nina cycle. The PDO seems to make the phases stronger if they match up (example: an El Nino is stronger [warm phase] when the PDO is also in a warm phase). In ’98 the PDO was slipping into a “cool” phase just as the El Nino was fading.

The Climate Prediction Center seems to follow the “dissipating Strong El Nino” connection for their May forecast: a better-than-normal chance of a cool and wet weather:

Monthly Outlook Monthly Outlook Precip

As for the next three months (May through July) the CPC is thinking a wet summer but “normal” temperatures. The chart (that you can’t read I know) shows that of the eight years where we had a strong dissipating El Nino six of the summers here in north Texas were both cool and wet. But the PDO relationship is mixed: of those six cool/wet summers three of them had a warm PDO in progress, two had cold PDO’s and one was flipping cold to warm.

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For what it’s worth, since 2006 the PDO was in “cool” phase but at the start of 2014 it returned to a “warm phase” (where we are now).