AAA  May. 22, 2015 5:51 PM ET
A strong El Niño could flourish by fall: 5 ways it could affect U.S. weather
  Jason Samenow
The Washington Post News Service
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The El Niño warm-weather pattern in the Pacific has reached a weak-to-moderate intensity and is growing stronger. Many computer models are now forecasting a strong to very strong event by the fall. Some simulations even suggest it could exceed the mighty 1997-98 event, the most powerful in modern records.

Although computer model forecasts of El Niño made in the spring for the fall have proven erratic and led forecasters astray at times in the past, they warrant considering the implications of a strong event.

A strong El Niño event, signified by much warmer than normal waters in the central and eastern tropical Pacific ocean, would have major consequences on the weather all over the world.

For residents of the United States, here are the five El Niño effects we'd likely hear most about:

1. Enhanced rainfall over California

Historically, moderate to strong El Niño events have increased California precipitation by about 10 percent. Given the state's extreme water shortage, such an increase in rain and snow would be welcome — although a lot more would be needed to end the drought

If the El Niño exceeded strong levels and reached "very strong" levels, it would likely go further in denting the drought. While the sample size is small, past very strong events have produced 174 percent of normal precipitation, on average, across California.

Of course, while no two El Niño events are alike, the rains from the last "very strong" El Niño event in 1997-1998 resulted in $550 million in damages and deadly flooding. So a very strong or "super" El Niño in California could be a case of too much of a good thing.

2. A depressed Atlantic hurricane season

El Niño events introduce hostile upper-level winds in the tropical Atlantic, which have historically greatly limited the number of tropical storms and hurricanes. Given this year's El Niño forecast, seasonal forecasters are predicting below-normal hurricane activity.

Despite the predictions for a relatively inactive season, forecasters stress it only takes one storm to cause tremendous harm and urge preparedness.

3. A milder-than-normal winter for much of the United States.

During strong El Niño events, the polar jet stream is usually displaced well north of its normal position, and mild air floods the central and northern tier of the United States. At the same time, an active southern storm track keeps the South cloudy, damp and cool.

The Climate Forecast System model's predictions for December and January portray a classic strong El Niño pattern, with above-normal temperatures in all but the southernmost parts of the nation.

4. Reduced tornado numbers

After one of the quietest starts to tornado season on record through March, activity has picked up — especially as of this May. However, the current count of tornadoes for the year is still about 25 percent below average.

The development of a strong El Niño would probably continue to hold tornado numbers down. Recent research has linked strong El Niños with below-normal tornado activity.

5. An extremely active Pacific typhoon season

The Pacific typhoon season is off to its busiest start on record. Typhoon Dolphin, which developed in the western Pacific last week, marked the seventh typhoon to form in 2015, the greatest number so early in the season on record. Three of those typhoons attained the intensity equivalent to a Category 5 hurricane (on the 0-5 Saffir-Simpson wind scale), setting the western Pacific on record pace for the number of such storms, according to Mashable's Andrew Freedman.

Due, in part, to the effects of El Niño, ocean temperatures in the western Pacific are about 4 degrees Fahrenheit above normal. Given such warm background temperatures and the prospect of El Niño strengthening, it is probable that typhoon activity will remain high, especially during peak season from July to November.

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The first four of these effects would have mostly beneficial outcomes for the United States: drought-assisting rain, fewer hurricanes, fewer tornadoes and, following two straight brutal winters in the northern United States, a welcome break from the bitter cold.

Of course, despite current model predictions for a strong event, the forecast for El Niño is uncertain and will evolve. But it's certainly worth paying attention to.

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