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Friday, August 14, 2015

Strong El Niño likely to peak late fall-early winter, NOAA forecasters say

Aug 14th 2015 5:47AM811264155


Forecaster consensus unanimously favors a strong El Niño at its peak in the late fall and early winter, according to an updated forecast released Thursday by the National Oceanic Atmospheric Administration (NOAA).


The prediction for a strong El Nino at its peak is based on the latest guidance from computer models, which all forecast a strong El Nino during that period of time. NOAA also said that all signs in the atmosphere and the Pacific Ocean point to a significant and strengthening El Niño.

El Niño now has an 85 percent chance of lasting into early spring 2016. This is an increase of five percent over NOAA's last El Niño update in July. NOAA also continues to say that there is a greater than 90 percent chance of El Niño lasting through the upcoming winter.

El Niño is an anomalous, yet periodic, warming of the central and eastern equatorial Pacific Ocean. For reasons still not well understood, every 2-7 years, this patch of ocean warms for six to 18 months.

The fact that El Niño is likely to last into spring is important for the United States since precipitation and temperature impacts from a moderate-to-strong El Niño are typically most noticeable during the colder months. We have more on what those impacts are later in this article.

NOAA reports that sea-surface temperature anomalies increased in July in the Niño 3.4 region. This is the middle portion of the equatorial Pacific Ocean that is most commonly used to measure the intensity of an El Niño event.

As mentioned above, NOAA's forecaster consensus is for a strong El Niño at its peak, perhaps the strongest since the 1997-1998 episode and, thus, may play a stronger role in your weather.

What does warm water have to do with the weather?

A 'Sea Change' in Atmospheric Circulation

Typically, easterly trade winds near the equator pile warm water into the western Pacific Ocean. Conversely, the resultant upwelling, or upward movement of deep, cold ocean water keeps the eastern and central Pacific Ocean cooler.

Thunderstorms require at least some degree of warm, humid air near the surface, so they're more numerous and persistent over the western Pacific warm pool, and much less so in the eastern equatorial Pacific.

During an El Niño, these trade winds weaken, and may at times reverse from west to east. Warmer western Pacific water then slowly sloshes back toward the central, even eastern Pacific Ocean in what's known as an equatorial-trapped Kelvin wave.

Therefore, the most persistent thunderstorms will shift from the western to the eastern and central Pacific Ocean in an El Niño.

This trade wind reversal and the resulting reorientation of thunderstorms changes the atmospheric circulation not just over this swath of the equatorial Pacific Ocean, but can also have far-reaching impacts on the atmospheric circulation.

Weather Impacts

First, keep two things in mind throughout this discussion of potential impacts:

1) El Niño is not the sole driver of the atmosphere at any time. Day-to-day variability in the weather pattern, including blocking patterns, forcing from climate change and other factors all work together with El Niño to determine the overall weather experienced over the timeframe of a few months.

2) No two El Niños are exactly alike. The intensity matters for impacts.

El Niño's clearest impact on northern hemisphere weather patterns occurs from late fall through winter.

Looking at past moderate-strong El Ninos, here are the upshots for temperatures and precipitation from late fall through winter in the U.S.:

- Wetter: Southern U.S. from California to the Carolinas then up parts of the East Coast

- Drier: Parts of the Ohio Valley, Great Lakes, Northwest and Northern Rockies

- Cooler: Desert Southwest, Southern Plains, northern Gulf Coast

- Warmer: Northern tier of states from the Pacific Northwest to the Northern Plains, Great Lakes, and Northeast

Note these are impacts that are typically expected, but they aren't always the rule.

Residents of the western states may remember the flooding that struck California during the strong 1997-98 El Nino. In February 1998, a series of storms caused an estimated $550 million in damage and killed 17 people in California. A total of 35 counties were declared federal disaster areas. This fits into the bucket of the wetter-than-average winter you would typically expect in a moderate or strong El Niño.

Interestingly, during the previous winter there was also major flooding in California and it was even more costly with a total price tag of $1.8 billion, according to Jan Null, a consulting meteorologist in California. However, El Niño was not present that winter and rainfall for the season was near average. The flooding was the result of excessive rainfall that fell in a short time period combined with snowmelt from late December to early January.

The weak El Niño in the winter of 2006-07 provided a totally different story than what we saw in the very strong 1997-98 El Niño winter.

California had its 23rd driest winter season on record when looking at the three-month period from December 2006 to February 2007. In Los Angeles, the entire water year from July 2006 to June 2007 was the driest on record with just 3.21 inches of rainfall.

So, those hoping for drought relief next winter in the Golden State shouldn't immediately draw a conclusion that significant rains are ahead in any El Niño year. The strength of the El Niño can play a role in the outcome. In addition, heavy rainfall can occur with or without El Niño present and that was the case in the winter preceding the strong 1997-1998 El Niño.

What About Hurricane Season?

There is a body of scientific evidence linking the occurrence of El Niño with increased wind shear in the tropical Atlantic Basin, which is one factor - along with dry air - that limits the development and strengthening of tropical cyclones.

As of this writing, we are seeing those hostile conditions in parts of the Atlantic Basin.

Dry air has dominated from the southern Gulf of Mexico to east of the Lesser Antillesduring the first two months of the hurricane season. Wind shear has been and remains strong from the Caribbean Sea and parts of the Gulf of Mexico to the far western and eastern Atlantic.

(MORE: Hostile Conditions in the Atlantic)

During the 2013 hurricane season, 14 storms formed, but only two reached hurricane strength. Neither of these hurricanes reached major hurricane status, which is defined as Category 3 or stronger on the Saffir-Simpson Hurricane Wind Scale.

When considering overall season activity via the ACE index, 2013 was the least active Atlantic hurricane season since 1994. There was no El Niño in place in 2013.

Now, consider the 2004 Atlantic hurricane season. Four hurricanes - Charley, Frances, Ivan and Jeanne - hammered Florida in less than six weeks. There were 15 storms and nine hurricanes that season, which is an active one by any measure, and it developed despite a weak El Niño.

Taken together, the five El Niño hurricane seasons since 1995 averaged about 11 storms, 5 hurricanes, and 2-3 major hurricanes, a reduction of four storms, 3 hurricanes, and 1-2 major hurricanes from the average since 1995.

Prior to the current active phase of Atlantic hurricane activity (pre-1995), there were several other relatively slow hurricane seasons: 1982, 1986, 1987, 1991 and 1994. The 1982 season was particularly inactive, with only six tropical storms and two hurricanes. The next year, despite one of the strongest El Niños on record finally fading by early summer, only four storms formed the entire season.

Exactly where the equatorial Pacific Ocean warms in an El Niño matters, as well.

Warming in the eastern equatorial Pacific: lower numbers of Atlantic tropical cyclones
Warming in the central equatorial Pacific: higher numbers of Atlantic tropical cyclones

As we've stated, El Niño is only one driver of the atmospheric circulation. Wind shear over the Atlantic Basin may be less on some days, despite being higher when averaged over an entire season.

While we have focused on numbers of storms and hurricanes, it's ultimately a storm's path that matters for impact.

"Even if we get a strong El Niño, that doesn't mean no U.S impacts (from hurricanes or tropical storms)," says Dr. Phil Klotzbach, a tropical meteorologist and researcher at Colorado State University.

Klotzbach notes that Hurricane Betsy hit both Louisiana and Florida in 1965 and Agnes flooded out the eastern U.S. in 1972, both during strong El Niños. All it takes is one intense, landfalling hurricane to make many forget an El Niño was even there.

Meanwhile, the eastern, central and western Pacific Basins remain very active. At one point in mid-July we had six active tropical cyclones across the Pacific Ocean at the same time.

In early August, Typhoon Soudelor hammered Saipan, Taiwan and eastern China. As of this writing, computer model forecast guidance indicates that two more typhoons are possible in the next few days.

Colorado State University tropical scientist Phil Klotzbach found tropical cyclones are about three times more likely to impact Hawaii in El Niño years vs. La Nina years.

So far this season, three named storms have formed in the central Pacific Basin. In addition, two named storms have crossed into the central Pacific from the eastern Pacific.

Those two systems that originated in the eastern Pacific have brought some impacts to Hawaii. Guillermo brought high surf, but otherwise passed well enough north to minimize rain and wind impacts. Hilda was forecast to bring enhanced rainfall chances to the Big Island in the next few days.

Ultimately, this El Niño will exert some influence on the numbers. However, all it takes is one tropical cyclone making landfall in a populated area to change perceptions of an active season.

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