NASA visualization of El Niño in the fall of 2009 – with telltale warmer than normal water in the equatorial Pacific, as indicated by yellow and orange shades. (NASA)

In a bulletin issued this morning, NOAA issued an El Niño Watch predicting a roughly 50% chance for development later this year. An “El Niño” is the abnormal warming of ocean temperatures in the eastern Pacific Ocean along the west coast of South America near Peru and Ecuador. It can have profound influences on weather patterns around the world.

NOAA’s bulletin says “sea surface temperature anomalies have recently increased near the International Date Line” as well as “in the central and east-central equatorial Pacific,” and that “many dynamical models predict El Niño to develop during the summer or fall.”

El Niño conditions are declared when the average sea surface temperatures in the eastern Pacific are at least 0.5°C above average for three consecutive months. These abnormally elevated sea surface temperatures allow for the atmosphere to warm and provide instability, leading to the development of thunderstorm activity.

Several computer model projections suggest equatorial sea surface temperature will warm to more than a half a degree  C above normal, which would indicate El Nino conditions later this year (NOAA/IRI)

The warmer temperatures caused by an El Niño tend to alter the jet stream in the Northern Hemisphere, allowing a subtropical jet stream to form north of Hawaii and extend across parts of the western and southern United States. Increased thunderstorm activity over the eastern Pacific allows moisture to rise into the upper atmosphere and leak into this branch of the jet stream, creating the opportunity for beneficial rainfall across the affected areas next fall and winter, especially in California where they are experiencing exceptional drought conditions.

During the summer, El Niño conditions tend to inhibit tropical activity in the Atlantic basin. Upper-level outflow from thunderstorms in the eastern Pacific vents out over the western Atlantic Ocean during the summer months, creating strong wind shear that kills tropical cyclones before they have a chance to form. The 1997 Atlantic hurricane season occurred during the strongest El Niño ever recorded – as a result, it only saw 8 named storms and 3 hurricanes.

The effects of El Niño on East Coast weather in fall and winter are complex and depend on its strength.  Strong El Niños tend to flood the East with mild, wet weather and, in winter, little snowfall.  Weak to moderate El Niños tend to be more favorable for snow.

“Interestingly, the median snowfall for the 14 El Niño events recorded since 1950 is 21 inches, or about 6 inches more than the climatological season average of 15.4 inches at [Washington, D.C.’s] Reagan National [Airport],” wrote Capital Weather Gang’s Rick Grow in an analysis last year.   The last El Niño  event in 2009-2010 coincided with Washington, D.C.’s snowiest winter on record.  But, in the super strong El Niño event in 1997-1998, only 0.1″ of snow fell – tying for D.C.’s least snowy winter on record.

Despite the predictions and the issuance of an El Niño Watch, NOAA cautions that there is still “considerable uncertainty” in the models as to whether or not an El Niño will actually develop. Discerning weather observers will remember that the last predicted El Niño in 2012 turned out to be a bust.

All in all, NOAA’s current forecast indicates that there is a 50/50 chance for an El Niño to form later this year, and as with any long-range forecast, significant uncertainties exist that warrant careful caution and observation.