Which parts of U.S. could benefit most from El Niño?

May 29

An El Niño is likely to develop within the next few months, and as we discussed in a recent post, this doesn’t necessarily mean widespread doom and gloom. In fact, some parts of the United States would actually benefit from the effects of El Niño, the anomalous warming of the eastern Pacific sea surface that influences weather patterns worldwide. Let’s take a look…

Reduced Atlantic hurricane activity

Warm water anomalies (red) developing in the Central Pacific Ocean during a strong El Niño. Winds that normally blow from east to west weaken, allowing the warm water to push up against the South American Coast. (NASA)
Warm water anomalies (red) developing in the Central Pacific Ocean during a strong El Niño. Winds that normally blow from east to west weaken, allowing the warm water to push up against the South American Coast. (NASA)

Trade winds generally steer hurricanes from east to west across the Atlantic Basin. When an El Niño develops, these steering winds slow and can sometimes even reverse the flow. The slower winds from the east interact with the El Niño-forced winds from the west from the Pacific Basin. These interactions increase wind shear (the variation of winds with height) across the Atlantic’s main hurricane development region, limiting tropical cyclone development.

Both Weatherbell Analytics and Colorado State University researchers have hinted at a higher probability of a less active Atlantic hurricane season this year. Still, all it takes is one strong storm to have a big impact, such as when Hurricane Andrew hit Florida during the 1992 El Niño.

Related: NOAA predicts below normal Atlantic hurricane season, but it only takes one!

Stretched out jet stream and storm track

Typical El Niño and La Niña jet stream positions. (NWS)
Typical El Niño and La Niña jet stream positions. (NWS)

During La Niña, the jet stream tends to exhibit deeper troughs and ridges. This typically leads to more intense short-duration precipitation events over relatively small regions along the meridional-like (north-south) jet stream, with extended breaks between events. Conversely, storm tracks during an El Niño year tend to migrate across the country in more of a zonal (east-west) flow. While the dominant storm track will be determined by the exact strength of any El Niño that develops later this year, a persistent zonal jet stream would promote a steadier flow of light-to-moderate precipitation events stretched out over time and over vast geographic regions.

Historical odds favor above-average precipitation for much of the U.S.

Many of the 48 contiguous states have higher probabilities for above-average precipitation during an El Niño year during both the summer and winter. Winter months in the U.S. tend to have more impacts from an El Niño than the summer months, when the polar and subtropical jet streams interact less. In the maps below, the meteorological winter months of December, January and February show clearer precipitation signals than do the summer months.

Climatological average precipitation risks during El Niño (NOAA Earth System Research Laboratory)
Climatological average precipitation risks during El Niño (NOAA Earth System Research Laboratory)

The winter season map on the right shows a higher risk for extreme wetness in the Southern and Central U.S. (dark green). Notice how much less colorful the summer map on the left is, which indicates higher probabilities for near-normal precipitation.

Late-season drought relief possible in Southwest

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For the drought-stricken areas in California and the Southwest, fertile soils from El Niño precipitation would be a welcome sight. Golden Gate Weather Services discusses the typical El Niño impacts for California:

“If just the five strong El Niño events are looked at then the rainfall has been above normal four of the five seasons, and all four were at least 140% of normal. However, if only the weak and moderate El Niños are examined then it is seen that six of the 17 years received below normal rainfall, six near normal (80%-120%) and five above normal.”

Californians will no doubt be hoping for a strong El Nino, which could benefit much of the rest of the country as well with moist, nutrient-rich soils. However, even though odds favor increased precipitation for California and especially the Southwest U.S., abundant rainfall is never guaranteed.

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Matt Ross · May 29