El Niño Conditions Now Official; Cold, Snow Take a Parting Swipe at East

More than a year after the prospect of a 2014–15 El Niño event first surfaced, NOAA’s Climate Prediction Center (CPC) issued a long-awaited El Niño Advisory on Thursday. We’re still a long way from a textbook example of El Niño: ocean warming is barely above the standard threshold, and the atmospheric response is not yet fully formed and consistent. One thing we do know is that sea-surface temperatures in the crucial Niño3.4 region of the eastern tropical Pacific have now met the El Niño threshold (0.5°C above average) across four overlapping three-month periods (Sept- Nov, Oct-Dec, Nov-Jan, and unofficially Dec-Feb). A total of five such periods are required before the title “El Niño episode” is bestowed by NOAA, so we could be there by April.



Figure 1. Weekly departures from average in sea-surface temperature (degrees C) across the Niño3.4 region of the eastern tropical Pacific. Image credit: NOAA Climate Prediction Center.


Along with the increased certainty of warm SSTs, the atmospheric response is gradually becoming more coherent, according to CPC forecaster Emily Becker. “For the last few months, we’ve been seeing some suggestions of borderline atmospheric El Niño conditions, but until this month we were below that borderline. This month, we’ve finally crept above it,” said Becker in a blog post at climate.gov. She points to a weakening of equatorial trade winds, as expressed in the Equatorial Southern Oscillation Index, as well as an increase in rainfall across the central tropical Pacific. NOAA’s advisory pegs the odds of El Niño conditions extending into northern summer at 50-60%. Weak El Niño events are much less likely to produce reliable effects on U.S. weather than stronger events, although there could be a El Niño–boosted enhancement of rainfall across the Gulf states this spring. With California’s rainy season winding down, there’s little hope of any major benefit from El Niño.

Seven of the eight global computer models surveyed this week by the Australian Bureau of Meteorology indicate that Niño3.4 temperatures will be above the NOAA threshold (0.5°C) in July. Predictions are most challenging during northern spring, when El Niño episodes are usually on the wane. If this event does mature and extends into next autumn, then 2015 could become one of the rare calendar years that sees continous El Niño conditions from January to December (as measured by the oceanic index described above). Going back to the start of NOAA records in 1950, the only other such years are 1953, 1969, and 1987. A prolonged El Niño event isn’t necessarily a strong one, but several models do push the Niño3.4 index above 1.0°C by this summer. An event under way in July typically strengthens by the end of the year, as climatology becomes more favorable.

Twin tropical cyclones possible next week
The incipient El Niño could get a shot in the arm from a major westerly wind burst that models are consistently developing next week around 5°S in the western Pacific. As they push against the prevailing east-to-west trades, westerly wind bursts can help nudge the ocean-atmosphere linkage toward the direction of El Niño. The atmospheric set-up that favors a westerly wind burst can also encourage the development of twin tropical cyclones to its north and south, and models are indicating this may also occur next week. In fact, multiple runs of the GFS and ECMWF models are suggesting that a tropical cyclone east of Australia could become one of the strongest on record for that region. It’s far too soon to take any model forecast literally, with the event still nearly a week out, but the consistency across multiple models lends credence to the idea of a powerful cyclone that could threaten islands in the southwest Pacific north of New Zealand sometime late next week.


Figure 6. Snowfall on Thursday, March 5, in Glasgow, Kentucky, east of Bowling Green. Much of central Kentucky saw more than a foot of snow from Wednesday into early Thursday. Image credit: wunderphotographer tightwad6972.

Winter’s last gasp back East?
March came in like the proverbial lion this week across the eastern U.S., taking one last swipe at the region before a much-anticipated warm-up. Heavy sleet and snow developed on Thursday from Texas to New York--including many of the same areas struck last week--within a band of moist upper-level flow extending from the tropical Pacific to the U.S. East Coast. The Dallas-Fort Worth airport received 2.5” of snow, yielding the snowiest March since 1947; amounts of 5+” were reported in the north DFW area. Central Kentucky was again buried in foot-plus accumulations: Lexington saw its heaviest 24-hour total (14.6”) and two-day total (17.1”) on record.


Figure 3. Temperatures ran the gamut across the Southeast as a strong cold front plowed across the region at 6 AM EST on Thursday, March 5. Image credit: WunderMap.


Parts of the Southeast got a brief taste of spring warmth before the cold roared back, leading to some unusually dramatic cool-downs. In Jackson, MS, the reading sank from a humid 81°F at 3 PM CDT Wednesday to a blustery, damp 28°F at 10 AM Thursday. On Friday morning, unprecedented cold for so late in the winter struck several locations, especially snowbound Kentucky.

All-time record lows for March:
Frankfort, KY: –10°F (old record –3°F, Mar. 7, 1960)
Urbana, IL: –7°F (old record –5°F, set Mar. 1906)
Paducah, KY: –6°F (old record –2°F, last set Mar. 6, 1960)
Pittsburgh, PA: –4°F (old record –1°F, last set Mar. 2, 1980)
Lexington, KY: –2°F (tied old record, last set Mar. 6 ,1960)
Harrisburg, PA: 0°F (old record 5°F, last set Mar. 10, 1984)

Toasty in Spain; howling in Italy and Croatia
Temperatures soared to summer-like readings along Spain's east coast early this week. On Monday, March 2, the towns of Sagunto and Elche both reached 30.4°C (86.7°F), a reading that’s close to the average daily high in August at the nearby coastal city of Valencia. These are the warmest temperatures known to have occurred on the Iberian peninsula (mainland Spain) so early in the year, according to weather records researcher Maximiliano Herrera. The nearest rival is the 32.4°C (90.3°F) reported at Almeria on March 3, 1987. Warmer temperatures have occurred in wintertime on Spanish territory beyond the peninsula, including 34.2°C (93.6°F) at Melilla (a Spanish enclave on the north African coast, adjoining Morocco) on Feb. 27, 2010.


Figure 4. Firemen inspect a truck overturned by heavy winds, near Split, Croatia, Thursday, March 5, 2015. Winds caused traffic disruptions in much of coastal Croatia. Image credit: AP Photo/Ivo Cagalj, PIXSELL.



This week’s heat in Spain preceded a strong Mediterannean cyclone that produced extreme bora (cold downslope) winds and extensive damage, including many downed trees, in Italy and Croatia on Wednesday night (see Figure 4, above). According to Herrera, a gust to 209 km/h (130 mph), one of Italy's strongest on record, was reported at Gigliana, Tuscany. Even stronger gusts were reported in parts of Croatia, with heavy mountain snowfalls. Severe Weather Europe’s Facebook page includes a number of dramatic photos and videos of pounding waves along the Croatian coastline.


This week’s WunderPoster: Lenticular clouds
The latest installment in our WunderPoster series (Figure 5, right) pays homage to the surreal structure of lenticular clouds. These clouds form when a strong jet stream impinges on a mountain range and the resulting wavelike motion is trapped within a layer of stable air. An isolated peak can produce lenticulars resembling a stack of dinner plates (or flying saucers). All WunderPosters can be downloaded in formats suitable for posters or postcards.

Now it’s your turn: help us create a WunderPoster!
To celebrate our 20th anniversary, we’re looking to you to provide inspiration for a new WunderPoster. Our “picture yourself here” website has all the details.

Bob Henson


Figure 6. These sunset lenticulars along Colorado’s Front Range were featured by the U.S. Postal Service in 2004 in its “Cloudscapes” stamp series. Image credit: Carlye Calvin.