The U.S. National Centers for Environmental Information (NCEI) national climate report for April shows how the La Nina winter storm track not only hung around last month, but also produced some noteworthy precipitation extremes and set the stage for difficult and damaging crop conditions.
La Nina is the cool phase of the El Nino-Southern Oscillation periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean. During La Nina events, sea surface temperatures are at least 0.5 degree Celsius (about 0.9 degree Fahrenheit) below average in the equatorial eastern Pacific. (Current values are from 1.0 to 3.0 degrees C or 1.8 to 5.4 deg F below average.) These cold waters in the Pacific Ocean push the jet stream northward. This jet stream track favors drought development int the southern United States along with heavy precipitation development in the northwestern U.S. and Canada.
That winter storm track duration is affected by the strength of La Nina. And during April, La Nina was a moderately strong event. The Australia Southern Oscillation Index (SOI), a barometric measurement of El Nino or La Nina, shows that La Nina strengthened significantly during April. The SOI averaged plus 20.01 during April -- almost nine index points higher than the March average of plus 11.93. (An index value of plus 7.0 is the threshold for La Nina.)
April precipitation reflected the influence of that La Nina-enhanced storm track. The NCEI reports that "Multiple late-season snow events contributed to a wet April for North Dakota, which reported its second-wettest such month on record. Oregon and Minnesota ranked seventh wettest. In contrast, New Mexico had its second-driest April on record and Kansas ranked third driest." USDA's weekly weather and crop bulletin noted "State precipitation rankings ranged from the second-driest April in New Mexico to the second wettest in North Dakota. New Mexico's only drier April occurred in 1972; North Dakota's only wetter April was observed in 1986. Kansas experienced its third-driest April, behind 1963 and 1989, while top-10 rankings for April dryness occurred in Colorado and Arizona. In contrast, top-10 rankings for April wetness extended beyond North Dakota into Minnesota, Oregon, and Washington."
Temperatures were also on either side of an extreme warm or cold extreme in many areas. "The real story was the north-to-south variation in weather conditions," the USDA bulletin stated. "For example, state temperature rankings ranged from the third-coldest April in Washington to the eleventh-warmest April in New Mexico and Texas. In Washington, only April 1955 and 2011 were colder. Top-10 rankings for April cold were also observed in Minnesota, Montana, North Dakota, and Oregon."
Impact on crop conditions and activities is dramatic. U.S. corn planting entered the second week of May with the lowest rate of completion in nine years. (https://www.dtnpf.com/…) Hard red winter wheat crop production estimates for Oklahoma are only half the total of last year because of drought. In northern crop areas, large numbers of prevented planting acreage are being discussed because of saturated ground and flooding.
The effect of the La Nina-enhanced storm track also contributed to an active wildfire season beginning. From Jan. 1 through May 3, the NCEI notes that 1.3 million acres have burned -- 160% of average for this time of year. In addition, April was an active severe storm month; the tornado count for April was above average, yet another metric that points to La Nina's influence.
The heavy northern precipitation and above-normal precipitation in the Midwest brought some drought easing. However, drought maintains its tight grip on the Southern Plains through the southwestern U.S. USDA notes that "more than 40% of the country experienced drought each week from Sept. 29, 2020, to May 3, 2022, an 84-week streak that has broken the U.S. Drought Monitor-era record (previously, 68 weeks from June 19, 2012 to Oct. 1, 2013)."
For more on flooding and prevented planted concern, see https://www.dtnpf.com/…
Wet conditions are also affecting the Canadian Prairies. See: https://www.dtnpf.com/…
More details on drought-affected wheat production are available here.
Bryce Anderson can be reached at Bryce.firstname.lastname@example.org
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