Summer 2018 Climate Forecast Reasoning: Great Plains
Based on a weak La Nina to weak El Nino transition combined with climate driven by a cool pool south of Greenland the JUN/JUL/AUG 2018 climate in the Great Plains looks dry for wheat but dryness avoids corn.
United States
The issue, of course, is whether drought becomes an issue for the 2018 warm season. More specifically, will drought across the Kansas/Oklahoma wheat belt strengthen/expand or weaken (as NOAA indicates). Also, will drought emerge in the Corn Belt later this warm season?
Analysts immediately charge to ENSO for the answer. I agree with this concept but probably for a different reason. Emerging as a new climate issue is the climate generated by moderate-to-rapid transitions of ENSO phase not just the actual presence of La Nina or El Nino.
The most recent dramatic example was the El Nino to La Nina climate transition occurring in May to September last year driving the most active and intense September on record for tropical cyclone activity in the North Atlantic. There is risk of an emerging weak El Nino later this warm season. Implied is the opposite regime of last year: Weak La Nina to weak El Nino. An analog of this transition must be produced.
The second climate influence especially for U.S. corn is the strengthening cool pool south of Greenland in the North Atlantic basin. The atmosphere above is also cooling sustaining a persistent upper trough. The trough expands westward over the still snow covered northeast Canadian areas and the southern periphery of this upper trough clashing with warmer air across the southern U.S. is driving a wet pattern affecting the Corn Belt including severe storms. This pattern is likely to hang on.
From an analog standpoint the approach is fairly simple. During the middle 1990’s to now climate cycle when ENSO/PDO has generally been in the cool phase and AMO has been in the warm phase what analogs show a La Nina to El Nino transition during the warm season combined with years when the cool pool was present south of Greenland.
Of course, there’s not a perfect (ENSO) analog during the past 20 years or so. Years starting in La Nina (using multi-variate ENSO index) and shifting toward or into El Nino were 2006, 2009 and 2012. The El Nino development in 2006 and 2009 are likely too strong for 2018 expectations while 2012 observed a quick transition to El Nino in late summer only to see warm ENSO fade as fast as it arrived.
When weak analogs emerge it’s not a good idea to look at temperature/precipitation averages. However, the general upper air pattern produced by combining the analogs is useful. Our goal? Where’s the upper ridge (Fig. 1)?
We’ll average that answer with analog years from the past 20 years or so when the cool pool has been present south of Greenland. The cool pool south of Greenland has varied in intensity but present each June from 2009 to 2017 based on ECMWF analysis. An average of the 500 MB anomalies across North America must be considered for summertime in that 9-year period (Fig. 2).
Fortunately for analysts the upper air patterns for each prediction method are similar rendering above average confidence. The upper trough is in the East and upper ridge in the West while the Great Plains lay beneath the two regimes. The combination of these analogs renders a Great Plains precipitation forecast for JUN/JUL/AUG 2018 indicating DRYNESS Kansas to Texas and eastern Dakotas to western Minnesota (Fig. 3). WET weather is indicated over the Ohio Valley.
Core of the dry zones are in the 4-5 in. less than normal rainfall for the 3-month period with similar indications (except wet) in the Ohio Valley. So…a confident projection of wheat crop climate for summer 2018 is drier while most of the corn growing regions misses a dry climate.
Fig. 1-2: 500 MB anomalies based on ENSO analog years (left) and North Atlantic cool pool years (right).
Fig. 3: The analog years combined and implications for precipitation anomalies in the Great Plains for JUN/JUL/AUG 2018.
