Longer term forecasts of what lies ahead for the Central Plains this fall and winter often start with a look at ocean temperature trends.
Sea surface conditions in the Equatorial Pacific continue to exhibit above normal temperatures, with a large pool of anomalously warm water underneath the surface that extends from South America to Indonesia. The strongest area of warmth below the surface is just west of central Equatorial Pacific. Therefore, there is ample warmth underneath the Pacific to maintain the surface warm pool.
Current outlooks by the Climate Prediction Center (CPC) call for this warm pool to continue through winter, with a peak warmth occurring before the end of the year. This warm event is commonly known as an “El Nino.” Although CPC indicates that this event will end by next spring, the warm pocket underneath the surface has yet to show signs of any cold pool development in the western Equatorial Pacific.
Until evidence of a cold pocket develops below the surface, anomalous warm water will continue to work eastward and reinforce surface warming in the eastern Equatorial Pacific. Over the next three months, climatologists will be watching the western Equatorial Pacific for development of a cold pocket. If this doesn’t happen by the end of 2018, the odds would favor El Nino conditions lasting through the summer and a high probability that a second-year event will unfold in fall 2019.
In addition to the warm equatorial Pacific, the Gulf of Alaska and the northern half of the Atlantic have both turned warmer than normal. The interaction of these two regions can give us an idea of where the most likely areas of below normal moisture will develop. Under a warm Atlantic and warm Pacific, dryness issues are most pronounced across the northern third of the continental U.S., with above normal dryness favored across the southern third of the U.S. from New Mexico through Georgia.
Under an El Nino, the northern jet begins to relax in the late summer and early fall. Precipitation usually tilts to the wet side from the southwestern desert area through the southern Mississippi River valley region. At the same time, dryness begins to develop across the Pacific Northwest through the western Dakotas as the eastern Gulf of Alaska upper air ridge begins to push eastward and the western Gulf of Alaska upper air low slides into the eastern half of the Gulf of Alaska.
As this process unfolds, the northern jet stream begins to weaken as more energy begins to strengthen the variable Pacific jet (subtropical jet). Pieces of energy from the Gulf of Alaska upper air low will slide underneath the upper air high pressure to the east of this low. This helps pull subtropical moisture northeastward out of the Equatorial Pacific and into the southwest coast of North American.
These types of conditions have shown signs of developing in the last 30 days as dryness has begun to expand eastward from the Pacific Northwest. Most of the western Dakotas, Montana, Wyoming, and the northwestern third of Nebraska have been well below normal precipitation. Just to the east of this area, excessive moisture continues to be a problem across the northern Mississippi River valley.
The preliminary 30-day outlooks issued by the Climate Prediction Center indicate cooler than normal conditions centered over eastern Nebraska and Iowa. CPC’s preliminary 30-day outlooks are most influenced by atmospheric conditions at the time of the forecast release. With the cool air moving into the upper Midwest during the second half of September, it would be reasonable to expect that cool conditions would be favored during the early stages of October, especially with the northern jet stream directing the upper air flow out of northern Canada into the upper and central Midwest.
As fall progresses, most of the Corn Belt will likely deal with periods of heavy rainfall, interspersed with warm and dry weather. Although weak El Nino conditions are developing, the northern jet stream is still strong enough to bring upper air troughs over the northwestern U.S. upper air ridge, then drive them southeastward over the northern High Plains. If we are in a true El Nino pattern, there should be a weakening of this process as the fall progresses.
However, if we look back at the last El Nino event, heavy snowstorm activity hit the eastern Dakotas and northern Minnesota during the second half of fall of 2016. El Nino conditions were exhibited off and on during the late summer and early fall, but were not dominant enough to weaken the northern jet. As the event strengthened during the first half of the winter, milder conditions developed across the northern Plains due to a weakened northern jet leaving this area well south of the mean northern jet position.
What happens if the subtropical jet fails to form as expected during El Nino conditions? Actually, approximately 3 of 10 El Nino events will fail to conform to typical patterns. Under these circumstances, fall and winter temperatures can be exceptionally brutal and stormy. Arctic air infiltration occurs regularly because the northern jet failed to weaken and the primary steering currents favor the northern half of the country east of the Rocky Mountains. In other words, a typical winter pattern.
