The Australia Bureau of Meteorology declared this past Tuesday that the El Nino event has ended as the parameters of sea surface temperature, southern Oscillation Index (SOI), trade winds and cloudiness near the date line have all returned to normal.
The question that is now being asked is what does a neutral Pacific mean for the current growing season in the U.S.? Also what would happen if sea surface temperatures drop to weak La Nina conditions over the summer as some models are indicating?
In trying to answer this question we will start with an assessment of where the pattern has been during the month of May. The pattern we currently see is behaving like an El Nino pattern with an active southern branch of the jet stream across the central and southern U.S. Subtropical high pressure has been virtually non-existent in the U.S. this spring, remaining off to the south over Mexico. This has allowed for an active rainfall pattern over much of the central and southern U.S. The northwest Midwest, Northern Plains and Prairies have experienced drier weather under the dominance of stationary high pressure, which allowed severe fires in northern Alberta.
As we head into the month of June we only expect to see some minor changes to this pattern as there is expected to be some lag time between the oceanic end to El Nino and the atmospheric end. We would expect to see some northward shift in the most active rainfall band across the U.S. This would be good news for some of the drier areas in the Northern Plains and Prairies but could lead to disruptions in soybean planting in the Midwest as well as some disease issues to maturing wheat and harvest disruptions in the Southern Plains.
During July and August, as the effects of El Nino fade and possibly La Nina develops we may see some drier weather developing in the north-central U.S. This is due to a couple of reasons. So far this spring, as we stated earlier, we have seen little in the way of subtropical high pressure building over the southern U.S., especially in the southwest U.S. where it often sets up during the spring. We have seen more high latitude blocking (high pressure) building southward into the mid latitudes which has promoted the dryness in parts of the north-central U.S. and Prairies. Also La Nina conditions tend to lead toward drier conditions in the north-central U.S. Therefore we may see a trend towards hotter and drier weather in the north-central U.S. during the July/August period. Due to the early planting of the corn crop in much of the north-central U.S. you would think that the corn crop would likely escape any major stress from a turn to hotter/drier weather due to early pollination. The soybean crop would appear to be more vulnerable due to the fact that the critical pod-filling stage of development does not occur until August.
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