The weak El Nino event in the eastern Tropical Pacific continues. Its affect on the atmosphere continues to be weak to negligible, although the recent few weeks have experienced several storms over the southwest US in a manner reminiscent of an El Nino. Figure 1 shows the 500-mb (~18,000 feet) geopotenital height anomaly over the past 30 days. Note the strong negative (low-pressure) anomaly over the Pacific Northwest, a stark departure from earlier this winter when no such anomaly was seen over western North America and the eastern North Pacific Ocean.
The previously moderate-strength El Nino event has weakened significantly in the eastern Tropical Pacific. Its affect on the atmosphere continues to be negligible thus far this winter season. Although sub-surface ocean temperatures remain warmer than normal along parts of the Pacific equatorial region, implying a chance for this El Nino event to re-strengthen, this is looking less likely at this time. This has little to no impact on the North Platte, but suggests a reduction in the chances of a very wet spring over the South Platte basin.
The moderate strength El Nino event continues in the Eastern Tropical Pacific. Its effect on the atmosphere has been negligible for a large part of this winter season. However, some changes in the atmospheric circulation were noted beginning in late December and into early January, characterized by reduced ridging in the western United States and more frequent disturbances traversing the southwest United States. However, while these changes brought cooler temperatures to the North and South Platte basin, precipitation was light.