Tony Lupo, chair of atmospheric sciences at the MU College of Agriculture, Food and Natural Resources, says that the hot and dry weather has two main causes: eastern Pacific ridging and blocking events, and lower sea temperatures in the central Pacific.
Eastern Pacific ridging is an elongated area of high atmospheric pressure. Ridges enhance summer heat and, depending on their strength and how fast they move, can bring record summer heat and stifling air pollution to a region for several days or weeks.
"The long-lasting ridging over the eastern Pacific showed signs of weakening earlier this month," Lupo said. "When this pattern diminishes, as I predict it will sometime by September, a more normal jet stream pattern will return and direct much-needed rainfall back to
parched California, Arizona, New Mexico, Oklahoma, Kansas and Texas."
Atmospheric blocking occurs when a powerful high-pressure system gets locked in one place. These motionless areas tend to cover a large area and low pressure areas tend to grind to a halt behind them. Rain-carrying storms aren't able to move in their normal west-
to-east pattern because of this blocking. Blocking events also can trigger dangerous conditions such as extreme weather. If hot and dry weather doesn't move, short- to long-term drought conditions can result.
"Only about 20 to 40 blocking events occur each year, making them among the rarest of weather events," Lupo said. "However, the greatest frequency occurs over the Atlantic and Pacific. Additional improvement will come to the west because of a change in the
equatorial Pacific sea surface temperatures that have a strong influence on weather patterns over North America."
For nearly four years, the Pacific Ocean has experienced La Nina or neutral conditions in the Pacific. During these periods the sea surface temperatures across the central Pacific Ocean can be lower than normal. These conditions typically direct the jet stream from the
Pacific on a northeast path over Canada. Lupo says storms follow the jet stream and leave the western United States dry.
"Recently, La Nina has weakened into a neutral state and is showing signs of transitioning into an El Nino state," Lupo said. "This will allow the jet stream to flow over the middle of the U.S., bringing much needed showers with it."
Lupo says that rainfall would be welcomed in the west, as California and Nevada are in their third year of what the U.S. Drought Monitor calls an "extreme drought," the second highest of six rankings, and about 10 percent of those areas are experiencing an "exceptional drought," the highest possible level. As of mid-May, approximately 38 percent of the contiguous U.S. suffered from drought according to the U.S. Drought Monitor.