Climatologist says weather conditions may change this fall, helping bone-dry Texas and California.
|Almost 40% of the U.S. is currently experiencing drought conditions. The red shading indicates the worst conditions. Source: Eric Luebehusen/U.S. Department of Agriculture.|
THE historic dry conditions for the American West and southern Plains will continue through the summer, but climate conditions may change this fall and bring some relief, according to a university climatologist.
Tony Lupo, professor and chair of atmospheric science at the University of Missouri's College of Agriculture, Food & Natural Resources, said two causes of the dry conditions — a series of persistent eastern Pacific atmospheric ridging and blocking events and lower sea temperatures in the central Pacific — will change later this year.
Lupo pointed out that long-lived ridging over the eastern Pacific was showing signs of weakening in late May. When this pattern dissipates, possibly sometime by September, a more normal jet stream pattern will return, directing much-needed rainfall back to parched California, Arizona, New Mexico, Kansas, Oklahoma and Texas.
Normal rainfall would be a welcomed occurrence as most of the West and middle third of the nation are beginning the summer season with an existing rainfall deficit (Map).
As of mid-May, drought covered approximately 38% of the contiguous U.S., the U.S. Drought Monitor reported. That number is up significantly from May 2013, when 14% of the U.S. was suffering from drought conditions.
Lupo said 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.
Lupo said there is evidence that the persistent ridges and atmospheric blocking over the eastern Pacific that have stopped or misdirected needed rainfall are beginning to diminish, restoring a more typical jet stream pattern.
A ridge is an elongated area of high atmospheric pressure. Ridges enhance summer heat. Depending on their strength and how fast they move, they can bring record summer heat and stifling air pollution to a region, Lupo said.
Ridges occur both at the Earth's surface and at higher altitudes. Upper-level ridges generally produce sunny, dry weather to their east. Air tends to sink to the east of a ridge, which inhibits clouds and precipitation. Extremely hot weather during the summer and unusually mild winter weather are often associated with a strong, slow-moving, upper-level ridge.
Atmospheric blocking occurs when a powerful high-pressure system gets locked in one place, Lupo explained. These motionless areas tend to cover a large stretch and "block" the following low-pressure areas that grind to a halt behind them — much like a backup of cars behind an accident on the freeway. Rain-carrying storms aren't able to move in their normal west-to-east pattern.
Blocking events can trigger dangerous conditions when tied to extreme weather, Lupo said. If hot and dry weather becomes parked, short- to long-term drought conditions can result. Blocking can also park rainstorms over one area, producing flooding.
Only about 20-40 blocking events occur each year, making them among the rarest of weather events, Lupo said. The greatest frequency occurs over the Atlantic and Pacific. In the U.S., the harshest winters on record are usually due to blocking.
Lupo said additional improvement will also occur in the West and southern Plains because of a change in the equatorial Pacific sea surface temperatures, which have a strong influence on weather patterns over North America.
For almost four years, there have been either La Nina patterns or neutral conditions in the Pacific. During this period, sea surface temperatures across the equatorial central Pacific Ocean can be 3-5 degrees C lower than normal, Lupo explained. This typically directs the jet stream from the Pacific on a northeastern path over Canada. Rain-producing storms follow the jet stream, leaving the West and southern Plains dry.
Recently, La Nina has weakened into a neutral state and is showing signs of transitioning into El Nino. This will allow the jet stream to flow over the middle U.S., bringing much-needed showers with it.