Whiplash weather: blame it on the Jet Stream [View all]

[pscot]

http://www.wunderground.com/blog/JeffMasters/show.html
(more at the link)

The hottest temperature of all on Tuesday was 108° at Tekamah, Nebraska. This is just 2° short of warmest temperature ever recorded in the state of Nebraska during May: 110° at Broken Bow in 1895 (exact day unknown.) Numerous all-time early season heat records were set on Tuesday, making the event the most notable May wave in the Midwest since a multi-day event in 1934. That heat wave was not preceded by unusually cold weather, which is what makes the May 2013 Midwest heat event truly extraordinary


The position of the jet stream is a critical controller of weather regimes across the U.S., Europe, and Asia. Precipitation-bearing low pressure system ride along the axis of the jet, and the jet marks the boundary between cold, Canadian air to the north, and warm, subtropical air to the south. On average, the jet flows from west to east, but the jet often departs from average. The jet is continually rippling with U-shaped troughs of low pressure that allow cold air to spill southwards. The troughs are counterbalanced by upside-down-U-shaped ridges of high pressure that bring warm air northwards. When these ridges and troughs grow to unusually large amplitude, record extremes of both cold and heat occur adjacent to each other. Often times, the jet will have multiple extreme loops that result in unusual extremes over large portions of the Northern Hemisphere. That was the case Tuesday in Europe, where an unusually strong ridge of high pressure was present over Western Russia, with a companion strong trough of low pressure over the U.K. Moscow, Russia hit 29.7°C (85.5°F) on Tuesday, and several locations within the city rose to 31°C (88°F). According to weather records researcher Maximiliano Herrera, this has never happened in the first half of May before. At the same time, up to 3" of snow fell in the U.K. on Tuesday, a rare occurrence for mid-May. Could climate change be a factor in the extreme gyrations of the jet stream this year? It could, and I discussed some of the possible connections in my April 2013 post, Unusually cold spring in Europe and the Southeast U.S. due to the Arctic Oscillation.



igure 2. Monday afternoon's jet stream shows the pronounced ridge (upside-down U-shaped curve to the jet stream) over the Midwest U.S., which led to all-time record high temperatures for so early in the year. At the same time, a sharp trough of low pressure (U-shaped dip in the jet stream) was present over the East Coast, which allowed cold air from Canada to spill southwards and set record lows--for example, 44° at Tallahassee, Florida on May 14.

Jeff Masters and Christopher C. Burt