Extreme Jet Stream Bringing U.S. Record Heat, Record Cold, and Flash Flooding

The jet steam is exhibiting unusual behavior over the U.S., a pattern we've seen become increasingly common in summertime over the past decade. There's a sharp trough of low pressure over the Central U.S., and equally sharp ridges of high pressure over the Western U.S. and East Coast. Since the jet acts as the boundary between cool, Canadian air to the north and warm, subtropical air to the south, this means that hot extremes are penetrating unusually far to the north under the ridges of high pressure, and cold extremes are extending unusually far to the south under the trough of low pressure. The ridge over the Western U.S., though slowly weakening, is still exceptionally intense. This ridge, which on Sunday brought Earth its highest temperatures in a century (129°F or 54°C in Death Valley, California), was responsible for more record-breaking heat on Tuesday. July 2. Most notably, Redding, California hit 116°, just 2° short of their all-time record. Death Valley had a low of 104°, the second hottest night on record since 1920 (the hottest was just last summer!) Numerous daily high temperature records were set in Arizona, California, Nevada, Utah, Montana, Oregon, and Washington. It was the opposite story in the Central U.S., where the southwards-plunging jet stream allowed record cold air to invade Texas. Waco, Texas, hit 58°F this morning (July 3), the coldest temperature ever measured in July in the city. Numerous airports in Texas, Nebraska, Arkansas, Louisiana, Kansas, and Missouri set new daily record low temperatures this morning. And over the Eastern U.S., the northward-pointing branch of the jet stream is creating a potentially dangerous flooding situation, by pulling a moisture-laden flow of tropical air from the Gulf of Mexico over the Florida Panhandle north-northeastward into the Appalachians. Up to five inches of rain is expected over this region over the next few days, and wunderground's severe weather map is showing flash flood warnings for locations in Florida, Georgia, South Carolina, and North Carolina.


Figure 1. Jet stream winds in the upper atmosphere at a pressure level of 300 mb on July 3, 2013. The jet had an unusually extreme configuration for summer, with a sharp trough of low pressure over the Central U.S., and equally sharp ridges of high pressure over the Western U.S. and East Coast. Image from the wunderground jet stream page.


Figure 2. Predicted precipitation for the 7-day period ending Wednesday, July 10, 2013. Image credit: NOAA/HPC.

Third extreme jet stream pattern of the past five weeks
This week's extreme jet stream pattern is the third time in the past five weeks that we've seen a highly amplified ridge-trough pattern that has led to extreme weather. The others:

1) The end of May and beginning of June, when the $22 billion Central European floods occurred. A high pressure ridge became stuck over northern Scandanavia, causing all-time May heat records--as high as 87°F--at stations north of the Arctic Circle in Finland. The high pressure ridge blocked low pressure systems from moving north, and a series of two low pressure systems dumped record rains over Austria and Germany, creating the highest floods ever seen on portions of the Danube River. The $22 billion price tag made it the 5th most expensive non-U.S. weather-related disaster in world history.

2) June 18 - 22, when a ridge of high pressure over Alaska broke all-time heat records in the state, with unofficial readings as high as 98°F. A low pressure system became trapped over Alberta, Canada, bringing the city of Calgary a $3 billion flood disaster. This was the most expensive flood in Canadian history, and third most expensive natural disaster of any kind for the country. The only more expensive disasters were a 1989 wildfire ($4.2 billion) and a 1977 drought ($3 billion.)

As I discussed in a March 2013 post, "Are atmospheric flow patterns favorable for summer extreme weather increasing?", research published this year by scientists at the Potsdam Institute for Climate Impact Research (PIK) in German found that extreme summertime jet stream patterns had become twice as common during 2001 - 2012 compared to the previous 22 years. One of these extreme patterns occurred in August 2002, during Central Europe's previous 1-in-100 to 1-in-500 year flood. When the jet stream goes into one of these extreme configurations, it freezes in its tracks for weeks, resulting in an extended period of extreme heat or flooding, depending upon where the high-amplitude part of the jet stream lies. The scientists found that because human-caused global warming is causing the Arctic to heat up more than twice as rapidly as the rest of the planet, a unique resonance pattern capable of causing this behavior was resulting. According to an email I received from German climate scientist Stefan Rahmstorf, one of the co-authors of the study, unusually extreme jet stream amplitudes likely played a role in the May - June Central European flooding event.

Tropical disturbance in the Gulf of Mexico
Yellow means caution: the National Hurricane Center (NHC) has drawn a yellow circle on their Graphical Tropical Weather Outlook around an area of disturbed weather in the Gulf of Mexico. In their 8 am EDT July 2 outlook, NHC gave the region a 10% chance of developing into a tropical depression or tropical storm by Friday. Satellite loops show only minimal heavy thunderstorm activity associated with the disturbance, which is suffering from high wind shear of 20 - 30 knots. Dry air due to the presence of an upper-level trough of low pressure over the Western Gulf of Mexico is also interfering with development. The upper-level trough is expected to weaken and pull to the north late this week, bringing more favorable conditions for development over the Southern Gulf of Mexico's Bay of Campeche by Friday. The atmosphere will moisten and wind shear may fall to the moderate range, 10 - 20 knots. The disturbance should move west to west-northwest, arriving near the Texas/Mexico border region on Monday or Tuesday. None of the reliable forecast models predict the disturbance will develop.

Elsewhere in the Atlantic
A large upper-level cold-cored low pressure system over the middle of the North Atlantic will move to the southwest during the week, and this low is expected to arrive in the Bahamas by Sunday and South Florida by Monday. Although the models do not show that this low will will acquire a surface circulation and develop tropical characteristics, it will be worth watching for development late this week.

Jeff Masters