A look at weather extremes and the big-picture climate connections.

Heat Wave Expands Again Before Retreating to the West

The grueling, protracted heat wave is still affecting much of the Upper Midwest and Plains on Tuesday, while utility crews have make progress restoring power to the Mid-Atlantic region after unusually severe thunderstorms tore across the region on June 29 in an event forecasters call a "derecho."

According to a Weather Channel analysis of climate data provided by the National Climatic Data Center, the 190 reports of all-time warm temperature records during the June 25 to July 1 period was enough to nearly double the total for the entire decade thus far.

Forecast high temperatures for Friday, July 6. Credit: NOAA.

Triple digit heat is in the forecast on Tuesday for areas along and to the southeast of a line from eastern Montana to Minnesota. Minneapolis-St. Paul is facing the very real prospect of a 100°F Fourth of July holiday, prompting heat watches and warnings. Such warmth is unusual in the Twin Cities, with just 63 occasions of 100°F temperatures or higher since records began there in 1873. 

Chicago came close to 100 degrees on Monday, with a high of 98°F, which was the second-hottest high temperature of the year so far, according to WGN-TV meteorologist Tom Skilling. Skilling noted that Chicago was warmer than New Orleans, Houston and Tampa on Monday. Very hot conditions are expected in Chicago through Friday, with high temperatures of 15 to 20 degrees above average.

The extreme heat also will affect states such as Arkansas, Illinois, Iowa, Missouri, and Mississippi on Tuesday. The heat will shift eastward on July 4, with 95 to 100°F temperatures encroaching on Washington, D.C. , Baltimore and Philadelphia once again.

Here’s how the National Weather Service put it in a technical forecast discussion on Tuesday: “WELL ABOVE NORMAL TEMPERATURES ARE EXPECTED FROM THE CENTRAL PLAINS EASTWARD TO THE OHIO VALLEY AND SOUTHWARD TO THE SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY.  AS THE RIDGE EXPANDS EASTWARD OVER THE NEXT FEW DAYS...MUCH OF THE EASTERN THIRD OF THE COUNTRY WILL SEE A RESURGENCE OF THE HEAT EXPERIENCED LAST WEEKEND.”

The heat has already made its mark on U.S. temperature statistics. During the June 26 to July 2 period, 2,120 daily high temperature records were set or tied in the U.S., along with 722 daily warm overnight low temperature records. More than 3,000 daily record highs were set or tied during June alone, and for the year so far, warm temperature records have been outpacing cold temperature records by a ratio of 7-to-1.

In a long-term trend that demonstrates the effects of a warming climate, daily record-high temperatures have recently been outpacing daily record-lows by an average of 2-to-1, and this imbalance is expected to grow as the climate continues to warm. According to a 2009 study, if the climate were not warming, this ratio would be expected to be even. Other studies have shown that climate change increases the odds of extreme heat events and may make them warmer and longer lasting.

Extended-range forecasts do show the potential for a shift in the weather pattern to take place during the second week in July, with the prospect of cooler weather for the East, and much warmer conditions in the West.

A "shelf cloud" seen at the leading edge of the severe thunderstorms as they swept across Indiana on June 29, eventually knocking out power to more than 3 million people. Credit: NOAA/Kevin Gould.

Meanwhile, storm cleanup continues in the Washington, D.C. area, where thunderstorms featuring hurricane-force winds uprooted trees and caused a severe disruption to the electric grid. As of Tuesday, the Washington Post reported that about 200,000 people in the Washington region were still without power, and patience with the local utility companies is running out.

The paper reported that four deaths have occurred in Maryland due to the heat wave, along with eight deaths in Virginia.

The Post published a scathing editorial criticizing officials and utility company executives for failing to adequately prepare for extreme weather events, among other potential threats.

“The storm gave rise to massive inconveniences and discomforts across the Washington area. Usefully, it also exposed the region’s absence of reliable fail-safes, spotty preparedness and sluggish response times in the face of emergencies. Now it’s up to leaders to identify and act on those shortcomings,” the editorial board wrote.

Climate change is expected to increase the risk of some types of extreme weather events, including heat waves and possibly severe thunderstorms as well. By adding heat and humidity to the atmosphere, manmade global warming may increase the instability that sets off severe thunderstorms, although the research into such small-scale storms is still at the early stages. An extremely unstable atmosphere, fed by all-time record heat above 100°F, was a key ingredient in the June 29 derecho event, as the Capital Weather Gang blog has detailed.

As a recent United Nations report pointed out, adapting to extreme weather and climate events is no easy task, and the derecho demonstrates that it is not just a challenge for population centers in developing countries. Even major cities in the U.S. are woefully underprepared for the extremes that already take place.

Note: I will be taking part in a Google+ Hangout with Stanford University climate scientist Noah Diffenbaugh on Thursday, July 5 at 4 pm eastern. We will be discussing the recent extreme heat and severe weather in the context of a warming climate. You can sign up for it here.

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