Midwest heat wave generates a heat index of 131° in Iowa; Bret no threat
A unusually intense, long-lasting, and widespread heat wave with high humidities continues to plague the Central U.S. The heat index--how hot the air feels when factoring in both the temperature and the humidity--exceeded 100°F in twelve states on Monday and thirteen on Sunday, with the dangerous heat extending from Texas northwards to North Dakota, Minnesota, and Wisconsin. At least thirteen deaths are being blamed on the heat in the Midwest. The heat index hit a remarkable 131°F at Knoxville, Iowa on Monday, and a heat index in excess of 120° was recorded at numerous locations in Iowa, Minnesota, and Illinois.
Figure 1. Predicted maximum heat index for Thursday, July 21, 2011. Portions of 33 states are predicted to have a heat index in excess of 100°F. Image credit: NOAA.
A 1-in-7-year heat wave coming for the Eastern U.S.
The extreme heat will shift slowly eastwards this week, peaking in Chicago on Wednesday, Detroit and Pittsburgh on Thursday, and New York City and the mid-Atlantic states on Friday. Temperatures near 100°F are expected in Detroit on Thursday and New York City on Friday. Detroit has hit 100°F 18 different years in its 137-year record, and New York City 22 years out of the past 140 years, so this heat wave is expected to be about a 1 in 7 year event. While that makes it a notable heat wave, the most remarkable feature of this July 2011 U.S. heat wave is the humidity that has accompanied the heat. A heat index over 130°F, such as was observed yesterday in Iowa, is very rare in the U.S., and extremely dangerous. According to Christopher C. Burt, wunderground's weather historian, the only place in the world where a heat index over 130°F is common is along the shores of the Red Sea in the Middle East. A major reason for the remarkably high humidities accompanying the July 2011 Midwest heat wave is the record flooding the region experienced over the past three months. As pointed out by wunderground's climate change blogger Dr. Ricky Rood in his latest post, with hundreds of thousands of acres of farmland still inundated by flood waters, and soils saturated over much of the Upper Midwest, there has been plenty of water available to evaporate into the air and cause remarkably high humdities. This makes for a very dangerous situation, as the human body is not able to cool itself as efficiently when the humidity is high. The extreme heat will last through Sunday in the mid-Atlantic, but should end on Monday, when a cold front is expected to sweep across the region. However, the latest models suggest the heat will re-amplify over the Midwest next week.
Figure 2. Tropical Storm Bret at peak intensity, at 5:15 pm EDT on Monday, July 18, 2011. At the time, Bret was building an eyewall and had begun to clear out an eye. Image credit: NASA/GSFC.
Tropical Storm Bret weakens
Tropical Storm Bret enjoyed a brief burst of intensification yesterday afternoon that brought the storm's winds up to 65 mph. However, Bret has sucked in a lot of dry air today, and is now a much weaker storm with winds of just 50 mph. Satellite imagery shows that Bret has not improved in organization this morning, thanks to dry air to the northwest that has been blown into the storm's core by upper-level northwesterly winds. Wind shear is a high 20 - 25 knots, and is expected to remain in the high range for the next three days. The combination of high wind shear and dry air nearby should act to keep Bret from strengthening, and the storm should slowly decay as it heads out to sea over the next few days. Bret is not a threat to any land areas.
None of the reliable models predict tropical cyclone development over the remainder of the Atlantic through July 26.
Weakened Typhoon Ma-on nearing Japan
Typhoon Ma-on has weakened to minimal Category 1 typhoon with 75 mph winds, and is battering a large swath of the south coast of Japan with tropical storm-force winds, as the storm slides northeast along the coast. Ma-on is expected to turn eastwards out to sea by Wednesday, and is not likely to bring tropical storm-force winds to the troubled Fukushima Nuclear Plant.