Intense heat wave bakes the Eastern U.S.

Intense heat seared large sections of the U.S. on Thursday, with dozens of new daily high temperature records adding to the formidable number of new records piling up this week. On Wednesday, 140 daily maximum temperature records were tied or broken, according to NOAA's National Climatic Data Center. This represents over 2.4% of all stations in the U.S., which is an exceptionally high number of records for one day. Over the past 30 days, daily high temperature records have outpaced low temperature records by more than 4 to 1, 1859 to 453, and by almost three to one over the past year. Daily high temperature records set yesterday included 100° at Detroit, the first time in sixteen years that city has seen the century mark. Two hyperthermia deaths were reported in the Detroit area, bringing the heat wave death toll for the U.S. to 24 for the week. Newark, NJ hit 103°, just 2° below that city's all-time record hottest temperature of 105°. That record may be challenged today, as the temperature in Newark at 11am was already 100°. Other notable temperatures yesterday included 101° in Syracuse, NY, only 1° below that city's all-time high of 102°; 95° in Binghamton, NY, 3° below their all-time high; 102° in Toledo, 3° below their all-time high; 102° in Raleigh, 3° below that city's all-time high of 105°. Accompanying the heat was high levels of air pollution, which also contributes to mortality. Air pollution reached code red, "Unhealthy", in Gary Indiana yesterday, and was code orange, "Unhealthy for Sensitive Groups" in thirteen other states.

The blast furnace-like conditions will continue today across much of New England and the mid-Atlantic, where high temperatures are expected to climb above 100° in Washington D.C., Baltimore, and New York City. Air pollution is expected to exceed federal standards and reach code orange, "Unhealthy for Sensitive Groups", in at least 18 states today, according to the latest forecasts from EPA. The pollution will be worst in Washington D.C. and Baltimore, where "code red" conditions--"Unhealthy"--are expected. The heat will continue in the mid-Atlantic states through Sunday, then ease on Monday when a cold front is expected to move through.


Figure 1. July temperatures in the lower 48 states between 1895 - 2010 showed a warming of about 1.2°F (red line) during that time period. The warmest July on record was 1936, with an average temperature of 3.1°F above average. The year 2006 was a close second, just 0.1°F behind. If model projections of an increase in U.S. temperature of 4 - 6.5°F by 2100 are correct, an average July in 2050 will have temperatures warmer than the record warm temperatures of 1936. Image credit: NOAA/NCDC.

The summer of 2011's place in history
July 2011 is on pace to be one of the five hottest months in U.S. history, but may have a tough time surpassing the hottest month of all time, July 1936. In that year, the dry soils of the Midwest's Dust Bowl helped create the most extreme heat wave in U.S. history during July. Wunderground's weather historian, Christopher C. Burt, has a look back at this great heat wave in his current post. I expect that by the time July 2011 is done, it will be a top-five warmest July on record, but will not surpass July of 1936 or July of 2006 (which holds second place, just 0.1° cooler than July 1936.) The summer of 1936 was also the hottest summer in U.S. history. That mark will also be tough to surpass this year, since June 2011 was the 26th warmest June on record, and June 1936 was the 11th warmest. August 1936 was the 4th warmest August on record. At this point, there's no telling how warm August 2011 will be, though NOAA's Climate Prediction Center is forecasting a much above average chance of warmer than average conditions over 95% of the contiguous U.S for the first week of August.


Figure 2. The 8 - 14 day outlook from NOAA's Climate Prediction Center predicts much above average chances of warmer than normal temperatures during the last few days of July and the first four days of August.

Climate change and U.S. heat waves
The heat index--how hot the air feels when factoring in both the temperature and the humidity--has been exceptionally high during this week's heat wave, due to the presence of very high amounts of water vapor in the atmosphere. That has made this heat wave a very dangerous one, since the body is much less able to cool itself when the humidity is high. The high humidities in the Midwest were due, in great part, to the record rains and flooding over the past few months that have saturated soils and left farmlands flooded. Today's extreme heat index values over the mid-Altantic are due, in large part, to near record warm ocean temperatures off the mid-Atlantic coast. According to the UK's HADSST2 data set, sea surface temperatures between 35° - 40°N and 75 ° - 70°W, along the coast from North Carolina to New Jersey, were 5.4°F (3.0°C) above average during June 2011. This is the warmest such temperature difference for any month in the historical record, going back to the 1800s. The most recent sea surface temperature anomaly maps from NOAA show that the July ocean temperatures have not been quite as extreme, but ocean temperatures in this region during July have averaged nearly 2°C above average, the second highest July ocean temperatures on record, behind 2010.

During the 1930s, there was a high frequency of heat waves due to high daytime temperatures resulting in large part from an extended multi-year period of intense drought. By contrast, in the past 3 to 4 decades, there has been an increasing trend in high-humidity heat waves, which are characterized by the persistence of extremely high nighttime temperatures. In particular, Gaffen and Ross (1999) found that summer nighttime moisture levels increased by 2 - 4% per decade for every region of the contiguous U.S. between 1961 - 1995. Hot and humid conditions at night for a multi-day period are highly correlated with heat stress mortality during heat waves.

Not surprisingly, the frequency, intensity, and humidity of heat waves is expected to increase dramatically in coming decades, if the forecasts of a warmer world due to global warming come true. A study presented in the U.S. Global Change Program Impacts Report, 2009, predicted that by 2080 - 2099, a heat wave that has a 1-in-20 chance of occurring in today's climate will occur every 2 - 3 years over 95% of the contiguous U.S. (Figure 3.) I estimate that this week's U.S. heat wave has been a 1-in-5 to 1-in-20 year event for most locations affected, so heat waves like this week's will be a routine occurrence, nearly every year, by the end of the century. According to a study published by scientists at Stanford University last month, though, this may be too optimistic. In their press release, lead author Noah Diffenbaugh said, "According to our projections, large areas of the globe are likely to warm up so quickly that, by the middle of this century, even the coolest summers will be hotter than the hottest summers of the past 50 years."


Figure 3. Simulations for 2080-2099 indicate how currently rare extremes (a 1-in-20-year event) are projected to become more commonplace. A day so hot that it is currently experienced once every 20 years would occur every other year or more frequently by the end of the century under the higher emissions scenario. Image credit: U.S. Global Change Program Impacts Report, 2009.

Arctic sea ice continues its record retreat
Sea ice in the Arctic continues to melt at the fastest pace in recorded history, as July ice extent has been averaging 5 - 10% less than the record low values set in 2007. According to the July 18 update from the National Snow and Ice Data Center, the rapid decline in the past few weeks is related to persistent above-average temperatures, and an early onset of the melting season due to especially low snow cover in Europe and Asia during May and June. High pressure and clear skies have dominated in the Arctic this summer, but that pattern is changing. The latest 2-week forecast from the GFS model shows that low pressure will dominate the Arctic for the next two weeks, bringing cloudier skies and less melting. This will likely slow down the melting enough so that sea ice loss will no longer be on a record pace by the 2nd week of August.

Tropical Storm Cindy
Tropical Storm Bret is dead, and Tropical Storm Cindy is moving over very chilly waters of 20°C, and does not have long to live. Cindy is not a threat to any land areas.

Invest 90L: an African wave worth watching
An African wave (Invest 90L) near 14N 55W, 400 miles east of the Lesser Antilles Islands, is moving west-northwest at about 15 - 20 mph. This wave is generating a limited amount of heavy thunderstorms due to the presence of a large amount of dust and dry air from the Sahara, and will spread heavy rain showers and strong gusty winds to the northern Lesser Antilles tonight through Saturday. The wave has a modest degree of spin to it, and is under low wind shear, 5 - 10 knots.

Dry air will continue to be a problem for 90L through Sunday, but once it finds a moister environment near the Bahama Islands early next week, it could develop. However, the expected track of the disturbance takes it over the rugged terrain of Hispaniola, which would inhibit development. Furthermore, wind shear is expected to rise to the moderate range, 10 - 20 knots, on Saturday, and could increase further by Monday, according to most of the computer models. Of the latest 00Z and 06Z runs of the four reliable models for predicting formation of a tropical depression, only the NOGAPS model shows development of 90L. The NOGAPS predicts the wave could attain tropical depression status on Tuesday, over the northwestern Bahama Islands just off the coast of Southeast Florida. The other models generally depict too much wind shear for the wave to develop. Right now, the deck appears stacked against development for 90L through at least Monday. NHC is predicting a 20% chance of development by Sunday. The eventual track of 90L next week has been trending more to the south in recent model runs, as they are generally depicting a weaker trough of low pressure developing over the Eastern U.S. This reduces the chances 90L will move up the U.S. East Coast, and increases the chances that it will enter the Gulf of Mexico.


Figure 4. Satellite image of Hurricane Dora taken July 20, 2011 by NASA's Aqua satellite.

Hurricane Dora in the Eastern Pacific weakening
Hurricane Dora in the Eastern Pacific put on an impressive burst of intensification yesterday, topping out as an impressive Category 4 storm with 155 mph winds, just 1 mph short of Category 5 status. However, high wind shear acted to knock a hole in Dora's eyewall, which has now collapsed, and steady weakening of the storm will occur today. Dora is expected to move parallel to the coast of Mexico, and should not cause any major trouble in that country. Dora is the second major hurricane in the East Pacific this year; Hurricane Adrian topped out as a Category 4 storm with 140 mph winds in early June. A NOAA P-3 aircraft will be investigating Dora over the next few days, to learn more about how Eastern Pacific hurricanes weaken when they move over colder water.

Vacation
This will be my last post until Thursday, unless 90L gets far more interesting than the current forecast. I'm headed up north to Lake Michigan to cool off and relax for a few days. In my absence, Angela Fritz will be handling the blogging duties, and she will have a post on the latest forecast for 90L on Saturday. Angela is on Pacific time, so her posts will be later in the day than I make them.

Jeff Masters