Forecast heat indices for Thursday, showing very hot conditions extending up the Eastern Seaboard (purple-pink indicates heat index of 105-115F) (NOAA)

A long-duration, widespread heat wave continues to bake virtually the entire central U.S., with a potentially deadly combination of high heat and humidity extending from Texas all the way to the Canadian border, oozing eastward during the course of this week.

During the weekend, the Central States and Upper Midwest suffered the most, with temperatures in the upper 90s to low 100s. When combined with dew points running in the mid-70s to near 80 - levels more typically experienced by residents of Louisiana than Wisconsin - heat index values soared into dangerous territory. And this heat event is not even close to being over, with a massive dome of high pressure sliding slowly east from the Plains States toward the Mississippi River Valley, eventually pumping warmer and more humid air into the mid-Atlantic.

The numbers tell the story of a brutally hot month so far. Through July 16, 969 daily high temperature records were either tied or broken in the country, including 12 all-time highest temperature records, most of which were set in Arkansas, Kansas and Oklahoma.

Nighttime lows have been extremely warm as well, with 1,924 records set or tied for daily highest minimum temperature. For example, on July 12, Richmond, Va., set a new benchmark for its warmest low temperature on record at 81 degrees Fahrenheit, breaking the old record of 79 degrees. This was the first time the low temperature failed to drop below 80 degrees since records began at that location in 1897.

The current heat wave threatens more record warm nights, a major concern since research shows that heat waves become much more deadly when overnight low temperatures stay very warm, depriving people of relief from excessively hot days. According to the National Oceanic and Atmospheric Administration (NOAA), in an average year, heat kills more people in the U.S. than any other weather-related hazard.

The heat has crossed the divide between the states that are suffering from an intense drought and those that have seen major flooding this year. Locations in Texas and Oklahoma - both drought states - have been roasting with temperatures at or above 100 F on a majority of days so far in July. In Stillwater, Okla., temperatures hit 100 F or above on every July day through the 16th, and the month is running nearly 9 degrees F warmer than average so far.

The heat in the drought region, particularly in Texas, Oklahoma and Arkansas, is tied to the dry conditions, since the absence of soil moisture allows more of the sun’s energy to go directly into heating the air. Of course, the hot weather further aggravates the drought by drying soils out even more, thereby making drought recovery more difficult.

U.S. drought monitor updated July 12, 2011, showing the extent of drought conditions across the country. (NOAA/NWS/NCEP/CPC)

In the wetter parts of the heat wave territory, such as Minnesota and Iowa, there is more water to evaporate from crops, swollen rivers and lakes, and therefore the heat is being paired with a fearsome dose of humidity. Dew points on Sunday afternoon were near 80 across the Upper Midwest, and the heat index - which indicates the combined effects of the air temperature and humidity levels - were reported to be as high as 126 degrees F. That is extremely perilous for human health, considering that the National Weather Service regards a heat index above 105 degrees as dangerous.

As CWG’s Jason Samenow wrote on Friday:

… Just to the north and northwest of where it’s been extraordinarily dry and soils are parched, it’s been extraordinary wet and soils are saturated. So when the huge heat ridge builds over the central U.S. over the weekend, intersecting the northern regions with high soil moisture, humidity levels will go through the roof in places like Minneapolis and Des Moines, even if actual air temperatures are slightly dampened.

When you add into the equation the abundance of “sweaty corn” around these same areas, humidity levels are going to be incredible. (Corn increases humidity by releasing large quantities of water into the atmosphere through evapotranspiration.)

Strong high pressure stirs up the heat. A sign of things to come?

An unusually intense, large dome of high pressure, which has parked itself across the nation’s midsection, is the proximate cause of the heat wave (preliminary data indicates the high may have already reached record strength for the central U.S.). Clockwise air circulation around this high is pumping warm, moist air up from the Gulf of Mexico. At the same time, sinking air associated with the high, from the top of the atmosphere down toward the surface, acts to squelch thunderstorm activity in all but its outer reaches, potentially leading to several days of severe weather in the northern periphery of the heat wave region while offering little chance at relief for the millions stuck in the middle.

Upper-air pattern from Sunday shows large dome of high pressure over central U.S. (NOAA)

National Weather Service hazards assessment outlining area to be affected by this week’s extreme heat. (NOAA)

To ensure that you and your loved ones are prepared for the upcoming heat, you may wish to read the Centers for Disease Control and Prevention’s heat wave preparedness guide, or this similar briefing page from FEMA. The bottom line: Use common sense, and avoid being outside (or even indoors without air conditioning) during the hottest period of the day to the greatest extent you can. Also, be sure to check up on friends and loved ones to ensure they do the same.