While setting hundreds of new heat records, the most notable aspect of the current heat wave gripping the central U.S. and spreading east is the associated humidity. In the upper Midwest Monday, the combination of heat and humidity brought widespread heat index values of 110+, with numerous 120 degree readings. The peak heat index occurred in Knoxville, Iowa, which reached a suffocating 131 degrees.
Animation of high temperatures from July 13-21 as the heat wave has progressed and is predicted to continue unfolding across the United States using the North American Mesoscale (NAM) model. (NOAA)
In addition to Knoxville’s 131, here are some other select heat indices from the Upper Midwest Monday (full list):
Freeport, Illinois: 124
Council Bluffs, Iowa: 126
Des Moines, Iowa: 114
Minneapolis, Minnesota: 117
Lincoln, Nebraska: 111
Pierre, South Dakota: 113
Madison, Wisconsin: 114
Weather Underground’s Jeff Masters noted Knoxville, Iowa’s 130+ reading is highly unusual in the United States.
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.
While nothing to sneeze at, Monday’s searing maximum temperatures (not counting the humidity) only resulted in 57 daily records. On the other hand, the high humidity levels made it very difficult for places to cool off which resulted in an astounding 288 record high minimum temperatures. Many record-setting locations did not drop below the upper 70s including Madison, Wisconsin. Minneapolis only fell to 80.
Heat indices today are forecast to range from 100-125 over much of the Midwest, with excessive heat warnings covering parts of 15 states.
Excessive heat and humidity levels reach the East Coast Thursday through Saturday. Heat indices are likely to reach 105 to 115 from Charleston, South Carolina through Washington, D.C. and into New York City Friday. Even Boston should experience heat indices near 100.
While dangerously high, the humidity won’t be quite as high along the East Coast as in the Upper Midwest Monday due to lower soil moisture levels. Also, locations in the Midwest see humidity levels elevated from the direct flow of air from the soupy Gulf of Mexico and due to the presence of corn fields which increase evaporation.
The actual air temperatures, while unlikely to reach historic levels, will still be quite hot. Jeff Masters predicts it will be a 1-in-7 year event for the East Coast:
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.
These conditions are sure to prompt widespread excessive heat warnings and heat advisories. In Washington, D.C. the National Weather Service is urging preparedness:
THE IMPACTS OF HEAT STRESS ARE CUMULATIVE...AND INCREASE EACH DAY THE HEAT LASTS. ANYONE WITH PLANS THURSDAY THROUGH SATURDAY OF SPENDING TIME OUTSIDE...OR INVOLVED IN STRENUOUS OUTDOOR ACTIVITY...WILL BE SUSCEPTIBLE TO SERIOUS AND POTENTIALLY LIFE THREATENING HEAT ILLNESSES. CITY AND COUNTY GOVERNMENTS SHOULD PREPARE FOR THE IMPACTS OF AN EXTENDED PERIOD OF EXCESSIVE HEAT
Post script: Some amazing heat records
Scanning the blogosphere and Twitter, I found the following records/statistics worth sharing:
* Wichita Falls, Texas has 54 days of 100+ temperatures in 2011 and 27 consecutive days reaching at least 100 (NWS Norman)
* In Tyler, Texas, readings have climbed to record triple-digits 32 days this year---21 of them in a row (Chicago Weather Center)
* From Paul Douglas (Minnapolis Star Tribune): “Oklahoma City temperatures have been 90 degrees or more for 47 straight days, topping a hundred nearly every day this month. With triple-digit heat possible through September, the city is on pace to break its record for such days (50, set in 1980).”
* Savannah, Georgia set a record for most consecutive days at or above 90 with 56 , breaking the old record by 12 days! (Capital Climate)
* “Mobile, AL had a record streak of 50 consecutive 90 degree days. It ended yesterday with a high of 82. The previous record 35 days in 1999.” (Paul Douglas)