Denver Matches All-Time High of 105°F; Heat, Pollution Spread East This Weekend

June will segue into July this weekend with much of the central and eastern U.S. enduring a blistering, dangerous heat wave that could extend into the July 4 holiday in some areas. Excessive heat warnings were already in place Friday morning for parts of 11 states from Kansas to Michigan, and heat advisories for the upcoming onslaught extended all the way to Vermont.

Ample low-level moisture—perhaps boosted by “corn sweat”—will add to the misery of the high temperatures in many locations. The heat index, a measure of the combined effects of heat and humidity, was predicted to soar as high as 120°F on Friday and Saturday across parts of northern Illinois as dew point temperatures approach 80°F. The heat index could be in the 105-110°F range in the New York City area from Sunday into Tuesday.

All-time record highs matched or toppled from Colorado to Scotland

The dome of heat building into the Northeast U.S. gripped the Rockies on Thursday. Denver’s high of 105°F on Thursday matched its all-time high in data going back to 1872. The other dates that saw 105°F in Denver were June 25 and 26, 2012; July 20, 2005; and Aug. 8, 1878. Two other nearby cities set daily records—Colorado Springs, CO (100°F) and Cheyenne, WY (99°F)—that were just 1°F below all-time highs. Amid the intense heat, a wildfire in the Sangre de Cristo range of southern Colorado surged to envelop more than 14,000 acres by Friday morning, closing a major travel route (U.S. Highway 160).

All-time heat records will be a bit less likely across the central and eastern U.S., but many daily record highs and record-warm minimums can be expected. Triple-digit highs aren’t out of the question by Sunday in upstate New York, where such readings are very uncommon. The last time Albany, NY, got up to 100°F was on Sept. 3, 1953. 

Parts of Europe are also suffering through an intense early-summer heat wave, especially the United Kingdom. Thursday was the first day since 2013 that all four U.K. countries (England, Scotland, Wales, and Northern Ireland) saw a temperature of at least 30°C (86°F). The airport observing site at Scotland’s largest city, Glasgow, notched the city’s highest official temperature ever recorded Thursday: 31.9°C (89.4°F). It was so hot that a membrane on the roof of the Glasgow Science Centre—designed to be “weatherproof”—began to melt. The capital of Northern Ireland, Belfast, also broke its all-time high on Thursday at the airport observing site, with a high of 29.5°C (85.1°F) beating 29.4°C (84.9°F) from July 10, 1934. In western Ireland, Shannon set its all-time high with 32.0°C (89.6°F). According to weather records expert Maximiliano Herrera, the reading at Shannon is the hottest temperature recorded anywhere in June in Ireland since 1976. In July 2006, a temperature of 32.3°C was recorded in Ireland.

Extremely dry conditions have paved the way for the heat across northwest Europe. The Netherlands are expecting their driest June on record, with the De Bilt weather station now at a record-low June rainfall total of 12.1 mm (0.48”). England’s “home counties” surrounding London are on track to tie June 1925 as their driest on record; they’ve averaged just 3.3 mm (0.13”) for the month so far—about 6% of normal. Near Manchester, an unprecedented burst of moorland fires is ravaging the normally moist peat-bog countryside.

Figure 1. Firefighters tackle the wildfire on Saddleworth Moor, England that continued to spread on Thursday, June 28, 2018, after the blaze was declared a major incident by Greater Manchester Police. Image credit: Danny Lawson/PA Images via Getty Images.

Heat is one of the most dangerous weather hazards

It’s estimated by the U.S. Centers for Disease Control and Prevention that 150 to 300 people are killed in the U.S. each year directly by heat, with heat contributing to hundreds more deaths in some years. The worst year of the 21st century for U.S. heat deaths was 2006, with 2012 a close runner-up. Air pollution during heat waves can be just as dangerous as the heat itself (see below).

For those without air conditioning, the most dangerous set-up is a multi-day stretch of hot afternoons combined with very high overnight lows that keep interior spaces (and people) from cooling off. Fortunately, the U.S. has made great strides in heat awareness and heat safety over the 20 years since the horrific 1996 heat wave in Chicago that led to more than 700 deaths. For example, neighborhood cooling centers are now common in larger Midwest and Northeast cities, and media are paying more attention to heat risks.

Figure 2. Predicted air quality index (AQI) for Friday, June 29, 2018. Regions colored in orange are expected to see ozone levels in excess of the federal standard, reaching the “Unhealthy For Sensitive Groups” range. Image credit: U.S. EPA.

Dangerous ozone pollution event underway

This week’s heat wave is bringing the worst ozone air pollution thus far this year to much of the Midwest and Northeast United States. An Ozone Action Day was declared for 24 U.S. cities for Friday, including Cincinnati, Detroit, Dayton, Denver, Indianapolis, Louisville, Baltimore, Pittsburgh, St. Louis, and Salt Lake City. On Saturday, Ozone Action Days are up for 18 cities, mostly in New Jersey and Utah. Ground level ozone, which has been blamed for approximately 12,000 premature deaths per year in the U.S. between 2010 and 2016, is created from chemical reactions between volatile organic carbon (VOC) compounds and nitrogen oxides in the presence of sunlight. The chemical reactions that create ozone happen faster at high temperatures, and the current heat wave can be expected to cause one of the deadliest ozone pollution events of 2018. Expect to see many areas with ozone pollution topping out in the “Unhealthy For Sensitive Groups” (orange) range on Friday, Saturday, and Sunday. At this level of pollution, people who are sensitive to air pollution can see negative heath effects, though the general public is not likely to be affected. If pollution levels hit the “Unhealthy” (red) range, people who are sensitive to air pollution are at increased risk of stroke, heart attack and breathing problems, and even healthy people may experience discomfort. On an Ozone Action Day, you are encouraged to:

• Conserve electricity and set your air conditioner at a higher temperature.
• Choose a cleaner commute—share a ride to work or use public transportation. Bicycle or walk to errands when possible.
• Refuel cars and trucks after dusk.
• Combine errands and reduce trips.
• Limit engine idling.
• Use household, workshop, and garden chemicals in ways that keep evaporation to a minimum, or try to delay using them when poor air quality is forecast.

We've seen big dust outbreaks before, but the past week is easily a top 10 dustiest for the tropical Atlantic going back 15 years. #hurricanes pic.twitter.com/mUFUzmqUdo

— Michael Lowry (@MichaelRLowry) June 29, 2018

African Dust arrives in U.S.

Residents from Texas to Wisconsin will have an unusual pollution concern this weekend—fine particulate matter (PM2.5) from African dust. A large and impressive outbreak of  Saharan Air Layer (SAL) arrived in Texas on Thursday, and NASA’s GMAO model shows the dust pushing northwards over the weekend, reaching Wisconsin and Minnesota on Sunday. While this dust will probably not elevate PM2.5 levels above the federal air pollution standard, it will cause hazy skies and decreased air quality.

Memorable severe weather on Thursday: Three derechos and a rash of tornadoes

From the Dakotas to the Florida Panhandle, three major complexes of thunderstorms (mesoscale convective systems) produced three separate long-lived clusters of high wind from Thursday into Friday that each appear to have qualified as derecho events, based on analysis from weather.com’s Jon Erdman, who also notes that this was the most active 24 hours of severe weather for the year to date. As of Friday morning, the NWS/NOAA Storm Prediction Center had compiled more than 600 reports of high wind for the period from 7 am CDT Thursday to 7 am Friday. Gusts above 70 mph were reported in several states, especially across parts of North Dakota and Minnesota, knocking down countless trees and limbs. Just after 4 am CDT, the airport at Grand Forks, ND, reported a wind gust to 76 mph.

WHOA! We could watch this timelapse on repeat for days. Check out the storms firing up to the northeast of our office before the shelf cloud rolls through right at sunset. Incredible. #mobwx pic.twitter.com/zLpS5zfGRs

— NWS Mobile (@NWSMobile) June 29, 2018

Tornadic supercells roamed the sparsely populated High Plains of southeast Montana and the western Dakotas on Thursday evening. Damage was minimal, but at least 7 tornado reports were compiled by SPC. One long-lived supercell dropped several tornadoes as it moved from extreme southeast Montana into South Dakota.

Dr. Jeff Masters co-wrote this post.

Watch how the low-level shear rapidly increases in the vicinity of Thursday's tornado near the Montana/South Dakota border. Around 00z, 0-1km SRH was a mere ~100 m2/s2, but two hours later it was around 400 m2/s2, which is extremely favorable for tornadic supercells. #mtwx #sdwx pic.twitter.com/sciHFspAtS

— Quincy Vagell (@stormchaserQ) June 29, 2018