The U.S. Summer is Off to a Record-Hot Start

Last month was the warmest June in 122 years of U.S. recordkeeping, beating out June 1933, according to the monthly climate roundup released on Wednesday by NOAA’s National Centers for Environmental Information (NCEI). Each of the 48 contiguous states came in above its average temperature for June, with Arizona and Utah setting all-time June records for heat. Thirteen other states had a top-ten-warmest June, stretching across the nation from California to Florida.


Figure 1. Statewide rankings for average temperature during June 2016, as compared to each June since 1895. Darker shades of orange indicate higher rankings for warmth, with 1 denoting the coldest month on record and 122 the warmest. Image credit: NOAA/NCEI.


Figure 2. Statewide rankings for average precipitation during June 2016, as compared to each June since 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 122 the wettest. Image credit: NOAA/NCEI.

The nation’s biggest weather calamity of June was the catastrophic flash flood in West Virginia that took at least 23 lives and destroyed more than 1500 homes. In contrast, the month as a whole was marked by unusual dryness across most of the country: it was the 14th driest of the 122 Junes on record. Only one state (Wyoming) had a top-ten-driest June, but the general lack of moisture was most evident across the northern Great Basin, the northern and central Great Plains, and the states from New Jersey and Pennsylvania northeastward. Grand Island, Nebraska, saw just 0.05” of rain, smashing its dry-June record of 0.43” from 1922.

Could this end up as the hottest summer in U.S. history?
In line with a global climate that’s being warmed by greenhouse gases, the contiguous United States has seen six of its ten warmest summers on record in just the last 15 years. On that basis alone, 2016 has a reasonable shot at becoming our hottest summer yet, especially with the head start provided by a record-warm June. On the other hand, there is plenty of inherent variability from week to week and month to month, even in weather that’s averaged across the country.


Figure 3. Top ten hottest summers (June-August) for contiguous U.S., with the rankings of each month and the summer as a whole for the period 1895-2015 (1 = hottest). Data courtesy NOAA/NCEI.

At right, Figure 3 shows how each month played out during our ten warmest summers on record. Each of those blazing summers had at least one month that fell below the top-ten list for that respective month, showing how difficult it is to maintain the kind of unusual warmth we saw in June for an entire summer.


Figure 4. Departures from average temperature (anomalies) across the contiguous U.S. for the period July 1-10, 2016. Image credit: NOAA Climate Prediction Center.

As shown in Figure 4 (right), the first third of July was relatively cool across the Pacific Northwest, Midwest, and New England, with above-average heat continuing across the South and much of the West. This week should continue on the mild side across the northern U.S. as an unusually potent upper-level low for midsummer progresses eastward. This low has already generated a wild variety of weather over the last several days, including a round of torrential rains, flash floods, and tornadoes on Monday centered in Minnesota and Wisconsin. A phenomenal 24-hour rainfall total of 10” reported on Tuesday morning at Wascott, WI, isn’t too far from the state’s 24-hour record of 11.72”, set in 1946. The storms were fueled by extremely high amounts of atmospheric moisture: the dew point at Sioux Falls, SD, hit 82°F on Monday, apparently setting a new all-time record high dew point for the city (h/t to Minnesota meteorologist Paul Douglas for this statistic). On its way to the Midwest, the upper low delivered a blitz of accumulating snowfall and freezing temperatures to the northern Rockies, including parts of Yellowstone National Park and the Grand Tetons above 7000 feet.

As for next week, the ECMWF and GFS models have been remarkably consistent on developing a very strong upper-level high sprawling across much of the nation during the latter half of July. It’s too soon to know exactly how fierce the heat will be, or where its epicenter will be located, but the models suggest that temperatures may challenge the 100°F mark as far north as the Dakotas by later next week, with 90s enveloping most of the nation east of the Rockies for what could be an extended period. The 8-14 day outlook from the NWS Weather Prediction Center shows high odds for above-average temperatures over the entire contiguous U.S. except for the Pacific Northwest, with odds favoring below-average precipitation for most of the Plains and mid-South. If the heat manifests as expected, it may be enough to counterbalance the northern mildness so far in July and keep 2016 in the running for warmest U.S. summer on record, particularly if August stays on the hot side.


Figure 5. An ominous cloud associated with fast-moving thunderstorms sweeps across Aberdeen, South Dakota, on Monday, July 11, 2016. Image credit: wunderphotographer stuswan.


Figure 6. Enhanced infrared image of Hurricane Celia (left) and the smaller Tropical Storm Darby (right) as of 1530Z (11:30 AM EDT) Tuesday, July 12, 2016. Image credit: NOAA/NESDIS.

Celia weakens; Darby develops
The procession of tropical cyclones across the eastern North Pacific that began with the formation of Tropical Storm Agatha on July 2, followed by Hurricane Blas (named on July 3) and Hurricane Celia (named on July 8), continued on Tuesday morning with the formation of Tropical Storm Darby. As of 11 AM EDT Tuesday, Darby was located about 500 miles south of the southern tip of Baja California, moving west at 10 mph with top sustained winds of 40 mph. Although it will be heading west atop very warm sea-surface temperatures, Darby may also encounter cold water churned up in the wake of Hurricanes Blas and Celia. The National Hurricane Center outlook brings Darby to Category 1 strength by Friday, then weakens it by the weekend.

Meanwhile, Hurricane Celia is now a Category 1 storm, located about 1300 miles west-southwest of southern Baja California as of 11 AM EDT Tuesday, with top sustained winds of 90 mph. Celia topped out as a Category 2 hurricane with peak winds of 100 mph on Monday night. Now angling toward the northwest at 12 mph, Celia is expected to gradually bend back westward while slowly weakening over cooler waters during the next couple of days. Based on long-range runs of the GFS and ECMWF models, there is still a chance that a weakened Celia or its remnants could pass just north of the Hawaiian Islands early next week, bringing some high surf and a chance of squalls, but it is too soon to assign any confidence to this possibility.

Both the European and GFS models show an area of disturbed weather will develop several hundred miles southwest of the coast of Mexico by the end of this week, and this disturbance has the potential to intensify into a tropical storm over the weekend. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave this future disturbance 2-day and 5-day odds of development of 0% and 60%, respectively. Both Darby and the next potential storm (which would be named Estelle) are expected to follow paths similar to Agatha, Blas, and Celia--generally west to west-northwest, away from Mexico. If the Eastern Pacific manages to spit out a Tropical Storm Frank before the end of the month--which is quite possible, given the long-range forecasts of the continued presence of the MJO over the Eastern Pacific into the end of July--this would give us six named storms for the month, which would approach the July record (from 1985) of seven named storms forming in the Eastern Pacific, according to NHC hurricane scientist Eric Blake.

Bob Henson and Jeff Masters