July 2018: Hottest Month in California History, Record-Wet in Mid-Atlantic

July was California's its hottest month in 124 years of recordkeeping, according to NOAA’s monthly summary of U.S. climate released Wednesday. For the contiguous U.S. as a whole, it was the 11th hottest July on record, with almost every state coming in warmer than average. The national average of 75.5°F was 1.9°F above the 20th-century norm, said NOAA.

California’s average for July of 79.7°F was 0.2°F above the state’s previous hottest month on record, July 1931. In addition, several communities in California and adjacent Nevada had their all-time hottest single month. These include:

Palm Springs, CA:  97.4°F (previous 97.2°F in July 2006)
Fresno, CA:  88.2°F (previous 87.8°F in July 2006)
Bishop, CA:  81.8°F (previous 80.8°F in July 2017)
Reno, NV:  81.8°F (previous 80.5°F in July 2017 and July 2014)
Tonopah, NV:  79.7°F (previous 78.9°F in July 2017)
Winnemucca, NV:  78.1°F (previous 77.5°F in July 2014)
Elko, NV:  77.2°F (previous 76.8°F in July 2013)
Blue Canyon, CA:  74.2°F (previous 73.7°F in August 2012)

Most notably, Death Valley recorded an astounding monthly average in July of 108.1°F. This is the highest monthly average known to have been reliably measured at any station in the world, based on the global archives maintained by NOAA’s National Centers for Environmental Information (NCEI). See our revised Cat 6 post, originally published July 31, for more details.

The blazing temperatures in California, combined with the normally parched summer climate, helped to stoke wildfires that are continuing to ravage the state. These include the Mendocino Complex, the state’s largest fire on record and the first to top 300,000 acres.

Figure 1. Statewide rankings for average temperature for July 2018, as compared to each July since records began in 1895. Darker shades of red indicate higher rankings for heat, with 1 denoting the coldest month on record and 124 the warmest. Image credit: NOAA/NCEI.

Eight states across the U.S. West had a top-ten-hottest July, including California. The nation’s other hot pocket was New York and New England, where each state had a top-ten-hottest July. Both Caribou, Maine, and Burlington, Vermont, saw their warmest average on record for July and for any other month. In Caribou, the July average of 70.9°F was the first time any month has topped 70°F in records going back to 1939. The 76.0°F in Burlington exceeded all months going back to 1892.

A soaker in the mid-Atlantic

The latter half of July was very moist across the Eastern Seaboard, thanks to a persistent upper low over the Ohio Valley and Appalachians—quite unusual for midsummer—that kept moisture streaming across the mid-Atlantic. Pennsylvania saw its wettest July on record, with a statewide average of 7.37” beating out 7.14” from 1992. It was the second-wettest July in Maryland records: the state’s 8.73” average was more than 0.90” above all other July totals, topped only by an amazing 10.70” from 1945.

Figure 2. Swatara Creek flooding envelops the Duke Street Bridge in Hummelstown near Hershey, Pennsylvania, on Wednesday, July 25, 2018. Days of relentless downpours left central Pennsylvania a soggy mess, closing roads and businesses, sending creeks and streams spilling over their banks and requiring rescues and evacuations. Image credit: Sean Simmers/The Patriot-News via AP.

Baltimore had its wettest July in 148 years of recordkeeping. The city received 16.73”, more than 5” above its previous record from July 1889. The only wetter month in Baltimore history was August 1955, when two hurricanes passed nearby.

Washington, D.C., ended up with its fourth-wettest July on record through a strange juxtaposition, as reported by Capital Weather Gang. The first half of the month was record dry, with no measurable rain; the second half of July was the wettest on record.

Dry conditions held firm across the U.S. West, including the Northwest. Idaho saw its sixth-driest July on record, and Seattle picked up just 0.05” of rain compared to its long-term July average of 0.70”. Unless a major pattern change occurs, the widespread Western dryness will leave the region vulnerable to wildfire for weeks to come. In its August 1 outlook, the National Interagency Fire Center projected above-average wildland fire potential in August for most of California, Oregon, Washington, and Idaho.

Figure 3. Statewide rankings for average precipitation for July 2018, as compared to each July since records began in 1895. Darker shades of green indicate higher rankings for moisture, with 1 denoting the driest month on record and 124 the wettest. Image credit: NOAA/NCEI.

The heat continues to accumulate in 2018

Boosted by the extreme warmth of May, the last three months have been the warmest May-to-July stretch in U.S. weather history, with an average of 70.9°F topping the 70.6°F observed in 1934.

For the year so far (the period from January through July), 2018 is the eleventh warmest year on record for the contiguous U.S. This is the warmest January-to-July in more than a century of recordkeeping for both Arizona and New Mexico, and it’s among the five warmest in California, Nevada, Utah, and Colorado.

Pennsylvania experienced its wettest January-to-July on record, and Maryland had its second wettest, as did West Virginia. The year to date has been wetter than average over a broad swath from Wyoming and Montana eastward across the Corn Belt and Ohio Valley into the mid-Atlantic, as well as for the lower Mississippi Valley and Florida.

Thanks to WU weather historian Christopher Burt for gathering data used in this post.