July 2018: Historic Extremes in the U.S.

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U.S. July 2018 Temperature/Precipitation Rankings: California hottest July on record while Pennsylvania observed wettest

Discussion: Mid-summer across the U.S. was hot! The national average temperature ranked 114 (of 124 years where 124 is hottest) in the 1895-2018 climatology for July. The West, Southwest and Northeast were much above average temperature with a record hot July observed in California (Fig. 1). The national precipitation ranking was a middle-of-the-road 60 but certainly an average of extremes featuring very dry conditions in the Northwest U.S. while Pennsylvania was wettest on record (Fig. 2).

The U.S. climate observations for July 2018 continue to produce an extreme signature due to the tendency of recent climate (past 5-10 years) to produce increasing number of weather patterns featuring stagnant high pressure ridge areas that cause a dry climate which dries soils and enhances attendant anomalous heat or stationary upper troughs causing excessive rainfall to produce historic flooding. This characteristic of climate will continue and is the result of a slower jet stream caused by lowering thermal contrast in the upper air that drives the jet stream. The lowering thermal gradient aloft in the middle latitudes is caused mainly by warming of the polar region.

During July 2018 the upper air pattern produced two impressive semi-permanent high pressure areas with one near New England and another stretching from the Gulf of Alaska to California (Fig. 3). The polar vortex was unusually strong during July at home base near the North Pole. Note the amplitude of these semi-permanent upper air features, a classic set-up to produce extreme weather and climate.

Climate forecasters use sea surface temperature anomaly (SSTA) patterns to help explain and predict climate regimes as their presence is driven by long-term climate patterns. Note that in July the high pressure ridge areas were across or just upstream from a very warm ocean surface (Fig. 4). The anomalous warm SSTA southwest of California was caused by the 2015 El Nino and despite changes since that time in the El Nino southern oscillation (ENSO) the anomalous warmth in the southeast North Pacific has remained. The tendency for high pressure ridging to occur across and downwind warm SSTA regions is responsible for long-term drought in California which was a significant contributor to the record hot July 2018.

In the high energy demand Los Angeles area excessive heat spikes have occurred with minimal recovery in-between each event accelerating the stress on the electric grid (Fig. 5). The first heat spike occurring July 6-7 produced record 104-108F heat. While following events were not as excessive there was essentially no break in the anomalous hot pattern which continues in early August including another heat spike August 7-8. As expected the super heat emerges following a long duration lack of rainfall (Fig. 6).

July 2018 started very warm in the Central U.S. but a pattern change brought much cooler air mid-to-late month. As the cool air mass shifted east the air mass stalled running into blocking high pressure. The upper trough causing the cooler Midwest air mass entrained subtropical moisture on its eastern periphery made more buoyant by the anomalous warmth of the ocean surface to the east. A perfect set-up for a 2-week period (Fig. 7-8) of heavy rainfall in the eastern U.S. causing a record wet month of July in Pennsylvania.

Fig. 1: U.S. statewide average temperature ranks by NOAA for July 2018.

Fig. 2: U.S. statewide average precipitation ranks by NOAA for July 2018.

Fig. 3: 500 MB anomaly analysis for July 2018 identifying two dramatic high pressure areas responsible for historic U.S. heat in the West and Northeast U.S.

Fig. 4: Global SSTA analysis for July 2018 identifies warm SSTA regions correlating with mid-summer 500 MB ridge areas causing anomalous heat in western North America and the Northeast U.S.

Fig. 5: The 90-day daily average temperature compared to normal in Los Angeles, CA.

Fig. 6: The 365-day daily accumulative precipitation compared to normal in Los Angeles, CA.

Fig. 7: Precipitation amount observations across the U.S. for July 15-21, 2018 identifying excessive amount in the northern Mid-Atlantic region.

Fig. 8: Precipitation amount observations across the U.S. for July 22-28, 2018 identifying excessive amount continuing in the Northeast U.S.