Change the month, change the weather pattern

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Fig. 1: July 1-24 500 MB anomalies across the northern hemisphere.

Fig. 2: July 1-24 temperature rate anomalies across the northern hemisphere.

Fig. 3: July 1-24 precipitation rate anomalies across the northern hemisphere.

Discussion: There’s an old adage in climate: “change the month, change the pattern”. August 2018 is likely to validate that generalization. One cause of the change which climate scientists say is underestimated by forecasters is seasonality. As the greatest influence of solar heating on the northern hemisphere middle/high latitudes during July descends in August the climate pattern should naturally change.

Climate Impact Company maintains one of the greatest influences on northern hemisphere climate during spring/summer 2018 was an unusually cooler than normal sea surface temperature anomaly (SSTA) zone in the North Atlantic mostly south and southwest of Greenland cooling the atmosphere above maintaining the wintertime polar vortex. To balance-out the presence of the cold polar vortex in eastern Canada to Greenland an upper ridge causing unusual heat and dryness formed over North America and Europe.

Mid-summer brought a pattern change. The polar vortex did what it should do retreating to the North Pole atop snow and ice and the cold ocean just to the south. To compensate for this concentrated area of cold air an elongated high pressure ridge extended across the northern hemisphere mid-latitudes and typical of mid-summer, where high pressure ridging and dry soil moisture conditions met unusually hot weather developed. This pattern has produced countless historic anomalous hot events across the northern hemisphere during July 2018.

Extremes breed (more) extremes. The anomalous hot/dry July 2018 dominance was countered by historic rainfall episodes (also). South of the high pressure ridge stretched across the mid-latitudes of the northern hemisphere historic rainfall events occurred caused by an enhanced wet monsoon in Southeast Asia and subtropical low pressure areas in the eastern U.S. and southern Europe.

August is approaching and forecast models indicate another hemispheric pattern change is ahead. In 14 days a collection of all model (i.e. mega-cluster) upper air forecasts offers 2 primary solutions both favoring a blocking high pressure area over the polar region to Greenland. This forecast is a dramatic change from the mid-summer pattern.

Fig. 4: Favored (58%) model mega-cluster in 2 weeks and the consensus 500 MB anomaly forecast.

Fig. 5: Second-favored (42%) model mega-cluster in 2 weeks and the consensus 500 MB anomaly forecast.

August is approaching and forecast models indicate another hemispheric pattern change is ahead. In 14 days a collection of all model (i.e. mega-cluster) upper air forecasts offers 2 primary solutions both favoring a blocking high pressure area over the polar region to Greenland. Arctic oscillation (AO)/North Atlantic oscillation (NAO) forecasts validate the evolving high latitude blocking high pressure pattern as each index descends into negative phase for August. The forecast is a dramatic change from the mid-summer pattern.

In general, the upper air pattern projected in 14 days by the model mega-cluster suggests a west-to-east arc of anomalous warmth across the northern U.S. while the warmer than normal Gulf of Mexico could become tropically active. The pattern indicates high energy demand in California and the Northeast U.S. due to anomalous heat and humidity. Dryness emerges across the northern states and possibly the Corn Belt causing some late season crop stress.

The super-hot/dry summer affecting much of Europe shifts to Western Europe. Southwest Russia/Ukraine are more temperate and susceptible to some rain showers. Much of Russia observes pattern change as an amplified ridge gives way to a broad cooler upper trough.

The Southeast Asia monsoon could also be quite wet depending on strength and influence of the Madden Julian oscillation (MJO). However, a broad upper ridge centered on Mongolia should cause the hottest anomalies in the northern hemisphere during mid-August likely affecting at least northern China.

Summary: A new month brings a pattern change. August 2018 is no exception. The polar vortex is intense near the North Pole in the current northern hemisphere pattern but fades completely by mid-August causing vast changes in most the northern hemisphere middle latitude climate.