Extratropical Highlights –June 2017

 

1. Northern Hemisphere

The 500-hPa circulation during June featured above-average heights over the western U.S., Europe, and north-central Asia, and below-average heights over the high latitudes of both the North Pacific and North Atlantic and also over western Russia (Fig. E9). This anomaly pattern projected onto the strong positive phase of the East Atlantic (+2.0) teleconnection pattern and also onto the negative phase of the Scandinavia (-1.4) pattern (Table E1, Fig. E7).

The main land-surface temperature signals during June included above-average temperatures in the western U.S., western Alaska, Europe and north-central Asia, and below-average temperatures in northwestern Russia (Fig. E1). The main precipitation signals included above-average totals in the southeastern U.S., northwestern Russia, and China, and below-average totals in the north-central and southwestern U.S.  (Fig. E3).

 

a. North America

The 500-hPa circulation during June featured an amplified wave pattern across the U.S., with above-average heights in the west and an amplified tough in the east (Fig. E9). This pattern contributed to anomalously warm and dry conditions in the southwest and Plains states, and also to above-average precipitation in the southeast (Fig. E3).

Rainfall totals in large areas of the southwest were less than 25% of normal (Fig. E6). Area-average totals in both the Inter-mountain and Plains states were in the lowest 20^th percentile of occurrences (Fig. E5). In contrast, area-average totals in both the southeast and Gulf Coast regions were in the upper 90^th percentile of occurrences.

According to the U.S. Drought Monitor, drought ended in southern Georgia and central Florida. However, extreme drought developed in North Dakota and eastern Montana, and severe drought developed across the northern half of South Dakota.

 

b. North Atlantic/ Eurasia

The 500-hPa circulation during June featured above-average heights over Europe and north-central Asia, and below-average heights across the high latitudes of the North Atlantic and western Russia (Fig. E9). This anomaly pattern projected onto the strong positive phase of the East Atlantic (+2.0) teleconnection pattern, and onto the negative phase of the Scandinavia (-1.4) pattern (Table E1, Fig. E7). This pattern contributed to well above-average surface temperatures across Europe and in north-central Asia, with departures in both regions exceeding the 90^th percentile of occurrences (Fig. E1). It also contributed to anomalously cool and wet conditions (Fig. E3) in northwestern Russia.

 

c. China

Large portions of China recorded well above-average precipitation during July (Fig. E3), with country-wide totals above the 90^th percentile of occurrences (Fig. E4). This above-average precipitation was associated with anomalous upper-level divergence (Fig. T24) located within an enhanced right-entrance region of the east Asian jet steam (Fig. T21). These conditions were associated with an above-average strength of the southeast Asian monsoon.

 

2. Southern Hemisphere

The mean 500-hPa circulation during June featured an anomalous zonal wave-1 pattern, with above-average heights across the high latitudes of the South Pacific and Indian Oceans, and below-average heights over the high latitudes of the South Atlantic Ocean and also over Antarctica (Fig. E15). This pattern contributed to well below-average precipitation across southern Australia, with many areas recording totals in the lowest 10^th percentile of occurrences (Fig. E3). The pattern was also associated with above-average surface temperatures in southern Africa and central South America (Fig. E1).

The west African monsoon extends from June through October, with a peak during July-September. During June 2017, the west African monsoon system was enhanced (Fig. E3) with area-average totals exceeding the 90^th percentile of occurrences (see Sahel region, Fig. E4)