North Atlantic Cool SSTA Pool Linked to May 12-18 U.S. Severe Weather Pattern

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Summary: Over the next 7 days (or more) a persistent frontal boundary aligns from the Midwest U.S. into the Northeast Corridor separating cool air across Canada linked to an unusually strong upper trough caused by cooling of the North Atlantic south of Greenland and a drought-induced upper ridge over the Southwest States. Many severe weather events develop along this frontal boundary beginning this weekend lasting through next week bringing gully-washer rains that cause flooding, damaging wind and possible tornadoes from severe storms and plenty of cloud-to-ground lightning from Chicago to Philadelphia.

Discussion: Since 2013 (with the exception of much of last year) a persistent cool pool of ocean water has appeared south of Greenland. The proposed cause of the cool pool is the fresh water flow to this region from Greenland ice melt. The cool pool is inhibiting the northeast flowing Gulf Stream from reaching the northern North Atlantic with usual intensity.

Across this wide area of cool oceanic surface water (Fig. 1) which has been intensifying the past 30 days (Fig. 2) the atmosphere also cools creating a persistent low pressure trough. The low pressure trough has been present much of spring but has recently intensified (Fig. 3). Occasionally, the upper trough expands westward to Canada while to the east a strong ridge pattern has dominated the springtime pattern in Europe.

Upper level features of this intensity can shape the prevailing upper features downstream around the particular hemisphere. Being aware of and tracking these regional cool (and warm) SSTA regions (or “blobs”) is a critical factor in assessing (and predicting) climate. Many cool and warm blobs have appeared in the global oceans in recent years and are significant contributors to extreme climate.

Fig. 1: Sea surface temperature anomaly (SSTA) analysis identify the North Atlantic cool pool south of Greenland.

Fig. 2: Sea surface temperature anomaly (SSTA) 30-day change analysis indicates the cool pool south of Greenland is strengthening.

Recent examples are the warm blob in the northeast North Pacific in 2013-15 causing drought in western North America and contributing to the wintertime “polar vortex” pattern during North America winter 2013-14 and 2014-15. A warm blob east of Australia helped produce record heat (and drought) in eastern Australia this past summer season. A warm blob southwest of California/Baja California present now has contributed to warmth and drought affecting the Southwest U.S.

Fig. 3: 7-day 500 MB anomaly analysis identifying a strong ridge-trough-ridge pattern from the U.S. to Europe. Strong 500 MB features lead to weather extremes.

The cool pool south of Greenland was first noticed in 2013 and persisted for several years before temporarily fading last year. The cool pool of 2013-16 was linked to the diminished number of hurricanes in the North Atlantic. Curiously, the cool pool faded last year and a near record warm North Atlantic contributed to an unexpectedly ferocious hurricane season.

There is an apparent link between the cool pool south of Greenland and SSTA conditions in the tropical North Atlantic. The North Atlantic tropics cooled significantly in early-to-mid spring as the cool pool south of Greenland also intensified. Forecast models are slow to warm the each region for tropical cyclone season which starts June 1. If the models are correct the 2018 tropical cyclone season may revert to the inactiveness (of hurricanes) observed 2013-16.

Fig. 4: The NOAA/WPC 7-day rainfall amount forecast indicates excessive amount across the Ohio Valley to the Northeast Corridor (and Florida).

The upper trough stretched across the North Atlantic cool pool is a semi-permanent feature and will contribute to general climate and increase the risk of extremes heading into summer. The most recent example of extremes is the super warm climate across Europe beneath the upper ridge pattern downwind the North Atlantic trough.

Another extreme event takes place in the U.S. the next week or so. The nature of this extreme event is gully-washer rainfall and severe thunderstorms. The NOAA 7-day quantitative precipitation forecast indicates 2-4 in. of rain from the Corn Belt to the Northeast Corridor (Fig. 4). These amounts may be too minimal as operational models indicate 5-6 in. near Chicago, IL and Albany, NY.

The persistent location of what may be 4 to 5 severe weather outbreaks over the next 7 days in this stretch is caused by a climate boundary between the cooler weather to the north caused by the upper trough present due to the cool SSTA south of Greenland and the drought induced Southwest U.S. ridge pattern.

Conclusion: Tracking regions of unusual warmth or coolness in the global oceans outside of the El Nino southern oscillation (ENSO) regime has become paramount to explaining and forecasting climate and especially extremes in recent years. The strengthening cool pool south of Greenland is the most recent active example. The atmosphere is affected such that dramatic weather events occur such as the extreme warmth of European spring and over the next week or so persistent severe weather from the Midwest to the Northeast Corridor.