U.S. Hail And High Wind Event

The Warm Tropical North Atlantic Index
04/30/2021, 9:08 am EDT
Big Warm-up Equatorial Upper Ocean Heat
05/03/2021, 12:20 pm EDT
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Highlight: Record hail size! High winds! Why?

Fig. 1: Severe weather reports from April 28 featuring a record hail size in Hondo, TX just west of San Antonio and following high wind gusts in the Northeast Corridor beginning April 29th and peaking April 30th.

Discussion: Scientists estimate the largest hailstone on record in Texas (over 6 in. across in diameter) occurred near Hondo, Texas last Wednesday night (Fig. 1). Three hail storms struck that night in Norman, OK, Dallas, TX and San Antonio, TX. Widespread damaging hail was reported…2-3 in. diameter hail was common. $1B damage is projected across the Dallas Metroplex due to this event. A single massive hail storm in this region is not unusual in this region at this time of the year. However, three massive hail storms are unprecedented.

The same storm caused an unusually widespread area of high wind gusts in the Northeast Corridor beginning the following day and peaking in strength last Friday. Wind gusts of 50-65 mph were reported at all major airports from Washington, DC to Boston, MA.

Can we say anything about the climate pattern causing these unusual events?

In April a persistent Greenland high-pressure ridge indicating presence of a strong negative North Atlantic oscillation (-NAO) emerged. -NAO patterns produce upper trough patterns over or near the Northeast U.S. On average, the past 30-days has featured a “wicked” upper trough to the east of New England (Fig. 2). Also typical of persistent and strong -NAO patterns are the inevitable evolution of a polar vortex pattern which emerged in late April (Fig. 3).

Fig. 2-3: Upper air pattern leading to the record hail/high wind event mid-to-late last week.

Causing the Texas/Oklahoma hail storms and following high wind event in the Northeast U.S. was the merging of the polar vortex and persistent trough off the East Coast. The thermal gradient (which drives accelerated wind speed) was dramatic between the late winterlike cold atmosphere over central Canada into the Northeast U.S. clashing with the early summerlike warmth across the far southern U.S. The energetic atmosphere created by these merging set of circumstances is responsible for the weather extremes.

While this explanation is synoptic meteorology-oriented. There is a significant aspect of climate that contributed to this event. During late winter/early spring Climate Impact Company made an observational comment that the ocean surface east of New England and south of the Canadian Maritimes had warmed to record levels for that time of the year. Ocean surface temperatures averaged 5-10F warmer than normal in this region with parts of the Gulf Stream 11-13F warmer than normal. The cold air flowing from Canada off the Northeast U.S. Coast into this region of unusually warm air caused the upper trough east of New England to be unusually intense.

The warm water east of New England and south of the Canadian Maritimes is the result of the North Atlantic Warm Hole (NAWH) climate pattern which intensified during this past decade causing an area of cooler waters south-southeast of Greenland slowing the northeastward charging Gulf Stream and forcing warm water to pile-up east of the Northeast U.S.

Currently, the SSTA pattern east of New England and south of the Canadian Maritimes remains quite warm (Fig. 4). However, the storms described above have caused widespread cooling the past 30 days in this sector of the North Atlantic (Fig. 5). SSTA trends are linked to climate forecasts. Consequently, the East U.S. is revised cooler for May in an updated report issued shortly.

Fig. 4-5: The western North Atlantic remains quite warm but the persistent trough(s) moving off the Northeast U.S. in April have caused vast cooling.