The year started out very variable with large swings in temperature between near record highs and record lows. Near the end of February a string of unusually mild and quiet weather persisted for several days with temperatures setting a new daily record high of 73.9°F (23.3°C) on the 26th, beating the previous record for the day of 73.0°F (22.8°C) set in 1975.
February’s warm and quiet spell was rudely interrupted in early March. An strong cold front brought very cold air to Bermuda, including the year’s only daily record low. This was part of a cold air outbreak that lasted several days. Gales and blustery showers with isolated hail accompanied a daily record low on the 5th March when the low reached 47.7°F (8.7°C) breaking the previous record of 52°F (11.1°C) set in 1968.
April through September was much quieter with smaller swings between extreme temperatures. Mild dry periods and the occasional cool rainy days with slightly higher than normal temperatures dominated this period.
As Autumn progressed, temperatures resumed their typical variability, but mainly swinging between normal and higher than normal temperatures. The year’s second daily record high temperature was set on the 7th of December when temperatures reached 77.4°F (25.2°C), beating the previous record high of 77°F (25.0°C) set in 1978.
The year started with a significant rain event on the 5th January on which a month’s worth of rain fell in a single day leading to some flooding. This 5.34″ (135.6 mm) of rain was the heaviest rain for a meteorological day at any time during the winter months (Dec-Jan-Feb), and the fifth wettest day on record.
Slightly below normal precipitation fell through the remains of winter and by mid-spring, extended dry periods were taking hold. April 2017 was the second driest April on record with only 0.73″ (18.5 mm) of rain falling in the entire month. This dry pattern continued through May and early June.
2.09″ (53.1 mm) of rain fell on the 11th June setting a daily record (beating the previous record of 0.53″ (13.5 mm) set in 1988) and signaling an end to the dry spell. From mid-June through the end of the year, near normal precipitation totals were observed. This maintained a year-to-date deficit of roughly 5.00″ (127.0 mm). Other daily records were 1.21″ (30.7 mm) on the 18th July (1.14″/29.0 mm, 1993), and 1.77″ (45.0 mm) on the 19th October (1.50″/38.1 mm, 1963).
Sea surface temperatures around Bermuda were far above normal for much of 2017. Light winds and prolonged periods of settled weather in spring and early summer limited the amount of mixing between the warm surface waters and the cooler water just below – priming the summer for above normal temperatures. This year no direct impacts from tropical cyclones was a welcome change to the pace set over the last decade, but also meant they were unable to provide the mixing that would cool the sea surface temperatures in late summer/early autumn.
The Bermuda Weather Service noted in monthly climate summaries that the monthly mean sea surface temperatures were among the five highest recorded since 1950 for September and October, and among the three highest for November.
Another part of this story is the presence of a nearby warm ocean eddy. This eddy lingered in the area for most of autumn. The eddy not only contributed to seas surface temperature anomalies, but also brought abnormally high sea surface heights. This came on top of elevated sea heights due to the astronomical Spring Tide phenomenon, and led to periods of generally minor coastal flooding throughout Autumn.