Fair Holiday Weather


After a full seven days of heavy, thundery and locally squally showers sparked by a dissipating stationary front in the area, the weather changed its pace and cleared out for the holiday. The Bermuda-Azores high is rebuilding a ridge in from the southeast and that is providing a return to fair summertime weather for the coming week. The Bermuda Weather Service measured 7.51″ of rain during that seven day period – more than a month’s worth of rain that brought the dry spell to an abrupt end! Highlights of this event include the 3.63″ of rain that fell on the 25th, near record low temperatures on the 25th and 27th, and the squall on the 27th. This unsettled weather brought July’s rain total to 8.94″ at the Bermuda Weather Service. See BWS for the latest official forecasts.

The Squall

Generally light winds were the theme for most of the 27th. However, just after noonday, a line of showers developed to the west of the island and began moving eastward. Just before the showers began to cross the island from west to east they became thunderstorms. The generally light winds that persisted into the early afternoon suddenly increased to strong with gale or near-gale force gusts as the torrential rain and associated rain-cool downdrafts of the thunderstorms passed overhead. Just as quickly as the weather came on, the rain and wind died off as the thunderstorms exited to the east of the island.

Squall
Temperature, Dew Point, Pressure, Wind, and Rain Rate from my PWS during the squall. Plotted with R. From about 12:30pm to 4:45pm 27 July 2015. Further brief but gusty showers were observed later in the evening.

At my PWS, 1-minute sustained winds increased from less than 5 mph (4 kts) to a peak of 23 mph (20 kts) just after 2:00pm local time. Peak gusts reached 36 mph (31 kts). This coincided with heavy rain with rainfall rates approaching 7 inches per hour and a dip in temperature from 80.3 F to 73.1 F. Further, as evidence of the downdraft, there was a spike in pressure by about 0.5 hPa during the heaviest rains and strongest winds. Similar squally conditions were observed by other PWS mainly in the central and eastern parishes around the island.

Squall : “A sudden increase of wind speed by at least 16 knots, the speed rising to 22 knots or more and lasting for at least one minute. It is often accompanied by showers or thunderstorms.” (BWS Glossary – search ‘squall’) While the conditions were just shy of this definition at my PWS, observations at the airport met the criteria for a squall: winds increased from 5 to 23 kts (an 18 kt increase) between 1:55pm and 2:12pm. The peak gust at the airport from this squall was 35 kts. Similar wind gusts were observed at the airport in other showers during the week’s heavy rains, but the increase in wind speed around the showers wasn’t sufficient to define them as squalls.

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