Unusual Late June Warmth

20150620-20150625 Heat Index
Temperatures and Heat Index as observed at the Bermuda Weather Service. Dips in the temperature and heat index near the right side of the plot are related to isolated rain showers briefly lowering the temperatures. The area highlighted in orange includes heat index values in the “Extreme Caution” category.

Persistent southwesterly and west-southwesterly flow around the western side of the Bermuda-Azores High in combination with much warmer than normal sea surface temperatures in the Southwest Atlantic has led to a string of above normal temperatures and high humidity. The most recent forecast from the Bermuda Weather Service keeps this persistent flow in place through the end of June so there is little to suggest any significant changes to the temperatures or humidity during that period.

The normal daily maximum temperature this time of year is around 83F and maximum temperatures have been running near 86F for the past several days. Record maximum temperatures are near 88F and the highest recorded June temperature since 1949 is 90F at the airport. However, the temperatures are not the entire story – high humidity has come along with those higher than normal temperatures. The Dew Point temperature – a measure of the humidity – has also been consistently high. During this string of higher than normal temperatures, the dew point has been around 75F. Dew point temperatures above 70F are generally considered uncomfortable to oppressive.

The combination of high humidity and high temperatures has led to heat index values between 95F and 100F. The following advice or qualifications are attributed to the corresponding heat index values by the US National Weather Service: 90-105F “Extreme Caution”, 105-125F “Danger”, and >125F “Extreme Danger.” It should be noted that the heat index does not include the apparent cooling effects of wind or the apparent heating effects of sun. Further, inland and sheltered areas around the island could experience higher temperatures than at the airport where official observations are taken making this heat index value even higher.

The only inklings of a cool off would be found in any isolated shower activity over the next few days as a weak cold front approaches from the northwest. The approach of this front is expected to enhance surface convergence in the area which is expected to lead to isolated showers before the front dissipates to our north and lifts away to the northeast signalling the return to dry but humid southwesterly flow.

For Bermuda’s latest official weather information see the Bermuda Weather Service.

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