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.
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.
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.
There are quite a few personal weather stations in Bermuda that broadcast online. Particularly during daytime, some of these stations record temperatures that are likely too high. Regardless, these should all be considered unofficial. The official observations for Bermuda can be found at the Bermuda Weather Service, and are archived on their Climate page. Here is a list of PWS and other weather stations in Bermuda as of 26 July 2015:
A weak front slowly approached the island from the north all week and brought daily showers to the island for the last week. Some of those showers came with gusty winds with gusts measured over 40 kts at the airport. Isolated thunder was also in the area at times.
As of 3 am local time 26 July, the seven day rain total was 5.57″ at the Bermuda Weather Service – 3.63″ of that fell in the 24 hours ending at 3am local time. Most of that 3.63″ fell in the morning and late at night on the 25th. Keep in mind that 1971-2000 climatology has normal total monthly rainfall for July at 4.94″. The rains from this week’s slow moving/stationary front have completely erased the year-to-date rainfall deficit which was over 5″ at the start of the week.
Further, the rain and cloudiness kept temperatures quite low – a welcomed break from the heat and humidity that had set in on southwesterly flow around the Bermuda-Azores high and ahead of the front. In fact, rain cooled air led to a near record low temperature for the 25th of 72.1F – the record low stands at 69.1F set in 2007. The 1971-2000 normal low temperature for the 25th is ~78F.
Below are some rainfall measurements from unofficial sources around the island (via Wunderground) compared to the rain measured at the Bermuda Weather Service for the 24 hour period ending at 3am local time on 26 July.
Trimmingham Hill, Paget
Tucker’s Town, St. George’s
Ocean View, Southampton
Bermuda Weather Service, St. George’s*
St. David’s, St. George’s
Hinson’s Island, Warwick
McGall’s Bay, Smith’s
Knapton Hill, Smith’s
Gilbert Hill, Smith’s
Chaingate Hill, Devonshire (my PWS)
With more showery rain with a possible thunder and gusty winds in the forecast in the coming three days leading up to the Cup Match holiday, pay close attention to updates from the Bermuda Weather Service, including any advisories, watches, and/or warnings.
Tropical Storm Claudette quickly became post tropical Tuesday night south of Newfoundland where it was devoid of deep convection due to a combination of high vertical wind shear and low sea surface temperatures north of the Gulf Stream. Thus the Atlantic returns to quiet and no tropical cyclone formation is expected within the next five days. In Bermuda, a welcomed increase in the chances of showers with possible thunder will begin on Friday and potentially last through the weekend and into early next week. See the Bermuda Weather Service for the official forecast.
Meanwhile, the Pacific continues its active streak with three hurricane strength tropical cyclones impacting land simultaneously. Category three Hurricane Dolores last night passed over Mexico’s Socorro Island in the Eastern Pacific. The southern eyewall of Dolores brought sustained winds measured up to 80 mph with gusts as high as 115 mph to that island. Pressure fell as low as 968 mb as Dolores made its closest approach.
In the West Pacific, Typhoon/Tropical Storm Halola passed south of Wake Island. Halola formed in the Central Pacific and tracked westward into the West Pacific where it became a typhoon. Peak winds measured by the National Data Buoy Center (NDBD) at Wake Island reached 36 mph with gusts to 51 mph. Central-to-Western Pacific crossovers are unusual because so few tropical cyclones form in the Central Pacific. However, the above normal sea surface temperatures associated with El Niño tend to lead to a more active Central Pacific. A recent notable Central Pacific tropical cyclone was Hurricane Ioke in 2006, one of the most powerful tropical cyclones of Central Pacific origin on record. Ioke also passed over Wake Island and several low elevation atolls as a major tropical cyclone causing extensive damage in its path.
Halola is expected to continue tracking westward, deeper into the Western Pacific and slowly strengthen into a major typhoon over the next five days.
Finally, Typhoon Nangka is making landfall in Southern Japan as a category one equivalent typhoon. Heavy rainfall, particularly in the more mountainous areas has been accumulating for at least 36 hours now and the risk of flash flooding and mudslides is escalating as Nangka slowly approaches the coast. Hurricane conditions are expected to begin shortly on Shikoku and slowly spread northward with the typhoon. The main concern here is flooding – Tropical Storm Talas in 2011 took a similar approach to the coast with a similar intensity as Nangka and produced serious flash flooding and mudslides that resulted in dozens of fatalities.
The National Hurricane Center has determined that the non-tropical low to the northwest of Bermuda has become a tropical storm. On Saturday night, the non-tropical low moved off the coast of Virginia and North Carolina. It then quickly lost its connection to the dissipating front that extended eastwards to Bermuda and westwards into the Ohio Valley. Over the warm Gulf Stream waters, the low developed a well defined, closed surface circulation with deep and persistent convection near the center while losing its frontal characteristics – meeting the criteria to be designated as a tropical cyclone.
Deep layered southwesterly and west-southwesterly flow along the east coast of North America between the Bermuda-Azores High over the Atlantic and a trough over the continent is expected to steer the storm generally northeastward. This will take Claudette toward the Canadian Maritime Provinces, possibly impacting eastern Nova Scotia and Newfoundland after becoming post-tropical over cooler waters and higher wind shear north of the Gulf Stream. The remnants of Claudette then turn back toward the north then northwest as it gets absorbed into an extratropical low over Quebec.
Claudette is not expected to bring high winds, surf, or even rain to Bermuda during its lifetime. For the latest official forecast on Claudette, see the National Hurricane Center.
After a wet start to the year, mostly in February, Bermuda has fallen behind in precipitation. The Bermuda-Azores high has held a persistent ridge across the western Atlantic for the second half of Spring. As of July 9th, the airport was 3.81″ behind the normal year-to-date total precipitation. The last several days have featured a few hit or miss, passing isolated showers – no tank rain. However, an approaching cold front may change that. Keep in mind that a month’s worth of precipitation is 3-5 inches, and a week’s worth is around an inch.
A cell of high pressure to the southwest of Bermuda briefly allowed a period of light northerly winds over the last few days as a weak cold front dissipated in the area. Isolated areas of convergence and less stable air in the area led to isolated showers and thunderstorms for the middle of this week. Winds have since backed to the west and increased to moderate.
This weekend, another weak front is expected to become stationary in the area rather than dissipate. By early-morning Saturday, the cold front to the near north should enhance local convergence enough to produce isolated showers, a band of showers with a chance for thunder is possible Saturday afternoon along or just ahead of the front itself – depending on how well the boundary holds together as it approaches. The front then becomes stationary near the island, oriented from west to east as a wave of low pressure develops offshore of Virginia on Saturday evening. This keeps a chance for isolated showers and a risk for thunder in the area through Sunday.
The general expectation is for around 0.50″ of rain to fall from showers on Saturday and Sunday, but over an inch of rain is possible depending on the exact track and location of the heavier showers and thunderstorms ahead of and along the front as it approaches and becomes stationary. Since it has been very dry lately, this rain is very welcome.
Winds shouldn’t become an issue around this front. Expect today’s moderate westerly winds to continue into Saturday morning, veering northwesterly Saturday afternoon, and becoming light behind the front Saturday night. There is a slight chance for winds to become gusty in and around showers or thunderstorms, mainly ahead of the front. Winds remain variable at times on Sunday, but mostly light.
Monitor the progress of this weather with the Bermuda Weather Service and keep an eye out for possible Small Craft Warning and/or Thunderstorm Advisory.
The tropical Atlantic continues a quiet stretch after mid-June’s Tropical Storm Bill – no tropical cyclone formation is expected in the next five days. Meanwhile, the entire Pacific ocean has become very active; earlier this week there were three typhoons simultaneously in the West Pacific, a tropical storm in the central pacific and four other tropical disturbances with potential for tropical cyclone development.
Part of this activity can be attributed to an unusually strong phase of the Madden-Julian Oscillation (MJO), another part to unusually warm equatorial Central and Eastern Pacific waters associated with El Niño, and yet another to the climatological upswing in West Pacific tropical cyclone activity this time of year. The Western Pacific is the most active region in the world for tropical cyclones and so it is not uncommon to have multiple intense cyclones at the same time. The MJO is a wave that circles the globe from west to east, and makes one revolution roughly every 30-60 days. It consists of a region of enhanced convective activity and a more inactive region. The amplitude of this enhanced and reduced convective pattern is typically highest over the Indian Ocean and West Pacific. Tropical cyclones are known to form more readily in the enhanced convective phase of the MJO. The warmer than normal equatorial Central and Eastern Pacific waters associated with El Niño is also known to aid in tropical cyclone development in those regions.
Linfa has since dissipated over southeastern China where it made a landfall as a category one equivalent typhoon. Chan-hom is approaching eastern China for a possible landfall near Shanghai and later likely impacting the Korean peninsula and Japan as a much weaker cyclone. Chan-hom became a typhoon near Guam, Rota and Saipan bringing torrential rains there, then passed between Okinawa and Miyakojima as a category four equivalent typhoon within near hurricane conditions on Okinawa. Nangka has weakened from a brief run at Super typhoon status yesterday thanks to a concentric eye-wall cycle and some increased wind shear. Nangka is expected to continue westward for the next few days before being steered northward – possibly threatening Japan.
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.