Active Tropics

The northern hemisphere tropics have taken another active turn. Starting in the Atlantic, after Danny dissipated in the northeast Caribbean, tropical storm Erika formed not too far behind in the central Atlantic. Erika then tracked mainly westward, following a similar track to Danny. Unlike Danny, however, Erika was a larger storm that struggled with wind shear its entire life. Erika tracked through the northeast Caribbean, crossed Haiti then Cuba where it degenerated into a tropical wave.

Erika’s heavy rains resulted in deadly flooding in parts of the northern Lesser Antilles islands. The remnants of Erika could cause hazardous flooding in parts of Florida and contribute to rainfall occurring across much of the southeastern United States over the next three days. As of 8am EST 30 Aug, the National Hurricane Center is giving the remnants of Erika a low chance of regeneration over the next 5 days as they track into the eastern Gulf of Mexico and then get drawn into an area of low pressure in the southeastern United States.

Tropical Storm Fred between the West Coast of Africa and the Cape Verde islands. Aug 30 2015 at 14:45UTC
Tropical Storm Fred between the West Coast of Africa and the Cape Verde islands this morning.

In an unusual turn, Tropical Storm Fred has formed to the southeast of the Cape Verde Islands. Typically it takes several days over the open water for African easterly waves to organize into tropical cyclones and by then they are to the southwest or west of the Cape Verde Islands. In Fred’s case, the robust wave emerged into the Atlantic and almost immediately became a tropical cyclone. Fred is forecast to track northwestward, through the Cape Verde Islands and steadily strengthen along that path. Fred is expected to be over the Cape Verde Islands by Monday afternoon. This has warranted the issuance of Hurricane Warnings for those islands.

Terra MODIS satellite imagery from yesterday showing the three Pacific hurricanes. Hawaii can be seen between Kilo and Ignacio.
Terra MODIS satellite imagery from 29 Aug 2015 showing the three Pacific hurricanes. Hawaii can be seen between Kilo and Ignacio.

Meanwhile, three major hurricanes are simultaneously churning up the waters of the Tropical Pacific ocean. From west to east, Hurricane Kilo formed in the Central Pacific and very slowly organized into a tropical storm then hurricane. Kilo is about to cross into the Western Pacific area of responsibility where it will be renamed a Typhoon. After that, Kilo is forecast to take a turn to the northwest, but then turn back towards the west by Wednesday, some fluctuations in intensity are expected as vertical wind shear fluctuates along the path. Kilo is largely in the open Pacific ocean away from populated land.

Hurricane Ignacio formed in the Eastern Pacific and tracked westward into the Central Pacific. Ignacio is expected to pass north of the Hawaiian Islands starting on Tuesday. Elevated surf seems to be the primary threat there, but strong winds and gusty showers are also possible and tropical storm conditions are possible in those higher elevations – tropical storm watches are in effect for several of the Eastern Hawaiian islands. Ignacio is expected to weaken as it passes north of the islands as vertical wind shear increases.

Finally, Jimena is tracking westward through the Eastern Pacific. On this track, Jimena is forecast to cross into the Central Pacific by Wednesday. Jimena is forecast to remain far from any land for the next five days.

All three of these hurricanes were at category four strength as of the 11am EST update from the National Hurricane Center and Central Pacific Hurricane Center.

None of the tropical cyclones out there are currently a threat to Bermuda within the next three days. Stay tuned to the Bermuda Weather Service for the latest official forecasts. For the latest on the above mentioned cyclones see: National Hurricane Center | Central Pacific Hurricane Center.

Danny Feels the Effects of Shear

Danny 23 Aug 15 1845UTC
Tropical Storm Danny continuing its approach on the Northern Lesser Antilles Islands. The center of the cyclone (~15.5N, ~58.5W) is occasionally exposed between intermittent bursts of deep convection. GOES-floater RGB satellite imagery at 3:45pm local time.

Last night the strong vertical wind shear that was forecast to begin hindering Hurricane Danny’s intensification started to take its toll on the small cyclone. Visible satellite imagery before sunset last night showed the surface circulation pushing westward while the deep convection weakened and drifted northeastward leaving the center exposed – as a result Danny was not able to maintain hurricane status and has weakened to a tropical storm. Intermittent bursts of deep convection continue over the center of Danny, but the surface circulation is still exposed at times. The hope is that a weakened Danny will be able to bring beneficial rains to the northeastern Caribbean where unusually dry conditions have been the theme this summer.

Last night, Danny tracked to the left (south) of yesterday afternoon’s forecast track; likely under the influence of a more westerly low level steering flow rather than the deeper layered west-northwesterly steering flow. This means that it is now more likely that Danny will pass through, not north of, the Lesser Antilles Islands where tropical storm watches and warnings are in effect, and then the Greater Antilles for early next week. So the combination of strong vertical wind shear and resulting dry air entrainment combined with forecast land interactions along this more southerly track means that it is very likely that Danny will degenerate into a tropical wave and become diffuse over the mountains of Hispaniola by Tuesday night. This land interaction also reduces the chances for redevelopment past Hispaniola to near nil.

two_atl_5d0 (1)
National Hurricane Center 5-Day Tropical Weather Outlook. For the next five days: A tropical wave in the central tropical Atlantic (red ‘x’) has a high chance of becoming a tropical cyclone as it tracks through the red swath.

Elsewhere in the Atlantic, the tropical wave that was following Danny has maintained an area of deep convection and is showing signs of organization. This wave is much larger than Danny was but is expected to follow a similar track through the next five days. The National Hurricane Center is giving this wave a high (70%) chance of becoming a tropical cyclone by Tuesday afternoon, and a high (80%) chance of becoming a tropical cyclone by Friday afternoon as the wave slowly organizes on its quick west-northwestward track. Dry and dusty Saharan Air to the north of the wave combined with an encounter with the same area of strong vertical wind shear that Danny has struggled with are obstacles to this wave’s development through day five. Threat to land is small at this time and is at least five days out. Further, any threat to Bermuda is still low and would be at least 8 days out. This will allow plenty of time to track changes in the wave’s development and fine tune its track.

Monitor the National Hurricane Center and the Bermuda Weather Service forecasts for the latest official information regarding the track and intensity of Atlantic tropical cyclones and weather in Bermuda.

Hurricane Danny Nears Lesser Antilles

A low formed to the southwest of Bermuda as forecast. However, it is becoming less likely that it will acquire sufficient tropical characteristics to be classified as a subtropical or tropical cyclone as it drifts northward past Bermuda to the east today. Earlier forecasts suggested that this low would pass to the west of the island bringing a slug of widespread shower activity and strong southeasterly winds for the weekend. However, that forecast evolved through the week and the low passed to the near east of the island overnight Friday into Saturday. This kept that area of widespread showers and strong winds to the east of the island and made for much more settled weather for the weekend with light and variable winds with only an isolated shower at times. Further, the threat for this non-tropical low to acquire sufficient tropical characteristics and become a subtropical or tropical cyclone is diminishing as environmental conditions become increasingly less favorable for that to occur.

NOAA Hurricane Hunter P3 Aircraft transmitted radar data during their mission in Hurricane Danny Friday afternoon at 16:21 UTC
NOAA Hurricane Hunter P3 Aircraft transmitted radar data during their mission in Hurricane Danny Friday afternoon at 16:21 UTC. Image from TropicalAtlantic.

Meanwhile, tiny tropical storm Danny strengthened quite stunningly into a major hurricane between Thursday and Friday morning. NOAA P3 Hurricane Hunter Aircraft investigated both the low-level structure of the hurricane and the surrounding upper atmosphere to better assess how conducive the surrounding environment is for Danny’s future. Hurricane Hunter aircraft transmitted some airborne radar data back along with the flight measurements in real time for public dissemination for what could be the first time. Another mission this afternoon will give an updated observational fix on the location and structure of the cyclone and the state of the surrounding environement.

Small hurricanes are often plagued by sudden and extreme swings in intensity. Low vertical wind shear in addition to favorable sea surface temperatures and a pouch of deep tropical moisture that was drawn along with Danny as it pulled northward out of the inter-tropical convergence zone allowed the hurricane to exhibit a burst in intensity that was somewhat greater than expected. The hurricane hunter aircraft found Danny as a category 3 hurricane with roughly 115 mph maximum 1-minute sustained winds and a minimum central pressure near 974 mb.

However, on the flip side, as small cyclones can quickly strengthen under ideal conditions, only slight deviations from ideal conditions can result in a sudden collapse in the cyclone’s convective structure and subsequently rapid weakening. Danny has since weakened from Friday’s intensity and is expected to continue a weakening trend on its approach to the Lesser Antilles Islands.

Despite the jump in strength, Danny is still expected to continue west-northwestward. During this period, an increase in vertical wind shear is expected to allow nearby dry air associated with the Saharan Air Layer to be entrained into the circulation. The dry air forces convection to collapse – this disrupts a tropical cyclone’s circulation and results in weakening. By Monday morning, Danny is expected to be a moderate tropical storm on the doorsteps of the northernmost Lesser Antilles Islands where Tropical Storm Watches are in effect. Danny should then pass near or north of the Virgin Islands and then Puerto Rico Monday night into Tuesday morning – continuing to weaken. Past Tuesday morning the fate of Danny is very uncertain.

[See Latest NHC Track]

Danny could succumb to the vertical wind shear, subsequent dry air entrainment, and land interaction and degenerate into a tropical wave before getting near or north of Hispaniola where environmental conditions begin to improve – the remnant tropical wave would likely dissipate if it were inland over the Dominican Republic or Haiti. Alternatively, Danny could maintain enough of a low level circulation to begin regenerating if it gets north of Hispaniola and avoids significant land interaction. For this reason we cannot rule out that Danny could become a threat to the Turks and Caicos Islands, the southeastern Bahamas Islands on Wednesday and Thursday, or possibly even Bermuda in the following week. It is therefore imperative that constant vigilance be paid to the official products from the Bermuda Weather Service and the National Hurricane Center as the forecast progresses, new information is processed, and confidence in Danny’s ultimate fate increases.

National Hurricane Center 5-Day Tropical Weather Outlook. For the next five days: The low near Bermuda (Yellow 'x') has a low chance of becoming a (sub)tropical cyclone over the next five days as it tracks through the associated yellow swath. A tropical wave near the Cape Verde islands (orange 'x') has a medium chance of becoming a tropical cyclone as it tracks through the orange swath. Finally, a second tropical wave over Africa has a low chance of becoming a tropical cyclone over the next five days as it emerges into the Atlantic near the Cape Verde islands and tracks west-northwestward through the overlapped yellow swath over the next five days.
National Hurricane Center 5-Day Tropical Weather Outlook. For the next five days: The low near Bermuda (yellow ‘x’) has a low chance of becoming a (sub)tropical cyclone over the next five days as it tracks through the associated yellow swath. A tropical wave near the Cape Verde islands (orange ‘x’) has a medium chance of becoming a tropical cyclone as it tracks through the orange swath. Finally, a second tropical wave over Africa has a low chance of becoming a tropical cyclone over the next five days as it emerges into the Atlantic near the Cape Verde islands and tracks west-northwestward through the overlapped yellow swath over the next five days.

Elsewhere in the Atlantic, a robust tropical wave has moved off the west coast of Africa and is showing signs of early organization near the Cape Verde Islands. This wave has been given a 60% chance of becoming a tropical cyclone in the next 5 days by the National Hurricane Center. This wave is forecast to become a much larger cyclone than Danny, but track westward or west-northwestward into the central tropical Atlantic similar to Danny. Another tropical wave, still inland over Africa also has potential to develop once it emerges into the far eastern Atlantic. Both of these systems will be far out to sea for at least the next five days, and likely past that and so neither of them are immediate threats and there is plenty of time to monitor them.

Active Weather Pattern Develops

Aug 19 15 1445UTC
RGB Satellite imagery at 14:45UTC showing the cloudiness associated with shower and thunderstorm activity in a band running from North Carolina to just south of Bermuda.

Tuesday morning saw locally squally thunderstorms as a north-south oriented line of thunderstorms crossed parts of the island dumping up to unofficial amounts of 2″ of rain in highly isolated spots, however some areas, particularly in the western parishes, did not pick up nearly as much rain. Gusts in and around these thunderstorms generally reached 30-40 kts, an increase from the light and variable winds that had preceded the storms overnight. My PWS recorded 1.12″ of rain for the 18th of August, the Bermuda Weather Service had the same amount.

The line of thunderstorms that dominated the weather scene Tuesday morning formed along a weak surface trough and was supported by an upper level low that was sat to the northwest of Bermuda. This upper level low is slowly drifting southeastwards, towards the island and the best upper level divergence to support thunderstorm activity is also drifting southeastwards. Low level convergence associated with the surface trough in combination with the upper level low will support clusters of showers and thunderstorms that could pass over the island producing locally heavy rain and gusty winds at times with clearer periods.

A more organized surface trough is then expected to develop to the west of Bermuda. At this point, the upper low is expected to still be in the vicinity of Bermuda. The developing surface trough is then expected to stall near to the southwest of Bermuda overnight on Thursday. An area of low pressure could form from this surface trough near to the island as it starts to lift out of the area to the north as we approach the weekend. The interaction of the developing surface low and weakening upper low is a recipe for a potential subtropical transition.

8am 19 Aug 2015
National Hurricane Center chart showing the area being monitored for possible tropical or subtropical development – the yellow area indicates a low chance of a cyclone forming. Link in-text below for the latest chart.

Regardless, the formation of a surface low to the southwest of Bermuda will result in winds likely settling out of the southeast and gradually increasing to moderate/strong with gusts near gale force at times starting Thursday night. The National Hurricane Center has given this potential system a low (30%) chance of becoming a tropical or subtropical cyclone in the next 5 days as of 9am BDA time. The Bermuda Weather Service is also monitoring this potential development and expect the forecast to be fine tuned as the weekend approaches and it becomes more certain whether a cyclone will develop.

Elsewhere in the Atlantic, Tropical Storm Danny quickly formed in the east-central tropical Atlantic out of an African Easterly Wave – a kick-off to the notorious Cape-Verde Season. The National Hurricane Center forecast for Danny indicates steady strengthening to a hurricane by Friday morning as the cyclone moves west-northwestward. This track takes Danny just to the east of the Windward Islands by Monday morning.  Danny is therefore not expected to pass near Bermuda in the next five days, but should be monitored for any track changes, especially for the period after Monday.

The Cape-Verde Season typically encompasses the majority of the tropical cyclone activity for the Atlantic in any given year and is loosely defined as mid-August through early-October. Many of the tropical cyclones that form during this period find their origins near the Cape Verde Islands from African Easterly Waves – this is why it is referred to as the “Cape Verde Season”. Tropical cyclones that form near the Cape Verde Islands tend to have over a week over warm tropical water before reaching any land in the Western Atlantic or Caribbean Sea.

Follow the Bermuda Weather Service and National Hurricane Center for the latest official updates.

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.

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.

Other PWS

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:

Name Site Notes
Chaingate Hill, Devonshire Wunderground Temp/RH/Wind/Press/Precip
Moore’s Lane, Pembroke Davis WeatherLink Temp/RH/Wind/Press/Precip
Strathmore, Paget Davis WeatherLink Temp/RH/Wind/Press/Precip
Cedarvale South VP9NI, Devonshire Davis WeatherLink Temp/RH/Wind/Press/Precip
Devon Heights/Elnoc, Devonshire Wunderground/Davis WeatherLink Temp/RH/Wind/Press
Nine Man Cave, Smith’s Davis WeatherLink Temp/RH/Wind/Press/Precip
Gilbert Hill, Smith’s Wunderground/Davis WeatherLink Temp/RH/Wind/Press/Precip
McGall’s Bay/The Aviary, Smith’s Wunderground/Davis WeatherLink Temp/RH/Wind/Press/Precip
Knapton Hill/Vraiment, Smith’s Wunderground/Davis WeatherLink Temp/RH/Wind/Press/Precip
Milton Cottage, Pembroke Davis WeatherLink Temp/RH/Wind/Press/Precip
Port’s Island, Warwick Davis WeatherLink Temp/RH/Wind/Press/Precip
St. David’s, St. George’s (Peter’s PWS) Wunderground/Davis WeatherLink Temp/RH/Wind/Press/Precip
Spanish Point, Pembroke Wunderground Temp/RH/Press
Civil Air Terminal?/Bermuda Esso Pier, St. George’s Wunderground/NDBC/Tides&Currents(NOAA) Temp/Wind/Press/SeaTemp/SeaLevel
Tucker’s Town, Hamilton Wunderground/PWS Weather Temp/RH/Wind/Press/Precip
Magnolia Hall, Hamilton Wunderground Temp/RH/Wind/Press/Precip
Trimmingham Hill, Paget Wunderground Temp/RH/Wind/Press/Precip
Hinson’s Island, Warwick Wunderground/Other Temp/Wind/Press/Precip
Laurel Lane, Sandy’s Wunderground Temp/RH/Press
Heydon Drive, Sandy’s Wunderground Temp/RH/Press
Commissioner’s House Dockyard, Sandy’s WindGuru Temp/Wind
Ocean View, Southampton Wunderground Temp/RH/Wind/Press/Precip
Bermuda Institute, Southampton Weather Bug Temp/Wind/Press
The Mid Ocean Club, Hamilton Weather Bug Temp/RH/Wind/Press
St. David’s Lighthouse (AWOS), St. George’s Other (requires Java) Temp/RH/Wind

This post will be adapted into a page in the future – stay tuned!

A Break in the Dry Spell

Visible Satellite Imagery at 8:45am local time 25 July 2015. Just as some of the heaviest showers of the morning were passing through.  GOES East Hurricane Sector Archives.
Visible Satellite Imagery at 8:45am local time 25 July 2015. Just as some of the heaviest showers of the morning were passing through. GOES East Hurricane Sector Archives.

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.

Place 24hr Rain
Trimmingham Hill, Paget 3.76″
Tucker’s Town, St. George’s 3.73″
Ocean View, Southampton 3.64″
Bermuda Weather Service, St. George’s* 3.63″
St. David’s, St. George’s 3.45″
Hinson’s Island, Warwick 3.28″
McGall’s Bay, Smith’s 2.97″
Knapton Hill, Smith’s 2.94″
Gilbert Hill, Smith’s 2.85″
Chaingate Hill, Devonshire (my PWS) 2.60″

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.