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.


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.

Rain in Sight

Monthly Actual vs. Normal year to date precipitation total for the airport. Normals based off of 1979-2000 climate period as is the climate records from the Bermuda Weather Service. Where there is brown, there is a deficit; where there is blue, there is a surplus of precipitation.
Monthly Actual vs. Normal year to date precipitation total for the airport as of July 9th. Normals based off of 1979-2000 climate period as is the climate records from the Bermuda Weather Service. Where there is brown, there is a deficit; where there is blue, there is a surplus of precipitation.

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 Tropics

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.

Japan’s Himawari 8 satellite, the newest suite of sensors operationally observing the Earth caught (from left to right) Typhoons Linfa, Chan-hom, and Nangka on July 8th at 530UTC in its full disc visible band. This geostationary satellite has the highest spatial and temporal resolution of the current operational meteorological satellites in orbit. New York Times Article. NOAA is expected to launch a similar satellite next March – GOES R.

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.

Stationary Front brings tank rain

Aug 10 14 1645UTC A
RGB Satellite imagery shows the convective activity along the stationary front near Bermuda this afternoon.

Hurricane Bertha merged with a stationary front along the east coast of the United States. As Bertha became extra-tropical in this merger, it passed to the north of Bermuda and a second area of low pressure formed to the south of Bertha. The extra-tropical remnants of Bertha and the secondary low pressure dragged the stationary front toward Bermuda as a cold front. This brought isolated breezy weather on Wednesday with isolated showers overnight. Heavy rains with thunder then moved in on Thursday. The cold front then stalled near Bermuda as both Bertha and the secondary low moved into the central north Atlantic. With the stationary front nearby isolated showers also remained in the area from Thursday night through Friday. They became more scattered on Saturday as an upper level disturbance approached from the west and the stationary front became more active. The same upper level disturbance pulled the boundary northwards enough for heavy showers to become widespread near Bermuda on this morning. Rain associated with the complex of heavy showers and thunderstorms to the near south of Bermuda has persisted into the early afternoon today (Sunday).

Bermuda Weather Service Radar image at 1:13pm this afternoon showing widespread steady rain over Bermuda with heavy showers and thunderstorms to the south and east of the island.
Bermuda Weather Service Radar image at 1:13pm this afternoon showing widespread steady rain over Bermuda with heavy showers and thunderstorms mainly to the south of the island.

Rain cooled air so far today has kept Bermuda in the low to mid 70s all day – very unusual as record lows are typically between 68F and 72F this time of year. This rain comes as Bermuda had been recovering from a dry start to Summer with below normal precipitation from early June into early July, with May typically being fairly dry. Some unofficial rainfall totals between Wednesday and Saturday are below. These are from personal weather stations found at Wunderground. An additional 1.00-2.50″ fell today with isolated amounts nearing 3.00″.

Location Wednesday Thursday Friday Saturday
Moore’s Lane, Pembroke 0.04″ 1.47″ 0.00″ 0.24″
Trimmingham Hill, Paget 0.02″ 2.52″ N/A N/A
Chaingate Hill, Devonshire 0.01″ 1.44″ 0.00″ 0.31″
Gilbert Hill, Smith’s 0.01″ 1.47″ 0.00″ 0.49″
Knapton Hill, Smith’s 0.02″ 1.72″ 0.00″ 0.51″

This stationary front remains nearby into Monday, but a wave of low pressure moving off the US east coast will pull the boundary northward across Bermuda with showers and potential for thunder Monday into Tuesday. Winds will also increase moderate to strong at times as this area of low pressure approaches and moves by Monday through Wednesday. Winds should begin easterly on Monday, then veer more southerly and southwesterly by Tuesday and Tuesday night as the boundary moves to the north of Bermuda.

For the latest official forecast for Bermuda see the Bermuda Weather Service.

A very unsettled end to August

Tropical Rainfall Measuring Mission (TRMM) Satellite Derived daily precipitation anomaly in mm/day. Bermuda's location is highlighted in red.
Tropical Rainfall Measuring Mission (TRMM) Satellite Derived daily precipitation anomaly in mm/day over the last 30 days. Bermuda’s location is highlighted in red. Satellite indicates that, for August, Bermuda saw about 5mm/day of rain more than normal. 

A slow moving frontal system interacted with several troughs in the western Atlantic near Bermuda to bring days of heavy showery rain and thunderstorms to end August. These were even squally at times with gusts as high as 40kts observed at the Bermuda Weather Service. August 2013 has ended as significantly above normal for precipitation with 10.43″ recorded at the Bermuda Weather Service; a full 4″ above normal for the month as compared to 1981-2010 climatology of 6.38″. This month’s surplus completely eradicated the year-to-date precipitation deficit. There were three days that each saw more than an inch of rain, that statistic combined with a near normal number of precipitation days indicates that when it did rain, a lot of rain fell at once. Interestingly, these thunderstorms came with significant cloud to ground lightning that was blamed for power outages on two separate occasions when utility poles were struck.

A result of the heavy showers was several record and near record low temperatures as the heavy rain cooled the air. The daily record low temperatures of 70.9°F on the 15th and 69.4°F on the 26th were found only in heavy showers and thunderstorms. And overall, temperatures were kept around 2°F below normal for August – likely with some relation to the unsettled weather. It is not unusual for a front to make it near Bermuda and then stall and dissipate or lift out of the area during August – but it is a bit unusual for several fronts to pass through Bermuda in August bringing a change in airmass. This is likely related to the Bermuda-Azores high averaging nearer the Azores than normal.

For official climate data, see it as provided by the Bermuda Weather Service here.

Bermuda-Azores High July 2013

Mean sea level pressures for the month of July show the location of the Bermuda-Azores high. ESRL reanalysis generated product.
Mean sea level pressures for the month of July show the location of the Bermuda-Azores high. Higher sea level pressures are in warmer colors. The extent of the ridge system is nicely defined by the 1020mb isobar in yellow.  ESRL reanalysis generated product.

For the month of July, the Bermuda-Azores high on average took the two-celled form with a center of high pressure near Bermuda, and another near the Azores connected with a ridge across the Central Atlantic. Both centers of high pressure averaged to be near 1022mb. Looking at anomalies for this month, it reveals that pressures were higher than normal near Bermuda and lower than normal near the Azores, suggesting that the Bermuda-Azores high pressure system was displaced further west than normal.

The higher than climatology pressures near Bermuda associated with the Bermuda-Azores high, and the cell of high pressure to the south of Bermuda during the month of July was very likely related to Bermuda having a very dry July. With 2.36 inches of rain falling in one day and only 0.75 inches for the rest of the month.  High pressure is typically associated with subsidence and limited cloud development and so precipitation is in turn limited. Additionally, July 2013 was near normal in terms of air temperatures as the mean monthly temperature was 81.4°F compared to 1981-2010 climatology of 81.1°F.

In addition, the light winds brought by the cell of high pressure near Bermuda allowed sea surface temperatures to run about 2°F above normal near Bermuda. This could have important implications if this warm pool persists into the peak of hurricane season.

The first half of August had a change in this pattern allowing more active weather into the area, with which significant thunderstorm activity with heavy rain was observed. 5.60 inches of rain have fallen so far this month – according to 1981-2010 climatology, August sees 6.38 inches of rain. We are well on our way to catching up with climatological year-to-date precipitation totals now that the weather has become more changeable. However, now with the Bermuda-Azores high ridging back into our area, Bermuda may have another period without much rain.

Stormy Start to weekend

Zoomed in Bermuda Weather Service radar image showing vigorous thunderstorm activity slowly moving across Bermuda, this is the last of three rounds of showers on Friday July 26th at about 5:16pm.
Zoomed in Bermuda Weather Service radar image showing vigorous thunderstorm activity slowly moving across Bermuda (marked by the white crosshairs), this is the last of three rounds of showers on Friday July 26th at about 5:16pm.

A series of weak surface troughs, enhanced by the semi-permanent Tropical Upper-Tropospheric Trough (TUTT) that typically sets up to the south and east of Bermuda, passed Bermuda Friday and today bringing heavy showers. These sometimes came with gusty winds and vivid lightning and thunder. Although generally isolated, they occasionally turned scattered in coverage at times meaning some areas of Bermuda got much more rain than others. Despite that, over the course of Friday and Saturday, 2-5inches of rain fell in Bermuda. This puts a huge dent in the developing dry spell and knocked high temperatures down nearer to climatological values.

These troughs are moving away from the island, taking their surface convergence with them, and the Bermuda-Azores high is retrograding to center itself in the central Atlantic as a dome of high pressure – this puts Bermuda to the west of the high, firmly under its western flank in deep southerly flow and more settled weather. Expect temperatures to return above normal as warm advection continues. For Monday into Tuesday – expect the Bermuda-Azores high to begin to weaken and flatten out into a ridge extending from Bermuda to the Azores rather than a dominating dome of high pressure. Light winds near the ridge axis could allow for convective showers to develop over Bermuda, possibly a Morgan’s cloud if the wind direction is right – something to monitor.

Wednesday into Friday sees the western side of the ridge become less defined as an easterly wave develops and meanders into the area – this may result in weather similar to Friday for the start of Cup Match. Keep monitoring the latest forecasts and advisory products from the Bermuda Weather Service for the latest official information for Bermuda.

Atlantic Analysis JULY 27 2013 2145UTC
My analysis of the northwest Atlantic showing the location of several troughs as red dashed lines, the blue arrows are rough streamlines of upper level winds indicating the location of the TUTT. The yellow circle shows the surface low that the National Hurricane Center is monitoring for tropical development with their latest chance of development in the next 48hours.

Meanwhile, Tropical Storm Dorian has bit the dust, so to speak. Succumbing to dry, stable air to the north, Dorian was not able to sustain its convection and so surface pressures near the center began to rise and, no longer producing westerly winds to the south of the center, Dorian degenerated into a Tropical Wave. Although there is a small window for Tropical Wave Dorian to redevelop into a Tropical Cyclone, it’s chances seem slim at this point.  The Tropical Wave is expected to continue following the well established flow south of the Bermuda-Azores high and pass north of the Caribbean. Although this means it avoids the islands, it also means the wave will enter an area of upper atmospheric winds from the TUTT that are not supportive of Tropical Cyclone development. Regardless of development, the wave could bring showers and gusty winds to the Windward islands, the Greater Antilles, the Turks and Caicos, and the Bahamas islands in the distant future if the wave holds together as a trackable feature.

Additionally, a non-tropical low has formed and is semi-detached from frontal boundaries over the warm waters of the Gulf Stream. The National Hurricane Center gives this area a 10% chance of becoming a tropical cyclone in the next 48hours. It is expected to quickly pass over the cooler waters of the Labrador current to its north before it has a chance to become tropical. Follow updates and products issued by the National Hurricane Center for the latest official information on tropical cyclones in the Atlantic.

Meanwhile, in the Pacific, Tropical Storm Flossie crossed from the Eastern Pacific into the Central Pacific, potentially threatening Hawaii as a weakening tropical storm or tropical depression in four to five days. An uncommon occurrence to follow as it develops.