Dry Early May Weather

Early May Weather
Observations from the Bermuda Weather Service. Temperature and dew point on top, wind direction on bottom. To the right of the yellow line, the relative humidity (indicated by how close together the temperature and dew point are) increases as winds settle out of a south-southeasterly (135-180°) direction.

April ended by clearing out a very warm and humid airmass that was accompanied by patches of fog that advected across the island. This left the start of May quite dry with northerly winds and a ridge of high pressure to the north. As the ridge shifted to the east of Bermuda, northerly flow gradually veered southerly bringing in more humid air. High pressure has remained centered generally to the east of Bermuda since the fifth of May maintaining humid south-southeasterly flow.

This setup is fairly common for this time of year: ridges of high pressure settling in near Bermuda bringing persistent settled and mild weather across the western Atlantic. The Bermuda-Azores high typically starts to build in during late-May/early-June to its July peak extent and intensity, prevailing winds come from the south and southwest with a warm and humid the norm.

This high pressure setup is generally effective at keeping rain-making frontal systems at bay. However, it appears the ridge extending across Bermuda might weaken enough to allow a weakening front to bring some showers to the island over Friday. High pressure then re-establishes itself with a return to more settled weather over the weekend. The weak front is unlikely to be able to bring any real change in the airmass and the warm humid weather should continue without much break.

May is typically one of the driest months in Bermuda. Long periods without much rainfall are characteristic of April through June. Water availability can become an issue when several such dry spells occur in one year – particularly so as water demand increases in the warm season.

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Quite A Squall

Between 5:15 and 5:30pm on Sunday April 8th, a severe thunderstorm blew across Bermuda. The squall brought heavy rain, severe wind gusts along with thunder and lightning. Gusts greater than 50 kts were observed at Pearl Island (62 kts), the Crescent (58 kts), Bermuda Esso Pier (54 kts), and at the official observing site at the airport where a peak gust of 71 kts was recorded.

The rain and thunderstorms brought 0.56″ (14.2 mm) of rain to the airport.

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From top to bottom: 2-minute sustained winds, wind gust, wind direction, air pressure, air temperature, relative humidity, and precipitation total. Around 5:20pm local time, the main squall moved through with peak gust of 62 kts, and a drop in temperature of about 4°C. (Source: Bermuda Weather Service)

 

 

Ship Tracks Around Bermuda

2018-02-23 060220 UTC
GOES-East Geocolor imagery centred on Bermuda at 06:02 UTC, 23 February 2018. This multi-spectral product highlights low clouds (e.g. stratocumulus) as light blue shades. The brighter light blues indicate thicker low cloud.

[Watch animation]

High pressure over the western Atlantic over the last few days has been associated with persistent stratocumulus cloud cover. This typically thin layer of low-altitude cloud is a fairly common feature over the eastern half of ocean basins, and its occurrence around Bermuda is not uncommon – particularly in the cooler months.

Strong high pressure comes with subsidence. Air is compressed and warms as it descends. The descending air, with origins in the mid/upper-atmosphere is also quite dry.  This warm, dry air cannot make it all the way to the surface because it doesn’t have enough momentum to push through the cooler layer near the surface (the boundary layer). This leads to a stable layering of air that manifests as warmer drier air over cooler more humid air.

The boundary layer is often well mixed with an even distribution of heat and moisture (and can become increasingly humid over the ocean). Meanwhile, the warm air above acts like a lid over the cooler boundary layer preventing mixing between the two layers. Moisture trapped in the well mixed layer can form a sheet of stratocumulus cloud given the right conditions.

Stratocumulus_Annotated
Potential temperature (the temperature that dry air would theoretically have if it was brought to the surface without external heating), Specific humidity (the mass of water vapour for every kilogram of air), and Relative Humidity (the ratio between the actual water vapour content and the water vapour content needed for saturation). Profiles measured by weather balloon at 00:00 UTC 23 February 2018 at Bermuda.

Exhaust from ships contains aerosols that act as cloud condensation nuclei. These are airborne particles that water can condense onto to form cloud droplets. The aerosols from the ships tend to form more, and smaller, cloud droplets making the clouds contaminated with ship exhaust appear brighter and thicker to satellite instruments.

The ship tracks in the animation above appear to advance from east to west or west to east, following the path of the ships. Meanwhile, another cloud enhancement appears to advance southeastward, starting from Bermuda. This track follows the wind direction in the boundary layer (unlike the ship tracks). It could be the result of aerosols from BELCO or the Incinerator acting to enhance the cloud brightness through the same mechanism as ship tracks.

High pressure and the settled weather it brings has been a theme for much of February 2018. Few mid-latitude cyclones have impacted the island so far this month. As a result, the cold air has stayed away and temperatures have remained far above normal. The lack of cyclones has also meant that precipitation totals are below normal and no days this month have experienced gale force winds. Sea surface temperatures are also far above normal as a result.

The month is expected to end with changeable weather as high pressure gives way. A cold front then takes the opportunity to push southeastward across Bermuda with some rain and showers followed by cooler air.

Further Reading:
Coakley, J. A., R. L. Bernstein, and P. A. Durkee, 1987: Effect of Ship-stack Effluents on Cloud Reflectivity. Science, 237, 1020-1022.
Hudson, J. G., 2000: Cloud Condensation Nuclei and Ship Tracks. J. Atmos. Sci., 57, 2696-2706.
Wood, R., 2012: Stratocumulus Clouds. Mon. Wea. Rev., 140, 2373-2423.

Weather Recap: Bermuda 2017

Air Temperatures

BermudaClimate2017_temps
Daily temperature data for Bermuda. The blue bars represent the observed daily temperature range from the minimum temperature at the bottom of the bar, to the maximum temperature at the top of the bar. The blue shaded region represents temperatures below the normal daily minimum to the daily record low, the brown shaded region represents the normal daily range, and the red shaded region represents temperature above the normal daily maximum to the daily record high. Data are sourced from NCDC and BWS and should be considered unofficial.

The year started out very variable with large swings in temperature between near record highs and record lows. Near the end of February a string of unusually mild and quiet weather persisted for several days with temperatures setting a new daily record high of 73.9°F (23.3°C) on the 26th, beating the previous record for the day of 73.0°F (22.8°C) set in 1975.

February’s warm and quiet spell was rudely interrupted in early March. An strong cold front brought very cold air to Bermuda, including the year’s only daily record low. This was part of a cold air outbreak that lasted several days. Gales and blustery showers with isolated hail accompanied a daily record low on the 5th March when the low reached 47.7°F (8.7°C) breaking the previous record of 52°F (11.1°C) set in 1968.

April through September was much quieter with smaller swings between extreme temperatures. Mild dry periods and the occasional cool rainy days with slightly higher than normal temperatures dominated this period.

As Autumn progressed, temperatures resumed their typical variability, but mainly swinging between normal and higher than normal temperatures. The year’s second daily record high temperature was set on the 7th of December when temperatures reached 77.4°F (25.2°C), beating the previous record high of 77°F (25.0°C) set in 1978.

Precipitation

BermudaClimate2017_precip
Daily year-to-date rainfall. The green areas are the observed rainfall for 2017. The yellow line represents the normal year-to-date rainfall. The dark green shaded region represents the surplus in year-to-date rainfall, while the brown shaded region represents the deficit in year-to-date rainfall. Data are sourced from the NCDC and BWS and should be considered unofficial.

The year started with a significant rain event on the 5th January on which a month’s worth of rain fell in a single day leading to some flooding. This 5.34″ (135.6 mm) of rain was the heaviest rain for a meteorological day at any time during the winter months (Dec-Jan-Feb), and the fifth wettest day on record.

Slightly below normal precipitation fell through the remains of winter and by mid-spring, extended dry periods were taking hold. April 2017 was the second driest April on record with only 0.73″ (18.5 mm) of rain falling in the entire month. This dry pattern continued through May and early June.

2.09″ (53.1 mm) of rain fell on the 11th June setting a daily record (beating the previous record of 0.53″ (13.5 mm) set in 1988) and signaling an end to the dry spell. From mid-June through the end of the year, near normal precipitation totals were observed. This maintained a year-to-date deficit of roughly 5.00″ (127.0 mm). Other daily records were 1.21″ (30.7 mm) on the 18th July (1.14″/29.0 mm, 1993), and 1.77″ (45.0 mm) on the 19th October (1.50″/38.1 mm, 1963).

Sea Temperatures

Sea surface temperatures around Bermuda were far above normal for much of 2017. Light winds and prolonged periods of settled weather in spring and early summer limited the amount of mixing between the warm surface waters and the cooler water just below – priming the summer for above normal temperatures. This year no direct impacts from tropical cyclones was a welcome change to the pace set over the last decade, but also meant they were unable to provide the mixing that would cool the sea surface temperatures in late summer/early autumn.

The Bermuda Weather Service noted in monthly climate summaries that the monthly mean sea surface temperatures were among the five highest recorded since 1950 for September and October, and among the three highest for November.

Another part of this story is the presence of a nearby warm ocean eddy. This eddy lingered in the area for most of autumn. The eddy not only contributed to seas surface temperature anomalies, but also brought abnormally high sea surface heights. This came on top of elevated sea heights due to the astronomical Spring Tide phenomenon, and led to periods of generally minor coastal flooding throughout Autumn.

Abnormal Tides

Bermuda Esso Pier Water Levels.png
19 Sep to 12 Oct 2017 Water levels from Bermuda Esso Pier, St. George’s Island. Verified water level observations in green, preliminary water level observations in red. Forecast water level due to the predicted astronomical tide in blue. Water level is with respect to mean higher high water, where positive values indicate normally dry land is inundated. Alternative reference water level marks are on the right.

Over the weekend, the combination of the spring tide and a high amplitude ocean eddy resulted in localized coastal flooding around low-lying areas of Bermuda. Tides were running around 1.5 ft above expected levels which were already higher than normal thanks to a spring tide.

The role of the Astronomical Tides:

The astronomical tides are driven primarily by the gravitational effects of the Moon on the ocean. When the Moon is directly overhead, the water rises in response to the Moon’s gravitational pull. When the Moon is directly underfoot, the water rises again to balance the pull of the Moon on the opposite side of the Earth.

During “Spring” tides, the gravitational pull of the Sun on the oceans acts in the same direction as that from the Moon. This results in higher than normal tides and tidal ranges. Conversely, during “Neap” tides, the gravitational pull of the Sun is acting perpendicular to that of the Moon and lower than normal tides and tidal ranges can be expected. Looking at the blue line in the above figure, higher tides associated with the Spring tide can be seen around the 20th September and again last weekend, while lower tides associated with Neap tide can be seen around the 28th September.

Additionally, the Sun and Moon have to be aligned in space for their gravitational pull to act in the same direction. This manifests as a New Moon when the Moon is between the Sun and the Earth, and a Full Moon when the Earth is in between the Sun and the Moon. Both New and Full moon are associated with Spring tides. The Lunar cycle (including one Full and one New Moon) repeats roughly every 29 days and so you can expect a Spring tide a little more than every fortnight.

[More on Spring and Neap Tides]

Finally, the Moon follows an elliptical orbit around the Earth and so is closer or further away twice per orbit. Every ~7.5 Spring tides, the moon reaches its closest distance to Earth during a New or Full Moon. When the Moon is closer to Earth (perigee), the tides are slightly higher than normal. The opposite is true for when the Moon is furthest from Earth (apogee). Tides during last weekend’s Spring tide were higher than the 20th September’s Spring tide because the Moon was near/at perigee last weekend, and not during the 20th September.

The role of Ocean Eddies:

2017280atsha
Sea surface height anomalies showing a positive anomaly greater than 30 cm near Bermuda (circled in black). This was associated with an anticyclonic eddy that intensified as it tracked southwestward toward Bermuda over the last two months.

Ever present in the ocean, eddies can manifest as regions of higher (positive) or lower (negative) sea surface height anomalies. The flow around these sea surface height anomalies is often close to balanced and so they can persist for a long time as they track across the ocean surface. These anomalies are typically small, less than 30 cm.

Typical flow around a positive sea surface height anomaly is clockwise (anticyclonic), and counter-clockwise (cyclonic) for a negative sea surface height anomaly in the northern hemisphere.

Over the weekend, a positive sea surface height anomaly associated with an anticyclonic eddy was tracking near Bermuda with amplitude estimated to be more than 30 cm (1 ft) via satellite measurements. Coinciding with the spring tide and Lunar perigee, this resulted in abnormally high water levels and some coastal inundation.

See some media mention of the tides here: Royal Gazette; Bernews (1); Bernews (2)

With sea level rise associated with climate change, it is reasonable to expect this mostly nuisance level of inundating events to occur more frequently as water level anomalies don’t have to be as extreme for flooding to occur.

Hurricane Maria

The peak of the 2017 Atlantic Hurricane season continues to be active. Another major hurricane has had significant impacts in the Eastern Caribbean. Hurricane MARIA made landfall in Dominica on Monday evening as a category five hurricane with maximum sustained winds near 160 mph, with higher gusts. Winds of this strength are capable of producing catastrophic damage. Early reports from Dominica confirm that widespread wind damage indeed occurred.

In addition to violent winds, tropical cyclones present serious water hazards. Coastal areas are flooded as the ocean is blown onshore, combining with astronomical tides – this is known as a storm tide. This flooding can be accompanied by large battering waves at the immediate shoreline. Furthermore, heavy rains intrinsically linked to tropical cyclones can result in life-threatening flash flooding and mudslides. Both of these hazards also affected land in Dominica.

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Météo-France Radar Imagery from Martinique and Guadelupe‏: Hurricane Maria passing over Dominica 18 UTC 18 Sep – 08 UTC 19 Sep 2017. The purple and blue colors indicate light rain, while the green and yellow colors indicate heavy rain. Source.

At landfall in Dominica, the core of category five hurricane MARIA was small. Hurricane force winds extended at most 30 miles from the center. But the spiral bands, packing torrential rains and damaging tropical storm force winds extended over Martinique and Guadelupe.

The high terrain on Dominica was able to weaken MARIA somewhat as it crossed that island. However, emerging over the high sea surface temperatures of the Caribbean and remaining in a favorable atmospheric environment, MARIA was able to quickly regain strength.

As is common with powerful hurricanes, particularly those that have small cores, an eyewall replacement cycle began on Tuesday evening. This is when spiral bands organize into an outer eyewall structure surrounding the original eyewall. The outer eyewall then intensifies at the expense of the inner eyewall. The inner eyewall eventually dissipates and the outer eyewall contracts. This typically ends with a slightly weaker hurricane with a larger core of violent hurricane force winds.

TJUA_2017-09-20_0344UTC_annotated
National Weather Service Radar Imagery from Puerto Rico as MARIA passes near St. Croix. On the right, the inner eyewall is highlighted in pink, and the outer eyewall in purple. In both images, the lighter rain is in blue and green with heavier rain in yellow and orange colors. Imagery at 0344 UTC 20 Sep 2017.

In the pre-dawn hours of Wednesday morning, the outer eyewall of MARIA was strengthening and the inner eyewall weakening. The resulting broadened core of MARIA was now wide enough to bring the violent hurricane force winds of the outer eyewall to St. Croix of the US Virgin Islands.

TJUA_2017-09-20_0950UTC
National Weather Service Radar Imagery from Puerto Rico just before the time of landfall in Puerto Rico. Here, light rain is blue and green colors, heavy rain in the yellow and orange colors. Imagery at 0950 UTC 20 Sep 2017. Radar imagery from Puerto Rico ended at this time for reasons related to the hurricane.

The typical progression of the eyewall replacement cycle was interrupted just before completion as MARIA came ashore in Puerto Rico near sunrise Wednesday morning, as a category four hurricane with maximum sustained winds near 155 mph – still capable of producing catastrophic wind damage, coastal flooding, and life-threatening inland flash flooding. Adverse weather spread across all of Puerto Rico through the morning, leaving widespread wind damage and floods.

The much more substantial terrain of Puerto Rico significantly disrupted the core of MARIA, and the hurricane has emerged into the Atlantic and has maximum sustained winds near 115 mph. This remains a dangerous major hurricane.

Remaining in a favorable environment of light wind shear and high sea surface temperatures, MARIA is now re-organizing as it tracks northwestward, offshore of the north coast of Hispaniola. The Turks and Caicos Islands are next at risk of direct impacts from the core of violent winds, while adverse weather extends across the Dominican Republic, Haiti, and the southeastern Bahamas Islands.

[See the latest Forecast Track for MARIA from the NHC]

MARIA is expected to pass near the Turks and Caicos Islands Friday Morning as a major hurricane capable of producing extreme to catastrophic wind damage, coastal flooding, and freshwater flooding. Late on Friday, MARIA is forecast to turn more northwestward, and then northward over the weekend. This takes the track of MARIA away from the Bahamas and the hurricane is expected to track between the US East Coast and Bermuda in the 3-5 day period.

Swells from IRMA, JOSE, and now MARIA have kept rough seas in Bermuda’s marine area for most of September. Bermuda should continue to closely monitor the progress of MARIA as it turns northward in the long range. At the moment, MARIA’s impacts on Bermuda appear to be a continuation of rough and hazardous seas in the marine area while winds may increase into the moderate-strong range as MARIA passes to the west on Tuesday.

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

Peak of Hurricane Season

 

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Multispectral GOES-East imagery of the tropical Atlantic 5th September 1815 UTC showing Hurricane Irma (center) and Tropical Storm Jose (lower right).

The 2017 Atlantic Hurricane Season is in full swing. According to climatology, this time of year typically sees the most tropical cyclone activity. The combination of a peak in sea surface temperatures, minimum in vertical wind shear, and a reduction in the extent of the dry and stable Saharan Air Layer this time of year all factor into why it is often very active.

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GOES-East color enhanced infrared imagery of Hurricane Irma, 5th September 2017 at 1845 UTC.

Hurricane Irma

This morning, Irma continued to strengthen. Near-continuous aircraft missions into the hurricane have found that maximum sustained winds have increased to near 185mph, making Irma a category 5 hurricane on the Saffir-Simpson Hurricane Wind Scale and capable of catastrophic wind damage where the core of the hurricane comes ashore.

[National Hurricane Center Track]

Short Term Forecast: Hurricane Irma is now beginning to turn toward the west-northwest as it tracks around the south side of the deep-layered Bermuda-Azores high. On this track, Irma will pass very near or over several of the Leeward Islands and Puerto Rico where Hurricane Warnings are in effect.

Potentially catastrophic wind damage is possible should the core of Irma’s strongest winds come ashore on any of these islands. However, as hurricane force winds extend up to 60 nautical miles from the center, widespread damaging winds can be expected through the Leeward Islands. Heavy rain could result in life threatening inland flooding, and a significant storm surge is expected to cause dangerous coastal flooding.

[Radar imagery of Irma out of Martinique via Brian McNoldy]

Irma is expected to remain in a very favorable environment with high ocean heat content, low vertical wind shear, and away from mid-level dry air. Irma is therefore expected to remain a very powerful hurricane with intensity mostly being controlled by internal storm structure as it approaches Antigua and Barbuda tonight. Irma will spend most of Wednesday passing through the Leeward Islands and should begin to pull away by Wednesday night.

Hazardous swell from Irma is forecast to reach Bermuda’s southern marine area on Thursday, with 6-10ft seas outside the reef. Otherwise Irma will pass more than 800 miles south of the island during this time.

20170905-1845UTC_JOSE
Color enhanced infrared imagery of Tropical Storm Jose, 5th September 1845UTC.

Tropical Storm Jose

Over the central Tropical Atlantic, satellite wind measurements have indicated that a tropical disturbance has organized into a tropical storm with maximum sustained winds near 40 mph winds, becoming Tropical Storm Jose.

Short Term Forecast: Jose is pulling west-northwestward out of the central Atlantic, feeling the same steering flow as Irma. The storm is in a mostly favorable environment and a general strengthening trend is expected. However, upper level outflow from Irma could introduce periods of strong vertical wind shear that could hinder strengthening at times. Jose is not a threat to land at this time, but could become a threat to the Leeward Islands over the weekend.

[National Hurricane Center Track]

Follow official updates at the National Hurricane Center, and Bermuda-specific updates at the Bermuda Weather Service.