Nicole and Matthew

As always stay abreast of the most up to date official information from the National Hurricane Center, and Bermuda Weather Service. The following are my unofficial thoughts:

Tropical Storm Nicole

No adverse weather is currently expected from Nicole in the next five days. However, Nicole is forecast to take on an erratic track starting Friday, keeping the storm within the ‘potential threat’ radius (400 nm) for at least the next five days while maintaining moderate tropical storm strength.

[Potential Threat: When the center of a tropical cyclone is expected to pass within 400 nm of Bermuda within 72 hours. – BWS Glossary]

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In the next 72 hours, Nicole is expected to come as close as 240 nm to Bermuda. However, both at and after that time there is potential for Nicole to track closer to the island. See the latest ‘Local Advisory‘ from BWS.

Further, the steering patterns that are responsible for Nicole’s erratic forecast track between Friday through Sunday are unstable. That means small changes in the steering pattern will result in large changes in the eventual track of Nicole. Adverse weather related to Nicole will therefore remain a possibility early Saturday, through and beyond Sunday.

Nicole originated from a tropical wave embedded in the inter-tropical convergence zone in the Central Atlantic. The wave tracked northwestward, staying northeast of the Caribbean. It experienced significant vertical wind shear from an upper level low nearby that slowed organization of the cyclone. Yesterday morning, the low became stacked with the axis of the weakening upper level low where vertical wind shear is locally lower, and convection was able to organize allowing a tropical cyclone to form.

On the 12pm forecast from NHC, Nicole will be over sufficiently warm waters and in a regime of gradually increasing wind shear. However, how (un)favorable the upper level winds are depends on the eventual evolution of the steering pattern, which is currently being forecast with lower than normal confidence and therefore, Nicole’s intensity forecast is lower confidence than normal. NHC is forecasting near steady intensity over the next three days, then a slight weakening.

Hurricane Matthew

Hurricane Matthew is currently exiting Haiti and Cuba where locally extreme damage was likely done. Matthew is about to begin a 36 hour track through the Bahamas impacting many of these islands with violent winds, life-threatening storm surge and heavy rain related flooding. Florida is then at threat from Matthew as the hurricane exits the Bahamas. Matthew too has an uncertain long-range track, with a small but non-zero chance for impacts in Bermuda.

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4th October 2016, Suomi NPP/VIIRS True Color imagery of Hurricane Matthew over the Gulf of Gonaives and Tropical Storm Nicole ~550 nm northeast of Puerto Rico.

Over the weekend, Hurricane Matthew rapidly intensified into the Atlantic Basin’s first category five hurricane since Hurricane Felix (2007) as it meandered in the eastern and central Caribbean Sea. Matthew then weakened due to a combination of inner core dynamics and some intermittent dry air entrainment, but remained an extremely dangerous category four hurricane.

[The eye of Matthew passed over Buoy 42058. via National Data Buoy Center]

At category four strength, Matthew made landfall in southwestern Haiti near Les Anglais on Tuesday morning, crossed the Gulf of Gonaives, and made a second landfall in eastern Cuba near Baracoa. Near the landfall locations, a combination of violent category four strength winds and significant storm surge likely caused locally extreme damage. Heavy rains, spreading far from the center of Matthew, are responsible for flooding across the Caribbean, particularly in Haiti, Cuba, and the Dominican Republic.

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National Hurricane Center 5-day forecast track at 3pm local time showing warnings in effect for Hurricane Matthew. See Latest.

Interaction with land over Haiti and Cuba has weakened Matthew only slightly to a category 3 hurricane. Matthew is currently in the south-central Bahamas. Violent winds and storm surge will make conditions extremely dangerous and locally life-threatening on many of the Bahamas islands as Matthew tracks through the island chain.

Further, Matthew is in a moist environment with low vertical wind shear and is over some of the warmest water in the Atlantic, as a result, Matthew is expected to at least maintain its category three intensity, if not re-strengthen as it moves through the Bahamas over the next day or so.

[Category what? See NOAA’s explanation of the Saffir-Simpson Hurricane Wind Scale]

By late on Thursday, Matthew is expected to be approaching the east coast of Florida as a major hurricane. Between day two and three Matthew is expected to parallel the east coast of Florida before turning sharply to the east on Saturday. However, small changes in the track of this hurricane could bring violent winds onshore, or keep them far off shore leading to an unusually wide range of possible impact scenarios despite a forecast with near average confidence through this timeframe.

After the eastward turn on Saturday, as with Nicole, the forecast becomes much less confident. Right now the most supported scenario appears to have Matthew beginning to turn to the south then west and threaten the Bahamas and Florida once again in the 5-10day range. However, Matthew could also continue eastward and become a ‘potential threat’ to Bermuda. At this time, no adverse weather is expected from Matthew in Bermuda.

Karl Passes with Minor Impact

Tropical Storm Karl failed to strengthen into a Hurricane as it passed Bermuda. Karl’s structure dramatically changed and deep convection waned in the final hours of approach. This was evidenced in aircraft reconnaissance missions that were ongoing into the storm into the early hours of Saturday. The region of peak winds shifted from the northwest quadrant to the southeast quadrant, and the center of the storm was surrounded by a gaping region of moderate winds.

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Karl about 85 nm south-southwest of Bermuda with deep convection near the center beginning to organize into an eye-like feature. 1:55am local time.
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Karl near closest point of approach, about 45 nm southeast of the island. deep convection redeveloping north and west of the center, but not as organized or as close to the center. 5:58am local time.

Karl passed about 45 nm southeast of the island just before 6am on Saturday with maximum sustained winds near 65mph. Winds around Bermuda peaked later in the morning as Karl moved away and winds backed to the north. Observations show sustained winds generally peaked in the 20-40 kts range with gusts up to 50 kts, particularly in elevated and exposed areas. Further, only these exposed locations saw sustained tropical storm force winds and even then, only in/around gusty showers.

Storm total rains from Karl officially reached 4.71″ at the airport. Automated personal weather stations around the island saw notably lower storm totals and this might be an artifact of measuring methods. Measuring rainfall accurately in tropical cyclones is notoriously difficult because of the accompanying high winds and I would suspect that the official total is likely closer to what actually fell.

Aside from a few delayed business openings, some transportation disruption, and isolated power outages, Bermuda fared well through Karl.

See a preliminary and unofficial list of observations around Bermuda from Wundground and BWS:

Location

Peak Gust

Storm Total Rain

Bermuda Weather Service (Official) 41 kts (47 mph) 4.71″
MAROPS, St. George’s 50 kts (58 mph) NA
Commissioner’s Point, Sandy’s 48 kts (55 mph) NA
Pearl Island, Paget 47 kts (54 mph) NA
The Crescent 45 kts (52 mph) NA
Gilbert Hill, Smith’s 43 kts (50 mph) 1.82″
Magnolia Hall, Smith’s 41 kts (47 mph) 2.39″
Town Hill, Smith’s 41 kts (47 mph) 3.03″
Hinson’s Island, Warwick 41 kts (47 mph) 1.59″
Cardinal, Southampton 41 kts (47 mph) 2.33″
Esso Pier, St. George’s 39 kts (45 mph) NA
St. David’s, St. George’s 37 kts (42 mph) 2.53″
Chaingate Hill, Devonshire (My PWS) 34 kts (39 mph) 2.43″
Wilderness, Smith’s 34 kts (39 mph) 2.22″
McGall’s Bay, Smith’s 33 kts (38 mph) 4.15″
Devon Heights, Devonshire 30 kts (35 mph) NA
Hamilton, Pembroke 30 kts (34 mph) 2.34″
Tucker’s Town, Hamilton 27 kts (31 mph) 3.29″
Moore’s Lane, Pembroke 24 kts (28 mph) 2.28″
Ocean View, Southampton 23 kts (27 mph) 2.36″
Devonshire, Devonshire 18 kts (21 mph) 2.46″

Karl now a Threat

For official Bermuda Weather Information, see: Bermuda Weather Service
For the latest Hurricane Information, see: National Hurricane Center.

While Karl struggled with strong vertical wind shear for a little longer than expected, and weakened into a tropical depression, Karl has since moved into a region with lower vertical wind shear. The upper level low that had been trailing Karl and keeping strong, shearing upper-level winds over the storm has continued moving off to the west while Karl has begun to turn to the northwest.

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Upper-level wind analysis from UW-CIMSS this morning showing the upper-level low (highlighted in red) to the southwest of Karl. Upper-level anticyclonic flow is also developing overtop of Karl.

This diminishing vertical wind shear across Karl is allowing the deep convection to organize near the center of the cyclone. Further, sufficient sea surface temperatures of ~29°C/84°F and a more moist environment are helping Karl sustain this deep convection. These factors support Karl strengthening steadily on its approach to Bermuda. Aircraft observation have indicated that Karl has continued to strengthen overnight with the minimum central pressures falling at a rate of 9 mb between 9pm last night and 9am this morning, a sign that Karl has begun this strengthening, and could strengthen more quickly today as its environment continues to improve. Karl could strengthen into a hurricane before passing Bermuda and additional warnings might be necessary later today.

Karl is expected to continue tracking to the northwest this morning, turning more to the north this afternoon, and north-northeast this evening/overnight. The 9am forecast track from the National Hurricane Center takes Karl about 50 nm to the southeast of Bermuda.

On this forecast track, Bermuda can expect strengthening easterly winds today, winds then back to the northeast and north this evening and overnight and peak as Karl passes to the southeast. Tomorrow morning, winds continue backing to the north and northwest and begin to subside. Northwesterly winds continue to diminish through the day on Saturday.

Steady, at times heavy, rain is expected to begin this afternoon, with isolated to scattered showers possible late this morning. Embedded squally showers are possible as early as this evening through overnight with a small but non-negligible chance for tornadoes.

Peak winds on island are expected to be strong tropical storm strength with gusts to hurricane strength, particularly in areas exposed to northwesterly, clockwise through  to easterly winds. Small deviations in Karl’s heading (ie. wobbles) and therefore track could result in very different conditions felt on Bermuda. For instance it is possible that Karl track directly overhead of Bermuda or to the west of Bermuda, in these scenarios, wind directions would be different or even opposite, exposing a different set of hillsides to the strongest winds and gusts.

Karl now a Potential Threat

For official Bermuda Weather Information, see: Bermuda Weather Service
For the latest Hurricane Information, see: National Hurricane Center.

Tropical Storm Karl is now expected to pass within 400 nm of Bermuda within the next 72 hours as of the 12pm advisory. This latest advisory from the National Hurricane Center takes the center of Karl about 260 nm to the south of Bermuda in the next 72 hours. Karl is forecast to pass closer to Bermuda between 72 and 120 hours from this advisory and adverse weather is possible from Karl.

At this time, Bermuda can expect dangerous surf and sea conditions over the weekend. Further, strong to possibly tropical storm force winds are to be expected beginning late on Friday through Saturday. The final track of Karl will determine the strength and direction of winds and whether or not any significant rains will fall.

wind-direction
Karl is expected to track northeastward past Bermuda. The latest NHC track  takes Karl east of Bermuda and the strongest winds would be felt from the northeast as a result (left panel). Conversely, should Karl pass to the west of the island, the strongest winds could be felt out of the southwest.

Karl has been steered to the west in the flow south of the Bermuda-Azores ridge of high pressure over the last few days. This steering is expected to continue for the next day or so. Karl then turns more to the northwest as the storm approaches a weakness in the ridge that is moving in from the west of Bermuda. By Friday, Karl begins to turn northwards, moving into this weakness in the ridge. Karl then becomes entrained in deep-layered southwesterly flow that accelerates the storm to the northeast.

Dry air surrounding the cyclone and persistent strong vertical wind shear have kept Karl weak and disorganized thus far. However, Karl is entering a more moist environment with higher sea surface temperatures. These factors should support more vigorous convection around the storm. Further, vertical wind shear is expected to weaken at the same time. This should allow deep convection to organize. Both of these factors support a strengthening storm in the coming days and Karl could be at or nearing Hurricane strength as it passes Bermuda.

Hurricane reconnaissance aircraft (ie. hurricane hunters) are scheduled to begin regular investigations into Karl this afternoon and this will give us a better idea of Karl’s current strength and structure, and how favorable the environment is.

As always, follow the official weather sources for the latest information.

Isolated Sunday Downpours

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Spatial distribution of unofficial rainfall reports around Bermuda yesterday, over 5″ of rain fell in some spots. Interpolated using data from WeatherUnderground, WeatherLink, and Bermuda Weather Service.

Late-morning, isolated showers popped up in the central parishes on Sunday dumping heavy rain from roughly 10am to 3pm with intermittent periods of thunder. The showers were largely stationary over the central parishes, at times spreading to the east. This left the western parts of the island mostly dry until isolated showers moved in from the west late in the afternoon.

While most places saw less than an inch of rain, where those showers and thunderstorms set up in the central parishes, unofficial rain totals of 2-5″ were reported. Light winds through much of the lower troposphere kept the showers that formed from moving much, and the heating of the island likely helped support the showers initiate.

A surface ridge to the south of Bermuda is maintaining light mostly westerly flow today. Meanwhile, a subtle surface trough embedded in that flow is helping to spark isolated showers and thunderstorms in the area.

Gaston Approaching the Azores

For the latest official hurricane information see the National Hurricane Center, for the latest official forecast for Bermuda, see the Bermuda Weather Service.

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Scattered showers around Bermuda, Tropical Storm Hermine in the Gulf of Mexico (tracking toward Florida), and Hurricane Gaston in the northeastern Atlantic, tracking toward the Azores. RGB vis-ir layered satellite image.

Hurricane Gaston

Hurricane Gaston continues across the northern Atlantic as a slowly weakening storm. As of the noon advisory from the National Hurricane Center, tropical storm warnings are in effect for the western and central islands of the Azores as Gaston quickly approaches from the west. Tropical storm conditions (sustained winds greater than 34 kts/39 mph) are expected to spread across the warned islands in the next 24-36 hours.

Gaston is moving over increasingly cool sea surface temperatures. As a result, a continued weakening trend is expected despite the otherwise favorable environment, and Gaston is expected to weaken below hurricane strength before reaching the western Azores on Friday and central Azores Friday night. Strong, locally damaging winds, and torrential rains can be expected, particularly on the windward side and at higher elevations on the western islands.

Hurricane Gaston is not a potential threat to Bermuda and is not expected to become a threat.

Tropical Storm Hermine

Meanwhile, TD9 has strengthened into Tropical Storm Hermine in the Gulf of Mexico. As of the noon advisory from the NHC, Hermine continues to approach Florida for an expected landfall just after midnight tonight. Hermine is expected to strengthen into a hurricane before coming onshore and hurricane warnings are in effect for parts of Florida as a result (see NWS).

Tropical storm conditions are expected to reach coastal Florida later this afternoon, and hurricane conditions are possible somewhere in the hurricane warned area overnight. Further threats from Hermine include: heavy rains and associated flooding, coastal flooding due to storm surge, and isolated tornadoes.

Hermine is now expected to track along the coast of Georgia, South Carolina, and North Carolina before exiting into the western Atlantic. Tropical Storm watches and warnings are in effect for the Atlantic coast of the US. Once offshore, Hermine is expected to become post-tropical and linger over the weekend and into the beginning of next week. Hermine is currently not a potential threat to Bermuda, but should be monitored closely as it tracks and lingers along or offshore of the US east coast over the weekend.

Active Tropics: Gaston, TD8, TD9, and Tropical Wave

The Atlantic has become fairly active over the last week or so, fitting well with the climatological upswing in activity this time of year. Follow the Bermuda Weather Service and National Hurricane Center for the latest official weather information and forecast.

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Hurricane Gaston to the distant east of Bermuda, TD8 approaching North Carolina/to the distant west of Bermuda, TD9 in the Gulf of Mexico, and a tropical wave near the coast of Africa. Visible satellite imagery from Weather Underground’s WunderMap.

Hurricane Gaston

Gaston strengthened into the Atlantic’s first major hurricane of the 2016 season over the weekend and on Monday as the Hurricane stalled to the distant east of Bermuda. Gaston has since picked up a slow east-northeast motion today – away from Bermuda. As of the noon advisory, Gaston is a category 2 hurricane with maximum sustained winds near 105mph.

The only impacts from Gaston have been and is only expected to be an increased swell for which a small craft advisory has been in effect. While no adverse weather conditions are expected on Bermuda from Gaston, this hurricane could threaten the Azores as early as Friday as a strong tropical storm or weakening hurricane.

Tropical Depression 8

Meanwhile, the remnants of Tropical Storm Fiona were absorbed into the remnants of a weak frontal system that brought several days of scattered showers and thunderstorms to Bermuda. A heavy, slow-moving, thunderstorm brought flooding to low-lying and poor drainage regions of Bermuda from the initial frontal system. This combined energy organized into a tropical low to the south of Bermuda on Friday that brought bands of gusty showers to the island.

That tropical low has since become Tropical Depression Eight (TD8) as it tracked westward, towards North Carolina. Strong southeasterly vertical wind shear over TD8 has thus far kept the depression from strengthening much on its approach to North Carolina. However, this shear is lessening, and the depression is tracking into a more moist environment and over the warm waters of the Gulf Stream, these factors should allow some strengthening today. TD8 should then turn to the northeast this afternoon/tonight, and become extratropical as it tracks into the North Atlantic.

As of the noon advisory, TD8 remained disorganized and weak with maximum sustained winds near 35mph. No adverse weather is expected from TD8.

Tropical Depression 9

Further, Tropical Depression 9 (TD9) formed in the Florida Straights Sunday afternoon. TD9 has been slow to organize due to strong northwesterly vertical wind shear over the last day or so. However, today that wind shear is lessening as the depression moves into the central/southern Gulf of Mexico. This should allow some steady strengthening as TD9 slowly moves northwards and then northeastwards over the next three days.

TD9 is expected to become a tropical storm, and possibly strengthen into a hurricane as it approaches the Gulf Coast of Florida on Thursday. Tropical Storm or Hurricane Watches will likely be required later today or early tomorrow for portions of Florida. As of the noon advisory, TD9 is forecast to continue northeastwards, crossing Florida into the southwestern Atlantic and potentially threatening Bermuda over the weekend as it passes to the northwest of the island and transitions to an extratropical cyclone.

Tropical Wave

Finally, another vigorous tropical wave is exiting the west coast of Africa in that infamous Cape Verde region. There is substantial model guidance that supports this tropical wave becoming the next Atlantic tropical cyclone as it tracks westward into the Central Atlantic. The National Hurricane Center gives this wave a 40% (medium) chance of becoming a tropical cyclone in the next five days. All tropical cyclones in the central Atlantic should be monitored as they could become a potential threat to Bermuda.