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.

Tropical Storm Fiona Approaches

Mid-August typically marks the start of the most active period for tropical cyclones in the Atlantic sometimes referred to as the Cape Verde Season. This part of hurricane season is so called because the upswing in activity in the Atlantic this time of year typically occurs from systems that form in the central and eastern Atlantic near the Cape Verde Islands. These type of systems are notorious for their long tracks that can favor stronger hurricanes once they reach the western Atlantic and threaten land. Right on cue, a series of tropical waves has begun tracking into the Atlantic from West Africa.

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Early season formation points (June 1-10) compared to formation points this time of year highlighting both the upswing in activity, and the shift into the eastern and central Atlantic, particularly near the Cape Verde Islands off the west coast of Africa. (Link)

Tropical Storm Fiona

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GOES Satellite floater un-enhanced infrared centered on Tropical Storm Fiona. (Link)

Fiona is currently a small tropical storm with maximum sustained winds near 35 kts. Its associated cluster of deep convection is supporting tropical storm force winds that only extend out to 70 nm away from the center of the storm. Fiona is currently embedded in the dry and dusty air associated with the Saharan Air Layer which is limiting the amount of deep convection the storm can sustain, while southwesterly vertical wind shear is limiting the organization of the deep convection Fiona does produce.

The Forecast

Bermuda should be monitoring Fiona carefully as the storm tracks generally northwestward, in the general direction of Bermuda. The latest forecast (as of 6pm local) from the National Hurricane Center (NHC) has Fiona continuing northwestward and weakening before degenerating into a remnant low on its final approach to Bermuda by Wednesday. With Fiona’s closest point of approach within the next 72 hours at 312 nm, Fiona is a potential threat to Bermuda (see: Local Advisory).

Fiona’s track to the northwest puts the storm over increasing sea surface temperatures; water near Bermuda is currently around 86°F (30°C) which is plenty warm to support a tropical cyclone. Aside from the increasing sea surface temperatures, Fiona’s environment is not expected to improve enough and re-strengthening is not expected.

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Map of a snapshot of sea surface temperatures yesterday – much of the Atlantic is warm enough to support a tropical cyclone using the traditional 79F (26C) threshold for formation (red contour). Warmer waters are generally more favorable for development. (Link)

However, it is important to monitor this system closely for any changes in this forecast as it approaches. In the past, marginal tropical cyclones like Fiona have struggled through dry/sheared environments, only to survive into the western Atlantic, finding it more moist and less sheared. This allowed quick reorganization and strengthening near Bermuda forcing short-notice warnings (eg. Maria (2011) and Gabrielle (2013)).

Otherwise, there are two other significant tropical waves to monitor as of 3pm local time. One such wave is southeast of Fiona, in the central tropical Atlantic. This wave is expected to track westward into the northeastern Lesser Antilles Islands/Caribbean Sea, and the NHC has given it a 60% (medium) chance for becoming a tropical depression or storm in the next 5 days. The second wave is now exiting the west coast of Senegal in Africa. The NHC is giving it a 70% (high) chance of developing in the next 5 days as it too tracks westward into the tropical central Atlantic.

Right now, showers are expected in Bermuda starting Sunday evening as a weak boundary moves into the area from the west. That boundary lingers as an area of troughiness that keeps showers and possibly thunderstorms in the region through Wednesday as Fiona, or Fiona’s remnants approach from the southeast. No adverse weather is expected from Fiona at this time.

As always, the best advice is to exercise constant vigilance with the tropics by following the latest official forecast updates from the Bermuda Weather Service and the National Hurricane Center.