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.

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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.

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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.

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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.

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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.

Emily, A Potential Threat?

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The Bermuda Weather Service Tropical Update Bulletin for this morning’s 6am update: Closest point of approach to Bermuda within 72 hrs (3 days) is forecast to be 236 nm to the NNW, 3 pm Thu, Aug 3, 2017. However, this system may move closer to Bermuda after this time period.

Yesterday morning, Tropical Storm Emily quickly formed in the Gulf of Mexico, just west of Tampa, Florida. The tropical storm made landfall south of Tampa a few hours later with maximum sustained winds near 40 kts. Through Monday evening and overnight, Emily tracked across the Florida peninsula, weakening to a tropical depression and emerged into the southwestern Atlantic early this morning. Tropical Depression Emily is a “Potential Threat” to Bermuda.

What does a “Potential Threat” mean?

PotentialThreatGlossary

This means that the center of a tropical cyclone (e.g. TD Emily) is expected to pass within 400 nautical miles of Bermuda in the next 72 hours. As of the 6am local time update from the National Hurricane Center and Bermuda Weather Service, Emily is expected to pass under 250 nautical miles away from the island on Thursday. It is important to note that these threat designations do not suggest that the tropical cyclone in question will result in adverse weather as it passes.

[See the BWS Glossary]

Another Emily?

Yes. The National Hurricane Center has six lists of names, and a new list is started for each hurricane season. The lists are then used in rotation, so every six years the same list of names is used again. A name is retired from the list is a storm is ‘so deadly or costly that the future use of its name on a different storm would be inappropriate for reasons of sensitivity’.  During the annual meeting by the World Meteorological Organization committee, representatives can suggest the retirement of a name and vote on the replacement name.

‘Emily’ has been a name on the Atlantic name lists since ‘Eloise’ was retired after the 1975 Hurricane Season. There have now been 7 tropical storms or hurricanes with the name ‘Emily’.

[More on Hurricane Names from the NHC]

What can Bermuda expect from Emily 2017?

Short answer: not much. As Emily passes on Thursday, the Bermuda Weather Service expects moderate winds (10-15 kts), slight seas (1-2 feet, inside the reef), and only isolated or scattered showers with a risk for thunder to dampen Emancipation Day, the first day of Cup Match.

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GOES-13 infrared imagery at 1045 UTC (7:45am local time) showing tropical depression Emily (red circle), a stationary front (red and blue line), and Bermuda (cyan circle).

Emily is not expected to change much in strength, possibly re-strengthening to a weak tropical storm at most. Strong vertical wind shear over Emily is allowing deep layered dry air to be entrained from the west. This is limiting the depression’s ability to maintain convection, organize, and strengthen despite being over the very warm waters of the Gulf Stream.

Showers and thunderstorms ahead of the stationary front to the west have already been the theme for Bermuda. Wind gusts reached gale force in and around some of those showers (official peak gust reached 35 kts at the airport yesterday, about the same as wind gusts currently around Emily). While showers and thunderstorms are expected to continue through Friday, high pressure building in from the east may allow dry (and even sunny) spells in between showers.

As Emily interacts with the front over the next 36-48 hours, it is expected to transition to a post-tropical cyclone. Emily is forecast to track northeastwards, along the stationary front over the next five days. The flow ahead of Emily is expected to gradually lift the stationary front away from Bermuda starting late today through her closest approach on Thursday. This allows high pressure to build in from the east, slowly reducing the coverage of shower activity and allowing winds to slacken. Emily then passes Bermuda and the flow behind her swings the front back towards Bermuda, bringing increased chances for showers on Friday morning.

Follow the Bermuda Weather Service and National Hurricane Center for any changes to their forecast.

Hurricane Nicole Post Storm Report Released

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NASA Terra/MODIS True Color satellite imagery of Hurricane Nicole near the time of closest point of approach 13 Oct 2016.

After each season, the National Hurricane Center prepares ‘post-storm reports’ on each tropical cyclone that formed in the North Atlantic and Eastern Pacific basins. These reports include data collected in real-time that may not have been available during operational analysis, and therefore can sometimes lead to revision of track or intensity. This was the case for 2014’s Hurricane Fay.

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Time trace of wind gusts (red ‘x’) and wind directions (green ‘triangles’) from the airport reproduced from the NHC report. We see the sharp decrease in wind gusts and sharp wind shift as the calm eye of Nicole crossed.

Today, the National Hurricane Center released its report on 2016’s Hurricane Nicole. Their analysis on observations from Bermuda suggest that widespread category one conditions occurred on the island with isolated areas seeing category two conditions. Aircraft reconnaissance measurements near the time of closest point of approach to Bermuda indicate that Nicole was still a category three hurricane with maximum sustained winds near 105 kts (120 mph). The island received impacts from the left-front quadrant of Nicole’s eyewall, missing these strongest winds (located in the right-front quadrant) and thus this was classified as a strike.

[strike, direct-hit, indirect-hitlandfall – Impact Terminology Defined]

Hurricanes are centers of extreme low pressure. Winds spiral inwards, towards centers of low pressure and in the northern hemisphere, this manifests as a counter-clockwise circulation.  Observations from Bermuda indicated that winds backed (turned counter-clockwise with time) from an easterly direction to a northerly, and then northwesterly direction suggesting that the center of circulation remained to east of the island and therefore did not make landfall, despite Bermuda entering the calm eye of the hurricane.

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Bermuda SRI Radar imagery showing steady and heavy rain rates persisting over Bermuda (red ‘+’) for hours before the drier eye and southwestern quadrant passed over the island. Reproduced from the NHC report.

Nicole’s impacts on Bermuda were also remarkable in that the hurricane made for one of the top-5 wettest meteorological days on record at the airport, and the Bermuda Weather Service was able to release a weather balloon in the eye that measured the highest precipitable water here since 1973 at 2.93″.

[What is Precipitable Water?]

Notable for an erratic early track and meteorological evolution (including two periods of rapid intensification), Hurricane Nicole will go down in the record books as the fourth early-October hurricane impact on Bermuda in three years. Once Nicole passed Bermuda, the cyclone underwent a complex transition into a powerful extratropical cyclone in the North Atlantic where it continued to produce storm force winds for several days.

Current Weather:

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Wind measurements at the Crescent, in the northern marine area via Bermuda Weather Service. Note peak in winds just before 9am 16 Feb 2017(sustained, to/gusts, middle).

Bermuda is now well into its ‘winter’ season where changeable weather associated with gales is commonplace. Yesterday’s cold front brought a brief warm and humid spell with southwesterly gales and thunderstorms that brought heavy rain to the island.

Locally severe wind gusts, mainly confined to the marine area, were also observed. The Crescent Buoy, in the northern marine area, measured a peak thunderstorm gust of 54 kts. Winds on island appear to have remained below severe levels (i.e. < 50 kts) with a peak gust of 43 kts measured at the airport prior to any thunderstorm activity.

Follow the Bermuda Weather Service for the latest official forecast, warnings, and observations for Bermuda.

Nicole Set for Direct Hit Tomorrow Morning

Follow the latest official updates from the Bermuda Weather Service and the National Hurricane Center. Below are my unofficial thoughts.

The core of now category two Hurricane Nicole is expected to pass near or over Bermuda early on Thursday. As of the 3pm advisory from the NHC, this could mean dangerous coastal flooding from a 6-8 foot storm surge, flooding away from the coast due to heavy rain of 4-8″, and isolated damage from possible tornadoes. Tropical storm conditions are expected to begin after midnight tonight and last through much of Thursday with a period of hurricane force winds beginning Thursday morning lasting through mid-afternoon. Sustained winds are expected to peak near 90 kts (105 mph), and will be higher in elevated and exposed locations (possibly near category three strength) with higher gusts.

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Hurricane Nicole just after 12pm advisory on RGB GOES multispectral satellite imagery, Bermuda can be seen at the top of this image to the north-northeast of Nicole.

Last night, Nicole quickly strengthened from a tropical storm to a category two hurricane, increasing winds by 30 kts and decreasing pressure by 21 mb in the 24 hours between 12pm yesterday and today. Nicole had stalled or drifted slowly to the west during this time as it interacted with the front left behind as the extratropical remnants of Matthew exited the Canadian Maritimes and left Nicole in a weak steering environment.

Nicole is poised for some additional modest strengthening as the hurricane sits in a region of warm sea surface temperatures (~28 °C), low vertical wind shear, and generally high moisture content. However, some drier and more stable air to the west, might be intermittently wrapping into the circulation and slowing the pace of intensification. Nicole is expected to be a strong hurricane and it is still possible for Nicole to become a major hurricane before reaching the island.

The track forecast philosophy has proved true for the last five days and hasn’t changed; Nicole has made the turn to the north and is still expected to pass close to or over Bermuda Thursday morning as it turns more northeastward. Wobbles either side of the track could make a huge difference in terms of conditions felt. These wobbles are due to inner-core dynamics are hard to predict with skill.

  • Track more to the left: easterly winds veer to the south, increasing to hurricane force as Nicole passes (east eyewall)
  • Track more to the right: easterly winds increase to hurricane force as Nicole approaches, suddenly become light and shift to the west, suddenly increase to hurricane force as Nicole passes (eye)
  • Track even more to the right: easterly winds increase to hurricane force and back to the north as Nicole passes (west eyewall)

Wind direction strongly controls who/where sees the most significant coastal flooding due to storm surge. For instance, should winds reach their peak out of the south, then south shore would take the brunt of any storm surge. Similarly, this changes wind exposure as windward hilltops see dramatically higher winds than the sheltered lee-side valleys. It is therefore imperative to both know your exposure and prepare for the scenario that exploits your exposure.

Expect scattered to widespread tree damage and power outages, isolated structural damage (mainly to roofing on elevated and exposed locations or along the track of any tornadoes, and along the coast if an inundating storm surge develops), and isolated flooding from rainfall in low-lying and poor drainage areas. Significant coastal erosion will likely begin to take shape tonight as seas inside the reef become rough, and isolated power outages are likely before dawn tomorrow as winds begin to reach 50 kts in exposed locations.

Nicole Begins Erratic Track

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:

Yesterday afternoon and overnight, Nicole put on a somewhat unexpected burst of organization and intensification peaking as a category two hurricane with maximum sustained winds estimated near 90 kts (105 mph). This morning however, Nicole has begun to take on the expected erratic track and has weakened due to stronger vertical wind shear. Nicole, however, remains a ‘Potential Threat’ to Bermuda. After drifting to the south and weakening over the next two to three days, Nicole is expected to accelerate northwards toward Bermuda, possibly re-intensifying and has a chance to become a ‘Threat’ to the island by early next week.

[Threat: When effects from any tropical cyclone are possible in Bermuda within 72 hours. – BWS Glossary]

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Most recent track forecast for Tropical Storm Nicole. See the latest ‘Local Advisory‘ from the Bermuda Weather Service’s Tropical Products page.

Nicole, after becoming stationary overnight and this morning, is expected to begin a slow drift to the south. In the short term, Nicole has therefore passed its closest point of approach to Bermuda which was about 285 nm south of the island at 6pm yesterday. However, Nicole is still a potential threat to Bermuda at about 290 nm to the south as of today’s midday advisory.

Nicole’s southward drift is expected to pick up pace today and gradually turn back to the west on Sunday as deep layered ridging builds to the north of the storm. Early next week, however, that ridging begins to move east of Nicole as another deep layered shortwave trough approaches the region from the west. This in combination with interaction from Matthew exiting the US east coast is expected to turn Nicole back toward the north. Nicole again approaches Bermuda early next week, possibly becoming a threat to the island by mid-week. The track forecast early next week is still more uncertain than average and it is not clear how Nicole will interact with both the shortwave trough and Matthew in the coming days.

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Hurricane Nicole yesterday afternoon rapidly intensifying  with eye feature and symmetric deep convection (left). Nicole after strong vertical wind shear rapidly weakens the cyclone, leaving it with asymmetric deep convection loaded to the south of the storm (right). See latest satellite imagery centered on Nicole here.

In terms of intensity, very strong vertical wind shear is currently weakening Nicole. These strong upper level winds have stripped the organized convection away from the low-level center of Nicole this morning. This has weakened Nicole dramatically from a category two with maximum sustained winds near 85 kts (100 mph) to a tropical storm with winds near 60 kts (70 mph) in six hours.

This strong vertical wind shear is expected to continue to weaken Nicole today to a minimal tropical storm by Saturday. These unfavorable upper winds are expected to persist at least through Monday, limiting Nicole’s intensity. Nicole could degenerate into a remnant low in this time, or could prove again resilient to large scale shear and maintain moderate tropical storm intensity. However, the environment is expected to gradually become more conducive for development as Nicole approaches Bermuda early next week.

The overall atmospheric set up next week is again very uncertain and therefore it is important to continue to monitor the progression of Nicole over the next five days and prepare accordingly should Nicole become a threat to Bermuda.

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.