As of the 6AM advisory from the National Hurricane Centre, Jerry has transitioned into a post-tropical cyclone. A tropical storm warning remains in effect for Bermuda as Jerry is still expected to bring tropical storm conditions to island this evening – a delay compared to earlier forecasts.
Jerry became completely devoid of deep convection yesterday as it succumbed to dry air and vertical wind shear. The storm was not able to take advantage of the brief period of lighter vertical wind shear and favourable upper-level dynamics as dry air prevented development. The storm has been unable to regenerate significant convection since then and has therefore transitioned into a post-tropical cyclone. Without deep convection, Jerry was steered further to the west and left caught in weaker low-level steering flow which delayed its approach to the island.
Jerry is now tracking east-northeastward, generally toward Bermuda. This motion is expected to continue until the centre of Jerry passes near or over Bermuda before midnight tonight. It is likely that Jerry will remain void of deep convection and continue to slowly weaken on approach as the surface low fills and decays.
However, Jerry is still expected to bring squally showers starting this morning, with a period of tropical storm conditions (sustained winds ≥39 mph) this evening just before the storm-centre passes the island. Expect peak winds to be out of the south through southwest. With weakened infrastructure following Hurricane Humberto, these winds will likely result in scattered power outages.
Karen a ‘Potential Threat’
Tropical Storm Karen is currently tracking northward away from Puerto Rico. In two to three days, Karen is expected to be in weak steering flow south of Bermuda and is expected to be within 400 nautical miles of Bermuda in the next 72 hours which makes it a ‘Potential Threat’. No direct impacts are currently expected from Karen. The current track forecast is at unusually low-confidence due to possible interaction with both a mid-latitude system and the remnants of Jerry, in addition to an environment marginally conducive for sustaining a tropical cyclone.
Not a week after Hurricane Humberto’s strike, Tropical Storm Jerry is a threat to Bermuda with a Tropical Storm Warning in effect. Tropical Storm conditions are expected in Bermuda as Jerry nears and passes the island sometime between Tuesday evening and much of Wednesday morning. However, impactful weather is expected well in advance of the closest approach of the storm, with the outermost rainbands moving in as early as late this afternoon with strong easterly winds.
Today through Tuesday Morning
Forecast: Moderate to strong easterly winds veer southeasterly this evening and overnight. Occasional showers or periods of rain. Then strong southeasterly winds with gusty, possibly thundery showers and periods of rain Tuesday morning.
The midday update from the National Hurricane Centre finds Jerry 345 miles southwest of Bermuda, moving slowly north-northwestward at 7 mph. Jerry currently has maximum sustained winds near 65 mph and a minimum central pressure near 991 hPa.
Jerry remains in an environment dominated by strong vertical wind shear. The upper-level flow is from the southwest, while the low-level flow is from the southeast. This change in wind direction with height is forcing all of the inclement weather to remain over the eastern half of the storm, limiting organisation and therefore preventing strengthening despite Jerry being over high sea surface temperatures of 28-29°C.
Jerry is generally moving slowly north-northwestward into a break in the sub-tropical ridge. This general motion is expected to continue over the next 24 hours.
On a technical note, a cluster of deep convection near the center of Jerry appears to be keeping the cyclone vertically coupled. As a result, Jerry is steered by a vector-average of the southeasterly low-level flow and southwesterly upper-level flow. The east-west components of the flow at these two levels roughly cancel, leaving a net southerly flow across the cyclone and a storm motion toward the north. Whenever the convection wanes, the low-level circulation becomes decoupled, follows the low-level flow, and drifts more northwestward. Whether Jerry can maintain the deep convection will determine how far to the west the storm drifts. This will have a knock-on effect for the timing (i.e. a drift further west could delay impacts) and potentially the severity of impacts to Bermuda (i.e. a more vertically-coupled cyclone could be stronger). Convection is currently waning over the centre.
Tuesday Morning through Wednesday
Forecast: Strong southeasterly winds veering southerly and increasing to tropical storm force (> 38 mph) by Tuesday evening. Showers, some thundery, become more frequent with tornado potential (always possible with nearby Tropical Cyclones). Winds veer westerly as Jerry passes in the pre-dawn hours of Wednesday morning. At its peak, expect strong tropical storm conditions with hurricane force (>73 mph) gusts in elevated and exposed locations. Conditions then quickly improve on Wednesday with tropical storm conditions subsiding by late morning.
The strong vertical wind shear is expected to diminish somewhat as the upper-level trough cuts off forming an upper-level low which backs away to the west in the 18-36 hour timeframe. As this occurs, the upper-level winds across Jerry decrease and the flow pattern becomes more diffluent. This could result in some short-term strengthening, but it is on the whole unclear whether Jerry will be able to take advantage of this short window of more favourable conditions. The most likely outcome is that Jerry maintains its intensity but restructures, and this is what the National Hurricane Center calls for.
After 36 hours, the deep layered flow shifts out of the west in response to a deepening trough over Atlantic Canada, steering Jerry eastward then accelerating northeastward. The centre of Jerry is expected to pass less than 100 nautical miles away from Bermuda in the pre-dawn hours of Wednesday morning.
As expected, Humberto has strengthened into a hurricane overnight Sunday. As of the latest advisory, Humberto had maximum sustained winds near 75 kts and a minimum central pressure near 978 hPa. The National Hurricane Centre’s latest forecast track for Humberto now indicates that the centre of the hurricane could pass 79 nautical miles (91 miles) away from the island on Thursday morning, and therefore impacts from Humberto are expected to commence in under 72 hours.
As a result, the Bermuda Weather Service now lists Humberto as a ‘threat‘ to the island, where the BWS’s Tropical Cyclone Threat listings are as follows (in order of increasing threat level):
A tropical storm or hurricane has passed its closest point of approach to Bermuda and is moving away under the influence of a well defined steering flow.
The centre of a tropical storm or hurricane is expected to pass within 400 nm of Bermuda within 72 hours.
Effects from a tropical storm or hurricane are possible within 72 hours and/or the centre of a tropical storm or hurricane is expected to pass within 100 nautical miles of Bermuda within 72 hours.
Tropical storm conditions are just about locked in for overnight Wednesday into Thursday as Humberto passes the island, and a tropical storm watch or hurricane watch is likely to be issued this evening. Watches are issued when adverse conditions from a tropical storm or hurricane are possible within the next 48 hours. They are upgraded to warnings if adverse conditions are expected within the next 36 hours.
However, the exact impacts are not yet so certain. The latest timeline from the BWS has showers with gusty winds moving in Wednesday morning. Showers become squally as tropical storm conditions (sustained winds 34-49 kts) spread across the island through Wednesday afternoon and evening with winds out of the south.
As with any tropical storm or hurricane, tornadoes are possible – primarily in these squally outer bands.
Southerly winds veer southwesterly then westerly, and strong tropical storm conditions (sustained winds 50-63 kts) develop overnight Wednesday into Thursday. Humberto makes its closest approach to Bermuda early Thursday morning (between 6 and 9 AM). During that time, winds peak and veer further, becoming northwesterly. Expect damaging winds (mainly vegetation damage, isolated property damage) and dangerous surf. Some minor coastal flooding is possible.
Humberto is expected to be a serious hurricane at its closest point of approach. Today and tomorrow, high sea surface temperatures ~29°C support intensification from its current category 1 strength to category 3. Increasing vertical wind shear and encroaching drier air could begin to degrade the hurricane’s core and begin a weakening trend on Wednesday into Thursday. However, favourable upper-level dynamics and continued high sea surface temperatures could offset some of these weakening effects. Altogether, the NHC currently expects a slowly weakening category 2 or 3 hurricane passing the island.
A track error of around 50 nautical miles, plus an intensity error of just 10 kts could bring major hurricane conditions to Bermuda Thursday morning. (Typical 72 hour track error is 102 nautical miles, and typical 72 hour intensity error is 20 kts.)
Tropical Storm Humberto formed just east of the Bahamas islands last night. As of the midday advisory from the National Hurricane Centre, Humberto has maximum sustained winds near 50 mph and a central pressure near 1005 hPa. Humberto is narrowly keeping the worst weather offshore of the islands of the northwestern Bahamas which were so severely hit by Hurricane Dorian at the beginning of the month. However, squally weather is still expected and tropical storm warnings are in effect for the northwestern Bahamas.
Confidence increasing for Bermuda impacts from Humberto
a. Now through Monday
Humberto is currently in a region of weak steering flow and it is thus stationary just east of the northwestern Bahamas. An upper-level low to the west of Humberto is imparting some vertical wind shear over Humberto and preventing significant organisation. This upper-level low is drifting away to the west, so vertical wind shear is expected to decrease today. This decrease in vertical wind shear in combination with Humberto embedded in a humid environment over sufficiently high sea surface temperatures is allowing steady strengthening today.
Tonight and tomorrow, southerly steering flow starts to impinge on Humberto as high pressure to the northeast of the storm begins to weaken and drift southeastward. As a result, Humberto is expected to track northwestward and then northward this evening and Sunday. The environment is expected to continue to become more favourable for Humberto and the storm is expected to become a Hurricane on Sunday.
By Monday, a deepening trough over Atlantic Canada begins to impart westerly flow over Humberto. The storm then begins to track toward the east, continuing to strengthen.
b. Tuesday through Thursday
On Tuesday, Humberto is expected to accelerate eastward and then northeastward around the southern edge of a trough positioned over Atlantic Canada. This takes a hurricane-strength Humberto near Bermuda overnight on Wednesday into the pre-dawn hours of Thursday. The environment on approach to Bermuda is expected to remain favourable for strengthening. It is possible that Humberto could be a major hurricane on approach to Bermuda.
While confidence is increasing for hazardous surf, rain, and wind from Humberto as early as Tuesday night in Bermuda, these impacts are not yet locked-in. In Bermuda, now is the time to start going over your hurricane preparedness plan and make a point of regularly checking for updates on Humberto from the Bermuda Weather Service and National Hurricane Centre.
Hurricane Florence underwent a remarkably well-forecast period of rapid intensification into a major hurricane yesterday evening. As of this morning’s 11am advisory from the National Hurricane Center, Florence is still a major hurricane with maximum sustained winds near 130 mph. Florence is moving west-northwestward, and is expected to continue that motion around deep-layered high pressure centered between Bermuda and Nova Scotia over the next day or so. Florence is expected to remain a very dangerous major hurricane during this time as it tracks through an environment conducive for hurricane growth.
Florence should make its closest point of approach to Bermuda mid-afternoon as it passes about 300 nm south-southwest of the island. Impacts on Bermuda should be fairly limited. Expect elevated and hazardous surf and rip currents along the South Shore from southeasterly swell generated by the major hurricane. Isolated showers are possible and winds could briefly increase to strong (20-33 kts) in the southern marine area. A Small Craft Warning is in effect for elevated surf and strong winds.
Florence is expected to continue west-northwestward today before turning northwestward on Wednesday through its final approach to the US East Coast. Florence is forecast to track over open ocean with sea surface temperatures near 29°C. Furthermore, environmental vertical wind shear is low and humidity sufficiently high. These ingredients suggest that Florence will continue as a major hurricane through its approach to the coastline on Thursday, and expected landfall Friday morning. Changes in the hurricane’s intensity are likely to be related to internal structural changes known as eye-wall replacement cycles. One such cycle is nearing completion this afternoon.
During such a cycle, the eye-wall (region of the hurricane with strongest winds and heaviest rains) is encircled by outer bands that form a secondary eye-wall. This has the effect of increasing the overall size of the hurricane. If the cycle is uninterrupted, the secondary eye-wall begins to intensify at the expense of the original, resulting in an overall weakening of the hurricane. Eventually, the original eye-wall dissipates and the secondary eye-wall assumes the role of the primary eye-wall, and the larger hurricane resumes strengthening.
Coastal and inland watches have been issued by the National Weather Service for parts of South and North Carolina in response to the wind and storm surge threat.
Increasing surf and dangerous rip-currents are expected to commence tonight or tomorrow along the US East Coast.
On Thursday, as Florence nears the coast of North and South Carolina, the hurricane is expected to enter a region of fairly weak steering. Florence should be near the southwestern edge of high pressure centered over the North Atlantic, and near the northeastern edge of high pressure centered over the Northwest Caribbean Sea imposing steering flow in opposite directions across the hurricane resulting in a weak steering regime. Both the track and intensity, and therefore the severity and duration of impacts from the hurricane depend strongly on the structure of the steering flow as Florence approaches the coast, and the timing with which the hurricane approaches this change in steering. While confidence is high that there will be severe impacts somewhere in the Mid-Atlantic region, exactly who gets what is still to be determined.
Residents should prepare for life-threatening storm surge inundation at the coast, freshwater flooding inland, and damaging winds – particularly at the coast, but extending some distance inland. See the National Hurricane Center’s Key Messages for more.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.