Isolated Downpours Last Night

A stationary front, lingering in the area since the 22nd has cleared to the east and southeast this morning. Winds shifted from the southwest to the northeast through the day yesterday with the main front pushing across the island with little fan-fare. In the evening, a line of showers developed along the island extending northeast and southwest from the island. A region of convergence between northeasterly and east-northeasterly flow helped spark these showers with isolated downpours that persisted for over an hour in spots.

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Distribution of rainfall from yesterday based on observations from the Bermuda Weather Service, Wunderground, and WeatherLink. This probably underestimates rain totals in Sandy’s parish as radar indicated heavier showers forming there first and being more persistent in that parish. You can really see the isolated nature of the downpours with <0.25″ across much of the eastern parishes, and over an inch in the western parishes.

Meanwhile, an area of low pressure is developing at the tail end of that lingering stationary front north of the Turks and Caicos Islands. The National Hurricane Center is monitoring this area for possible tropical cyclone formation. Their latest updates at 10am indicate a 70% chance for the formation of a tropical or subtropical cyclone in the next 5 days. However, direct impacts to Bermuda from this system appear unlikely at this stage.

Follow the Bermuda Weather Service and the National Hurricane Center for the latest official information.

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Significant Gales on the 8th

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Bermuda Weather Service radar imagery for Bermuda showing spiral bands of showers around the deep low as it made its closest point of approach on the afternoon of the 8th. 

A deep low passed to the near north of Bermuda on Friday brining gale force southwesterly winds to the island for several hours before veering to the west and northwest and diminishing into the evening. This followed a day of steady, moderate rains that accumulated around an inch islandwide. The low responsible had a minimum central pressures below 990 mb and maximum sustained winds around 55 kts.

Peak gusts officially reached 51 kts (59 mph) at the airport. Other unofficial gusts were measured at 75 kts (86 mph) at Commissioner’s Point WindGuru Spot, 59 kts (68 mph) at MARops via Skylink-Pro, and 37 kts (43 mph) at my PWS. These winds resulted in some isolated power outages and transportation disruption.

Following the Shapiro-Keyser Cyclone evolution model, the low fits the mature “warm seclusion” stage. This is fairly common in ocean cyclones but is more typical further north in the Atlantic.

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Infrared Satellite imagery showing the clouds associated with the low near Bermuda and accompanying fronts to the north and east.

However, because the low formed so far south and was headed east it is remaining over fairly warm sea surface temperatures. Its thermal structure, plus a detachment from fronts and continued convection around the center of the low could allow a transition to it being sustained by more tropical processes. This would allow the low to become a subtropical or tropical cyclone. Fortunately, it is tracking into the depths of the Central Atlantic and isn’t expected to pose further threat to land for at least five days. The National Hurricane Center is monitoring this low for a 40% chance that it transitions to a subtropical or tropical cyclone in the next 48 hours as it continues to produce storm force winds over the open Atlantic.

Active Weather Pattern Develops

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RGB Satellite imagery at 14:45UTC showing the cloudiness associated with shower and thunderstorm activity in a band running from North Carolina to just south of Bermuda.

Tuesday morning saw locally squally thunderstorms as a north-south oriented line of thunderstorms crossed parts of the island dumping up to unofficial amounts of 2″ of rain in highly isolated spots, however some areas, particularly in the western parishes, did not pick up nearly as much rain. Gusts in and around these thunderstorms generally reached 30-40 kts, an increase from the light and variable winds that had preceded the storms overnight. My PWS recorded 1.12″ of rain for the 18th of August, the Bermuda Weather Service had the same amount.

The line of thunderstorms that dominated the weather scene Tuesday morning formed along a weak surface trough and was supported by an upper level low that was sat to the northwest of Bermuda. This upper level low is slowly drifting southeastwards, towards the island and the best upper level divergence to support thunderstorm activity is also drifting southeastwards. Low level convergence associated with the surface trough in combination with the upper level low will support clusters of showers and thunderstorms that could pass over the island producing locally heavy rain and gusty winds at times with clearer periods.

A more organized surface trough is then expected to develop to the west of Bermuda. At this point, the upper low is expected to still be in the vicinity of Bermuda. The developing surface trough is then expected to stall near to the southwest of Bermuda overnight on Thursday. An area of low pressure could form from this surface trough near to the island as it starts to lift out of the area to the north as we approach the weekend. The interaction of the developing surface low and weakening upper low is a recipe for a potential subtropical transition.

8am 19 Aug 2015
National Hurricane Center chart showing the area being monitored for possible tropical or subtropical development – the yellow area indicates a low chance of a cyclone forming. Link in-text below for the latest chart.

Regardless, the formation of a surface low to the southwest of Bermuda will result in winds likely settling out of the southeast and gradually increasing to moderate/strong with gusts near gale force at times starting Thursday night. The National Hurricane Center has given this potential system a low (30%) chance of becoming a tropical or subtropical cyclone in the next 5 days as of 9am BDA time. The Bermuda Weather Service is also monitoring this potential development and expect the forecast to be fine tuned as the weekend approaches and it becomes more certain whether a cyclone will develop.

Elsewhere in the Atlantic, Tropical Storm Danny quickly formed in the east-central tropical Atlantic out of an African Easterly Wave – a kick-off to the notorious Cape-Verde Season. The National Hurricane Center forecast for Danny indicates steady strengthening to a hurricane by Friday morning as the cyclone moves west-northwestward. This track takes Danny just to the east of the Windward Islands by Monday morning.  Danny is therefore not expected to pass near Bermuda in the next five days, but should be monitored for any track changes, especially for the period after Monday.

The Cape-Verde Season typically encompasses the majority of the tropical cyclone activity for the Atlantic in any given year and is loosely defined as mid-August through early-October. Many of the tropical cyclones that form during this period find their origins near the Cape Verde Islands from African Easterly Waves – this is why it is referred to as the “Cape Verde Season”. Tropical cyclones that form near the Cape Verde Islands tend to have over a week over warm tropical water before reaching any land in the Western Atlantic or Caribbean Sea.

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

Where do Atlantic Tropical Cyclones Form?

The official start to the 2015 Atlantic Hurricane Season is fast approaching on June 1st. With that in mind, a question I want to answer today is: Where do tropical storms and hurricanes form?

To answer this I turned to the National Hurricane Center’s Best Track Data in their HURDAT2 dataset The hurricane seasons for the period 1950-2014, and only tropical and subtropical cyclones with top 1-minute sustained winds greater than 34kts were included. The HURDAT2 dataset contains 6-hourly data for Atlantic Tropical cyclones. Once it was determined that the tropical cyclone had fit the above criteria (1950-2014, at least TS,) its genesis coordinates were recorded (the latitude and longitude of its first entry as a tropical or subtropical cyclone).

These coordinate locations were split up into cyclone groups based on their peak intensity as follows:

– Peaked at tropical storm strength (34-63 kts)
– Peaked at minimal or moderate hurricane strength (64-96 kts)
– Peaked at major hurricane strength (over 96 kts)

Using an R spatial statistics package {spatstat} the spatial density of the genesis locations was calculated and plotted on a map of the Atlantic Basin. The resolution for these plots is roughly 0.664 x 0.291 degrees or about 76 km for the diagonal across each pixel.

Some interesting patterns are immediately evident in the distribution of genesis location density. For ‘All Tropical Cyclones,’ there are two main regions:

1. The open Tropical Atlantic known as the ‘Main Development Region – MDR’ (the area between the West Coast of Africa and the Caribbean)

2. The Southwest Atlantic (the area north of the Bahamas, the northwest Caribbean, and the Gulf of Mexico)

In region 1, the main source of tropical cyclones is likely African Easterly Waves, inverted troughs that move from east to west into the Atlantic associated with disturbances in the West African Monsoon. In region 2, some of these African Easterly Waves don’t encounter an environment favorable for development until they get steered into or North of the Caribbean where they have another chance to develop. Less frequently, midlatitude weather systems (extratropical cyclones) get steered southward into the subtropics and tropics where they encounter relatively weak steering flow and begin to transition into subtropical or tropical cyclones over the warm waters. This is more common north of the Bahamas. A further mechanism for development, particularly in the Northwestern Caribbean and Gulf of Mexico, is related to the monsoon trough over Central America. This trough, present mainly during the summer months, can be displaced northward into the Northwest Caribbean or Gulf of Mexico where, if it persists under favorable conditions, it can organize into a tropical cyclone. The number of cyclones that form in region 2 is similar to that in region 1.

All Tropical Cyclones

For those tropical cyclones that did not intensify to become hurricanes, ‘Just Tropical Storms,’ most form in the Gulf of Mexico or north of the Bahamas. This area takes up most of region 2 from above. This is likely because cyclones that form in the Gulf of Mexico are so close to land that they either make landfall and dissipate, or interact with dry air that disrupts their circulations.

For the cyclones north of the Bahamas, this is also a two-pronged possible explanation; cyclones that form here are typically quickly pulled poleward into the midlatitudes where they encounter wind shear and cooler waters, or they are formed from transitioning extratropical cyclones. The midlatitudes are a hostile place for tropical cyclones because of the strong wind shear associated with the jet stream, the cooler waters further north, and dry air. Dry air and wind shear from the midlatitudes is often nearby transitioning extratropical cyclones and this often slows the transition and limits intensification.

Region 2 is also active through the entire season whereas region 1 is active mainly during the peak of the season. Less than ideal early and late season conditions also likely play a role in limiting the intensity of cyclones here.

Just Tropical Storms

Probably for similar reasons, minimal-moderate hurricanes form most frequently in the Northwest Caribbean as seen on ‘Minimal Hurricanes Only.’ Here, they quickly interact with land limiting their intensification. Meanwhile, on the ‘Major Hurricanes Only’ plot, we see that the most common formation area is off the west coast of Africa. Cyclones that form in this region are notorious for their longevity and intensity and are informally known as “Cape Verde-type” Hurricanes. This region not only becomes most active during the peak of the season when the tropical Atlantic is most favorable for tropical cyclone development, but with a typical track towards the west-northwest, there is no land to interact with for over a thousand miles – plenty of open ocean to intensify over.

Minimal Hurricanes OnlyMajor Hurricanes Only

A more subtle pattern that becomes most evident in the ‘All Hurricanes’ plot are the two maximum of formation frequency west of the Cape Verde islands. This could be due to diurnal variations in convective intensity that help to organize the African Easterly Waves (ie. showers and thunderstorms flare up around the disturbance at around the same time each day – think afternoon thunderstorms.) Since each of these waves move westward at similar speeds, the maximum near the Cape Verde island could be related to the first diurnal maximum in convection, and the maximum in the central tropical Atlantic (further to the west) could be related to the second diurnal maximum. (ie. the disturbance becomes a tropical cyclones after one day over water or after two days.)

All Hurricanes

Wind shear and stronger trade winds in the Eastern Caribbean limit tropical cyclone formation. The wind shear disrupts the organization of deep convection – a key element of tropical cyclones. Similarly, strong trade winds keep the initial disturbance for forming a closed circulation which prevents the tropical cyclone from sustaining the deep convection it needs. Overall, conditions are not as favorable for tropical cyclone formation in the Eastern Caribbean despite the adequately warm waters.

It should be noted that while more major hurricanes are born in the Cape Verde region than anywhere else in the Atlantic, cyclones that form here do not necessarily impact any land and can therefore go unnoticed. The cyclones that form in the Southwestern Atlantic are much more likely to impact land, particularly when they form in the Caribbean and Gulf of Mexico, but they less frequently become major hurricanes. The most intense hurricane on record, Hurricane Wilma (Oct, 2005), formed in the Northwest Caribbean. Bermuda has also been impacted by major hurricanes originating from both of the active regions of the Atlantic. Hurricane Fabian (Aug-Sep, 2003) formed in the Main Development Region, while the Havanna-Bermuda Hurricane formed in the Northwest Caribbean (Oct, 1926.)

Where do landfalling cyclones and cyclones that impact Bermuda typically form? See a follow-up post for the answer to this!

The Best Track Data can be found here.

Ana Landfalls in South Carolina, Weakens

Tropical Storm Ana

Ana transitioned from a subtropical to a tropical storm on Saturday at around 6am Bermuda time. Tropical Storm Ana then slowly moved northwestward while weakening for a landfall in South Carolina near Myrtle Beach at around 7am Bermuda time this morning.

Tropical Storm force gusts were observed in isolated spots of South and North Carolina, mainly along and near the coast, for much of the morning leading up to and following landfall. Heavy rain continued as the now remnants of Ana drift through North Carolina with drenching rains of over 4″ already being reported near where Ana made landfall. Minor flooding concerns continue because of Ana’s continued slow motion this evening.

Meanwhile, mostly dry weather is expected in Bermuda for the next few days. A weak cold front will enhance the chances for precipitation as it passes on Thursday. Expect winds to slowly veer from southeast to southwest through Tuesday, then sharply veer from southwest to northwest on Thursday with the frontal passage. Generally moderate winds could become strong for a time ahead of Thursday’s front.

Subtropical Storm Ana Forms

The area of troughiness north of the Bahamas earlier this week has drifted northwards and organized a closed area of low pressure, cut off from fronts, and is sustaining organized deep convection. As a result of these structural changes in the low (confirmed via Air Force Hurricane Hunter missions,) the National Hurricane Center has determined that this low has become a Subtropical Storm.

Ana is expected to meander over the Gulf Stream, just offshore of the US east coast, for two to three more days before being picked up and taken northwestward into South or North Carolina by increasing southerly flow ahead of a mid-latitude weather system late in the weekend/early next week. This mid-latitude system is also expected to bring heavy snow to the Wyoming, Colorado, Nebraska and South Dakota – in addition to a “moderate” risk for severe thunderstorms (including hail, damaging winds and tornadoes) in the southern Great Plains as it pushes eastward.

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Counties highlighted blue for Winter Storm Watches from NWS at 10:45am local.
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Storm Prediction Center convective outlook for Friday showing a moderate risk for severe weather in red over Oklahoma and Texas.

Meanwhile, troughiness related to what were the fronts that Ana shed to become subtropical in addition to a cold front approaching from the northeast are bringing chances from showers and thunderstorms this afternoon through early Sunday morning. As a result, the Bermuda Weather Service has issued a Thunderstorm Advisory. The cold front doesn’t quite make it across Bermuda, dissipating on Saturday, so winds remain light to moderate and southeasterly through the beginning of next week thanks to Ana to the distant southwest this weekend and high pressure to the northeast behind that weak front early next week. While the cold front never really made it to Bermuda, slightly less humid air will still filter in on return flow from the southeast associated with that high pressure to the northeast of Bermuda early next week.

Subtropical Storm Ana circled in red, motion indicated by a red arrow. Troughiness near Bermuda circled with a dashed red line, and an approaching cold front highlighted with a blue line.
Subtropical Storm Ana circled in red, motion indicated by a red arrow. Troughiness near Bermuda circled with a dashed red line, and an approaching cold front highlighted with a blue line. Overtop RGB Satellite imagery at 10:45am local time.

For the latest on Ana see updates from the National Hurricane Center and the Bermuda Weather Service – Tropical Products. For the latest official forecasts including watches and warnings for Bermuda, also see the Bermuda Weather Service.

Possible Sub-Tropical Cyclone Near Bahamas/Gulf Stream

A weak surface trough over the Bahamas and Cuba is interacting with an upper level trough over the Gulf of Mexico. Over the next 3 – 5 days there is potential for the surface and upper level systems to align in such a way that allows the surface trough to organize and sustain deep convection and become a sub-tropical cyclone.

The area of troughiness is currently moving northwards into the southwest Atlantic. Over the next 3 – 5 days, the trough and its associated convection are expected to then track northwestward – towards the South Carolina coast. An area of convergence to the far east of this trough/sub-tropical cyclone could bring scattered shower activity to Bermuda in 3 – 5 days as a weak cold front approaches the island from the north.

In the meantime, fair weather is expected in Bermuda for the next 2-3 days. Overall, little if any noticeable impact to Bermuda is currently expected from this disturbance.

For the latest updates see the National Hurricane Center, and see the Bermuda Weather Service for official forecasts for Bermuda.