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.

A Weak Cold Front Passage

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RGB Satellite image at 10:15am local time showing the cloudiness over Bermuda associated with this morning’s front.

A weak cold front passed between 8:30 and 9am this morning bringing isolated showers followed by patchy rain and drizzle. Westerly winds veered sharply to the north following the passage of the front. Expect slightly cooler and less humid weather over the next few days with persistent north-northeasterly flow.

North to north-northeasterly winds continue this afternoon and strengthen overnight, becoming moderate, as a low develops along the front far to the northeast of the island, and a trough rotates around it, increasing the pressure gradient and, in turn, winds for Friday. As this low slowly drifts away to the northeast and high pressure builds in from the west, this moderate north-northeasterly flow is maintained through Saturday. By Sunday morning, high pressure will have moved in with light northerly winds. However, winds start backing westerly ahead of another weak frontal system, and increase again, moderate to strong, by Sunday night. This next system could bring showers and rain for Monday evening.

For the official forecast and weather information for Bermuda see the Bermuda Weather Service.

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.

Strong Morning Storms

HUIR151140845
Infrared Satellite imagery showing a band of clouds associated with thunderstorms oriented northeast to southwest across Bermuda at 5:45am (local) Friday April 24th 2015. Shortly after this time, the thunderstorms brought gale force gusts to the airport.

Early Friday morning, strong thunderstorms brought gusty winds to the island along with heavy downpours. Winds gusted as high as 45 kts at the airport, and unofficial wind gusts over gale force were widespread around the island as these thunderstorms crossed the island.

The morning storms were associated with a cold front. Friday evening saw the passage of a post-frontal trough with more showers – however, these showers were largely just in the vicinity and not as intense as the morning’s thunderstorms. Following the post-frontal trough strong northwesterly winds came in with gusts over gale force keeping cool weather in place for the first part of the weekend. The high temperature on Saturday only reached 66.9F at the airport, compared to a normal high near 72F, similarly the low got to 62.2F compared to a normal low near 64F.

A trough associated with an area of unsettled weather to the north of the island produced some heavy showers in the area for Sunday, but could keep showers in the area through Tuesday. Expect winds to generally be strong and southwesterly through northwesterly through Tuesday.

PWS Meteogram for April 24th TS
Trace of Temperature, Dew Point and RH (Top), Wind Speed, Wind Gust, and Wind Direction (2nd from Top), Temperature, Inside Temperature, Heat Index, Wind Chill (2nd from Bottom), Pressure, rainfall, and rain rate (Bottom). Around 5:45am winds jump from ~5mph to ~ 25mph with gusts as high as 37mph, temperature falls from ~70F to ~62F, and rain rate spikes to over 5 inches/hr, if only briefly.

March ends Dry

With several notable cold snaps and warm spells, March temperatures balanced out to be near normal. However, lengthy dry spells in between meager rain events led to a much drier than normal month. The Bermuda Weather Service measured 2.87″ of rain for the month, with mean monthly temperature of 64.8F. The period of record (POR*: 1949-2014) normal March rainfall is 4.31″ with a median of 4.20″; the 1981-2010 climate period normal March rainfall is 4.69″. The period of record normal mean March temperature is 64.3F, and the climate normal mean March temperature is 64.8F.

April and May are typically the two driest months for Bermuda with POR normal (and median) precipitation of 3.48″ (2.87″) and 3.44″ (3.03″) respectively. The spring transition from wintertime frontal rains to summertime rains from tropical/easterly waves often features extended dry periods where dry Canadian high pressure settles over the Western Atlantic. February’s more than 9″ of rain was roughly two months worth of precipitation and should somewhat make up for the less than normal March rains, and we are indeed still running more than 1.50″ above normal year-to-date precipitation. Remember to conserve water!

* POR has missing data between 1968-1970 for March.