Tag Archives: 90L

NHC Monitoring Two Areas for Tropical Development

The National Hurricane Center (NHC) was monitoring two areas for possible tropical development on Thursday afternoon.  A strong tropical wave was moving through the southeastern Caribbean Sea and the wave was designated as Invest 90L.  At 2:00 p.m. EDT on Thursday the center of Invest 90L was located at latitude 12.2°N and longitude 65.8°W which put it about 160 miles (260 km) east of Bonaire.  The tropical wave was moving toward the west at 18 m.p.h. (29 km/h).  The maximum sustained wind speed was 35 m.p.h. (55km/h) and there were gusts to 45 m.p.h. (75 km/h).  The minimum surface pressure was 1011 mb.

Another tropical wave moved off the coast of West Africa on Thursday and NHC designated that wave as Invest 99L.  At 2:00 p.m. EDT on Thursday the center of Invest 99L was located at latitude 10.2°N and longitude 20.9°W which put it about 370 miles (595 km) south-southeast of the Cabo Verde Islands.  The tropical wave was moving toward the west at 18 m.p.h. (29 km/h).  The maximum sustained wind speed was 30 m.p.h. (50 km/h) and there were gusts to 40 m.p.h. (65 km/h).  The minimum surface pressure was 1010 mb.

The tropical wave over the southeastern Caribbean Sea will have the most immediate impact to land.  The wave could bring gusty winds to Aruba, Bonaire and Curacao within 24 hours.  The circulation of Invest 90L is elongated in an east-west direction and that is probably because of strong easterly winds blowing in the lower atmosphere.  There are some indications of a counterclockwise rotation on loops of visible satellite imagery, but it is not clear if the rotation extends all the way down to the surface.  The tropical wave is generating winds to near tropical storm force in the northern portion of the wave.  There are numerous thunderstorms developing along the axis of the wave.

Invest 90L is moving under the western end of an upper level ridge.  The ridge is producing southerly winds which are blowing over the top of the tropical wave and those winds are contributing to moderate vertical wind shear.  The strong easterly winds in the lower levels are also contributing to the shear.  Invest 90L could develop into a tropical cyclone when it moves farther west.  The shear could diminish and the Sea Surface Temperatures in the western Caribbean Sea and Bay of Campeche are very warm.  NHC is indicating that there is a 20% probability Invest 90L will become a tropical cyclone during the next five days.

The tropical wave over the far eastern Atlantic has a better chance of developing into a tropical cyclone.  Invest 99L is moving over water where the Sea Surface Temperature is near 28°C.  It is moving under the eastern end of an upper level ridge.  The ridge is causing northeasterly winds which are blowing toward the top of the tropical wave.  Those winds are producing moderate vertical wind shear.  When the wave moves farther west, it will move under weaker winds and the wind shear will decrease.  NHC is indicating that there is a 70% probability that Invest 99L will become a tropical cyclone during the next five days.

Low Pressure Develops East of the Bahamas

An area of low pressure developed east of the Bahamas and the system was designated as Invest 90L on Sunday afternoon.  At 5:00 p.m. EDT on Sunday the center of Invest 90L was located at latitude 24.0°N and longitude 71.0°W which put it about 260 miles (420 km) east of the Bahamas.  Invest 90L was moving toward the northwest at 11 m.p.h. (17 km/h).  The maximum sustained wind speed was 40 m.p.h. (65 km/h) and there were wind gusts to 50 m.p.h. (80 km/h).  The minimum surface pressure was 1009 mb.

The circulation of Invest 90L consists of a large asymmetrical low pressure system.  Most of the showers and thunderstorms are occurring northeast of the center of circulation.  Those showers and thunderstorms are being generated by convergence of winds from a large high pressure system over the north Atlantic Ocean into the area of low pressure.  A swirl of low clouds has emerged on the southwestern side of the low, but it is unclear if this is the actual center of circulation or is just a transient mesoscale feature.  A strong pressure gradient between the high pressure system and the low is producing an area of winds to tropical storm force northeast of the center of the low.  The winds are weaker in other parts of the circulation.

Invest 90L is in an environment that is mostly unfavorable for tropical cyclones.  It is moving over water where the Sea Surface Temperature is near 25°C.  So, there is less energy in the upper ocean to support tropical development.  An upper level trough over the southeastern U.S. is producing strong southwesterly winds which are blowing across the top of Invest 90L.  Those winds are producing strong vertical wind shear.  Marginal Sea Surface Temperatures and strong vertical wind shear make the classical development of a tropical cyclone unlikely.  However, the temperature in the upper troposphere is also cold and there may be enough instability in the atmosphere to produce thunderstorms.  If the wind shear decrease, then more thunderstorms could develop closer to the center of circulation and a subtropical cyclone could form.

The high over the north Atlantic is blocking northward movement of Invest 90L and the high is steering it toward the northwest.  A general northwesterly motion is expected to continue for another day or so.  Eventually the high will move off to the east and Invest 90L will start to move toward the northeast.  On its anticipated track Invest 90L will stay east of the U.S. and the Bahamas.  Invest 90L could pass close to Bermuda and it has the potential to bring gusty winds.

Invest 90L Organizing North of Panama

The structure of the low pressure system designated as Invest 90L became better organized on Saturday over the southwestern Caribbean Sea north of Panama.  At 7:00 p.m. EDT on Saturday the center of Invest 90L was located at latitude 10.8°N and longitude 80.9°W which put it about 220 miles (355 km) east-southeast of Bluefields, Nicaragua.  Invest 90L was moving toward the east at 3 m.p.h. (5 km/h).  The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (70 km/h).  The minimum surface pressure was 1006 mb.

More thunderstorms developed closer to the center of circulation on Saturday.  A band of stronger thunderstorms formed south of the center and a second band of storms developed northeast of the center.  Other rainbands appeared to be forming in other areas of the circulation.  Storms in the band northeast of the center began to produce upper level divergence that was pumping mass away.  An improved convective structure could be a indication that Invest 90L is developing into tropical depression.

The atmospheric environment became more favorable for tropical development on Saturday.  Upper level southwesterly winds which were blowing across the top of the low pressure system weakened on Saturday  and the vertical wind shear lessened.  Less shear allowed more thunderstorms to develop and persist.  Since Invest 90L has meandered over the same part of the southwestern Caribbean Sea for several days, it has mixed slightly cooler water to the surface.  However, the water in that part of the Caribbean Sea is very warm and Invest 90L is moving over water where the Sea Surface Temperature is still near 29°C.  The environment is favorable for development of a tropical cyclone and Invest 90L could become a tropical depression or a tropical storm at any time.

Invest 90L is located underneath an upper level ridge and the steering currents are weak.  The low pressure system drifted toward the east on Saturday and some models are suggesting that Invest 90L could make a slow counterclockwise loop over the southwestern Caribbean Sea during the next few days.

Invest 90L Meandering Over Southwest Caribbean Sea

An area of low pressure designated as Invest 90L has meandered over the southwestern Caribbean Sea during the past several days.  At 7:00 p.m. EST on Thursday the center of Invest 90L was located at latitude 12.3°N and longitude 81.3°W which put it about 170 miles (275 km) east of Bluefields, Nicaragua.  Invest 90L was moving toward the west at 3 m.p.h. (5 km/h).  The maximum sustained wind speed was 25 m.p.h. (40 km/h) and there were wind gusts to 35 m.p.h. (55 km/h).  The minimum surface pressure was 1006 mb.

Invest 90L consists of a broad area of low pressure over the southwestern Caribbean Sea.  There is a definite area of counterclockwise rotation, but the is no well defined core at the center of the low.  There is a swirl of low clouds and showers at the center of Invest 90L, but there are no thunderstorms near the center.  Thunderstorms form and dissipate in bands around the periphery of the circulation.

The environment around the southwestern Caribbean Sea is not as favorable for the development of a tropical cyclone as it was earlier this week.  Invest 90L is moving over water where the Sea Surface Temperature is near 29°C.  However, northerly winds blowing across the Gulf of Mexico have transported drier air over the northwestern Caribbean Sea.  The drier air has not reached Invest 90L, but the drier air is just to the northwest of the low pressure system.  An upper level trough over the Gulf of Mexico is producing southwesterly winds which are blowing over the top of Invest 90L.  Those winds are generating moderate vertical wind shear which is inhibiting the organization of the low pressure system.  Even with the inhibiting factors the National Hurricane Center is indicating that the probability of the formation of a tropical cyclone during the next five days is 70%.

The fact that there is only a low level circulation means that Invest 90L is being steering by winds closer to the surface.  The winds in the lower levels have been pushing the low pressure system very slowly toward the west.  Guidance from numerical models suggests that Invest 90L could meander over the southwestern Caribbean Sea for several more days.

Subtropical Storm Alex Transitions to Hurricane, Threatens the Azores

The structure of Subtropical Storm Alex evolved into the structure associated with a tropical cyclone and the National Hurricane Center reclassified it as Hurricane Alex on Thursday morning.  At 10:00 a.m. EST on Thursday the center of Hurricane Alex was located at latitude 31.5°N and longitude 28.4°W which put it about 490 miles (790 km) south of Faial in the Central Azores.  Alex was moving toward the north at 20 m.p.h. (32 km/h).  The maximum sustained wind speed was 85 m.p.h. (140 km/h) and there were wind gusts to 105 m.p.h. (170 km/h).  The minimum surface pressure was 981 mb.

A Hurricane Warning has been issued for Faial, Pico, Sao Jorge, Graciosa, and Terceira in the Central Azores.  A Tropical Storm Warning has been issued for Sao Miguel and Santa Maria in the Eastern Azores.

Alex has developed the structure associated with a small hurricane.  An eye is clearly visible on satellite imagery.  A ring of thunderstorms surrounds the eye and there are spiral bands rotating around the core of the circulation.  Latent energy released by convection around the core of Hurricane Alex generated a warm core in the middle and upper troposphere.  That convection is also generating upper level divergence which is pumping out mass, especially to the north and east of the center.

Hurricane Alex is over water where the Sea Surface Temperatures (SSTs) are near 20.5°C.  Those SSTs are usually considered to be too cold to supply enough energy to promote the development of a hurricane.  However, it is January and the temperatures in the middle and upper troposphere are also cold.  So, there is clearly enough instability to generate convection and create a hurricane.  The thunderstorms are in Alex are not as tall as they would be if the SSTs are warmer.  The shorter thunderstorms do not extend into the stronger winds in the upper troposphere and Hurricane Alex is not experiencing as much vertical wind shear as might be expected.   The combination of more instability and less vertical wind shear allowed Alex to transition from a subtropical storm to a hurricane.

The upper level divergence could allow Alex to intensify a little more during the next 12 hours.  Hurricane Alex will move over SSTs that are even colder and at some point the structure of Alex will change again.  It will develop fronts and transition into an extratropical cyclone.

Alex is being steered northward by an upper level trough to its west and that general motion is expected to continue for the next several days.  On its anticipated track Hurricane Alex will move over portions of the Azores on Friday.

The Hurricane Intensity Index (HII) for Alex is 12.7.  The Hurricane Size Index is (HSI) 7.0.  The Hurricane Wind Intensity Size Index (HWISI) is 19.7.  Those indices suggest that Hurricane Alex is capable of causing localized minor damage with isolated areas of serious damage.

Alex is the first Atlantic hurricane to form during the month of January since 1938.  Alex is the first tropical cyclone be a hurricane in January since Hurricane Alice in 1955.

Subtropical Storm Alex Develops over East Atlantic

A low pressure system over the eastern Atlantic Ocean has been classified as Subtropical Storm Alex by the National Hurricane Center.  At 4:00 p.m. EST on Wednesday the center of Subtropical Storm Alex was located at latitude 27.1°N and longitude 30.8°W which put it about 785 miles (1260 km) south-southwest of the Azores.  Alex was moving toward the northeast at 14 m.p.h. (23 km/h).  The maximum sustained wind speed was 50 m.p.h. (80 km/h) and there were wind gusts to 65 m.p.h. (105 km/h).  The minimum surface pressure was 990 mb.

Subtropical Storm Alex evolved out of an extratropical cyclone that formed off the southeastern coast of the U.S. late last week.  The extratropical cyclone cycled through the typical stages exhibited by low pressure systems in the middle latitudes as it moved eastward across the central Atlantic Ocean.  It eventually became an occluded extratropical cyclone over that region.  The low pressure system moved south on Tuesday, which placed it over slightly warmer Sea Surface Temperatures (SSTs).  As it began to extract more energy from the warmer ocean, the fronts attached to the low pressure system began to dissipate.  The low began to take on a more circular shape and convection began to intensify around the center of circulation.  A more circular shape with no fronts and taller clouds prompted the National Hurricane Center to classify the low as a subtropical storm.

A subtropical storm is a hybrid low with some characteristics of a tropical cyclone (circular shape and no fronts) and some characteristics of an extratropical cyclone (cooler air aloft).  Subtropical Storm Alex has a well organized low level circulation.  There is a clear area in the center that looks a little like the eye of a hurricane.  The center is surrounded by a ring of taller clouds.  Spirals bands of clouds are rotating around the core of the circulation.  However, the vertical structure is different from a tropical cyclone.  Subtropical Storm Alex is under an upper level trough and the upper level divergence is limited.  Also, the clouds in Alex are not growing as tall as the clouds in a tropical cyclone.

The environment around Subtropical Storm Alex would normally be considered to be unfavorable for intensification of a tropical cyclone.  The SSTs are near 23.5°C.  However, since Alex is near the center of an upper level trough, there is not much vertical wind shear.  So, there could be a chance for some intensification during the next 24 hours.  Subtropical Storm Alex will move over cooler SSTs and it is likely to make a transition back to an extratropical cyclone in two or three days.

The upper level trough is steering Alex toward the northeast and a general northward motion is expected to continue for the next several days.  On its anticipated track Subtropical Storm Alex could approach the Azores in about 36 hours.  It could bring strong winds and locally heavy rain when it moves over the Azores.

Subtropical Storm Alex is the first tropical or subtropical cyclone to form over the North Atlantic Ocean in January since 1978.  It is only the fourth tropical or subtropical cyclone on record to form over the Atlantic Ocean in January since 1851.

Strong Low Spins Up Off Southeast Coast

A strong non-tropical low pressure system spun up off the Southeast Coast of the U.S. on Thursday and there is some possibility that it could eventually develop subtropical or tropical characteristics.   At 4:00 p.m. EST on Thursday the low pressure system was centered near latitude 29.0°N and longitude 74.0°W which put it about 425 miles (685 km) west-southwest of Bermuda.  The low was moving toward the east-northeast at 18 m.p.h. (29 km/h).  The maximum sustained wind speed was 60 m.p.h. (95 km/h) and there were wind gusts to 75 m.p.h. (120 km/h).  The minimum surface pressure was 994 mb.

The low pressure system exhibits the structure of an occluded extratropical cyclone.  An occluded front coils away from the low until it splits into a warm front and a cold front.  Drier air is being pulled around the western and southern sides of the low and a dry slot has formed in the southeastern quadrant of the storm.  However, upper level divergence is pumping mass away from the center of the low and the surface pressure decreased quickly on Thursday afternoon.

The low is in an environment that is not favorable for the classical scenario of formation of a tropical cyclone.  It is moving over water where the Sea Surface Temperature (SST) is near 25°C and strong upper level winds are creating significant vertical wind shear.  However, research has shown that it is possible to get a transition to a subtropical or tropical cyclone over cooler SSTs, when a low pressure system moves into an area where the wind shear is less.

A trough over the southeastern U.S. is steering the low pressure system toward the east-northeast and that general motion is expected to continue during the next day or two.  An upper level ridge is forecast to move north of the low pressure system in a couple of days.  The upper level ridge is expected to steer the low pressure system toward the east-southeast early next week.

If the low pressure system takes the expected track, it could eventually move over slightly warmer SSTs and into an area where there is less vertical wind shear.  The more favorable environment could cause the structure of the low to change into a more subtropical or tropical form.  The National Hurricane Center is giving the low a 10% chance of transitioning into a subtropical cyclone during the next 48 hours and a 30% chance that it makes that transition during the next five days.