Monthly Archives: August 2017

Tropical Depression Eleven-E Forms West of Mexico

Tropical Depression Eleven-E formed west of Mexico on Friday.  At 11:00 a.m. EDT on Friday the center of Tropical Depression Eleven-E was located at latitude 17.7°N and longitude 109.2°W which put it about 335 miles (535 km) west-southwest of Manzanillo, Mexico.  The depression was moving toward the west-northwest at 15 m.p.h. (24 km/h).  The maximum sustained wind speed was 30 m.p.h. (50 km/h) and there were wind gusts to 40 m.p.h. (65 km/h).  The minimum surface pressure was 1009 mb.

The circulation of Tropical Depression Eleven-E is not particularly well organized.  The depression does have a distinct center of circulation which can be seen on visible satellite images.  Most of the showers and thunderstorms are occurring in the western half of the circulation.  The rainbands east of the center of circulation are thin and consist mainly of lighter showers.  The thunderstorms west of the center of circulation are generating only a little upper level divergence which is carrying mass to the west of the depression.

Tropical Depression Eleven-E is in an environment that is only marginally favorable for intensification.  The depression is moving over water where the Sea Surface Temperature is near 28.5°C.  So, there is sufficient energy in the upper ocean to support intensification.  However, an upper level ridge centered over northern Mexico is producing strong northeasterly winds which are blowing across the top of the depression.  Those winds are causing significant vertical wind shear.  The shear is probably the reason for the asymmetrical pattern with all of the thunderstorms in the western half of the circulation.  The shear is forecast to continue and if it does, there will be little or no intensification.  If the shear is less, then some intensification could occur.

Tropical Depression Eleven-E is being steered to the west-northwest by the ridge to its north.  A general west-northwesterly motion is forecast to continue for the next several days.  If the vertical wind shear increases and the depression weakens to a low level system, then winds closer to the surface would steer it more toward the west.  On its anticipated track the depression is expected to move away from Mexico.

Typhoon Noru Threatens Southwestern Japan

Typhoon Noru posed an increased threat to southwestern Japan as it slowly moved closer on Thursday.  At 11:00 p.m. EDT on Thursday the center of Typhoon Noru was located at latitude 28.6°N and longitude 131.3°E which put it about 145 miles (235 km) south-southeast of Kagoshima, Japan.  Noru was moving toward the west at 8 m.p.h. (13 km/h).  The maximum sustained wind speed was 85 m.p.h. (135 km/h) and there were gusts to 105 m.p.h. (170 km/h).  The minimum surface pressure was 962 mb.

Although Noru is still a powerful and dangerous typhoon, it is not quite as well organized as it was several days ago.  A very large eye with a diameter of 95 miles (150 km) is at the center of circulation.  A broken ring of thunderstorms surrounds the eye.  There are breaks on both the eastern and western sides of the eyewall.  A broad rainband wraps around the southern and eastern sides of the circulation.  Thunderstorms in the core of Noru are generating strong upper level divergence which is pumping mass away from the typhoon.  There are few showers and thunderstorms in the northwestern quadrant and there may be drier air in that part of Typhoon Noru.

Winds to typhoon force extend out about 40 miles (65 km/h) from the center of circulation.  Winds to tropical storm force extend out about 185 miles (295 km/h) from the center.  The Hurricane Intensity Index (HII) for Typhoon Noru is 12.7.  The Hurricane Size Index (HSI) is 16.4 and the Hurricane Wind Intensity Size Index (HWISI) is 29.1

Typhoon Noru will move through an environment that may be somewhat favorable for intensification.  Noru will move over water where the Sea Surface Temperature is near 30°C.  An upper level ridge northwest of Noru is producing northeasterly winds which are blowing toward the top of the typhoon.  Those winds are creating moderate vertical wind shear.  The typhoon could also pull in drier air on the northwestern side of the circulation.  Even though the environmental factors are mixed, some intensification may be possible during the next 24 to 48 hours.

Typhoon Noru is approaching the western end of a subtropical ridge which has been steering the typhoon toward the west.  Noru is forecast to slow as the steering currents weaken.  Typhoon Noru is forecast to turn slowly northward during the next 24 hours.  In about 48 hours westerly winds in the middle latitudes will begin to carry the typhoon more quickly toward the northeast.  On its anticipated track Typhoon Noru could approach the northern Ryukyu Islands on Friday.  It could bring strong winds to Amami-O-Shima.  The center could pass near Yaku-Shima and Tanega-Shima during the weekend.  In a little over two days Typhoon Noru could reach southwestern Kyushu.  Typhoon Noru has the potential to bring strong winds and heavy rain to parts of southwestern Japan during the next few days.  The heavy rain could also cause flash floods.

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.

Typhoon Noru Turns Toward Southwestern Japan

Typhoon Noru turned toward southwestern Japan on Tuesday.  At 11:00 p.m. EDT on Tuesday the center of Typhoon Noru was located at latitude 25.6°N and longitude 136.2°E which put it about 525 miles (850 km) south-southeast of Miyazaki, Japan.  Noru was moving toward the northwest at 6 m.p.h. (10 km/h).  The maximum sustained wind speed was 120 m.p.h. (195 km/h) and there were wind gusts to 150 m.p.h. (240 km/h).  The minimum surface pressure was 942 mb.

Typhoon Noru has a very symmetrical circulation that looks similar to what are sometimes called annular hurricanes.  There is a large circular eye at the center of Typhoon Noru.  The eye has a diameter of 35 miles (55 km).  The eye is surrounded by a wide ring of strong thunderstorms.  There are few bands of showers and thunderstorms outside the core of the typhoon.  The thunderstorms in the core of Noru are generating strong upper level divergence which is pumping mass away from the typhoon.

Typhoon Noru will be moving through a favorable environment during the next several days.  Noru will move over water where the Sea Surface Temperature is near 30°C.  The upper level winds are weak and there is little vertical wind shear.  Typhoons and hurricanes that have an annular structure like that of Typhoon Noru tend to maintain a fairly steady intensity.  Given the favorable environment, Noru could strengthen during the next few days.

A subtropical ridge is steering Typhoon Noru slowly toward the northwest.  The ridge is forecast to strengthen and to steer Noru more toward the west-northwest during the next several days.  On its anticipated track Typhoon Noru could approach southwestern Japan in about three days.