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Tropical Depressions Form South of Key West & Southeast of Cape Hatteras

Two tropical depressions formed near the U.S. on Sunday.  Tropical Depression Nine formed south of Key West, Florida and Tropical Depression Eight formed southeast of Cape Hatteras, North Carolina.  Both depressions could have an impact on the U.S., but Tropical Depression Nine could pose a greater risk to the southeastern U.S.

At 5:00 p.m. EDT on Sunday the center of Tropical Depression Nine was located at latitude 23.7°N and longitude 81.7°W which put it about 60 miles (95 km) south of Key West, Florida.  It was moving toward the west at 9 m.p.h. (15 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 1009 mb.

At 5:00 p.m. EDT on Sunday the center of Tropical Depression Eight was located at latitude 31.8°N and longitude 70.9°W which put it about 355 miles (570 km) southeast of Cape Hatteras, North Carolina.  It was moving toward the west at 9 m.p.h. (15 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 1010 mb.

The circulation of Tropical Depression Nine is still organizing.  A NOAA plane found a distinct center of circulation, but most of the stronger thunderstorms are south and east of the center.  There is a broader counterclockwise rotation with numerous showers and thunderstorms indicated by the National Weather Service radar in Key West.  An upper level low east of Florida is contributing to easterly winds that are blowing across the northern side of the depression.  The vertical wind shear is inhibiting the development of thunderstorms north and west of the center.

The circulation of Tropical Depression Eight was more well organized earlier today.  Strong easterly winds from the same upper level low that is affecting Tropical Depression Nine are creating significant vertical wind shear.  Those winds and the shear they caused blew the upper portion of the depression’s circulation west of the low level center of circulation.  The low level circulation is presently exposed as seen on visible satellite imagery and by an Air Force Reserve Hurricane Hunter aircraft.

The intensity forecast for Tropical Depression Nine is challenging because it is moving through a complex environment.  The depression is moving over water where the Sea Surface Temperature is near 31°C.  So, there is plenty of energy in the upper ocean.  However, as mentioned above, an upper low is causing vertical wind shear which will inhibit intensification.  Tropical Depression Nine could move into a region where there is less shear when it moves over the Gulf of Mexico.  Tropical Depression Nine has a good chance to intensify into a tropical storm, but it is unclear if it could become a hurricane before it reaches the coast.

There is also significant uncertainty in the track forecast for Tropical Depression Nine.  A ridge over the southeastern U.S. is steering the depression toward the west and that general motion should continue for several more days.  The ridge is forecast to weaken during the middle of the week and that should allow Tropical Depression Nine to turn toward the north.  The timing of that turn and how sharp it will be are still uncertain.

The upper low is expected to continue to generate wind shear over Tropical Depression Eight.  If the upper levels winds remain as strong as they are now, the depression will slowly weaken.  If the upper level winds weaken, then Tropical Depression Eight could strengthen into a minimal tropical storm.  The upper level low is steering the depression west and that general motion is expected to continue in the short term.  The depression could stall just off the coast of North Carolina.

Tropical Depression Nine could intensify and bring wind and rain to the Gulf Coast later this week.  The Gulf Coast is also vulnerable to storm surges.  Given the uncertainty in both the track and intensity, interests along the coast should monitor official sources of information for updates on Tropical Depression Nine.  The primary effects of Tropical Depression Eight are likely to be higher than normal surf, rip currents and some beach erosion along the North Carolina coast.  People along the Mid-Atlantic coast should pay attention to the depression in case the forecast changes.

Disturbance Near Yucatan Could Develop Over Gulf of Mexico

A tropical disturbance over the northwestern Caribbean Sea near the Yucatan peninsula is moving northwest and it could develop into a tropical cyclone when it moves over the southern Gulf of Mexico.  The National Hurricane Center (NHC) has designated the disturbance as Invest 93L and NHC is putting the probability of development into a tropical cyclone during the next two days at 80%.  At 8:00 p.m. EDT the center of Invest 93L was located at latitude 18.5°N and longitude 87.5°W which put it about 170 miles (270 km) south of Cancun, Mexico.  Invest 93L was moving toward the northwest at 11 m.p.h. (17 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 1005 mb.

Invest 93L consists of a broad area of low pressure at the surface.  Several bands of thunderstorms are occurring in the eastern and northern parts of the circulation.  There is no well defined center of circulation and there are fewer thunderstorms in the inner portion of the circulation.  The strongest winds are occurring in the bands of thunderstorms north and east of the center.  There is some upper level divergence to the east of Invest 93L.

The environment around Invest 93L is not totally favorable for development.  The Sea Surface Temperature in northwest Caribbean Sea is near 30°C, which means there is plenty of energy in the upper ocean.  However, an upper level low near Texas and an upper level ridge east of Invest 93L are combining to generate strong southwesterly winds that are blowing over the top of the disturbance.  Those winds are creating significant vertical wind shear which is inhibiting the development of Invest 93L.  The upper level ridge is enhancing the upper level divergence to the east of the disturbance which allowed the pressure to fall several millibars on Saturday.  The upper winds are lighter near the axis of the ridge which is why the stronger bands of thunderstorms are persisting in that area.

Invest 93L is forecast to move toward the Yucatan peninsula on Sunday.  If the broad area over low pressure moves over land that will prevent it from developing until it moves back over water.  Numerical models are guiding that the disturbance will develop into a tropical cyclone when the area of low pressure moves over the southern Gulf of Mexico.  The upper level ridge is forecast to extend over the southeastern Gulf and if that happens the wind shear could decrease enough to allow development to occur.  Invest 93L could become a tropical depression or a tropical storm over the Gulf of Mexico.

The southwesterly flow over the Gulf of Mexico is forecast to turn Invest 93L toward the northeast.  On its anticipated track the system could reach the west coast of Florida near Cedar Key late Monday or early Tuesday.  The primary risk is likely to be locally heavy rain.  However, the coast in that area is very susceptible to storm surge and there could be some rise in the water level.  The National Hurricane Center has tentatively tasked a reconnaissance plane to investigate Invest 93L on Sunday, if necessary.

Tropical Storm Bills Forms Near Texas

The National Hurricane Center (NHC) classified a low pressure system over the western Gulf of Mexico as Tropical Storm Bill.  At 11:00 p.m. EDT on Monday the center of Tropical Storm Bill was located at latitude 27.1°N and longitude 94.2°W which put it about 160 miles (260 km) east-southeast of Port OConnor, Texas and about 155 miles (250 km) south-southeast of Galveston, Texas.  Bill was moving toward the northwest at 12 m.p.h. (19 km/h).  The maximum sustained wind speed was 50 m.p.h. (85 km/h) and there were wind gusts to 65 m.p.h. (105 km/h).  The minimum surface pressure was 1005 mb.  A Tropical Storm Warning has been issued for the portion of the coast from Baffin Bay to High Island, Texas.

Bill formed as a result of the interaction between a broad surface low pressure system and an upper level low over the Gulf of Mexico.  Thunderstorms began to form near the center of circulation on Monday afternoon and the structure of the system changed to a more tropical cyclone like pattern.  The thunderstorms near the center began to warm the middle and upper troposphere and create a warm core.  The thunderstorms became substantial enough to generate upper level divergence from the core of the circulation and NHC classified it as Tropical Storm Bill.  Bill is moving over water where the Sea Surface Temperature (SST) is near 29°C.  The vertical wind shear over the system decreased on Monday and Bill has a few hours during which it could intensify before it makes landfall in Texas.

An upper level ridge over the southeastern U.S. is steering Bill northwestward toward the coast of Texas and that motion is expected to continue.  Bill is expected to make landfall in Texas on Tuesday.  Although Bill will bring some wind when it makes landfall, heavy rainfall will be the biggest hazard.  Heavy rain on top of wet soils will create the potential for new flooding in some locations, especially in Texas and Oklahoma.  Winds blowing toward the coast will also create some water level rises and Coastal Flood Warnings and Advisories have been issued for portions of the coast of Texas and Louisiana.

Low Pressure Organizing Over the Southern Gulf of Mexico

A complex low pressure system over the southern Gulf of Mexico is showing signs of greater organization.  A reconnaissance aircraft investigated the system on Sunday afternoon and found evidence of a broad area of low pressure at the surface and winds to tropical storm force northeast of the low.  The official designation for this system is Invest 91L.  At 11:00 p.m. EDT on Sunday night the approximate center of the broad low was located at latitude 22.7°N and longitude 90.2°W which put it about 540 miles (870 km) south-southeast of Galveston, Texas and about 540 miles (870 km) east-southeast of Brownsville, Texas.  The low was moving toward the northwest at 14 m.p.h. (23 km/h).  The maximum sustained wind speed was 40 m.p.h. (65 km/h) and there were gusts to 50 m.p.h. (80 km/h).  The minimum surface pressure was 1007 mb.

Invest 91L is a complex system which includes an upper level low over the southern Gulf of Mexico and a broad area of low pressure at the surface.  Clusters of thunderstorms are forming intermittently east and north of the center of circulation.  The pressure gradient between a strong high pressure system centered over the Atlantic Ocean and the developing surface low is causing the strongest winds to be found in the northeastern part of Invest 91L.  The upper level low is causing the system is to exhibit a more subtropical mode of formation rather a typical tropical genesis.  However, more thunderstorms are beginning to develop closer to the center of circulation and the National Hurricane Center is giving Invest 91L an 80% chance of becoming a tropical cyclone.  The low is moving over water where the Sea Surface Temperature (SST) is around 27°C to 28°C, which is warm enough to support the development of a tropical cyclone.  The upper level low was generating vertical wind shear, but if the surface low develops under the center of the upper low, then the wind shear will have less of an effect.  If more thunderstorms form near the center of circulation and a warm core develops, then the low could be classified as a tropical storm on Monday.  Some intensification is possible because of the warm SSTs.

An upper level ridge centered over the southeastern U.S. is expected to steer the system toward the northwest and it could make landfall on the coast of Texas in 24 to 48 hours.  The primary risk will be locally heavy rainfall, although a minor storm surge could occur in some coastal locations.

Possible Tropical Development in Western Gulf of Mexico

An area of showers and thunderstorms near the Yucatan Peninsula has shown signs of a cyclonic circulation, although the rotation may be in the middle troposphere and not at the surface.  The formal designation of this system is Invest 91L.  At 2:00 p.m EDT on Saturday the center of Invest 91L was located at latitude 20.0°N and longitude 88.4°W which put it about 100 miles (165 km) southeast of Merida, Mexico and about 760 miles (1220 km) south-southeast of Galveston, Texas.  Invest 91L was moving toward the northwest at 5 m.p.h. (8 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 1009 mb.  The National Hurricane Center is now giving Invest 91L a 50% chance of developing into a tropical depression or tropical storm in the next five days.  An additional reconnaissance plane has been scheduled to investigate it on Sunday afternoon.

The circulation of Invest 91L is not very well organized.  Almost all of the showers and thunderstorms are occurring in the eastern half of the circulation.  Visible satellite images indicate a possible center of cyclonic rotation over the Yucatan peninsula, but it is not clear if the rotation extends all of the way down to the surface.  Counterclockwise winds around an upper level low over the Gulf of Mexico are generating significant vertical wind shear over Invest 91L.  However, the upper level winds east of the upper low are diverging, which is enhancing rising motion and is supporting the development of the showers and thunderstorms in that area.  The upper level divergence is also causing the surface pressure to decrease slowly, which could lead to the formation of a surface center of low pressure.

When Invest 91L moves into the southern Gulf of Mexico, it will move over water where the Sea Surface Temperature is near 27°C to 28°C.  So, there is sufficient energy in the upper ocean to support the development of a tropical cyclone.  The key factor will be the interaction between the upper low and any surface center that forms.  If the surface center forms too close to the upper low, then vertical wind shear will likely inhibit development and the system will remain fairly weak.  If the surface low develops a little farther southeast of the upper low, then there will be less shear and more upper level divergence.  That could allow for more intensification.

Invest 91L is expected to move with the upper low as the upper low moves northward.  Most guidance from numerical models moves the system toward the north-northwest toward the Upper Texas Coast.  However, there will be above normal uncertainty in the track until a well defined center of circulation forms at the surface.

Interests in the western Gulf of Mexico should monitor the progress of Invest 91L.