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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.