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Tropical Update | Invest 99L Tagged – Gulf Coast Storm Center


Figure 1. IR satellite image of the North Atlantic from 12:00 UTC 9/21/18 | Credit: CIMSS

There are still four systems being monitored for development across the Atlantic Basin: Invests 97L, 98L, and 99L, and a non-tropical area of low pressure that will soon form over the far North Atlantic between Bermuda and the Azores.


Invest 99L: The wave being monitored over West Africa has emerged in Atlantic, and is now invest 99L.

99L is already showing some signs of organization. Vorticity has been on the increase since “splash down.” Convective activity has also been persistent.

Overall, gradual development/organization appears likely. The Saharan Air Layer (SAL) is well to the north of 99L. There is also a pocket of low shear along the westward path 99L will take. The GFS and ECMWF continue to only forecast some weak development near the Lesser Antilles Tuesday/Wednesday, while the UKMET is consistent with 99L becoming a moderate to strong tropical storm early next week.

There continues to be some support for development from the ECMWF ensembles (EPS). There is now support from the GFS ensembles (GEFS) too.

In their 8 AM EDT tropical weather outlook (TWO), the NHC gave 99L a medium chance (60%) of developing sometime over the next 5 days, saying, ” a tropical depression could form early next week.”

note: It is too early to say what will happen with 99L beyond the middle of next week.


Invest 98L: Convective activity has decreased significantly since yesterday. This is due to a combination of  dry air and increasing shear. Because of the aforementioned unfavorable conditions, there likely won’t be any tropical/subtropical development for at least the next 3 – 4 days. By early to mid next week conditions could become more conducive. Most of the model guidance is currently showing weak development.

Figure 2. Track guidance for 98L from 12:00 UTC 9/21/18 | Credit: NCAR

98L will make a anticyclonic loop (see figure 2). There is potential for a track fairly close to the East Coast mid week next week; t is too early to know how close, though. Should we get a tropical/subtropical cyclone from 98L, it should remain rather weak.

The NHC placed the chance of development at 20% over the next 5 days in the latest TWO.


Invest 97L: Westerly shear has increased, meaning the chance of getting anything to develop from 97L is very near zero. The impact on the system can already be seen on satellite, as the low-level center is now racing ahead of the convection.

Figure 3. Visible satellite image of 97L from 13:50 UTC 9/21/18 | Credit: Tropical Tidbits

None of the model guidance predicts development of 97L in the short-term or long-term.

In the latest TWO, the NHC gave 97L a 10% chance of developing over both the next 2 and 5 days.


Area of low pressure expected to form between Bermuda and the Azores: This will be brief, as there doesn’t appear to be any threat to land.

A non-tropical area of low pressure is going to form midway between Bermuda and the Azores tonight. Environmental conditions are going to be conducive for this low to develop. The ECMWF, GFS, and UKMET support development. A tropical/subtropical cyclone is likely to form this weekend or early next week.

The NHC placed the odds of formation at 70% over the next 5 day in their 8 AM EDT TWO.


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The post Tropical Update | Invest 99L Tagged appeared first on Gulf Coast Storm Center.



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