The Push North and the Wave Belt – Tropics Tamer to End September

Valid September 28, 2017 at 9:00 AM CT

Note: Last week’s blog described Lee as having little chance of re-development, in line with the slim chances the National Hurricane Center gave for the storm to reorganize. To the surprise of many, Lee did re-intensify, and into an even stronger storm. Perhaps more caution is necessary considering the uncertainty of the Atlantic basin this season.

There has been at least one named storm active in the Atlantic hurricane basin since the beginning of August. The month of September is just two days away from ending with an active hurricane on each day. But could the tropics be finally quieting down some heading past the peak and into the final home stretch of the season.

A Maria and Lee Marriage?

Tropical Storm Maria and Hurricane Lee continue to churn over the Atlantic Ocean, but a change and close encounter are in the forecast for these two long-lived systems. Maria, once a powerful major hurricane that destroyed much of the infrastructure in Puerto Rico, has been degraded to a tropical storm off the North Carolina coast. Lee, meanwhile, remains a Category 2 hurricane over the open Atlantic, posing no threat to land in its current state.

National Hurricane Center - Google Chrome 2017-09-28 09.02.04

Figure 1 – Maria and Lee continue to be the two active storms over the Atlantic basin, but pose little threat to the United States.

Both storms continue to push north to northeasterly, but are expected to be influenced by a strong cold front coming off the east coast of the United States. The front will help turn Maria and Lee to the east, pushing them further away from land. Being picked up by this front should also help both storms weaken significantly over the north Atlantic, as suggested by the HWRF and GFS models. Both may still hold tropical storm strength through the weekend, but should become extratropical by the end of the day Sunday.


Figure 2 – NOAA’s National Forecasting Chart showing the location of a strong cold front, which should help Maria and Lee turn further out to sea.

As Maria and Lee continue to weaken and become extratropical over the north Atlantic, expect the moisture to remain with these storms. While total precipitable water levels have dropped as both storms weakened, amounts remain particularly strong near the center. The remnants of these systems are expected to bring rain to the United Kingdom and Ireland to begin the week, potentially arriving in time for the second NFL London game of the year.


Figure 3 – Total precipitable water across the Atlantic basin, showing high levels of TPW still near the centers of Maria and Lee.

Future Development Sites

The National Hurricane Center only identifies one area of potential development, with a disturbance just south of Cuba with a twenty percent chance of intensifying in the next 48 hours. Those chances increase to 40 percent in the next five days. However, there are three tropical waves identified on the latest surface map across the Atlantic basin.


Figure 4 – Surface map of the western hemisphere showing three tropical waves, along with Maria and Lee.

The tropical wave with likely the least chance of development sits very close to the Yucatan Peninsula of Mexico. With its close proximity to land and the prevailing pattern for this wave to track westerly over land, it will more than likely be torn apart. While sea surface temperatures remain favorable over the waters around the peninsula, wind shear remains neutral to unfavorable for development.

The remaining two waves will be worth watching over the next few days for any potential development. The one closest to the Lesser Antilles sets up in a ripe spot for development, very close to areas where Irma and Maria both strengthened. It will also be interesting to watch if the wave closest to the African coast will be influenced at all by an outbreak of the Saharan Air Layer.


Figure 5 – Zonal wind anomalies (top) and velocity potential anomalies (bottom) from August 29 to October 3.

The Kelvin-filtered 200 hPa velocity potential and zonal wind anomalies are consistent with the eastward propagation of Kelvin waves. Anomalously elevated levels of velocity potential and zonal wind are nearly collocated from September 3 to September 23. The eastward propagation is then validated by the lower levels of each to the west.

Other Tropical Tidbits

There is an area of investigation in the western Indian Ocean, just northeast of the island of Madagascar. No activity is expected in the next 48 hours over the eastern Pacific, or the neighboring central Pacific. Ditto for the western Pacific, with the Japanese Meteorological Agency having no storms of tropical storm strength or greater. Finally, our thoughts remain with the people of Puerto Rico, as a humanitarian crisis not seen in a U.S. state or territory since Katrina is playing out.

The tropical update blog is part of a weekly assignment for Dr. Kim Wood’s Tropical Meteorology class at Mississippi State University. Post created for academic purposes.


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